The Stovebolt Site Forums The Shop Area Engine Shop Rough idle when warm

My step van has a 261 and was sounding great. I drove it around town and noticed it was starting to idle a little rough at stop lights after it got really warmed up. Could anyone suggest things to check that would make the engine idle rough when warm. I've decided to get a heat insulator for the updraft carb just in case it's getting a little too hot down there. I'm going to check timing again tomorrow. I also noticed it wasn't starting right back up as easy after shutting it down after being warm. Other than that, I'm not sure. Thank you.

A slightly high float level can cause fuel to spill over into the venturi and cause an over-rich idle mixture when hot, and also create a flooding condition that requires extra cranking time to clear the flood if the engine is restarted within a few minutes of a hot shutdown. It's more common on a downdraft, but I've seen the same kind of symptoms on tractors with updraft carbs.
Jerry

if it has a heat riser, make sure its opening up, if not the manifold will overheat and cause poor idle..

The heat riser valve plate needs to close as the engine warms up, not open up.
Carl

My heat riser does not work, but it is open.

The more I read about the heat riser, the more it may be likely causing the problem. The updraft manifolds are not easy to find. I wonder if I could find a two bolt riser to put on before the exhaust pipe.

No.

I just finished checking the timing and noticed the vacuum needle was at normal but jumping a little. I also noticed that when I revved the engine I got a burst of exhaust in the engine bay. Does this all seem to point to a leak.

Sounds like a good time to take the manifolds off and separate it to fix the heat riser valve and check the condition of the flanges which mate to the head.

I've seen other models that have the heat riser at the end of the manifold before the pipe. Lincoln says no. I wondered why. Seems like a viable option since mine is missing completely and updraft manifolds are difficult to find.

V8's have a passageway through the cylinder heads and intake manifold that allows some exhaust gas to move from one cylinder bank to the other, under the carb, and exit out the other manifold when one side is partially plugged off by a closed heat riser valve. That's impossible on a stovebolt six with either an updraft or a downdraft intake. Just take the manifold assembly off and fix the heat riser- - - -it's an easy job. I usually use stainless steel for the shaft and flapper valve, install a new bimetal spring, and fix it once and for all.
Jerry

I don't think that you understand how a heat riser operates on an inline six. There is a valve inside of the heat riser box which directs hot exhaust towards the underside of the intake manifold to heat it up to improve operation while engine is cold. Once the engine warms up, the bi-metalic spring relaxes allowing the counterweight to move the valve, redirecting the exhaust away from the intake and straight out through the exhaust manifold.
I am guessing the heat valve which you are referring to is what was used on V8s. That was located on the right exhaust manifold at the exhaust pipe. When this one is closed, it redirects exhaust through a port in the intake manifold then out through the left exhaust manifold. While doing this, the exhaust heats up the intake manifold for better cold engine performance.
Putting one of these on your inline 6 exhaust pipe would be like stuffing the tail pipe with a potato.
Carl

I see a riser for a 285 I think it may work.

whoops-265

Ok, I get it. Thanks Carl