[bob d]

On a cold start with a full choke about what RPMs should the tach be showing? Warm engine is about 700 RPMs. Thanks Bob

[carbking]

In 1965, no specification was given for tripower, but 4 barrel cars had a fast idle specification of 2500 RPM.

Jon

[steve25]

It’s actually best in terms of parts longevity to to set the cold “high “ idle as low as you can and still have the motor not stall out when you put the trans into drive if your the type that starts and drives off.

[59safaricat]

You don't have a location but custom set it like Steve says. I adjust mine around 1800-1900 RPM's but live in a warm climate. If you live in the north, may have to set it a bit higher. Bunch of other factors come into play if the engine it modified, especially camshaft selection.

2500 seems crazy high!

[VCho455]

2500 seems high but that may be the rpm used when putting the high idle cam into play when the engine is hot. The dealership wouldn't want to wait for the engine to cool down if it was hot from a test drive. So there had to be a rpm to set the high idle at when the engine is hot. That high idle number most likely varied for different areas of the country. Obviously LA would have a different high idle setting from Detroit. Thus the different rpms quoted in different literature.

On my daily driver Burban my cold idle rpm setting is about 800 rpm when in neutral and the weather is below 50 degrees. In gear it is virtually the same cold as hot. It takes a dozen re-adjustments over a week or so to get a carburetor choke dialed in like mine is. The numbers given in the manuals and other sources are just the baseline starting points.

Just my opinion which along with a sawbuck will get you a cup of coffee most anywhere.

[59safaricat]

Originally Posted by VCho455

On my daily driver Burban my cold idle rpm setting is about 800 rpm when in neutral and the weather is below 50 degrees. In gear it is virtually the same cold as hot.

Not a fan of a high idle? I guess it's easier on the transmission if it's an automatic and you don't wan to wait around for the fast idle cam to be completely off.

Pretty good skills if you can make the transition seamless.

[steve25]

As I look thru the reproductions that Ames has for the emissions decals they do say the fast idle should be 2500 rpm!
The only reason I can see for this is to get the motor off the of the very rich and ware inducing choke on period as fast as possible.

To me it seems better to set it only as high as needed to not stall out and then drive off on your journey which will heat the motor up faster to pull the choke off.

[steve25]

Here’s the engine tuning data from a basic 400 and a 1968 RA400, the RA 400s high idle speed called for is freakin 2800 !!!

[Kenth]

Once you have understood the instructions for setting the cold start speed with a hot engine and with the fast idle cam in the highest position, the recommended rpm setting data is reasonable.
Some brands uses the lower part of the fast idle cam to set the cold start speed (with hot engine) and obviously have a lower rpm setting.
The 2800 rpm setting done with hot engine does NOT equal 2800 rpm´s at cold start.

As for the Tri-Power there is NO cold start rpm setting, it is what it is after the curb idle speed is set, and the choke rod adjustment is correctly set.

FWIW

[Schurkey]

Originally Posted by Kenth

The 2800 rpm setting done with hot engine does NOT equal 2800 rpm´s at cold start.

As for the Tri-Power there is NO cold start rpm setting, it is what it is after the curb idle speed is set, and the choke rod adjustment is correctly set.

FWIW

Kenth is correct about carb issues.

HOW are idle speeds adjusted on the center carb? I don't know much about Rochester Tri-Power carbs. All my experience with Tri-Powers involves the Holley 2300s, and vacuum secondaries. (Mopar 440 +6 carbs)

The 2GC carbs I'm familiar with use an idle-speed cam for base and fast idle; if the base idle is set properly, the fast idle is handled by the geometry of the idle speed cam. Base idle, and fast idle speeds can't be adjusted separately.

Knowing GM, though, they've made some carbs that do have separate adjustments. I'd like to see a photo of the idle speed screw arrangement of your center carb.



2500--2800 rpm cold idle seems scary-fast, but you'd kick the gas pedal as soon as the engine rpm stabilized, a few seconds after starting. That drops the fast-idle cam to the second-highest step, or if there are no discrete steps on the cam, it'll roll the fast idle cam to a specified location (often identified by a stamped in "arrow head" ">" which significantly reduces the idle speed before you'd drop the auto trans into gear.