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MG MGB Technical - timing chestnut

Apology’s in advance for what I am sure is a subject that goes around and around but here goes, I have just bought a timing light with an advance dial so you can imagine I’m straight into the garage to check the timing, warm up the engine then check and find that it is indeed running at the apparently prescribed 10 degrees before TDC, for my piece of mind I then loosened the distributor and advanced the timing to where I thought the engine sounded a lot happier and revving more freely, went on a short run and found that the car ran much better than before and with better acceleration , even labouring the engine in 5th gear (type 9) didn’t result in pinking, having checked the timing I now find that it is running at tick over (about 850-900 revs) at 17 degrees before TDC , I have no problem with this but I’m hoping that someone can tell me if it is going to result in future problems such as overheating etc., thanks in advance for any input, vin
Vin Rafter

You don't say what year the car is?
Paul Walbran

Assuming you have a later car with a high compression engine - it should run 15 degrees btdc at 1000rpm. Depending on your head gasket/whether the head has been skimmed/your fuel and the accuracy of your timing marks, I would trust your ear !
Don't run her much too lean 'though - or much over 5000rpm.
Roger W

Also, what is maximum advance?
Dave O'Neill2

Sorry I should have said, the engine is an 18v, I couldn’t say what year it’s from, it was from a uk rebuild company called ivor searl, and it’s fitted in my mga, I am led to believe it is a standard engine with an unleaded head, also how do i find maximum advance, thanks again for you comments Vin
Vin Rafter

Sounds interesting Vin Rafter. I also have an Ivor Searle reconditioned engine in my 77 roadster. The spec is 18V 846. You write 17 degrees btdc at 1000rpm. What have you got at 850 revs? I set mine at 10 degrees btdc (with vacum adavance tube disconnected) as specified but have thought it has not been running correctly. No problems as such but seems a bit sluggish. I had expected more up and go from a 1.8. But then I'm no expert.

What are the thoughts on this?
/Moss
Moss

I run my 18v high compression engine around 18degs at 1000rpm also. I use non-ethanol fuel, runs very smoothly to me, I can detect no pinking under load and it starts easily.
M F L Sherrit

Vin

Find max advance by steadily increasing engine speed until it stops advancing. Max figure should be between 30 and 34 degrees depending on fuel. It is the most important one to get right, as it's harder to hear any grumbles or pinking from the engine at high speed.
Ideally, if the engine has the correct full advance it will also be correct everywhere else.

However, we live in the real world of old cars where things have got worn over the years, so most cars need some work to restore the original advance curve. If this is not done, then compromises have to be made as to where to set the timing.

While advance has a big effect at idle, the engine is much more tolerant of wrong timing then. Under full power wrong timing can build up heat really rapidly and lead to all sorts of expensive failures.

I've covered all this, including how to simply get the advance curve back to very near original, in more detail on my website at http://www.mgparts.co.nz/advice/technical-notes/setting-ignition-timing/ and http://www.mgparts.co.nz/advice/technical-notes/distributor-advance-rate-adjustments-compensating-for-high-mileage-wear/
Paul Walbran

Hello moss, im no expert but it sounds very much like my situation, why not try advancing the timing a little to see if things improve, if you dont have a timing light mark the position of your dizzy against the block so you can allways get back to where you are now, Vin
Vin Rafter

Assuming your ears are good enough to hear pinking, then if it isn't pinking now it is advanced then it will probably be running a bit cooler and definitely should have better performance and economy.

That was often the case when the cars were new and running on leaded, as the manufacturers specs for timing took into account a worst-case combination of component tolerances, plus a safety factor, to run without engine damage. Many engines could benefit from more advance, as mine did in the day. Some couldn't, like my roadster when I got it in 1989, so I just used the book figure.

With very different fuels these days, and unleaded especially 95 octane, many are going to have to retard from spec to run without pinking. My roadster is hopeless on 95, and runs much hotter on the gauge, but even with V-Power 99 it still has to be a bit retarded to control pinking - 14 degrees at 1000rpm instead of 19 degrees.

Assuming you have a high compression engine then without a lot of time on a rolling road tuning your curve for your engine and fuel the only other option is to run with as much advance as you can without pinking at any combination of throttle, revs and load. And be aware that you might get pinking at part throttle, but not at full throttle, because the bigger charge will cool the combustion chamber more, which reduces the chance of pinking.
PaulH Solihull

If you go by the numbers you could be seriously mislead by two things.
1. You are assuming that the dial on your timing light is accurate. i Found my Sealey item was 5 deg out.
2. we all assume the marks on the timing cover and the pulley are accurate. Not always the case.
SO go by performance. Use as much Advance as the engine will take without pinking. Pinking, by the way, is the explosion of the "end charge". i.e., the last bit of the mixture in front of the flame front which "pings" due to the pressure.
Allan Reeling

distr woes
mine is an 18V 847 engine in a 79 B.
All of a sudden my engine started to miss after about half an hour of running when the throttle was opened yet settled down with no load when I backed off pedal to constant throttle.
Suspecting the vac advance on a worn distr, I purchased a new electronic one. No better.
Put the old one back in (at specs and cleaned), replaced the coil and leads and started on the timing with a light. Setting the timing at 10 deg, (midway on the marks on the timing chain cover), no vacuum at idle resulted in badly missing engine. With vacuum connected from the carb no real improvement. So threw the specs away, moved the vacuum pickup to the manifold, and listened for its sweet spot. It is unbelievably off the cover at idle. I suspect it is around 30deg btdc, yet it is running nearly sweetly with lots of power, and cooler. But it has an irregular miss or fluff when running at power, not enough to hesitate the car but not completely smooth either. According to the timing light the vacuum advance is working regularly. My question: is it possible for the timing chain to slip a tooth resulting in my ridiculously advanced setting where it seems happiest.? Tappets were adjusted, and there is no obvious timing chain noise. Will appreciate any help or do I just leave sleeping dogs lie.
Paul , Australia
P G Ritchie

The timing chain cannot slip. Check the TDC mark is TDC, sometimes the RB front pulleys break up and the outer with the timing marks moves relative to the crank and gives false timing. Also make sure you do not have a n other fault such as a big air leak, sticking carb piston etc etc which may require 'peculiar' timing to run. Ckeck power to coil.

Peter
Peter Burgess Tuning

Manifold vacuum gives near maximum vacuum at idle, and usually that means maximum vacuum advance at idle, which is added to the centrifugal advance. Some distributors, including the 45D4 41610 for that engine, give up to 24 degrees vacuum advance.

Have you changed the points or condenser recently? I have heard of cases where too long a screw was used which fouled the weights at certain rpms.
P Hunt

Peter, not just RB pulleys - I've found it in CB and A series pulleys too.
Paul Walbran

This thread was discussed between 15/04/2013 and 30/04/2013

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