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MG MGB Technical - Timing 72B - How to Read Indicator
|I thought I'd start a separate thread, to narrow down the discussion.|
Having dealt with so many topics to get my 72B running better, yesterday I got down to checking timing and distributor.
My 1st test was to check the function of vacuum advance. I removed cap and removed vacuum hose from intake manifold to see if I could suck on the hose hard enough to cause the distributor's vacuum advance to move the plate in distributor. Nothing happens.
To me I think I can blow air to vacuum advance unit as well as suck air from, as I cannot create a vacuum when sucking on hose. Which to me implies the diaphragm is not working in the vacuum housing.
My 2nd test was to reinstall cap, start engine and check timing. Timing was on center mark of indicated 10 degrees @ 600-800 rpm. Disconnecting or connecting vacuum hose had no affect on timing.
Which further confirms that there is no vacuum advance working on distributor.
My 3rd test was to see what effect turning distributor had on idle/smoothness, etc. I turned the distributor counter clockwise 1/4" and engine idle decreased and backfired if giving it throttle.
I turned the distributor clockwise about 1/4" and timing dropped down to 20 degrees. (Turning distributor clockwise moves timing mark counter clockewise.) This also increased idle to around 1500 rpms.
I eventually locked in the timing at 20 degrees around 600-800 rpms.
But if I use my Haynes manual and the sketch of the timing marker (5 points w/each space = 5 degrees), the manual appears backwards. The sketch indicates TDC be at the bottom of marker, and 20 degree advance would be at top, when in reality, the 20 should be at the bottom. Or is this another one of those, "I'm looking at the world upside down" things.
Anyway, setting timing to 20 degrees (from 10) at 600-800 rpms has a very nice affect on engine. Drive yesterday was flawless, no pings, pretty of zip.
I did not check what timing gets to at 3000-4000 rpms, but timing does advance with rpm increase. It is just that the vacuum has no affect on timing.
So it sounds like I need a new vacuum advance unit, which I think run $50-75; car appears to run fine without it. What might I gain with a functioning vacuum advance? Should I leave timing at 20 degrees advance at idle?
|If you can suck (or blow) air through the vacuum advance capsule then the diaphragm is punctured, and will be weakening the mixture as well as not applying advance.|
But if you *can't* suck air through, simply can't move the points plate by sucking (blowing will do nothing in any event), then something is stuck, seized or jammed. You should be able to move the points plate clockwise relative to the rest of the distributor with physical force, but it does take some effort, as does sucking on the tube (maybe I suck more than most ...).
With a carb vacuum source removing the vacuum pipe will have no effect as there is no carb vacuum at idle.
With a manifold vacuum source connecting and disconnecting the vacuum pipe should definitely make a significant different to engine idle speed. If disconnecting the pipe from the inlet manifold and leaving the port open makes no difference at all, then the implication is that either the port on the manifold is blocked, or the diaphragm is punctured. With a working vacuum advance system disconnecting the pipe and blocking the manifold port should cause the idle revs to drop. Disconnecting the pipe and leaving the port open is a little more problematic as the vacuum leak cased by the open port will cause idle revs to increase, which could well counteract the idle reduction caused by the loss of vacuum advance.
With an unblocked port but a punctured diaphragm you have a continuous vacuum leak, and no vacuum advance, hence no change whether it is connected or disconnected.
Turning the distributor anti-clockwise will retard the timing, which generally slows the idle unless over-advanced to begin with. This should *reduce* the amount of timing advance as seen with a timing light on the pointers.
Turning it clockwise advances timing, and should increase the amount of advance as seen with a timing light on the pointers.
Early timing marks were below the pulley, later ones above and angled towards the alternator, but the effect is exactly the same. The Workshop Manual shows the later pointers going up to 20 degrees and equal sized, but I'm pretty sure my 73 only goes up to 5 degrees and has different sized pointers as per the earlier lower ones. I don't understand what you mean by TDC at the bottom and 20 degrees at the top, when you use clockwise and anti-clockwise earlier. Whatever, remember the last pointer the crank pulley groove passes is the TDC mark, the ones it passes before that are *before* TDC - first 20, then 15, then 10, and finally 5 degrees.
