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MG MGB Technical - New ignition timing

Even though I looked through some of the archived articles I couldn't find anything on this subject although I may have been looking in the wrong places. Decided to check the timing on my 67B today. It was set at 10 D BTC. I wasn't happy with the way it was idling so decided to advance it a little using a strobe light. Idling speed immediately increased and I settled at about 16 degrees Timing notches only go up to 15 so was only an estimation. When I increased revs I estimated it was about 30 D. I am running a Luminition ignition system through a D45 dizzy which I have fitted. Started looking on the net and found an article about timing and it said that the non leaded petrol burns slower than the leaded as was the fuel when the 67 was born.Just interested in comments from those more learned than me in such matters
Rod Pemberton
R J Pemberton

The criteria for timing these days needs to be does it pink or not, as that will vary with the fuel used as well as engine spec and general condition.

The original specs were based on new engines using 4-star leaded which was about 97 or 98 octane I believe, and standard unleaded is 95 in the UK, with 97 to 99 as a more expensive alternative. But as engines are pretty variable and some pinked more easily than others the factory timing was 'conservative' so none of them should suffer damage. For that reason some engines could be advanced from book without pinking, and get the benefits of improved performance and mileage. Haynes says this, and to run at full throttle from 30 to 50 in 4th gear, and set so barely perceptible pinking is evident. One of my engines in the 70s fell into that category, but my 73 roadster couldn't run any more than book or it pinked. I also have found that my engine can pink at less than full throttle, but not at full throttle, so I go by pinking at any combination of throttle opening, revs and load. Changing to unleaded pinked more easily, even on higher octane, so I have to have it slightly less than book now. When I've been touring in remote areas and could not get the higher octane, only 95, I've had to back off the timing there and then as pinking was so bad.

Advancing the timing at idle WILL increase the idle speed as more of the burn is occurring with the piston higher in the bore so gives a harder 'push'. But pinking at idle is very unlikely - unless you go ridiculously advanced. It's caused by spontaneous combustion when cylinder pressure reaches a certain point, and centrifugal and vacuum advance are designed to advance the timing when that is less likely, and vice-versa.
paulh4

Hi Rod, timing on your year should be 14-16 deg at idle, you you are in this range.
Max should be 30 deg, arriving originally at 2300 RPM, though some replacement distributors get there a bit later.
Some replacement distributors don't stop advancing till over 40 deg thanks to sellers adopting a one size fits all approach. Check yours isn't, as this. Much advance will damage the engine eventually.
Paul Walbran

I didn't notice any significant change in advance requirements on cars set-up on the rolling road before and after unleaded came in here during the 90s provided the octane was the same. If the burn rate was different then the advance would have been different.
As Paul says, octane will have an effect on timing needs. What are you running?
Paul Walbran

On the 98 octane we have here, of whatever flavour, I find that (unmodified) engines set up to the advance curve used up to 1973 don't pink. Our fuel is quite a different composition from UK fuel, so may not directly translate. Best intermediate way point on that curve is 25 deg at 2000, idle and max advance as above. we have found a bit of extra power at 32 deg sometimes but don't take them past that.
Paul Walbran

Hi Paul W
Yes I have heard that our fuel has optimistic octane rating when compared to say the States. We only get 95 or 96 down here. I bought the ignition second hand ( Luminition ) and was told it was off a B or it may have been off an 1800 Marina as the guy has bought a engine out of one to replace his B engine. Also told it was a standard fitting. Has the separate box that is bolted to firewall or in my case radiator mounting plate.
My timing is a guestimate as I do not have any other scale apart from timing marks on the cover
By the way I am doing the VCC Targa time trial this year starting in Taupo. Trying to preempt any break downs
Rod
R J Pemberton

Rod, your ignition set up is the same as my 79 B. Yes the label under the hood (bonnet) states 10-d BTDC @ 1500 RPM. That is the spec for USA cars.

As Paul posted, the question is "at what setting does your engine "pink"?"

My engine is no longer "factory" spec due to a rebuild .030 overbore and a 270 cam plus a Luminition 45D dizzy. My engine seems happy without "pinking" around 17-18-d BTDC @ 1500 RPM.The 10-d BTDC is now merely a starting reference point to your car's timing.

