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MG Midget and Sprite Technical - confused about what data to use for checking timin
|I have already checked my ignition timing using what I thought was the correct specifications but now I am having second thoughts or at least I want to understand what the book numbers referr to. I have three sources and none of them contain the correct numbers:|
(1973 midget with stock 1275)
Factory workshop manual:
Stroboscopic ignition timing: (vacuum pipe disconnected) = 16 deg. B.T.D.C. at 1,000 rpm
crankshaft degrees/speed (vacuum pipe disconnected):
0-2 deg at 600 r.p.m.
2-6 deg at 800 r.p.m.
4-8 deg at 1,000 r.p.m.
10-14 deg at 2,000 r.p.m.
20-24 deg at 5,600 r.p.m.
Ignition timing (dynamic, vacuum line disconnected):
9 deg BTDC at 1500 rpm
Moss Motors (USA) catalog tech specs:
13 deg BTDC @ 1000.
Please tell me how to reconcile these numbers. I am especially interested in understanding why the factory book says on one line that it is 16 deg BTDC at 1000 and then below that it says that it is 4-8 deg at 1000.
What numbers should I use for a basic dynamic ignition timing check?
|The factory manual figure is a total measurement to be seen when the engine is running (vac pipe disconnected)|
the 2nd set of figures is how much mechanical advance the particular distributor gives for a set rpm. you would to add the 4-8 deg figure to any static advance it to give 16 deg advance at 1,000 rpm if the dizzy gives 8 deg advance at 1,000 rpm you would set the static timing to 8 deg advance.
it's all very irevelent though! if you have access to a strobe timing light, disconnect (and plug) the vac advance pipe and set to 16 deg @1,000 rpm. It would also be prudent to rev the engine to ,say 5,000 rpm and note what advance you are seeing - you should see 28-32 deg
|If this is the case then my timing is set way too retarded because I used the set of numbers that were to be added to the static ones to get total static advance as strobe timing numbers.|
I set it using a timing light using the second set of numbers. The previous owner apparently had the same confusion because it appears that they also set it at about 5 deg BTDC.
So I should do it again and advance it to 16 deg BTDC at 1000 rpm with the vacuum pipe disconnected (and capped off).
to re-iterate, you agree correct?
If so, why does the Haynes manual says 9 deg BTDC at 1500?
The MOSS catalog says 13 deg BTDC at 1000. That would be in the range (7 static plus 6 centrifugal which would still agree with what you say).
So the only remaining mystery is what the Haynes manual says. Is it just wrong. They use 1500 rpm for the reference rpm. Since that is higher than 1000 I would expect the it to be a lot more advanced than the 1000 rpm number.
Thanks for your comments.
Yes, when the spark is happening whilst the engine is running is the important bit! set to 16 Deg BTDC @ 1,000 rpm, no vac, and you won't be too far off if everything is as standard - what fuel do you use? if you are using 95 octaine without any additive then that opens a can of worms! ( but set the timing to 14 deg BTDC )
I'm sure better qualified opinions will be along soon!
|This is not difficult and is as follows|
If the distributor is the original item then the following will be correct according to the figures above
Factory figure is 16 degrees BTDC at 1000RPM (vac disconnected)
This is 4 to 8 degrees mech advance plus about 8 degrees static making a total of about 12 to 16 degrees at 1000RPM
This 8 degrees static is then added to the 20 to 24 degrees at high revs to make 28 to 32 degrees
Not too accurate is it and how does that compare to Haynes manual??
God knows is the answer which is why the smart way to adjust timing is to set it at maximum revs with vac disconnected and set it at 32 degrees BTDC plus or minus 2 degrees, then it will be spot on no matter which dissy is fitted.
Max revs in this case is when the mechanical advance is fully in generally around 4.5K to 5000 RPM.
If you use this method then the timing will be optimised to your distributor and the engine will run fine, I can not think of any other method that ensures this to happen!
|Bob Turbo Midget England|
|thanks for the helpful info. this is exactly what I was looking for. Unless my timing mark is in the wrong place, I have been running too retarded.|
|it was fascinating to study the different engine specs shown in the Bentley '67 ~ '74 book, for the different years. The advance curve changes each year (and the differences between different markets (US vs. Europe) during the same years) helped me to understand how much the amount of advance could be altered to meet different priorities:|
- lowest emissions (more advanced lowers some pollutants, but risks detonation)
- engines supplied with different compression ratios (lower CR allows more advanced with less risk of detonation)
- also, see how the carburetor needle was changed in those different engines to correlate with those timing changes.
Be careful, some numbers in books refer to the initial advance, and then refer to advance as a number that should be added to that, and some refer to a number that has already been added to that.
In the end, the "never go above 32 total, so check it at 5000RPM" is the single most useful bit of advice, and should apply to any engine as a good cross-check. Then the initial "static" number will be the result of that 32 minus your distributor's advance amount.
Something you might consider if the engine has been moddified/tuned much beyound basic, is to call jeff shuluem at the place you purchased the timing tape / pointer and have him "recurve" the dissy, he does excellent work, dont hold back on info to him, the more he has the better the performace
remember you probably dont have the orginal dissy as they like to wear out and cheap to replace over 40 years nor is the engine probably stock as it came from the factory
the right dissy curve for the engine thats in the cars engine bay hole makes alot of differance
|I am still discovering the condition. I have no reason to believe that it has been modified yet. Thanks for all the advice. I am taking the day off friday to adjust valves and set the timing.|
I am having some engine trouble that I am still debugging. After I get these things right then I will be in a better position to decide what to do next.
This thread was discussed on 28/07/2010
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