Vacuum advance is very beneficial as it improves light-throttle cruising mpg, and sharpens part-throttle acceleration. At the very least you must seal off the manifold port if the diaphragm is punctured or otherwise leaking, and reset the carbs for correct mixture. Adjusting them at idle with a vacuum leak will result in a rich mixture at larger throttle openings.
As to what timing is best it is very much down to trial and error as the original figures related to the fuels available at the time, which are probably very different to what is available now, even if everything to do with the engine, ignition and carburation is to original specifications i.e. no wear or non-standard replacements. Easy on high-compression as you simply run with the most advance you can without pinking at any combination of throttle, revs and load. Not so easy with low compression as too much advance can stall the starter, but still not cause pinking when you do get it running. Just set it to what seems to runs best with the fuel you use and the conditions you use it under.
|Paul Hunt 2010|
|As Paul has stated, the vacuum advance adds to fuel economy and light throttle performance. If your's is not functioning, and it appears that it is not, you can simply plug the port at the carb for now. You can live without vacuum advance, but replacing the vacuum capsule, sometime down the line, is a wise decision. RAY|
|In trying to determine what was and wasn't working, I completely forgot about the implication or complication of their being an apparent vacuum loss at the distributor vacuum advance unit. As evidenced by my sucking on hose connected to unit and getting air not a vacuum.|
Yes I do have vacuum at the hose when it is connected to intake manifold. Yes, idle drops if the hose is left unplugged.
So, while leaving the hose open causes obvious vacuum leak and corresponding idle decrease, the vacuum leak when the hose is connected to vacuum advance unit must be less of a loss of vacuum, as it simply returns engine to the same idle as if hose were disconnected and plugged.
For now I'll disconnect vacuum hose and plug it or plug the port on the intake manifold until vacuum advance is addressed.
This may point to a partial explanation for my bad gas mileage.
But the car sure prefers 20 degrees at idle, with no vacuum advance influence. Oh, and it cranks just fine, and still starts with the slightest tick of the starter. Ran without hiccups on the 20 mile drive to work this morning, and strong hill climb speed/power.
|There is nothing wrong with 20 degrees at idle - as long as max advance doesn't exceed 36 degrees - you will have to check that with a strobe. (Ignore vac advance for this purpose). EXCEPT that it may indicate dizzy wear.|
It depends on the type of advance weights used in the dizzy (they are marked with their advance, ones marked 10 will give 20 degrees of crank advance)but also on the condition of the springs - weak ones will reduce the total advance achieved (because they will be part way up their profile even at idle).
So if your weights are marked 10 then the max static advance should be 16 degrees to give a max of 36
If they are marked 12 then max static advance should be 12 degrees.
If you set the advance statically and then check what you get at the various rpms as per the manual, then you will have a good idea of the condition of your dizzy.
|Chris at Octarine Services|
|Bob, Chris and all -|
"as long as max advance doesn't exceed 36 degrees" might be true for UK cars, but it is not for US ones. I'm pretty tired of hearing "universal maximum numbers" given, that usually are in the 30-35 BTDC range @ 3000=3500. It's a good and safe base point for trying things out on what used to be "standard" high performance tuning, but it is WAY off for strangled motors.
US 72 spec dist 41370 is:
20 @ 1700-2000
35 @ 3300-3900
41 +/- 2 @ 4800
Plus vac advance of 6 +/-2 @13" hg
Plus any static you add in.
These cars run like pigs if the total is anywhere close to 36 max. You absolutely cannot expect reasonable results on any of the emission cars with this setting. Further, you cannot figure anything out or use the factory recommendation with a distributor that is not perfect, ie, totally rebuilt by a competent rebuilder with a distributor machine. These things were built to meet very specific smog requirements, and the setting of base timing on a very steep part of the advance curve was and is a brain dead error - but it is where the certification tests required. You will also note that the specs are very loose. Also, they vary by year; 73 are 2-4 deg less mech with +4 vac, speced at slightly different RPM (so you can't compare directly!). Later cars are worse - up to 37 mech @ 4500 plus 24 +/- 2 vac, plus any static - total as high as 63 BTDC over 4800 RPM. And the specs get looser as the cars get newer. The combustion is so bad that it is extremely difficult to tell what is happening even with 15-20 degree changes, something unheard of on early engines.