The other issue arises when you shut the engine down. Will it "diesel" due to an excessive timing advance? So, somewhere between "pinking" and dieseling" is where your engine needs to be.

Cheers

Gary
79 MGB


gary hansen

My ignorance is showing. I've never understood the relationship between timing and dieseling. It seems to me that when you turn the key to off you've eliminated the ability to make sparks so the timing that the sparks would have been happening had the ignition system been on would have no effect.

I've heard, but certainly don't know, that dieseling is due to hot spots in the combustion chamber or heat and compression causing ignition. If so, again it seems that the phenomenon would be unrelated to where the spark timing was set.

Please enlighten me.

Thanks.

Jud
J. K. Chapin

Rod
Don't get caught out comparing US fuel to UK and here, the US fuel is rated using a different method

91 Premium in the US= roughly equal to our 98

The mechanical advance of these cars as standard starts advancing as soon as the engine fires up
The spec. in the book for a 67B is 10btdc (static)but as soon as the engine starts the timing is away and around Paul W's 14-16 is usually where it will be at idle-
A 79B as noted by Garry runs less timing at 10@idle but these are pollution equipped cars there and the dizzy runs more total advance and a bigger vac pot so they can run less timing and more throttle at idle to get heat into the cat convertor
With an improved camshaft, any increase in camshaft duration can be matched up with more base/low speed ign timing but as Paul W warns the total ign timing must not go past 32

The danger in setting up your engine by the pinging method is with the amount of ethanol in the fuel,not the fact that it's unleaded, some cars on some fuels just won't ping whatever you have the timing at, so there is a danger there--There are two types of detonation, the one you hear, and the one you don't, You have to be very carefull setting the timing just by pinging by ear at low to middle revs without checking total advance--It's ok (just) to do it like this if you're chasing absolute fuel economy but it is fraught with danger---If timing is too high at higher revs you won't hear it detonating but it will be--
Your bank manager will fix it up later
willy

William Revit

I've never really understood why Dieseling only occurs at switch-off, i.e. when the spark is no longer present. The cylinder pressures rise rapidly as soon as the mixture starts to burn, and if that rises above the self-combustion point of the unburnt fuel that is when you get pinking. So why does it Diesel with lower pressures? The 'explosions' then are far noisier than pinking. Higher idle speeds make it more likely, which I found I needed with the change to unleaded.

Mine always Dieseled slightly on leaded, but go so much worse on unleaded I had to do something about it. I bought an expensive valve off the MGOC that dumps air into the brake servo line but that had no effect on it at all. I'd studied the North American system which works by sucking the fuel out of the carb jets at switch off, which is so effective that no one realised until some time later that these cars had an ignition wiring error which meant that when you turned off the key the ignition kept working, it was the anti-runon system that stopped the engine.

Any road up as they say round these parts, I modified the MGOC valve to do the same thing by connecting inlet manifold vacuum to the carb overflow pipes, and that has worked perfectly for 20 years now. Subsequently I discovered that with the valve connected direct to the inlet manifold it worked as it should have done in the first place. In either position it makes for a nifty immobiliser.
paulh4

Rod, with a fixed timing light a useful way to read the advance past the end of the timing marks is to add a second one 15 degrees ahead of tdc. 15 degrees is 17mm on the early pulley and 19.7mm on the larger pulley used on rubber bumper cars and the Marina. The extra mark should be clockwise from the tdc mark. To avoid confusion, make the tdc mark more prominent somehow, there are several ways to do so.
This will give you certainty about reading max advance, at 30 deg your new mark will line up with the 15 deg pointer.
Paul Walbran

BTW, whenever I needed one, I used to step the second mark off the pointers, until one day I couldn't find my cam degree wheel and stepped off every 20deg right round the pulley. And found it went only 8 times. Did a double take, rechecked, yes only 8. Thought it was also meaduring/marking on my part, re-did it, same result.
Measured the distance between pointers on the cover, did some maths: 20 deg pointer actually 22.5 deg from the tdc mark! This was the small pulley/20 deg top marks timing cover, but I have never trusted any since.
Paul Walbran

Just to underline the point of the above, there is a real risk after fitting a different distributor that you will have too much advance at engine-damaging levels. Always check, never assume. Unless, as Willy says, you have a generous bank manager.
Paul Walbran

