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Advancing ignition for MGB

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MGB1974

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Love this forum and am posting my first thread! I am working on my 1974 1\2 ( rubber bumper ) B, and hope someone can enlighten me on the theory of advancing ignition. Mine is set now at about 15 degrees 'before Top dead centre' at 2500 rpm, but I have read lots on the differing specs for different MG engines., and also mention of 'total advance degrees' being upwards of 30 at high rpms. How is it possible to measure this using a strobe? as the crankshaft pointers only indicate 20 degrees in 5 degree intervals.

Paul in Toronto
 
Hi, Paul, and welcome to the party!

For MG-specific posts, please post over in the MG section - plenty of folks on hand to help out.

:cheers:
Mickey
 
A common figure for MGB timing is 32* at around 3500 - 4000 RPM, with the vacuum line to the distributor disconnected and plugged. To set this, you need a dial-back timing light. With that, you set the timing light to 32* and when the mark on the pulley lines up at the 0 mark you are actually at 32*.

The high RPM assures that you're seeing the max advance. You disconnect the vacuum line to keep that out of the equation.

Wiser folks than me will tell you if that setting (32* full advance, no vacuum) is correct for your car. It is what is recommended for mine, but mine is an earlier car with a Lucas 25D distributor, not sure what the optimum setting for your car is.
 
As for timing marks, you can do two things. The first is simply to use some trigonometry and make new marks on the pulley. The second is to get an electronic timing light where you can program in the desired advance, then you just line up the TDC marks--the timing light automatically delays or advances the pulse so that everything is right when the timing marks at TDC line up.

You don't have to time these cars at speed, though; you can time them at idle, and that's usually the factory spec.

A time-honored way to set the timing is to advance it until you get knocking on moderate acceleration, then back off until it disappears.

When I had my TR4A, back in the 60s, I only timed it using static timing. That was the factory recommendation, and there was so much jitter that I couldn't do it right with the timing light anyway.

BTW, welcome to the asylum!
 
Sarastro said:
As for timing marks, you can do two things. The first is simply to use some trigonometry and make new marks on the pulley. The second is to get an electronic timing light where you can program in the desired advance, then you just line up the TDC marks--the timing light automatically delays or advances the pulse so that everything is right when the timing marks at TDC line up.

You don't have to time these cars at speed, though; you can time them at idle, and that's usually the factory spec.

A time-honored way to set the timing is to advance it until you get knocking on moderate acceleration, then back off until it disappears.

When I had my TR4A, back in the 60s, I only timed it using static timing. That was the factory recommendation, and there was so much jitter that I couldn't do it right with the timing light anyway.

BTW, welcome to the asylum!
Click to expand...


Many Thanks to all with the quick responses and suggestions and the re-direct to the correct forum \ asylum!

A Timing light with a dial back feature may be in order.

Further experimenting today, I found that my vacuum advance did not seem to do anything when either connected to the port at the carb or disconnected and plugged. I was able to provide suction on the line by mouth and move the timing mark to about 35 degrees, but no change seen when connecting \ removing suction tube at idle of 1000...does this raise any eyebrows out there?

...meanwhile the car runs smooth. All my searching comes after I noticed my heat gauge slowly climbing to almost touching the 'N' mid-point after 25 min of highway speed, and feeling a lot of cabin heat. I have since replaced all my cooling components, rad, hoses, pump, stat, changed to colder spark plugs, refurbished the SU HS4 carbs and made the mixture little more rich, so the timing \ advance is where I have arrived in the troubleshooting chain. ( ignition is all new electronic, with 12V ballasted coil )

My only other suspicion is a manifold leak but testing with propane around the carbs and gaskets I get no uptick in the engine...any thoughts of what might be tried?

ps
today was a balmy 75F and perfect 'top-down-overdrive-driving-day...the whole reason I work on it all winter!

PW
 
If you were connecting it to a ported vacuum source on the carb, there would be no vacuum present until the throttle blade opened past the vacuum port. That's normal for that port.

