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MG MGB Technical - Need SU Needle Suggestion for 70 B with mods
|I have a 1970 MGB with a Fast Road Peter Burgess head on it. I have been trying to tune by car by the Su tuning manual. Which states 2 full turns on the mixtures nuts and then slight adjustment after that. I had to adjust the nuts more then 2 full turns richer to get it to run.|
I am leaning toward that I need differant jet needles in my stock SU carbs??? If so which one?
|Also forgot to mention I have the matched Cam to the head also. The exhuast is down to just the single back muffler and when up in pipe diameter one size. Otherwise car is all stock.|
|No, you have another problem. The needle profiles are all pretty close at idle; differences come in off idle. |
I normally find that two full turns is slightly too rich, which is good for first startup, but I now normally set the carbs 10 flats down and only go one or at most two flats either way from that.
You have an air leak, something wrong with the crankcase breather setup, float levels low, fuel starvation, timing errors, etc.
Has the car idled/run well ever?
What is the cam?
Obviously, Peter is the man to ask for recommendations. He's on here pretty often, try putting his name in your post title.
I don't think you need worry about the number of turns so long as it runs smoothly. If you are worried,
Peter will have a view, alternatively you could use the factory's recommended rich needles from your handbook or if you want advice locally (moss for example) they will need to know which version of SU and the number of your engine and the cam specs.
I suspect the cam change may have had the more significant effect on the engine's breathing and thereby fuel consumption. I hope someone has been here before you and can advise. We did not change the cam when we fitted the Burgess head to to an 18V engine but the engine to which I fitted a skimmed Downton head wanted special needles - fortunately Downton knew which ones - the result on 100 Octane was dramatic ... so keep looking until you are happy.
What filters are you running?
|Peter Burgess Tuning|
The following question has come up on the MGA board, relative to changes in carbs and throttle response: Can your marvelous new toy read acceleration under load between rpm numbers at various throttle openings? IE, load it at equivalent 40 mph in top gear, then open the throttle to a fixed stop point like half open, and pick out response times? Idea is to simulate a real world driving situation, like passing.
There have been several very long threads on this.
Steve Gyles created some very nice stub stacks, and Steve Ash had a bunch made. Some guys did some RR trials, with noticeable effects, but they were top endish. Everybody feels they made a big difference in part throttle running and throttle response, but we wants numbers!
|My car to a similar spec ran fine on the factory No 5 ie standard tune after the mods and before set up. When Redline RR it they put in rich no 6 and that perked it up a bit, but it used to soot plugs. I then got the closest = to AAA for fixed needles from Burlen and thse were an improvement. It was on an RR again at Aldon on the MGB register day this year. They found it was running s bit lean just off of idle and that i had it rich at idle to balance this out. They took some brass off the offending area reset the idle and the car was transformed. You cant beat having the kit plus someone who knows how to use it.|
|I have the stock air filters.|
The car runs good and has for several years. This is the first time I really tuned the carbs though. I feel there is some room for improvements.
|I'm with FRM, there is very little different in needle diameter at the idle end, nearly all are .890, with a very few .010 thicker or thinner than that.|
"Which states 2 full turns on the mixtures"
You did this two full turns down from being flush with the bridge, didn't you, and not as high as they will go?
How are you judging whether the idle mixture is weak or rich?
Did you have to richen both carbs by the extra two turns, or were they different?
|A couple of things strike me, namely why a gas flowed head, tuned cam and sports exhaust AND paper air filters???? Let the head breath with some free flow filters. Irrespective of it's idle performance it should need slightly richer needles.If the cam, head and exhaust, and hopefully the filters, allow more air to move, you need extra fuel to keep the mixture right. AAA might be a bit too rich, ABD should be better.|
|Wear and tear on the needles/jets/floats etc is hard to guess at. To be honest stock needles should be fine if all is well. Approx 60 thou down for the jet as a starting point.|
Re the dyno. If you had a way of controlling throttle opening (slacken cable till it gives the throttle opening rquired?) then yes you can do runs and time it.
Example of full throttle runs attached as bhp against time. Red lines after tuning and blue lines before. Solid lines bhp, dotted lines torque. Say you need 60 bhp to overtake....accelerating to 60 bhp takes 5 secs before tuning and 3.5ish after. Is this what you had in mind?
|Peter Burgess Tuning|
|I think I need to go over them again. I was doing them together on the mixture. I also turned the nut all the way up till it wouldnt turn any more. Which I think is not correct. Just needs to be flush with the bridge or air flow path. |
I was judging by the trick of raising the dashpot slightly. I kept making it richer and it kept wanting to die when i did that.
|I set jets .065 below the bridge as a start usually. 60 if it is hot and 70 in colder weather. Original parts give .0064 per flat. Rarely go more than one flat from these. Measuring is especially good as I hear that new jet adjusting bearings/nuts may have different threads.|
Just so, thanks. Now I am going to try to get Steve G or somebody with the stacks over to bother you!
