The Stovebolt Site Forums The Shop Area Engine Shop Change in idle speed when warm

Last weekend I had the great fortune to go for a drive with my 41 Chevy. Later that day, I decided to take my 42 out as well. I noticed that the idle speed on both trucks slow slightly when they are fully warmed up. Both are 235's, the 41 has a Carter, the 42 a Rochester carburetor. Both carburetors have been rebuilt, both have manual chokes, and the choke/throttle cables work as originally designed.

So why does the idle speed change? I suspect it is a combustion chamber temperature relationship. Any wisdom to share?

It could be an "age thing". I have noticed that when fully warmed up, I tend to slow down, and I am not quite as old as your trucks (don't miss it much ).

Seriously, it would be useful to know which Carter and which Rochester you have. The service replacement Carter and some of the Rochesters have a fast idle circuit, such that when the choke is activated, even slightly, the curb idle is increased.

If you have either of these, and were engaging the choke, the idle speed would decrease when the choke is completely disengaged.

Jon.

Jon,

I was hoping you would happen to read my inquiry.

A little more information:
41 truck has a 1954 truck 235 using 1941 intake/exhaust manifolds and a Carter W-1. Carb rebuilt by reputable long time rebuilder (as stated from my engine rebuilder).

42 truck has a 1958 2 ton 235 truck engine and a Rochester B. Carb rebuilt by me.

Both have manual chokes, and both experience the lowering of idle speed with the choke all the way open at all times. Both carburetors are of unknown pedigree, and neither came with the engines they are currently on now.

I notice the condition on the 41 most as it gets driven more, and for longer durations. I don't have a tachometer but the idle seems approximately 50-100 RPM lower when fully warmed up, and most noticeably immediately after a long high RPM run on the highway (truck has 6.67 gears so it is pretty wound up even at 40 MPH). There is no real problem with the issue it has, but I just noticed that the idle is "different", and cant really understand why there would be a change in RPM. The fact that both trucks do the same thing in spite of different carburation peaked my interest. I hope you can help me (and others on the Stovebolt) understand what is happening.

Thanks in advance,

Paul

Just to make sure I understand, the idle speed slows not immediately after pushing choke in, but at some point later when engine is fully warmed?

Just throwing crap at the wall here....maybe something to do with the manifold heat riser closing? Not real sure why that would change idle speed, but it would be a common element in the engines with two different carbs.

When the idle drops after fully warmed up, is the idle as smooth as silk like it was before it drops?

Sorry for the lag time in reply...sometimes work gets in the way of play.


Yes, your understanding is correct. Choke all the way open idle speed at approximately 5 minutes run time is faster than choke all the way open idle speed more than after 30 run time (after actually driving, not just sitting and idling). You may be on to something with the heat riser. Both vehicles do have functioning heat risers.

Carl, idle is smooth with both conditions. No performance issues on either, they actually operate surprisingly well for 1930's technology.

-Idle changes upon reaching operating temperature.
-Smooth idle under both temperature related conditions.
I'd say JW51 nailed it with the heat riser doing its job.
Proper setting of idle speed requires a fully warmed up engine.
In your case, adjusting the idle speed after fully warmed up will result in a slightly higher idle on a cold engine, which is desirable for speeding up the warm-up process.
Carl

Took my 41 for a 48 mile run yesterday. Paying closer attention, I noticed the drop in RPM thing occurred at the second traffic light at about 20 minutes into the drive. I think it may be the heat riser. JW51, I think you win a cigar! Perhaps the lesson learned is that tuning a stovebolt after an all idling "warm up" is different than a driving "warm up".

As much as I love winning the prize, I still struggle with the manifold heat riser actually changing the idle speed. Smoothness, yes. But speed?

The heat riser applies heat to the intake when the heat riser spring is cold. As the spring heats up, it gradually reduces the amount of heat to the intake until the plate closes.
The heat riser is designed to allow the engine to run smoothly throughout this slow opening process.
While it may remain running smoothly, there are some significant changes in air/fuel conditions during this transition.
It is plausible to me that the final idle speed would be different after the the engine is fully warmed up/normalized.
I would not be surprised if by adjusting the idle mixture screw setting either richer or leaner, that the final idle speed would change (maybe higher than when the engine was cold).
Carl

Well, out of the 246 views on this topic, you seem to have the "best" answer.

JW51,
From one imaginary internet friend to another imaginary internet friend, please accept my imaginary offer of the imaginary prize of 47 imaginary internet dollars for your contribution to this discussion.
Congratulations! (Real, not imaginary.)
Carl