With the vacuum hose detached, were you able to see the timing advance as the engine sped up? That's the function of the mechanical advance inside the distributor and if that does not work, there is some maintenance or repair required.
 
right - thanks M6 - the connection is to a ported source on the carb so it makes sense why I did not see any change because motor was idling maybe 800 rpm. As to the mechanical advance, yes that seems to work fine - ie; I had buddy rev the motor to 2500 and could see an advance of maybe 35 degrees BTDC ( with vacuum tube plugged )

Today I drove 1 hr @ highway speed, heat gauge needle was approaching 'N' and stopping, I made a small adjustment of the carbs ( richer by 1\6 turn ) and found that heat was slightly better for rest of trip.

Meanwhile the advance is still 20 degrees BTDC at 1000 rpm, which I think is too much by 5 or 10 degrees. I will adjust and road test again tomorrow.

Q: can someone explain the theory behind the rule of thumb that 'lower octane gas results in engine overheating?' I have trouble finding higher than 91 and highest on market is 94...
 
I think your timing is way too advanced. Are you getting any knocking when you accelerate from a stop? I'd be surprised if you aren't.

If your centrifugal advance isn't doing the job, that's something to deal with. The problem in timing at ~3500 RPM is that if you're not getting enough advance, you end up with way too much initial advance. The factory spec is for the initial advance, so I'd time it for that.

That said, I time my Porsche 912 for advance at 3000 RPM, but I have a modern, electronic distributor that has the precise, factory advance spec programmed into it. I can't do it at idle, because it adaptively changes the timing to smooth the idle, so it bounces around a lot.

Remember, these cars do indeed run well when the parts work right and are adjusted right. You don't have to do anything special to make them work properly. Tweaking the carbs and such to prevent overheating is not addressing the fundamental problem, and really shouldn't be necessary.

I'd suggest bringing the timing back to 5 degrees (or whatever the factory spec is) and I'll bet it runs cooler. From there, you can tweak it a little to compensate for the differences in today's fuel and such, but you shouldn't have to make big changes.
 
Steve thx - I did back the advance down to 15 BTDC @ 1000 rpm. ( I will soon have the use of a dial back strobe to be able to see the total advance degrees - the number 32 seems to be what I will be looking for as per other good folks on this forum ) Meanwhile, I drove the car about 200 miles @ 65 mph and was smooth as silk and running cooler - which may have been result of the decrease in timing initial advance, plus balancing the carbs, during which I noticed one throttle was not opening same time as other and I adjusted the linkage.

But - as you say I still have some knocking when putting the pedal down, so timing is not there yet ( centrifugal advance should be fine as its a new distributor )

by the way - is knocking a pre-ignition noise? Could this be due octane of 91 which is all I could find.

stay tuned
 
I don't have the specs for an MGB, but I'd be pretty sure that 15 degrees initial advance is still too much. The spec for initial advance of all these cars is usually around 5 degrees. Just back it off to whatever the spec is at idle, and that should take care of the preignition/knocking (which are two words for the same thing). You might have to make some minor carb adjustments when you change the timing a lot. I suspect that 91 octane is OK unless the engine has been modified for high compression. We all have access to the same fuel, and it seems to work--at least, I don't hear much complaining about it from people. But in any case, you can't draw any conclusions until you have everything set up right.

As for balancing the carbs, did you do this with a synchronizing tool? that's the only way to do it accurately. If they're off, that could be part of the problem.
 
someone recently remarked that the same can be achieved using a vac guage. Merely setting the distributor until the highest reading is shown.
Any thoughts on this method?
 
Start a new thread when you find one ten years old. Your idea is just a refinement on the old and valid shade tree mech. method. You don't need a gauge. Advance the timin to highest RPM; Retard to lose 100 or so RPM; drive it at 30 in 4th and see if you hear valve ping; if you do, retard a little more.
Bob

Addition: Floor the gas pedal at 30 in 4th to check for ping.
 
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