My thought is to fit an adjustable throttle stop to get repeatable part throttle settings. More ambitious types could use a TPS sensor in the carb linkage, so if that could be fed into your computer? This has always been the mysterious "drivability" or response issue that was formerly strictly a "driver feel" item.
This was all on MGA carbs with stock filter cans, and the reports are that the stacks helped the idle and in traffic transition a bunch. I keep telling them to get the inside support studs away from the stacks, or thin them down, as a further improvement.
As a reasonable check see what happens around idle. Rev the engine and let it come down to idle, if it dips below idle speed then recovers it is probably too weak. If it comes down sweet then slowly dies away it is probably too rich.
I think I might need some obd stuff to use tps or get a friend of mine to cobble something up. The inertia testing seems pretty close to street driving to me. Holding full loads seems more like a stationary engine requirements. It is interesting you talk about driver 'feel', say I ran a motorbike on the rollers and we measured the power curve accelerating in direct top ( best for coast down tests), then we fit a lighter flywheel,chain, sprockets, wheel and maybe tyre, even with all the gearing the same we would record more bhp on the next power run. This because we measure rate of acceleration not hold steady wot loads. The engine has less resistance to spinning up so it looks like it has more bhp, what it is doing is allowing us to use the power better. On the road this bike will accelerate quicker even though the bhp on wot load holding testing would be the same in both cases, the rider would definately 'feel' the difference, the inertia testing gives us an onjective measurement of the driveability which can backup a subjective, seat of the pants method......it has been a whole new leasrning curve for me trying to get my teeth around the testing and realising that bhp figures from dynos are not what they seem, especially when you see the calibration of the eddy brake change with temperature and changing air gap on the brake discs! The old Clayton water brake held steady temperatures! We tend to only use the eddy brake to hold full load checking for pinging/detonation or a partial load to ensure turbos spool up.
|Peter Burgess Tuning|
Apologies for stealing your thread! Where in
Ohio are you? I am 50 mi east of the line in Pa, 50 mi S of Erie. Perhaps I can make it up to you.
Sounds like just the thing. You are correct, dynos used to be pretty much constant state devices, and people were always saying that things would show up to the driver that could not be measured, so I reckon we got some progress.
The TPS idea is just for reference; I think throttle stops would answer my questions. Trouble is, I'm too far away!
To give an idea of what I am blathering about -
Steve Gyles' Original thread (MGA archive 2011) was:
Improved Airflow Stubstacks
Neil McGurk's dyno results and commentary:
Carb stub stacks
Steve's comments after fitting:
"I have just fitted the second stub stack and been for a test drive. I was very, very impressed. The car felt as though it was skipping and dancing on the throttle; very lively. It was so much more responsive on throttle opening. I have to admit that I felt a little nervous once or twice with the handling until I got used to it. You get used to driving your car in a routine sort of way on your local roads, then do a mod like this and it requires a slightly altered driving style.
In summary, a big tick for throttle response. Choke needed for less time. Engine pick-up on the still cold engine (without choke) very much improved. Love it."
"Peak power increased from 88.03HP at 4,641rpm to 92.31HP at 4,601rpm and peak torque increase from 104.4ft-lb at 4,350rpm to 107.4ft-lb at 4,350rpm
There were a couple of runs on each (without, then with stacks) and these were the best of both. As soon as the second set started the tester said there was a definite difference.
It is possible to tell the difference on the road too. Definitely a more "lively" feel is how I would describe it."
Dyno charts in thread.
So, the transition range is what I'm interested in, especially if we can get AFR readings as well. I am attempting to get somebody to visit you!
|We have got to the point where we can just tweak the idle mixes and get a difference in acceleration and crispness.|
|Peter Burgess Tuning|
|"I also turned the nut all the way up till it wouldnt turn any more. "|
That would explain it.
I am located in Cincinnati. I often thought about a dyno tune, the problem is not to many places speciallize in British Car tuning around here and I would need half a day to dial it in.
Thanks for the suggestion. I will try that tomorrow and also redo the mixture tuning.
I guess I need to order some hi flow air filters. I was trying to keep the stock appearance is the reason I never went with aftermarket.
That's about as far away as is possible and still be in OH-ten!
You do NOT need hi flow filters. The stock MGB filters are near the best stock filters ever supplied on a car, perfectly adequate for anything less than full race use. Many cars can use "free flow" filters, and all manufacturers of such like "free flow cash" and boy racer endorsements. Many aftermarket filters flow worse than the OE, especially if they do not use the original base.
If you want to spend ca$h, send your distributor off to Jeff Schlemmer this winter, will be far more useful than filters.
|Ok so I got around to checking the jet realative to the bridge, height wise. I was at .170 and .155 for each carb. I reset it to .065 for both of them and the car will not start with out the choke on or run with out the choke ingaged. So I am running way lean at .065. |
Im guessing I have another problem not related to the carbs??
