425 nailhead does not idle

kbalser

Member
I bought a rotted out but running and driving 66 wildcat for parts for my project. This was a 401, didn't run great but was able to cruise with it a bit. car idled ok. When I brought the engine in for rebuilding the block was bad. Found a 425 block and did a complete rebuild to stock. Used the heads and manifolds from the 401. I have a pertronics kit in the distributor. Ignition timing is set. Cam is stock replacement, cam timing checked and correct. Carb is a rochester 4GC. I rebuilt the carb myself. I used a well-recommended kit and followed the manual. I am not a professional carb doctor, but I have done a number over the years with good success. Carb has been apart 3 times to make certain everything is correct. Engine starts right up and runs great unless you try to get it to idle down. In neutral, about 700-800 is the best it will do. Any less, it will start surging and die. If you put in gear, it will die. Idle must be set at about 1100 for the engine to stay running in drive. It's like the idle circuit is not working. I figured at first it must be a vacuum leak but have not been able to find any. I even pulled the intake manifold off and pressurized the exhaust section, to see if there was a leak from there into the intake. With the rpm up, the engine runs great and has plenty of power. I have don't know what to try at this point. If anyone has any ideas, I'm listening.
 

suntreemcanic

Active Member
I have a 425 and I did not have an old carburetor. As per Russ at Centerville Auto Repair, he specializes in Buick Nailheads, he recommended an 825 CFM Edelbrock carburetor. It runs perfect right out of the box. Starts at 0 degrees and 100 degrees. No hesitation, almost as good as a modern fuel injected engine.
 

kbalser

Member
What is the initial timing, and is the vacuum advance connected and working?
I have played around with the timing, mostly with the vacuum advance disconnected. No help. I think right now the timing is at about 0 degrees with vacuum disconnected. The vacuum advance does work and does not leak. I should also mention that the crankshaft dampener has been checked for correct alignment.
 
I have played around with the timing, mostly with the vacuum advance disconnected. No help. I think right now the timing is at about 0 degrees with vacuum disconnected. The vacuum advance does work and does not leak. I should also mention that the crankshaft dampener has been checked for correct alignment.
The vacuum advance should be connected to manifold vacuum so that it adds ignition advance at idle. That will help it idle in gear.
 
I have had it connected as well, makes no difference.
One thing is for sure, it will not want to idle with no (o*) ignition advance. It will also run hot. If the vacuum advance is hooked to manifold vacuum, it will get vacuum anytime the engine is running. That will give you 14-18* of advance. It will idle easier with advance. If you hook the vacuum advance to ported vacuum, there will be no vacuum at idle, only when you open the throttle. Make sure the vacuum advance canister actually works. It could have a bad diaphragm.
 

kbalser

Member
I have sprayed around the carb area with starting fluid without result. throttle shafts are nice in the bores. not loose. I have been up and down with the timing both with the vacuum connected and blocked to know that I'm not going to solve the problem with timing. At this point I am thinking I will put an Edelbrock on it and see what happens. Thanks, guys, for your interest.
 
For interests sake, before you take the carburetor off, get the engine running as well as you can as close to an idle as you can. Then when you take it off, have a look and see how the butterflies are sitting relative to the idle transfer slot.
 
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