Note that the vac advance is absolutely critical on these smog engines - mileage and part throttle response and power all sink like a stone without it.
I start at static 5 BTDC and have moved up as high as 15 BTDC, on 72-73 US engines. Sounds to me that Bob is about midrange of what I use, but his vac is DOA. Anything less than 5 static usually gives a dying pig effect.
I will also note that the amount of air leaked into the engine through a dead advance unit is negligible, after the restriction of the long small diameter tube.
|Fletcher R Millmore|
|Sorry Fletcher, I was forgetting you poor people suffer from the low compression engines - my comments on max advance are based on the UK high compression engines.|
The limiting factor is detonation and the LC engines will take much more advance than the HC ones.
The vac advance isn't an issue here as it really only comes into play under light throttle conditions.
Nevertheless, my points about dizzy wear are valid - set the timing statically and go from there - setting it dynamically with a worn dizzy is a waste of time and effort.
|Chris at Octarine Services|
|Your vac advance capsule may be OK - have you checked that there are no leaks from the vacuum tube and fittings? My vac advance wasn't working but both ends of the vacuum advance tube were cracked, therefore no vacuum advance. New fittings and she works like a charm now.|
|"Yes I do have vacuum at the hose when it is connected to intake manifold. Yes, idle drops if the hose is left unplugged."|
This actually indicates vacuum advance *is* working! Introducing a new or larger vacuum leak where vacuum advance is not functioning would normally be expected to *increase* the idle speed, as the engine isn't doing as much 'work' with the leak present and you haven't changed the timing. Removing the effects of manifold vacuum at idle is what would reduce the idle speed by retarding the timing. Removing vacuum advance *and* introducing a vacuum leak at the same time is a bit hit and miss as the one would tend to counteract the other, so it could go either way or in the case of a fluke remain exactly the same.
|Paul Hunt 2010|
|I guess I should have been a little more clear in my above "clarification".|
My comment that I have vacuum at the hose going to the distributor advance unit is correct. It is a good strong vacuum coming from the intake manifold.
What I need to further clarify is that when the vacuum line is connected to the distributor, there is no change in timing. When I had timing set at 10 degrees at 600-800 rpm, the timing did not change with or without the vacuum hose connected.
I've since, as stated above, plugged the vacuum hose port on intake manifold, even if it was a negligible vacuum loss through advance diaphragm. I'm looking to minimize vacuum loss everywhere I reasonably can.
I've also, as stated above, set the timing at idle to 20 degree BTDC.
A 100+ mile trip in 90 degree weather on Saturday, noted good power on highway at all speeds. Good starting too.
Oh, and I've turned up the carb settings 1 more flat, making that a total of 5 flats up on SU HF carbs over the last few weeks, without any loss in power or starting or cruising.
And for those following my many discussions as I busily seek out a smoother and more reliable ride this summer, with better gas mileage. I no longer note any hiccups since installing the non-vented oil filter cap.
And I suppose I better clarify for the Brits and the Yanks. I am not running the majority of the smog equipment on my US version 72B. I have the charcoal canister still in the loop with vapor canister from trunk, float bowls from carbs, and valve cover. With the PCV in the intake manifold and the crankcase hose connected to the PCV valve.
I think this means I'm still a low compression engine though, and I'm running that wonderful 10% ethanol blend.
So I'm down to either replacing the vacuum unit or getting a new distributor from a MN shop that tunes them rather nicely for the MG market. I'm just trying to figure out all the cost/benefit angles first.