These carbs are brand new with maybe 1k miles on them. Im going to change out the fuel filter and check timing to see if that does anything.
The car should not start without the choke, unless it was just shut off. If it does, it is too rich.
It should run at half choke once it starts, and without the choke after a minute in this weather. If it does not, it is too lean (a lot).
Check for vacuum leaks. Loose manifold, loose carbs, broken heat shield/gaskets, core plug fell out if manifold balance pipe.
"These carbs are brand new " What does this mean? New by who?
Check float levels.
Go here and check fuel pump:
|The car was up to running temp when I tried restarting it. I drove it into work today and after I got home I started working on it. So it was running for about 30 minutes. It usually starts right up on first crank with no choke once it has been ran at somepoint during that same day. |
The carbs are brand new from Moss SU carbs for the 1970 model year. Does having to adjust the jets down a total of .165 or so seem odd to you??
I went ahead and replaced the fuel filter with no luck.
One question I have is the dist. seems off. The vaccum advance port is almost to the point if I advance it anymore I will not be able to remove the dipstick. Seem odd??
I rebuilt this engine about 6 years ago. It has roughly 2k miles on it. The car ran good last year but never really felt any gains from the Fast Road head. Well when I rebuilt the engine, I have a strange feeling that I got the dist gear in out of wack. Its also the Orginal ontouched dist except for Petronix Electronic Ignition.
Thanks for the help!
|Also some more information:|
Checked all manifold and carb bolts. They were tight. Checked for vaccuum leaks and didnt see anything. Only vaccuum Im pulling is for the vaccuum advance everything else is plugged up (been for several years). Pulled all 4 plugs and they looked good. Tan color.
I have a aftermarket solid state fuel pump. New 12 years ago when I bought the car.
"The car was up to running temp when I tried restarting it. I drove it into work today and after I got home I started working on it. So it was running for about 30 minutes. It usually starts right up on first crank with no choke once it has been ran at somepoint during that same day."
>>>If it feels warm to touch, it should start no choke. If it is at ambient temp, 70deg or less, choke is normal.
"The carbs are brand new from Moss SU carbs for the 1970 model year."
>>These carbs should have vent ports. If the vents are open, or the wrong oil cap is fitted, or fuel evap system has been removed or modified, it amounts to a big vacuum leak. That would require excessive rich adjustment at idle, but be far too rich above idle.
" Does having to adjust the jets down a total of .165 or so seem odd to you??"
>>> Very. I have said .065 repeatedly.
"I went ahead and replaced the fuel filter with no luck."
Was it clogged? Changing parts without diagnosis will drive you crazy.
"One question I have is the dist. seems off. The vaccum advance port is almost to the point if I advance it anymore I will not be able to remove the dipstick. Seem odd??"
>>>Indeed. See Matt's diagnosis next item.
"I rebuilt this engine about 6 years ago. It has roughly 2k miles on it. The car ran good last year but never really felt any gains from the Fast Road head. Well when I rebuilt the engine, I have a strange feeling that I got the dist gear in out of wack."
>>> You are one tooth out. The vac advance is normally just forward of straight up. Reove dist, drive sleeve, and screw a long 5/16-24 bolt in the gear. Pull it out until it just clears the cam gear and turn it one tooth anticlockwise. Reassemble.
>>>The head and cam should be very noticeable. It's not right.
"Its also the Orginal ontouched dist except for Petronix Electronic Ignition."
>>>Quoting meself from above: "If you want to spend ca$h, send your distributor off to Jeff Schlemmer this winter, will be far more useful than filters."
You gotta place for a traveling mechanic to sleep" I might go to NAMGAR in Dayton in July.
|I'm considering rent/loaning-through the mail- my Innovate, sniffer, and gauge out to any of youse guys that want to dial in your AFR at idle and anywhere else. Example: I started the new HIF at the recommended 3.5 turns out from the stop using the AAE supplied. Mix was good ( 13-14:1) to mid range then fell off leanish to 16-18:1 at the top end.I went to AAA which was nice and fat all the way to wide open when it leaned out( 18:1). The ABD provided 14-12:1 AFR up to about stage 10 or 11 then went lean= 15:1. I ended up with the mix adjust screw set at 2.5 turns out and the old standby #6 needle, showing a cool burning fattish AFR to about half needle, then getting fatter at 12.8 at 3500 rpm, and then real fat at tip of needle at WOT, about 10.5-11.|
It just ain possible to get that level of tune data from lifting the piston at idle. Cheers, Vem
|>It just ain possible to get that level of tune data from lifting the piston at idle.|
Lifting the pin at idle assumes the hard work has already been done (i.e. you've already got the /right/ needle). ;-)
That's a very generous offer on the analyzer -- I might have to take you up on it!
This thread was discussed between 09/06/2012 and 04/07/2012
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