I just bought this car 6 months ago. Since the cooler weather set in, I’m getting a very rough idle for the first 2-3 minutes of a cold start. When the engine warms-up, it starts and runs fine.

I thought the problem was the carbs [427-tri-power], maybe the choke or something. But I noticed that my heat riser may be a dummy spacer, it doesn’t have the latch on the side like stock heat risers.

During the warmer weather, the start-ups would typically be a high rev for a few minutes then settle down to about 800 RPMs. In the cold, I need to feather the gas to keep a high idle; then it smooths out when the temp gauge hits around 100 degrees, then it’s perfectly normal.

Question: Would the heat riser spacer cause a very rough idle on cold starts?

 

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You can try increasing the fast idle speed slightly - as long as when the choke is fully open, the fast idle cam still clears the arm so the idle is controlled by the regular idle speed screw. Your manifold heat passages may be blocked off which can delay the intake warming up and the idle settling down. Bur most likely it's just the combination of compression, aluminum intake, aggressive cam all needing a bit of feathering on a cold start until a little heat builds in the engine.

 

Yes.
Cold start trauma is the result of a cold engine with thick cold oil trying to run on a unpredictable unevenly lean mixture caused by much of the fuel introduced by the carburetor condensing back to gas on cold manifold walls. Since the exhaust crossover many of us eliminate has its purpose in preventing this very problem we should not be surprised or indignant at having to feather the throttle until she is warm enough to handle things on her own. I have a long ram 440 with two 600 CFM AFB's no under-carb heat and no chokes. Requires about 60 seconds of foreplay before she will cooperate at 55 degrees.

Dan

 

Wayne:

Besides everything that has been discussed so far, do you perhaps still have a tank of "summer gas" in the car. This gas does not have as much butane in it and will not vaporize as easily until engine and carb have a bit of heat.

I typically run with heat crossover open (as factory stock) but with heat riser wired open or replaced with a spacer. This reduces forced heat to the carb and reduces pass side exhaust back pressure, but still keeps the crossover open for some heat transfer and improved exhaust sound (i.e. balance tube).

FWIW.

Larry

 

I believe the choke on a tripower setup is set via a bimetallic coil. Is the rod length properly adjusted so that the cam is on the fast idle position when you push the pedal to the floor one time?

 

A spacer in place of the heat riser won't have any effect on cold starting or achieving a steady state fast idle speed a few seconds after it starts. Look at your CSM and verify that the choke valve, choke vacuum break, and fast idle are properly set. IIRC the fast idle, which is set with the engine hot is 2200 (L-71, but you didn't state if it's a L-71 or L-68), so initial cold fast idle should be in the range of 1900-2000.

The lack of a heat riser valve might have a negative affect on initial drive off if you do this immediately after a cold star, and will definitely have a negative effect if the heat riser passage is blocked, so let it fast idle for a minute or two before driving off.

If the heat riser is blocked, the exhaust will have equal, but uneven pulses and sound different than if the heat riser is open. It's tough to describe, but if you compare a 365 and 375HP FI engine idling side by side you can definitely tell a difference in the exhaust sound because the FI system has no heat riser passage.

The advantage of blocking the heat riser is that it reduces carb temperature, which reduces the incidence of excess fuel percolation, but the downside is that the engine may have poor drivability during warm-up, like a bad stumble starting off from the dead stop before it's fully warmed up.

Duke

 

I agree with Duke, the choke is not adjusted right.

I will say the heat in the manifold is your friend under most circumstances as it will aid in fuel vaporization and allow running a leaner fuel mixture if you are so inclined. I use the manifold heat plus the carburetor heat on my 300hp 67 but I have a wire hook for the heat riser that can be connected if It's to hot and I run manifold vacuum advance so exhaust temps stay down.

 

Some interesting answers in here.
The car is a L71, the Holley carbs are new, with less than 1K miles on them, the PO installed them. I’m not very familiar with carburetors, the last carbureted car I had was over 30 years ago. So there’s lots I don’t know [or forgot].

I haven’t touched these carbs accept for adjusting the idle screw up a notch. This rough startup began in late October, and has happened at every cold start since. So I suspect it’s temperature related.

The car runs great, accept for the few minutes during cold startup; I took the car out for a 45 mile run 2 days ago, and it ran like a champ. So it’s not a big problem, just a nuisance.

If anyone knows of a good tri-power instructional, let me know. I’ve searched but came up with nothing good.

 

Take the air cleaner lid off and set the choke with your foot pedal and before you crank the engine see if the choke blade closes completely.

Next, start the engine and look at the choke blade to see if it opens approx 1/4". While the engine is running rough further open the choke blade with your finger to see if the engine smooths out, don't stick your face over the carburetors for safety sake. If you find it works, the choke pull off rod needs to be adjusted.

 

The 1967 Chevrolet Chassis Service Manual has everything you need to get all the carb adjustments dialed in.

Duke