engine idle RPM

[johnzajc@gmail.com]

Hi---I have idle speed [head light on--A/C off] on Shelby gt 500 set at 1000 RPM---however read the article on 428 Cobr Jet tune instruction that it should be at 600/650 RPM--Any opinion on what ic correct--THNKS

[Bob Gaines]

Hi---I have idle speed [head light on--A/C off] on Shelby gt 500 set at 1000 RPM---however read the article on 428 Cobr Jet tune instruction that it should be at 600/650 RPM--Any opinion on what ic correct--THNKS

6-700 is typical. If a auto 1000 rpm is way too high.

[johnzajc@gmail.com]

Thank U--- I had carb. done by WOODRUF , GREAT restoration ., car literarily runs perfect, I guess I will just have to twick   idle to somewhere around 650---thanks again

[Drew Pojedinec]

Sometimes is hard to get CJ carbs idled down enough.

Engine really and truly will tell you what idle speed is needed more so than a book will.

[shelbydoug]

Sometimes is hard to get CJ carbs idled down enough.

Engine really and truly will tell you what idle speed is needed more so than a book will.

750 is more realistic. The ethanol in the fuel effects this.

Sunoco 106 racing gas...650 idle...yes. Maybe even lower. Don't forget these engines were built and tuned for 103 leaded.

[johnzajc@gmail.com]

I am using CHEVORON 94 that has no Ethanol, I can increase octane value with octane adatives, last resort 100/110 LL aviation which is easy to get but hard on walet and actually baned for street vehicles although we use it mainly for aicraft tugs with 428 engines

[shelbydoug]

I can't tell you the exact formula other then the purple Sunoco LEADED racing gas alone will drop the idle rpm in my car about 200 to 250 rpm as soon as it is in the carb.

I think it is called 106 now but at one time it was sold as street gas as Sunoco 260.

It's the lead in it that does the trick but it will leave white deposits on the plugs and grey dust in the exhaust pipes. You will also smell the difference as well but it is what the FE's were tuned to run on new and I think may be a large portion of the issue here.


Don't bother with the SUNOCO uleaded racing gas. It is called 102 octane here. I have a station here that has it in the pumps but it has absolutely no advantage in my car that I can identify. None. Nada. Ziltch.


Interestingly enough, during the first fuel shortage, "we" found that even the 289hp's would run OK on even unleaded regular with little or no change but I do remember some of the 289 Cobra guys discussing the inability to get them to idle down to 750.

This really is only mostly an issue with automatics. The manual trans cars really don't care that much other then it is annoying idling at 1,000 rpm.


Ethanol in the gas is only an issue if you go over 15%. Other then that it runs almost the same as unleaded regular. There is no advantage/disadvantage to it.


Scientifically, 89 octane "Regular" produces more heat calories then the higher octane fuels do. What the higher octanes are for is to prevent or reduce detonation and they do that by additives.

You are in fact loosing horsepower by using them. Regular will make more power but depending on various factors could allow detonation, or "dieseling". That is a danger to the engine. So you run "Super" to prevent that.

"Dieseling" being defined as untimed combustion without spark. Diesel engines work by compressing the fuel without a spark until it explodes. Diesel fuel has much lower octane rating in order to facilitate the process. No spark plugs in Diesel engines.


You MAY need to drop the idle advance down to 2 to 4 degrees and slow the advance in the distributor as part of the solution. You could also bleed off some of the idle vaccum down to 12-14 inches as part of the solution.

[johnzajc@gmail.com]

Thank U for the lesson on fuel---good think I can print the copy, my brain is in information overload mode--thanks again---thanks to SAAC FORUM to make it possible to share it

[NC TRACKRAT]

Drew is correct...The engine will tell you the "sweet spot".  An old trick to find it is to extend your radio antenna (if you have one), then adjust fuel and idle to get the least vibration.

[Bob Gaines]

Thank U for the lesson on fuel---good think I can print the copy, my brain is in information overload mode--thanks again---thanks to SAAC FORUM to make it possible to share it

I have used Octane Supreme fuel additive for years. It has real tetraethyl lead in the ingredients.  You can doctor the gas from 2 -16 points claims the MFG. I was able to doctor fuel to accommodate a 12 to 1  high compression engine gas with it . I couldn't get there with others that I tried. It is more economical to buy by the case . Shop around because I see that prices vary.  I am no chemist but it seems to work for about the same or a little less then race fuel on that high compression engine without the hassle of sourcing and storing that. It will be less of course with a engine that doesn't need as high of octane. It works for me. 

[Drew Pojedinec]

Drew is correct...The engine will tell you the "sweet spot".  An old trick to find it is to extend your radio antenna (if you have one), then adjust fuel and idle to get the least vibration.

Richness and distribution help too.
Most 4v my car needs to idle 1000rpms
2x4 it can idle 875-950
3x2 it can idle 825-850

[Bob Gaines]

A 4 speed idling at 1000+ is no problem but not so much fun driving a stock automatic with engine idling at 1000+ rpm. At least with a stock torque converter. abnormal hard clunking into gear from park and holding the brake pedal hard at stops etc.  It might not seem like much but it gets old compared to the smooth operation of normal automatic operation . Been there done that.

[shelbydoug]

Isn't the 650 with an automatic supposed to be when the car is in Drive, not Park?

[greekz]

Isn't the 650 with an automatic supposed to be when the car is in Drive, not Park?

That is where I have my 67 GT-350, automatic.  It seems to like that idle speed when warm.

[shelbydoug]

Isn't the 650 with an automatic supposed to be when the car is in Drive, not Park?

That is where I have my 67 GT-350, automatic.  It seems to like that idle speed when warm.

I find multiple carbs more difficult to idle down then single carbs contrary to Drew's observations.

I interpret that, correctly or incorrectly,  as more fuel available at idle and you can just shut down the idle circuits so much, not infinitely.


No doubts that this also has to do with idle signal strength to the carbs which also is involved with available manifold vacuum which involves cam selection.

There is more then one reason that a station wagon cam profile is used as stock in a 67 GT500 automatic.