OH, and to wrap all my postings into this one. The 100+ drive on highway in 90 degree weather had no noticeable volume of water leaking out of the radiator or the water pump. Don't ask me why! It was all leaking a week ago.
|Reading your various explanations/statements....|
"To me I think I can blow air to vacuum advance unit as well as suck air from, as I cannot create a vacuum when sucking on hose. Which to me implies the diaphragm is not working in the vacuum housing."
"My comment that I have vacuum at the hose going to the distributor advance unit is correct. It is a good strong vacuum coming from the intake manifold."
So this vacuum is at the distributor end of the vacuum advance tube or at the manifold port? Have you actually checked the integrity of the complete vacuum advance tube and end fittings?
|"I've also, as stated above, set the timing at idle to 20 degree BTDC."|
As a matter of curiosity and reference, what is the static setting?
"Oh, and I've turned up the carb settings 1 more flat, making that a total of 5 flats up on SU HF carbs over the last few weeks, without any loss in power or starting or cruising."
"Flats" refers to the sides of the hexagonal adjusting nuts on earlier carbs, which HIF do not have. So, how much have you leaned it out? (always assuming you are leaning it!) You said "HF" above, and HS4 in another thread, and say "UP" in reference to adjustment, HIF would be "in or out".
Does it still start cold without choke?
Perhaps better to ask if you indeed have HIF as 72 should, or earlier HS type, particularly given my next point.
"With the PCV in the intake manifold and the crankcase hose connected to the PCV valve."
70-on HS and all HIF setup does not use a PCV valve; the crankcase vent goes to the ports on the carbs.
With an unvented oil cap, you have no crankcase ventilation at all, which will kill your engine eventually, and cause oil leaks.
Does this thing have the correct engine? Carbs? Pics and numbers (engine, carbs, distributor) might help/
Vital that you know before buying the advance unit, or better, sending the distributor to Jeff for proper rebuild.
What's your mileage like now?
|Fletcher R Millmore|
|If you can suck/blow air though the vacuum capsule *itself* then it is punctured and ideally should be replaced. But they aren't cheap, and you need to get one with the correct markings on the side which indicate when vacuum advance starts, finishes, and its maximum level (in distributor degrees which are half crankshaft degrees).|
But it's still not clear as to whether lack of vacuum advance is due to a leak or blockage in the vacuum part, or something seized in the mechanical part under the cap.
|Paul Hunt 2010|
|Apparently my "clarifications" caused further confusion.|
For the record my 72B has SU HS4 carbs, not SU HIF4 carbs. So I have flats (surface on side of adjustment nut) on the bottom of the carbs that are turned up into carb to lean carb. I hate it when the typos are my fault !
For the record my 72B has a vacuum hose (new) running from distributor vacuum advance unit to the intake manifold. I have vacuum in the hose. Hose is not leaking.
When I say I am blowing air into or sucking air out of the vacuum advance unit I am doing this with the vacuum hose disconnected from the intake manifold, and the outlet on the intake manifold plugged, for the test.
Blowing or sucking air into the vacuum unit on distrubtor has no affect on distributor. That being the case, connecting the vacuum hose to the vacuum advance unit has no affect on distributor or timing. Since I can blow air into the vacuum unit, I'm assuming there is no blockage in vacuum advance unit.
When I say I've advanced timing from 10 degrees at 600-800 rpm to 20 degrees at same rpm, this 20 degrees is my static timing, too.
I have not checked to see what the timing is at 4000 rpms.
Yes, I always use the choke to start engine. But once it fires up, choke goes in partially immediately. And slowly decrease choke further as I drive away.
And the timing is steady, slowly increasing with corresponding rpms; no boucing about to imply excessive wear in distributor.
I've gone from a repeated 15.9 mpg (US) to 1-20.9 and 2-22.5 mpg, with all my tinkering. And engine temp has dropped off 2-3 needle widths.
I have an 18V585Z engine according to the plate on engine, w/SU HS4 carbs. The floats bowls are vented to the carbon canister, as is the vapor expansion tank from trunk, with fresh air entering bottom of carbon canister and all exiting through hose from carbon canister to valve cover. Once I put the non-vented oil filler cap on the valve cover, I no longer have hiccups in engine.
