It’s got a thick gasket and it’s the correct Autolite power valve. Plus if it was bottoming out I think it would cause an issue at part throttle not at idle.
Had a play whilst waiting for a vac gauge. It like 16 BTDC best at idle. At this point I can’t get the rpm below about 900 as the screw is all the way out, need to see what is limiting the throttle blades. Covering the primary and the engine gets slower and dies so don’t think there’s a vac leak. Covering the secondary doesn’t do anything.
Do those carbs have a secondary throttle plate adjustment screw? Maybe second’s are open and into transfer slot???
Yes they do and they’re closed, I double checked by putting my hand over the secondaries and it had no effect.
And if powervalve leaking thru you will not see it in venturis because power valve vacuun channel is in bottom between bores. And i just look one of my spare 4100 and it has total of 6 air bleed holes in booster assembly.
That’s the vacuum signal orifice, fuel will go through the venturis, it was also dry in there so I know the pv isn’t blown.
I guarantee, if power valve leaking thru, it wil suck raw gasoline via that vacuun signal orifice. But you dont have to believe me and you are sure power valve is not leaking.
I see what you mean now, it’s definitely not leaking through there as it was completely dry when I took it apart.
I am wondering if the fuel is bad. It’s a bit difficult to tell as it’s US fuel and it doesn’t smell the same.
How does a blown power valve affect idle “only” (cruise yes) when all it does is feed more fuel via a bypass to below the main jet? This is where the fuel is drawn from for the idle circuit which in and of it’s self is a secondary metering via idle adjustment screws.
I think what he’s suggesting is that if there is a hole in the diaphragm of the power valve it will allow fuel past into the vaccum signal hole and then into the engine. But it’s definitely not the power valve.
Sometimes its better take break with difficult malfunction and just bolt temporarily known good holley 600 in place of autolite. Just wake this beaty up at my barn where it was sitting two years.
I know that 6 month old fuel in my roadster (600 Holley) won’t idle, and with no choke doesn’t want to start below 70 degree F. Siphon it out, new fuel, problem goes away. The old stuff looks fine, smells fine, runs fine in my daily. I've always blamed it on the ethanol and/or the more volatile stuff evaporating off. Doesn’t really act like your problem.
Wouldn’t immediately strike me as bad fuel either but if I set it up to run on bad fuel rather than the fuel went bad maybe it is.
When you rebuilt the 4100, did you remove the plates from the shaft ? Primaries and secondaries are the same size, but different. The primary plates have a taper that the secondary plates don't. There is also a right side up for both. Get them in wrong and they won't close correctly.
One other odd thing now I’ve got a known good gauge it always stays at 16-17”hg, unless you rev it over 2000rpm when it’ll climb to 20 and then start to drop off. Change the timing, change the mixture, change the idle speed still 16-17” up to the point it just dies
I just rebuilt one of these and i had issues with it. the first issue i had was after bolting it to the manifold i cranked it and it was leaking all over the manifold. after a bunch of dicking it around i finally figured out that the power valve cover was warped and letting fuel leak. after figuring that out i started wondering why it was leaking fuel because there should not be any fuel that far down past the valve. i would suggest that you remove the power valve cover and the top of the carb on the bench, pour fuel in the bowl and check to see if fuel leaks through the valve or the gasket on the valve. the reason it runs rich is the fuel drains through the slots from no mans land between the bowl and the throttle bores. i hope this helps i wasted a lot of time figuring out the problem.
Ok, let's remember that a lot of carb problems are electrical and a weak or improperly timed spark can cause a rich mixture... Next, 10 BTDC is the place for timing a car in the 1970's running leaded fuel. Try this as an experiment: set your timing at 16 BTDC (with the vacuum disconnected) and see if the problem resolves. Modern fuel has additives for high pressure fuel systems, and this fuel takes longer to burn at low rpm/dynamic compression levels. This has been increasing over the last 30 years, as it has been since the 50's when fuel liked TDC at idle. My guess is that tuning the carb will feel a lot more responsive once you advance the timing.
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