I have a 305 freshly rebuilt with a 600 cfm holley carb. It ia hard starting rough idle I mean real rough I pull 14"of vac it runs pretty good in the higher rpm ranges. I replace cap wires rotor and the distributor is at 10 degrees advance. I cant seem to get this carb to idle well. The cam isnt that big and the timing advances with an increase in RPM. So I dont think it is a timing issue but I am not sure. Some help here would be appreciated. Would the secondaries not cracked open cause this problem?

 

Sounds like a vacuum leak.

Look for vacuum ports all around the carb that aren't used or capped off. For testing purposes, you could remove all the lines and put plugs on every fitting, and see if it's better.

If it has ever backfired through the carb, even once, you may also have toasted the power valve(s). It only takes a couple of minutes to replace those, you might want to try that first; put a 8.5 in the primaries, and a 5.5 in the secondaries if there is one.

Rough idle is not usually caused byanything about the secondary throttles.

 

I pulled 4 plugs and they all looked the same like they were foulde with carbon with the slight odor of gas. I think this is because my timing was too retard for the cam I have and maybe still. Its at ten now and I plan to advance it another4-6 degrees from some of the post with similar cams they seem to like alot of advance It was running a 6.5 power valve the valve had a lot of corrosion on it I dont think this would be normal. the powervalve is 6.5" and I am not sure if you can tell if it is blown by looking at it or not. The jets are 65 my guess is the carb was never touched from when it was bought. I got it used. I didnt find a vac leak. I did find that the choke fast idle screw was backed way out and not even working. Do you think this cause the rich condition? I think it did. The car does stall when the idle circuit lean/rich screws are turned all the way in for now I have them backed out 1 and a half turns. Is there anythig else I can look for at this point to cause my rch condition on my plugs?

 

Was that carb on your old motor and it ran fine till the motor was rebuilt?

 

No it was something I picked up while rebuilding the motor.
I should have rebuilt it when I had it off. Other than the corrosion on the power valve it looked to be in pretty good shape but I plan to rebuild it tomorrow. Any words of wisdom on this

 

I grew up with Quadrajets had a few holleys but never had them apart. I would run it through a carb vatt if you have access to 1. If not and you have the extra cash I bought that 5 gallon pail of carb cleaner that has the basket it works pretty good too but cost about 45ish, just a little easier then manual scrubbing. Anyways to me that sounds like your problem you may want to check on the carb board for the proper size jets for your 305 combo

 

Thanks for the reply I am trying to find the part number for the tuning kit for holley carbs the only one I can find is for the double pumper though. I think the jets and a slightly smaller numericly power valve will get me in the ball park along with the proper choke settings too oh and not to forget about the timing too.

 

Well, although I did not find a visible vacuum leak, I decided to change the intake gaskets and rebuild the carb......let me tell ya what a difference. 18" of vac at a 800 idle and 12 degrees advance no popping spuddering or stalling. I can not believe how easy it is to rebuild a holley 600 cfm vac sec carb. If I could do it anyone can. I wanted to thank everyones help here especially rb83, ede, hotroddn, f-bird88, although he probably thinks I am a pain in the a$$ by now lolol Oh instead of using felpro gaskets I use victor reinz they were cheaper and seamed toi be a much higher quality gasket. It went together very nicely.

by the way does anyone know an easier way to adjust the fast idle screw on the choke for 4160 holley vac sec carb?