I have a feeling everything is pointing to a vacuum leak, but I cannot find it. 13" at idle in gear > 16" at idle in park. Engine will only idle with primary butterflies open way beyond .030" of transfer slots...more like .090". Idle that shakes the truck. I disconnected all vacuum accessories...nothing changed. I've sprayed starting fluid all around the manifold, all fitting at the manifold, carb base, distributor clamp...no change. The truck did die immediately when I hit the distributor cap with the starting fluid. Is there anywhere else I can look?

 

Sounds more like a timing or ignition problem to me.

 

Not sure if its timing, at idle it is at 13. I run ported advance. My Holley 4160 is using reverse idle mixture screws (cw=rich & ccw=lean). According to holley base for idle mixture screws is at one turn out. Idles best with screws seated.

 

A vacuum leak would require less throttle opening and richer mixture screw settings. I agree with ignition timing or maybe a timing change jump.

 

Spraying anthing on the dist cap and having it die sort of points at bad plug wires, hence the shaking and the need to keep the idle set high.

PS: I suggest you use something other than starting fluid. Spraying that around an engine with leaky plug wires, hot exhaust, etc., could give you a nasty surprise. Maybe try WD40 to find air leaks.

 

So, with the idle screws all the way in, which means RICHER.

Float levels OK?

I would agree though, with Argess, why would it die with starter fluid at the cap?

 

Well to be more specific, I was spraying right where the distributor seal was. Then, being trigger happy, I sprayed right at the bottom of the cap. Spray must have ventured into the cap and killed the engine. The wires are new, but the cap and rotor are not. I will replace cap and rotor and see if it makes any difference.

 

You have a blown power valve. Replacing it will put things back to normal.

 

This carb is new (1 week). Previous carb 80457 exhibited the same behavior. I checked the power valve on the old one and it was functional.

 

There are only 2 things that allow and engine to idle with the mixture screws all the way in. Float level so high that the bowl flood over or a blow power valve.

Unless this is one of those screwed emissions carbs that has the idle screws metering air and not fuel.

 

Bear, you are correct, it is the emissions carb. In earlier post I mentioned "reverse idle mixture screws". Holley tech told me that one turn out is the base, but the engine just runs better with screws seated all the way in...less air. Holley mentioned that the only way to make it run richer in idle is to start drilling through the idle feed restrictors. I think I'll save that task for later, until I find the real culprit.

 

What's the base timing at idle at? If it's too low, you won't have a lot of vacuum, which means you'll need to open the throttle plates more to get it to idle, which means the idle ports won't pull enough vacuum to pull enough gas, etc. etc.

I know you said you have 13 inches of vacuum, but ... let's start at the beginning and get that initial timing posted.

 

In this case, you most definitely have a vacuum leak. Finding it is the hard part. You could even have a bad intake gasket leaking from the crankcase. Bad PCV valve. Bad vacuum canister on the dist. Bad vacuum brake booster.

FYI, I am not a fan of the ***-backward emission carbs, they create more problems than they solve and they inhibit performance.

 

Krewat: Base timing at 600 RPM is at 13. Mechanical total w. ported disconnected is around 35 or so.

Bear: PCV valve is brand new. Canister at the distributor does not affect idle with it connected or disconnected. Same thing with the brake booster and transmission modulator.

Question...presuming if it were leaking between the intake and crankcase. I am running edlebrock dual plane. Since each cylinder has its own port shouldn't the leak affect cylinder at that port location and not rest of the cylinders? I've checked all my plugs and did not see any difference between them.

On the other hand I did post some time ago that one plenum caused the engine to shake. https://www.ford-trucks.com/forums/9...ough-idle.html
this problem persisted even with a new carb, in this case all ports in one plane would have to share that leak...and that would be really bizarre.

This engine was supposedly rebuilt by the previous owner and I do see some evidence pointing in that direction. Could I just have a milder cam instead and try to match its performance to stock engine?