Engine idle pretty lumpy

I got the 425 nailhead started on an engine stand, got the timing set and water leaks under control so I broke in the cam, held it at 2300RPM for 30 minutes varying the RPM a little, using 15-50 break-in-oil it was running 36 pounds oil pressure and about 185 degrees. I am using a carburetor that has set for 15 years, it did alright at high RPM but when I tried to idle at 650 RPM it would not idle very long, it has a pretty lumpy idle. I am not sure if it is carburetor or cam. I bought the cam from Rus at Centerville Auto Repair, I let him pick it out. I have read about Buicks having a pretty aggressive cam and that was the need for the switch pitch transmission. Now it is time to hook up the transmission and slip it into the car. All in all I was very satisfied with the run in no oil leaks and no heating problems.

 

Congrats on the first run.

What is the vacuum at 650?

Several things can have that effect.

Lean or rich condition at idle, vacuum leak.
Mechanical advance issues with springs, or worn distributor shaft, points not adjusted correctly(Gap/Dwell), plugs not gapped correctly.

Pull the plugs, since you were not idling, but running at a decent RPM for a while, you should have a good read on them. Post pics if you can.

 

Adjust the idle mixture screws

 

where is your timing set at.

 

MY vacuum is a steady 17 inches and my timing was set at 6 degrees advance and my dwell was 30 degrees and also steady.

 

There are several initial curves for nailheads, from 2.5 deg BTDC to 12 deg BTDC.
You should be 10-12 initial. (if you have the correct distributor for the 425) Also, make sure the rubber bushing in the advance is not missing or "hardened" (often deformed) as that can result in too much advance at higher RPM.

With 17 inches, it does not sound like any vacuum issue.
Did you set the timing with a vacuum gauge at the highest point?
I would set the timing to 12, then, as Sean stated, adjust the idle mixture screws.

Work slowly, and let the engine "settle". It can take a minute or two to the change in fuel/air for the engine to get stabilized. Otherwise, you can be chasing the effect of the changes.

The carb may need a bit of adjusting to match the changes introduced with the cam.
What carb do you have?

 

I am assuming this is for a 1x4 carb. & intake manifold??? Did you put in the stainless spacer/heat shield under the carb??? IF you did not you are sucking exhaust into the intake. Reason for a rough/lumpy idle.

 

Tom I blocked off the exhaust from getting under the carburetor. I am using an Edelbrock carburetor currently. I have not decided on a fuel delivery system yet. I have a dual four barrel intake manifold, a '66 quadrajet manifold and a four barrel from a '59 401 that I am using right now. I will adjust the idle mixture.

 

Sounds like you have good vacuum so it doesn't sound like too big of a cam. What are the exact specs of the cam, .050" I/E, LSA, ICL, and such? Depending on what cam you have 650 rpm may be a bit too low for it to idle reasonably smooth. Of course all the above suggestions should be looked at but once I go with anything much bigger than a stock cam(in any engine) the idle speed needs to go up.

Are you using ported or manifold vacuum for your vacuum advance?

 

I drove the Vets to the hospital today so I did not work on my engine. I do not have the cam specs in front of me but I told Rus my 52 Buick weighs 5,000 pounds empty, I wanted to use 3:08 rear end gears and I was after torque. My vacuum was taken from the manifold and I live at 3500 ft elevation.

 

I adjusted the carburetor idle screws and was able to get it down to 670 RPM and it will idle all day there. Now at 2,000 RPM I have 20 inches of vacuum. With initial timing at 5 degrees with a new vacuum advance disconnected my mechanical weights advance 10 degrees at 2000 RPM and with the vacuum advance hooked up, I get a total of 25 degrees not counting the initial 5 degrees. I rev the engine to 2500 and there is no change. The mechanical advance starts at 1500 RPM and is all in at 1750 RPM. The vacuum advance stars at 1000 RPM and is all in at 1800 RPM. I am using a points ignition, a 1959 distributor. The plate under the mechanical advance weights reads 118 CCM (I am sure that "M" should be a W). The stud that limits the mechanical advance is severely worn. But over all I am real happy with my engine, oil pressure, cooling system, no oil leaks yet, it idles real smooth , no vibrations for just being bolted up to a solid steel engine stand no rubber mounts. I am going to have the distributor rebuilt.

 

I have one of those adjustable timing lights and the degrees is measured at the crank. With the vacuum advance disconnected, when I rev the engine up I get 10* advance at the crankshaft at 2,000 RPM. My vacuum advance provides 15* advance for a total of 25* .

 

Yes I understand that is why I said I am going to have the distributor rebuilt. I realize the mechanical advance is not doing its thing. There is a little more sideways movement in the shaft than I would like and the 8 lobes that the contact arm follows are a little rough so it is more than installing some different weights, experimenting with springs and a new bushing.

 

That distributor is meant for 12 degrees initial timing setting.

 

Thats right I looked up timing on a '59 and it was 12*. But looking up at that pin in the picture my pin does not have a bushing on it and it is plain worn out and the rest of my distributor has wear so I am going to get it rebuilt with the correct advance. The '59 engine that I recovered was in a station wagon upside down in a field. I still have the distributor cap was rusty colored inside, I am sure there is rust in between the advance plate and the plate with the hole that limits advance.

 

From 1954-63 Chiltons