QuestionQUESTION: Hi Barrie:
First let me answer the questions: (1) Yes the vacuum is taken from the Weber carburetter. (2) The number stamped on the dizzy is 17. I have attached a picture of the number because I do not know what the "action plate" is. The number is from the OPUS that was on the car when I bought it, which had the ugly bulbus attached amplifier. As previously stated I now have a Pertronix FlameThrower installed. The "advance" measurement did not change with the installation.
I was really surprised at the "advance" differences between the US and UK cars. Distributors do not determine the burn rate, they speed compensate for it. Are the differences due to engine design or fuel chemistry?? Is it possible for the person who rebuilt the engine in my car to have done something to slow the burn rate?
ANSWER: Hi Bob.
You gave me the chassis number. From that I know you have a USA LHD sports car made during the 1980 model year (September 1979 - August 1980).
What I do not know is the engine number. The fact that you have an earlier front cover makes me suspicious that you no longer have the original engine. Look between spark plugs 2 and 3. There should be a small alloy plate rivetted to the top of the block. If it is still there, please tell me the number.
Also, the distributor could be standard USA or Californian spec. The only difference is the vacuum unit. Look around the rim of the vacuum capsule for some very small letters stamped into it. They will either say 3-11-12 or 5-11-7. This tells me the vacuum advance (12 or 7) and the amount of vacuum needed to operate the capsule (e.g. starts at 3 inches of mercury, maximum advance at 11 inches).
The number 17 that you found on the action plate tells me that the maximum mechanical advance is 17 degrees at the distributor, which corresponds to 34 degrees on the crankshaft because the dizzy only rotates once for every 2 rotations of the engine.
Similarly, if the vacuum unit says 12 then that will add another 24 degrees of advance.
If you were then to set it at 30 degrees static advance, the theoretical limit would be 30+34+24 = 88 degrees of advance. At that setting, the engine would be seriously damaged the first time you drove it.
One last question, do you still have a catalyst fitted to the exhaust, and are they mandatory where you live?
............... Just re-read your previous correspondence.
If the engine number has been removed, then we have no idea what engine is fitted to your car. The very last engines had a cylinder head which was treated to accept unleaded fuel. It can be recognised by 4 un-drilled extra lumps on it, one alongside each spark plug hole. They might have been intended for diesel injectors, because these engines were also used in the LDV commercial van. It is not definitive, because the rebuilder might have re-used the head but not the block. Also, the camshaft might be non-standard. The list is endless. One thing you might try is a compression test. A standard UK engine should give 140 - 150 psi on all 4 cylinders. A lower compression USA setup might be 120 psi. A modified engine might be higher.
Keep digging
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RHS of engine
QUESTION: Hi Barrie:
Gad, how do you know all these numbers!! Here are my numbers.
As you can see from the attached figure the engine number and one of the rivits is missing. I do not know why. This is probably not the original engine.
Around the rim of the OPUS vacuum capsole are the numbers 5 11 7. There is also a long number; likely a sereal number. Using your calculation the limit would thus be 30+34+14=78 BTDC! That would require a very slow burn to occur ATDC. Not likely.
The engine appears, see attached figure, to have the "extra lumps" on it.
To prove that this engine has been really fiddled with, here are the cold engine compression measurements I just made. Cylinders from 1 to 4 are 185, 190, 181, 184 psi.
Thanks for hanging in here like this.
ANSWER: Hi Bob
We now know that you have a high compression engine and a distributor designed for Californian emissions regulations and Californian fuel quality. Not a good combination.
I am concerned that we do not know anything about the camshaft. It could be the original one, or it could (for example) be a Crane cam.
Does the engine pull well from low revs, or is it lumpy below say 1500rpm? Can you tell if the power comes in higher up the rev range?
Also, do you have a cat fitted to the exhaust system?
If the law allows it I would ditch the cat, fit a good quality sports exhaust system, and fit a distributor with the right characteristics to suit the state of tune for your engine. If you get a Lucas 45D4 unit you should be able to transfer your Pertronix Igniter to the new dizzy.
Looking at the Moss catalogue, there are several to choose from depending on your state of tune. The significant difference between HS4 and HIF4 carbs is the location of the vacuum take-off. Your Weber has it on the carb, so you probably need a dizzy designed for HS4 carbs.
Let me know how you get on
Regards,
Barrie Jones
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QUESTION: Hi Barrie:
I was required to put a catalytic converter on the car some years ago. The car presently is mildly lumpy below 1500-1800 RPM; which smooths out with any modest acceleration. Quite smooth above that engine speed. Note that since I installed the Pertronix FlameThrower distributor (not just the "igniter" points) I have set the advanced for a comfortable start and idle. I have emailed Pertronix for information on the FlameThrower advance curves but have received no reply. Their Home-Page advertises a "Tailored advance curve for optimal performance and driveability." I can and will measure the mechanical advance, however I do not own a vacuum pump for the vacuum advance.
Your camshaft query motivated me to dig through some old "rebuild" documentation. The "rebuild" included a Crane camshaft and lifters. No part number was identified. If I guess at the part number the duration @ 0.050" int/exh is 222/232; which is larger than the MG values of (I think) 198/214. Qualitatively the higher compression should reduce the "advance" but the longer duration should increase the required "advance." Another possible "rebuild" source of increasing the "advance" would be reduced fuel induction efficincy as well as my tendency to keep the idle mixture on the lean side of optimum.
An explanation for the crazy static advance I need is my central question. While it is near impossible to be quantative about these "sources" they could be the explanation.
I am not, at the moment, motivated to change distributors because I really like the way the Pertonix distributor cleans up the engine compartment. I will proceed by measuring the advance curve and fiddling with the advance setting to best suit my driving patterns. Thanks for the help. You have been great.
Bob
AnswerThanks, Bob
I guess we pretty much got to the bottom of it. Sounds like you have a nice engine, with a half-race cam and raised compression.
You say you have a Pertronix distributor, but your earlier question referred to a Lucas Opus distributor.
Don't worry too much about the vacuum unit, it only operates when the throttle is closed (or only partially open).
My final advice to you is to take your car to a good rolling road. They should be able to optimise all your settings (mixture and timing), and tell you whether your distributor has the right advance curve.
