My engine builder pointed out to me that when u pull the dist vac tube off the carb, the idle drops noticably. He said it should not do that and that carb distrib vacuum port is not functioning right.

The result apparently is that I have full advance at idle.

Recommendation: a non vacuum distributor.

Does all this sound correct?

 

No, don't do that as it will trash your gas mileage. What brand of carb, on what engine and is the motor hot rodded? The carb is either adjusted wrong, as the throttle plates are to far open at idle letting the port opening see manifold vacuum at idle. Try readjusting the the carb per the manual. It is amazing how screwed carbs get when adjusted by self proclaimed experts. If you have a hot rodded motor, you make have to drill some holes in the throttle plates so you can close them properly at idle. The other possibility is the wrong port on the carb is being used (very easy to do on the Edelbrock carbs).

 

Its a fairly new holly 750 vac and the dist is a stock unit in my 1968 390 with a 519 comp cam

 

Is the advance unit hose connected to the passenger side of the carb or to the base plate? I'm stupid, how above stock is the cam, duration and lift?

 

Hi, the tube is connected to the passenger side near the top of the carb...definatly not at the base plate.

I'm no expert but I think a stock cam is closer to 425 lift. I have a healthy idle.

 

Then you either have a maladjusted carb or you will need to drill it. I suspect a proper adjustment job will cure it as I've run some pretty high lift cams in FEs and never had to drill a plate yet.

 

The engine builder adjusted the floats and idle circuits.....what other adjustments can u do to a holley? (BESIDEs idle speed)

 

He maybe an engine builder but it doesn't mean he did the tuning adjustments right. You need to recheck them yourself. It's not rocket science and the Holley web site has all the instruction to do it. I have reset carbs that were done wrong by any number of different high dollar engine builders when their engines wouldn't idle properly, suddenly run fine after a twik job. Go here and find your carb # and follow the instructions.

http://www.holley.com/TechService/Instructions.asp

 

cool thanks!

 

Are you aware that there is a mechanical advance under the point plate? The OEM distributor has spring tangs that regulate the advance curve. The tangs under the plate are bent with a screwdriver to limit mechanical advance by spring tension and must be "dialed" in to establish the curve. When the idle falls is the vacuum hose plugged? Or wide open? A vacuum leak will lean the motor and raise the idle speed abit. If the hose is plugged, then it sounds like the diaphram has a leak. If the hose is open, the idle should rise, not fall. Is everything inside the distributor in order? Nothing sticking. Where is the timing, with and without the advance connected? And at what RPM is full advance and how much?.......aMP

 

aMoneypit, His primary problem is full vacuum advance at idle which means a carb problem first. If you are running manifold vacuum and this is what is happening with the carb throttle plates to far open at idle, this is the what you get and there is no way the mechanic advance has anything to do with this problem.

 

AND, how much advance is that, "full vacuum". There is more going on here than a ported vacuum outlet being screwed up. Is this an adjustable V/A? I have seen several Ford dist. that people have done all sorts of things to trying to get the detonation out of the motor, including adjustable advances. Thats why I asked about some actual timing numbers, and if the hose was plugged when the idle dropped. The advance system is designed to work with the throttle plates open while underway, and I don't see how the ported vacuum outlet would be that much affected if the plates were cracked a little too much at idle. If anything, the actual vacuum reading would be low if the plates were not sealing. Again, what are the actual timing numbers when this happens? Hose connected, and disconnected, open, and plugged........aMP

 

I will double check everythong tomorrow..please stand by

 

Matt, the timing should be set with the V/A disconnected and plugged. Set the initial to specs, then re connect the V/A and lower curb idle to specs with the idle speed screw on the carb. I think the problem may be that the initial was set with the V/A connected, so the timing drops drasticly low when disconnected. Just a thought..........aMP

 

Money , timing was set at 6 with the vac discon and plugged