I know of no ports on the carbs to which I would be connecting the 1/2" hose from the side cover of engine. Air is sucked out of crankcase by PCV valve.
I'd offer photos but I only have high resolution at moment. I will try to retake at lower resolution so they can be uploaded later.
I think I answered all the inquires.
I did not know all vacuum advance units weren't the same. I need to look closer at numbers on my distributor, I thought I had a 45D. Which is for a newer MGB. I've set the gap in my plugs at .035 assuming it was a 45D. Well its always something.
All I know it is it sure is running better and gas mileage is noticeably up, can't wait to get the vacuum advance resolved.
|Oh, and I can't send distributor to Jeff yet, as I need it for all the summer events. We have something almost every weekend. I realize simply having it all rebuilt to modern specs is the best alternative, albeit most expensive; and who's counting my time and everyone from this board !|
Unless you are trying to keep your car original, check out the 123 distributor, at www.123ignition.nl .
There are two dizzys that would do, the one with factory MG curves, 123/MG-4-R-V, and the generic one, 123/GB-4-R-V, with curves that better suit today's fuels. Several of the guys from my MG club, and myself have used the generic one with good results.
The only downside of the 123 is that changing the curve requires removal of the dizzy, to get at the adjuster, pain in the butt.
I would imagine that they would be around $US500.
|22.5 mpg (US) isn't too bad for a mixture of mostly city and some country/highway/easy cruise conditions. |
I'm getting the euivalent of 25 to 27 mpg (US) - and that's with more than 50% easy cruise conditions in the mix.
My engine is in fairly good condition using 98 RON unleaded fuel (not ethanol blend) without hard driving.
"Apparently my "clarifications" caused further confusion."
But we is gainin' onnit!
"For the record my 72B has SU HS4 carbs, not SU HIF4 carbs."
OK, now you get to figure out which HS4s.
" I know of no ports on the carbs to which I would be connecting the 1/2" hose from the side cover of engine. "
They are 3/8 brass tubes that stick out at 45 deg upwards, just outboard from the throttle shafts. If you do not have them, your carbs are pre 70.
If your carbs have spring loaded biased mixture needles, they are 69 onward; earlier are fixed needles.
This all matters because you need to know what needles it has, and/or should have - v dificil on a "bitsa" motor. Your moving of mixture by 5 flats without a really obvious effect indicates serious needle wear, and possibly jets as well if they are bias needle carbs.
I think you've established that the vac unit is DOA.
"When I say I've advanced timing from 10 degrees at 600-800 rpm to 20 degrees at same rpm, this 20 degrees is my static timing, too."
This is most unlikely, unless the mech advance is frozen at least partially - which would really kill mileage and performance. AFAIK, all common distributors have significant advance by 800rpm, generally at least a few degrees by 400.
Pre 71 are 10 @400-600; 71 is 10@900-1100; 72 are 20@1700-2000; 73 are 16 @ 1800-2250. The later two can be safely assumed to have half the advance in at half the spec speed for rough purposes. Consequently, your static cannot be the same as your 6-800 rpm figure.
"And the timing is steady, slowly increasing with corresponding rpms; no boucing about to imply excessive wear in distributor."
Tends to indicate the advance is NOT stuck, thus a procedural error in determining the static setting, in view of the above.
"I've gone from a repeated 15.9 mpg (US) to 1-20.9 and 2-22.5 mpg, with all my tinkering. And engine temp has dropped off 2-3 needle widths."
Dat gud, but can get better!
"I have an 18V585Z engine according to the plate on engine, w/SU HS4 carbs."
Correct for 72 OD car, carbs are not. Of course, you have no clue what bits are in the engine.
" The floats bowls are vented to the carbon canister, as is the vapor expansion tank from trunk, with fresh air entering bottom of carbon canister and all exiting through hose from carbon canister to valve cover. Once I put the non-vented oil filler cap on the valve cover, I no longer have hiccups in engine."
OK, so you do have airflow via the canister - good.
Air is sucked out of crankcase by PCV valve.
