I have a '69 with a 427. The old carb was so gummed up that I, bought a new Holly 4160 which fixed the majority of the problems I was having. The only problem left is that I can't reduce the idle speed - it idles at about 1100 RPM. I checked the float level and it is right. I searched for vacuum leaks everywhere with a hose and funnel to my ear and can't find any. The primary pump is adjusted correctly. With all this, when I unscrew the idle speed screw - the idle speed won't decrease below 1100 (but will increase when I screw it in). I've thought of adjusting the idle adjustment needle to reduce the gas flow, but that would only be covering up the problem - not fixing it.

Anyone have any suggestions as to what might be causing the problems?

Thanks in advance.

Ryan

 

Did you check the choke fast idle screw?

 

Secondary stop set too far open?

 

Throttle cable binding?

 

Close down the secondarys a bit. The screw us upside down so either remove the carb or bend a small flat bladed screwdriver to 90 deg and work it that way.

 

thanks for the suggestions. The engine is warmed up, and the carb drops down from fast idle. The cable is loose and there is basically no pressure on the pump. I didn't think about air being sucked through the secondary. From initial look, the secondary looks pretty closed though.

Am I right to think that screwing the idle adjustment needle in is not the right move? I'm sure the engine speed will drop, but I will have a too lean mixture.

 

Take the carb off and turn it over. The secondary slots need to be just showing below the butterflys. Adjust the primarys the same. Reinstall and try it, you will be surprised.

 

The secondaries were completely closed. The lever was just barely touching the adjustment screw, and unscrewing it dosn't close the secondaries any more - it just moves the screw away from the lever. The primaries can be closed a little more, but I would expect if that was the problem - I would see a very high high-idle speed (high idle now is about 1500 rpm).

 

Hose and funnel? Buy some starting ether and spray around the base of the carb.

 

A hose and funnel make a surprisingly good stethescope - makes a vacuum leak sound like a freight train (so you don't have to worry about the inconvenient side effects of ether - like explosion). You can also hear vacuum leaks through the walls of the vacuum hoses.

I'm planning to just replace the intake manifold gasket because unless anyone has any other good ideas - I'm basically out of options. I suppose it is also possible to be leaking through the head gasket. Maybe I'll get lucky.

Is it unusual that a brand new carb would come out of the factory with the idle speed screw set all the way out? Maybe the carb was machined wrong?

 

Mitch, try one more thing. I had the fast idle problem on my 70 vette 350 stock for years. I finally solved it. It was an ignition timing issue, not carb or vacuum leak. Here's why.
My TCS (trans controlled spark) solenoid quit years ago so someone bypassed it, hooking vacuum advance hose directly from carb to vac can on distributor. When I set my initial base timing, I did so as the book says, with vacuum hose disconnected and pluged. In this mode, I got a nice idle, at say 750 rpm. When I hooked the hose back up, my idle went up to say 1000. I could never adjust carb to lower that.
Ultimately, I restored my TCS system, which blocks vacuum to the distributor in all but 3rd and 4th gear, basically when cruising at 40 mph or more. Then I got back my nice idle.
My take on this is that the vacuum can in the distributor was advancing the timing say 5 degrees even at idle. The vac can is too aggressive in my car to allow it to operate at idle.
So, remove you vac advance hose. If the idle goes down to where you want it, you can do one of two things. Change the vacuum source to one where there is less vacuum at idle, such as a ported source on the carb above the butterfly. OR, change the vac can inside the distributor to one that will not give any advance at your idle vacuum. Lars has a paper on this site that gives part numbers of all vac cans and shows that they come in at different vacuums, and give different maximum advances. My ultimate conclusion is that GM put an agressive vac can in my 70 because it was designed to have no vacuum at idle because of the TCS system. Post back if ignition advance was responsible for your fast idle.

 

I've looked at timing, and without the vacuum advance I am about 8 deg BTDC. I don't have a TCS solenoid - the vacuum advance pulls vacuum directly from the carb. Since it is a manual xmsn - I woludn't have a signal to control the solenoid even if I was supposed to have one. From the sounds of it I may have some tweaking to do with the advance in the future - but I don't think it is the source of the problem. Thanks for the suggestion.

 

I rebuilt the carb in my boat, and I put the main gasket on backwards. The idle was about 1100. Turned the gasket around, and all was well.

 

I replaced the intake manifold gasket hoping to fix an unfound leak. I found some other things, but the car still idles a little over 10,000 RPM. I know the engine was rebuilt before I got it. Could a different cam and compression ratio be causing this? I'm basically all out of ideas.