I have my timing set at 6 @ 900rpm. I then set my mixture using a vacuum gauge. I took measurements from the distributor vacuum advance port on the intake (no vacuum advance on the distributor), is this an ok location to take the reading from? I got between 7-8 inches of vacuum @ 900rpm depending on the mixture but the vacuum needle was steady. The carbs have #66 jets with no power valve. When I rev the engine is sounds good, but if I test drive it under any kind of acceleration it sputters and coughs out the exhaust. It even backfired softly out the exhaust. I am just wondering if anyone has any ideas where to go from here. I also want to thank all those who have already helped me.
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Re: More L-71 Problems
Gary,
I would start with a fresh set of spark plugs. High C/R big blocks eat plugs in a hurry and the result is often the exact problem you have described. Your timing numbers are at least close enough that there shouldn't be any problem caused fron the few degrees that it's off from factory specs. If you drive your car around town and seldom get it out on the road to run hard, I would consider moving up ONE number hotter in the plug heat range. The spark plug change would be a good place to start and will more than likely cure the problem. Looking at the removed plugs will also tell you what's happening inside the combustion chamber. If the plugs are black or oil fouled, you may have a carburetor or oil consumption problem fouling your plugs. Both are common also. -
Re: More L-71 Problems
Gary,
Just read your post again and caught the part about "no power valve". I'm not 100% sure because it's been a few years since I've had those carburetors apart but I seem to remember a power valve in the center carburetor. If it's calibrated to have one and yours doesn't, that could be a very lean condition that would alas cause engine miss/backfire. Hope some of the more current 435 guys can tell us if their supposed to have it.
Just found the info. According to parts book, there is supposed to be a power valve in the primary carburetor.Comment
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Re: More L-71 Problems
The vacuum advance signal line will NOT provide a valid manifold vacuum reading at idle because it is a "ported" vacuum advance.
Tee your vacuum gage into the choke vacuum break line to read true manifold vacuum.
DukeComment
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Re: More L-71 Problems
Thanks for the help guys. I should have thought to check the plugs, I will do that. I had a feeling the advance port would not give a true reading. Would it be better to set the timing further back to 4 or 5?
About the power valve, Since the carb has #66 jets, shouldn't that be big enough jets to not need a power valve? Thanks for the help, I will try to get a chance to work on it tomorrow.Comment
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Re: More L-71 Problems
Gary,
Removing/plugging the power valve was an old racer trick that never did work at all for street operation and really wasn't even a good idea for racing. The whole theory of the power valve is to add the necessary additional amount of fuel when the throttle is opened and vacuum drops below a fixed vacuum point. (the value of the power valve) At that time, the engine can definitely use the extra fuel but when the engine is idling or running at cruise with light or normal load, that extra fuel produces a mixture that is entirely to rich and plug fouling, among other things, is sure to result. All hot rod shops sell this junk and even Holley has them listed in their catalog but steer clear. It's just another "hot setup" from hell.
I've been looking for my Holley carburetor specs book but haven't found it yet so I don't know what size the original jets should be. I'll keep looking unless someone else happens to post it for you. These cars ran amazingly well just the way they came out of the box and most of the "trick of the week" stuff was a waste of time and money. If you get your entire carburetion and ignition systems bone stock and working properly, you be impressed with the results.Comment
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Re: More L-71 Problems
If you can run up to 12 degrees initial timing without detonation, go for it.
Be sure to disconnect the vacuum can and plug the line while setting the initial timing - even though it has ported advance, and also be sure that you set the initial at lower than starting speed for the centrifugal. You can test by bringing the revs up from the speed you set to timing at and observing at what speed the timing begins to advance. You should set the initial timing at least 200 revs below the speed where the centrifugal starts to add advance.
DukeComment
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Re: More L-71 Problems
Twelve degrees initial timing on today's fuel is not a good decision for a 435 with original C/R, especially if the car has side exhaust. You'll never hear the rattle until it's too late and piston ring lands are coming out of the tailpipes. It's ok for idle but at around 2000-3000 RPM under heavy load, the timing would be quite a bit higher than it should be.
A better idea would be to use Dukes method of full vacuum to the vacuum advance unit at idle and part throttle, but only around 4 to 6 degrees (MAX) initial. Big blocks on the street are happy with around 36 degrees total. More when the engine is still cold but with the coolant temps that these engines run at, 36 is plenty.Comment
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Re: More L-71 Problems
Mike is absolutely correct. A power valve is the only thing that fills the transition between the idle circuit and the main circuit. The jets do little or nothing until vacumm builds up again after opening the throttle. The lack of a power valve will cause a huge stumble and a backfire as the engine gets real lean. Some times people get way off the mark and crank up the idle to compensate and actually start fuel running out of main circuit prematurely causing a whole bushel basket of other problems. Start the jetting and power vales at stock settings and adjust one thing at a time. Never change more than one thing at a time so you know if that change is for the good or bad. This means you can always go back one step and be back to normal. BY THE WAY, HOLLEY POWER VALVES ARE PRONE TO BLOWING WHEN AN ENGINE BACKFIRES. SO MAKE SURE THAT YOU GET A GOOD POWER VALVE IN THE CAR. I run leaner than normal jets and power valves. I took out some mechanical advance and cranked the inital timing a 12 degrees, which equals the total stock advance. I use extended plugs that are two ranges hotter. If you want to learn about carburetors get a Holley Caburetor book by HP books. It is the Holley bible.Comment
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Re: More L-71 Problems
I agree it will be better to go back to a stock setup with a power valve and #63(?) jets I believe. Who ever had the carbs before me did use them for drag racing so that probably explains the setup. They also had mechanical linkage to open all three at once. I have gone back to vacuum secondaries. Thanks for the help.Comment
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