"I did not know all vacuum advance units weren't the same. I need to look closer at numbers on my distributor, I thought I had a 45D. Which is for a newer MGB. I've set the gap in my plugs at .035 assuming it was a 45D. Well its always something."
Indeed. Check the number, 5 digits. There will also be the model 25D or 45D, and a 4 digit week/year code (like 25 72), which might help. 25D has a round shouldered cap, 45 is square shouldered and bigger OD.
"All I know it is it sure is running better and gas mileage is noticeably up, can't wait to get the vacuum advance resolved."
Yah, It might help if you realize that since the thing is a bitsa, the "correct" distributor won't be. So, you can try anything that fits. I'd suggest any low compression engine, like a late 1275 Spridget. I had wondrous results throwing an Austin America dizzy at a similar engine once.
|Fletcher R Millmore|
|Okay I've confirmed I have a 25D distributor. It is cast into body of dizzy, with arrows pointing at it, for those having difficulty knowing where to look. I still get a kick out of the arrows.|
It was the fact that I have a regular looking cap that made me think it was a 45D, but I see the 25D had 3 different caps, one of which is the shoulder style.
Short of pulling the dizzy out, I can't find any other numbers on the dizzy, but there are numbers on the vacuum advance, that even bent over upside down with a magnifying glass are hard to read, but for now they appear to be a 5, 6 or 8 then a A or 4 - Then there appears to be a 23989 to the side of that.
As to just starting over with a proper dizzy may eliminate a lot of compatibility issues and get me all working components.
VB offers the 25D for $120, Advanced Distributor (according to someone at a recent MMGG picnic) are $165 - and the AD dizzys are supposedly curved, etc. to the specific car. All well below the $500 above.
And no I'm not trying to keep the car original. I'm concentrating, on a limited budget, with addressing mechanical first. This is almost a daily driver now, never intended for show - there is that budget afterall.
And about my "static" timing comment - I've not needed a timing light for my regualar cars in many many years, and only rescued my timing light from mice for the MG. I'm rusty on my terms. I should have said "dynamic" timing, as the engine is running when I set the timing. Having also worked in a tire shop, with static and dynamic wheel balancing, one would think a light would have gone off, when I wrote "static" about something moving.....
I guess some of my "terms" have to be taken with a grain of salt. I spend a lot of time looking at something on the car, and saying to myself - I used to know this stuff.... Getting a refresher on a British car, is taking a while to sink in. My Honda is so simple...
So for now we have a 72 MGB with a MG engine and MG carbs and MG distributor. Its just whether all the parts were meant to be mixed.
Thanks again for assistance.
I'll try to figure out which SU HF4s. I'm still working uploading a photo.
|I have similar issues with my car. A 78 ex US B with the smog stuff removed, HIF carbs and some random 25D dizzy. I then went and replaced the LC pistons with HC ones and added K&N air filters. I have no idea where to set the advance to so I set it to 15 degrees at idle which I set at 750 rpm. Seems to run. Still seems to run hot though (but MUCH better after I put the thermostat in the right way up)! If it is running well, well, I don't know and don't have the experience to tell! |
This morning I filled the tank to the top (and the car feel so much better with a heavier bum!) and will see what MPG I am getting. I have a feeling it's not good so there is probably more tweaking to be done.
|Curves a a really iffy thing. Fair enough to keep the originals if all components are to original spec and you are using the original fuel, but these days that is highly unlikely. As for the very expensive 123 option it speaks volumes that the generic version is recommended for the MGB by one or two suppliers and not the version with the MGB curves! It's also a real pain to have to keep removing it and retiming it each time you want to try a different curve, I don't know why it couldn't have been accessible from under the cap. Probably any 25D4 or 45D4 distributor will work in any MGB, but some curves will work better than others. Short of spending much time and money on a rolling road determining just what the ideal curve for your engine is, then tweaking a distributor to deliver it, the best you can do is stick with the original and experiment to find the advance that works best for you. With a high compression this is easy, you run with the greatest advance short of pinking at any combination of throttle, revs and load, which will usually be less than the original even on the higher octanes. More difficult on a low compression as you could advance it to the point where it stalls the starter and still not get pinking, so as I say experiment to find what works best.|
The important thing on the vacuum capsule is a number sequence like 7-13-3 which indicates vacuum advance starts at 7 in Hg. is at a maximum at 13 in Hg, and that maximum is 3 distributor degrees which equates to 6 crankshaft degrees. This would be the capsule on a 25D4 41370 which would have been the original for an 18V 585.
