Another carb/choke question

[Joel T.]

Hi All;

I'm in the process of getting the '65 ready for a PV up in Mass. in September and am still having a problem with the high idle setting... here are the particulars.

Carb: Holley 4150 on my '65 L78.. has black choke housing and heat tube from right exhaust manifold. Carb was rebuilt (service replacement) and it appears that the choke spring was replaced. Everything under the hood is straight stock.

Problem: I'm trying to get the choke to set properly when cold and to get the correct high idle per the specifications... If I turn the choke spring two notches rich, the high idle cam will set on the highest lobe and I will get high idle... however the butterfly will not open sufficiently upon engine firing so I get the over-rich shake... as the car warms, the butterfly opens more, the shake goes away and high idle is reached... If I set the choke spring at center or one or two steps lean (per the manual), the butterfly will open more upon engine firing but the high idle cam will not be on the highest lobe but rather what appears to be second lobe.... hence I get no high idle at all...

Question: Is this at all correct??? I would assume that if it were working properly, I should be able to set the choke spring to the two marks lean and have the high idle cam engage at the highest lobe.... I could probably adjust the high idle set screw to work on that second cam lobe but that does not seem to be correct... Any suggestions are appreciated...

Thanks,

Joel

[Michael W.]

Re: Another carb/choke question

Not to put a stick in anybody's spoke, but I believe there's no GM spec for cold idle RPM with full choke. The process used during Flight Judging has been 'reverse engineered' and frequently leads to situations like yours where the car runs fine but won't pass the judging spec.

I believe that the GM spec calls for manually setting the high idle speed with a hot engine and no choke.

Or maybe I'm FOS again.

[Jack H.]

Re: Another carb/choke question

"I believe that the GM spec calls for manually setting the high idle speed with a hot engine and no choke."

You are CORRECT! And, the actual, choke engaged, 1st fire, cold engine RPM will vary GREATLY depending on actual ambient temperature.

That's why savvy judges (certainly those who are PV qualified), have the owner cold-start the car, and let it RUN without touching the accelerator pedal! They do NOT look at the tach upon cold start, and go do other PV checks in the car while the engine is warming up with heater operation being one of the LAST things checked/verified.

Once there is good HOT air coming out of the heater box AND the temp gauge has moved appreciably off cold to something approximating fully warmed engine conditions, the tach is read and RPM level assessed. After that, the judge asks the owner to 'kick' the accelerator and a determination is made on the carb's cold to warm engine kick-down adjustment for the PV.

Scrutiny is nowhere near this level during ordinary Flight Operations Check testing. But, not to worry, the PV qualified judges DO know what they're doing and have a set routine for the sequence of what they check + when they check it!

[Joel T.]

Re: Another carb/choke question

Jack/Michael;

Thanks!! Based upon what you guys have described here, I am good to go... After a few minutes of warm up, she will hit the magic number...

Back in the "old days" working on my Mopar, with a little fiddling around, I could get that motor to achieve that high idle setting pretty much from start up... without much warm up time... ... or perhaps my memory is not as good as I think it is!!

Regards,

Joel

[Michael H.]

Re: Another carb/choke question

Hi All;

Problem: I'm trying to get the choke to set properly when cold and to get the correct high idle per the specifications... Joel

Michael is exactly right. There's no such thing as a "specification" for cold engine fast idle speed. What is the definition of cold? (aaaaahhh... the absence of heat?) An engine that is at ambient temp is not cold. It's colder than normal op temp but if the air temp at the time of judging happens to be at or above 70*, there's a good possibility that the choke may NOT set to the highest step of the fast idle cam. If that's the case, the engine will not reach the RPM called out in the GM specifications.

The ONLY way to check the fast idle speed is on an engine that has reached near operating temperature, or at least warm enough to produce a stable idle. (it's the only constant) And the only way to do that is by shutting the hot engine off and manualy closing the choke blade to set the cam on the fastest step. When the engine is restarted, it should run at the RPM shown in the specifications.

At 70*, it's likely that a choke will not close completely, which would not set the high idle cam to the fastest/highest step.

As mentioned, I think (hope) the former "cold RPM test" has been eliminated from the judging manuals.

Below is the link to the original discussion from 2004.

https://www.forums.ncrs.org/showthread.php?t=26746&highlight=fast+idle+speed&uid=3162