1972 400CI engine miss

[Ralph Close]

I changed camshaft using Comp Cam (and lifters) just above stock specs. Engine is otherwise stock. I changed dist cap, wires and plugs at same time. Carb is 600 edelbrock. Problem is, engine runs fine on cold startup. As engine temp rises, an intermittent miss occurs. As operating temp is achieved it becomes a dead miss. Can't determine which cyl by pulling wires, nor by laser thermometer on header runners. No miss detected by watching timing lite on individual wires. All ignition components including coil (electronic ign) have been switched again. Engine comp. equal at 125 lb. Sprayed for vacuum leaks--none. No indication of sticking valves.
Any suggestions?

 

[Paul_G]

Have you adjusted the idle mixture after installing the new cam?

Check vacuum hoses, especially if you have power brakes check that hose. PVC sticking open, check hose.

You say it's not just one cylinder. How about if you choke a a little while it is warm, will the mis stop?

 

[727]

Can you verify that the choke is opening all the way? Had a similar issue and that was my problem

 

[PurpleBeeper]

Hmmm.... OK, then we can rule out ignition as a source of the miss. An engine can also miss if the fuel mixture is way off. The carb & vacuum line double check is in order.

Also, since you just changed the camshaft, is it "possible" that one or more of your pushrods are just a little bit too long? I'm just thinking out loud here. When an engine heats up, all the metal expands, including your pushrods/rockers. If your pushrods were already on the long side & got just a little bit longer when the engine heats up, that might cause one/more valves to hang open causing a miss. This is a long shot, I'll admit, but just a thought.

Paul G has some good tips IMO

 

[Ralph Close]

Hmmm.... OK, then we can rule out ignition as a source of the miss. An engine can also miss if the fuel mixture is way off. The carb & vacuum line double check is in order.

Also, since you just changed the camshaft, is it "possible" that one or more of your pushrods are just a little bit too long? I'm just thinking out loud here. When an engine heats up, all the metal expands, including your pushrods/rockers. If your pushrods were already on the long side & got just a little bit longer when the engine heats up, that might cause one/more valves to hang open causing a miss. This is a long shot, I'll admit, but just a thought.

Paul G has some good tips IMO

 

[Ralph Close]

Thanks for the several responses. My answer to all suggestions is yes I have checked those, with the exception of the "too long pushrod". I'll try loosening the rocker shaft bolts about 1 turn and see what happens.

 

[Ralph Close]

Have you adjusted the idle mixture after installing the new cam?

Check vacuum hoses, especially if you have power brakes check that hose. PVC sticking open, check hose.

You say it's not just one cylinder. How about if you choke a a little while it is warm, will the mis stop?

 

[Ralph Close]

Thanks.........checked all those

 

[Paul_G]

The only other thing I can think of then is to fully warm it up and do a compression test. See if all cylinders are equal and in good health. Other than that test the ignition coil. It may be failing when it gets hot.

How does it drive on the street?

 

[sebring+]

My vote is for ignition breakdown at hot temps.