Your Honda is simple? Just wait until it goes wrong (and they do).
|Paul Hunt 2010|
|I've exchanged emails with Jeff (Advanced Distributors) here in MN and I'm turning the dizzy over to him in Aug for rebuilt.|
In the meantime I'll keep tinkering with all the other problems.
|Even with an after market distributor on my supercharged B, I had great difficulty in setting up the timing correctly. So, I went back to an old method of setting the timing by feel. This is commonly called "road timing". You loosen the distributor clamp just enough to allow you to move the distributor. Now, take the car out for a drive and stop. Advance the timing a little bit. Drive the car some more. Stop the car and advance the timing a little more again. Repeat this until you can get the engine to start to ping when under load. Now, retard the timing a bit until the pinging stops. Lock down the distributor clamp. This will serve you well enough until Jeff can set up your distributor on his Sun machine. Ford actually recommended this procedure in the 70s as no 2 engines were the same. We also used this approach on boats, with inboard engines. We'd run the engine at full throttle and keep advancing the timing until the engine began to ping and the we'd retard the timing until it stopped. This gave the best power and fuel economy. My supercharged B gets over 27 mpg. RAY|
|Here is a silly question, having never experienced it how easy is it to tell an engine is pinging in an MG? If If try Ray's technique will it be obvious when it happens?|
|The pinging described above is something along the line of what a desiel sounds like all the time under acceleration. It is a different sound that the lifters clanging about, or other sounds. It may be hard to hear under some road, or urban conditions as there may be too much other noise around. But the desiel pinging is a close approximate comparison for sound.|
Or an elf with a tiny tin hammer hitting a metal object very fast under the hood, trying to get your attention to let him out.
Or for the ole' rock & roll folks, I think there is a Ventures song "Tequilla" that has a series of sounds at the beginning of the song (that mexican insturment with the beans rattling in it ??), just before the work "Tequilla" is shouted.
Use whichever is more familiar to you as your reference.
|"in the 70s as no 2 engines were the same"|
Very true. The book timing was to cater for the 'worst case' component tolerance scenario plus a safety factor to avoid warranty claims. I could retime my 73 Marina several degrees in advance of the book figure (I used a vacuum gauge) and got significantly better performance and economy as a result, with no pinking. It was under warranty so I had it serviced by the dealer and it always came back running like a dog (it seemed to me) till it got it back to my setting again. OTOH with my 73 MGB I found using the vacuum gauge ended up with the book figure, so I never bothered using the gauge again. That was on 4-star leaded, when we went to unleaded even the higher octanes I got/get pinking so it has to run slightly retarded from the book figure.
In my experience pinking is a noticeable tinkling or rattling sound like small stones falling onto concrete. I have my timing set so that it doesn't pink on roads where I live which are relatively flat, but that does mean it pinks in hillier and slower territory. You should be able to start and stop it on the throttle, or stop it by changing down a gear with the higher revs. I've also found it will stop when flooring the throttle (if it starts on part-throttle) which may be a factor of the larger cooler charge, ironically. I've also got a TCSA solenoid on a manual switch so I can turn vacuum advance on and off at will, which also helps under some circumstances.
Bear in mind you may *not* get pinking with a low compression engine unless the fuel grade is low.
|Paul Hunt 2010|
|When we were setting cars up to pass an emissions test, we always retarded the timing a few degrees. The cars would pass with ease, but had no power. After the test was over, we reset the timing to where it was before to regain the lost power. You should be able to hear an engine ping, when under load, quite easily. RAY|
This thread was discussed between 28/05/2010 and 04/06/2010
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