Rod my B timing is set up the same but I used a vacuum gauge to get there - again by following the method described by Paul Hunt. Totally different car compared to when set at standard 10deg.
steve livesley

You do have to be careful using a vacuum gauge as that sets timing at idle. With an old distributor - or one of the wrong spec for your engine - the rate of advance could more than was originally intended e.g. from tired springs, which is why going by pinking (or more correctly no pinking) is probably safer.
paulh4

Paul W
Adding that extra mark makes a lot of sense. Good No. 8 technology.
R J Pemberton

Update
Using Paul W method of marking the bottom pulley 17mm before TDC and running the motor at 2400 rpm the advance is 30 degrees. May knock the advance back slightly. Not sure how accommodating my bank manager is let alone the other half
Rod
R J Pemberton

When set up on the rolling road after its most recent rebuild my car didn't benefit from more than 9 degrees static advance. It doesn't pink at all. It has an early 25D4 dissi with an ambitious unleaded premium curve. These cars have a lot of vacuum advance as well.
It tended to pink more with the Oselli cam, especially when climbing hills at moderate RPM. Flooring the throttle and so killing the manifold depression and thus the vacuum advance always worked. It doesn't run on at all but it did when it was standard. Maybe this is an extra plus from the Peter Burgess head?

Stan Best

Dieselling or run on is caused by hotspots, usually glowing carbon deposits, igniting the fuel mixture - the engine can run backwards as the mixture is ignited well before TDC.

Dumping air into the manifold weakens the mixture so much that it does not ignite. As I have said many times before - flooring the throttle as you turn off the ignition stops the engine dead.
Chris at Octarine Services

Chris - that's what I've always understood, but with the engine running backwards because of ignition well before TDC, why doesn't it affect normal running?
paulh4

Inertia and speed I reckon - under normal running at 1000 rpm the 4 stroke cycle takes 1/8th of a second and the inertia of the engine keeps it going the right direction.

When you turn the ignition off, the engine slows to the point where an incoming charge fires while the piston is well before TDC and it kicks the engine backwards which is why you get that judder before the engine keeps firing in reverse and jerking about.

With the throttle shut any suction from the cylinders pulls fuel out of the jets even at low rpm - opening the throttle kills that suction and with low airflow over the jets the engine gets almost pure air and thus stops firing.
Chris at Octarine Services

Thanks Chris. I guess my previous understanding was pretty close to right. I rev the engine before shutting down but I have not been keeping the throttle open during shutdown. I know the function of the throttle is to control the flow of air but childhood memories of "step on the 'GAS'" persist causing my intuition to suggest that opening the throttle introduces more fuel. Wrong, I know but childhood beliefs are hard to dispel even 60-70 years later.

Jud
J. K. Chapin

This getting further and further away from my understanding of what happens.

When idling the butterfly is almost closed and the small amount of suction that gets past it pulls a small amount of air through and a matching amount of fuel with it.

Open the butterfly and that presents more suction to the carb, but the damper delays the rise of the piston, so the increased suction pulls more fuel from the jet without much increase in air flow, until the piston has a chance to rise. So if anything opening the throttle at switch-off floods the engine to some extent.

Ignoring the action of the damper the more you push the throttle pedal down the more you open the butterfly and the more air and fuel is sucked into the engine. So opening the throttle DOES introduce more fuel. How else would you accelerate?
paulh4

Ford here had a couple of goes at stopping the carburettored 6 cyl Falcons from running on/dieseling
First effort was 2 idle speeds--base idle was set at 500 rpm max. and the engine wouldn't run on at that but would stall easily especially autos when they were put into gear, so they introduced a small adjustable solenoid as a throttle stop that operated with the ign. This was set at 750rpm which fixed the stalling problem, and when the ign. is turned off the solenoid dumps the throttle lever down to the lower setting and no run on
Second effort was a little dual throat downdraft Weber carb. which had a fuel cutoff solenoid in the idle jet circuit---perfect
Of course, when fuel injection became more common (here anyway) this solved any run on problems as the fuel supply is turned off with the ign.key
William Revit

This thread was discussed between 16/08/2019 and 23/08/2019

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