Carb adjustments , need some help please.
Thread Starter
Instructor
Joined: Mar 2003
Posts: 235
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Carb adjustments , need some help please.
I have been atempting to tune my carb. Its a Road Demon 625cfm with Vacuum secondaries. I have between 10.5-12in of manifold vacuum at idle in drive. I put in a 9.5 power valve. I was told that you want a power valve thats approx. 2in less in vac than the vac reading. Currently I am trying to lean out the car some to get some mileage back. Also the car feels like it is running rich, you can also smell the gas in the exhaust, but the plugs have a light brown color to them. I was wondering what jets some of you are running in your holley, or demon carb on a small block chevy with specs similar to mine below. I know its alot of trial and error, but I think knowing what some of you are running might also help. Currently this is what is in the carb, 68 primary jets/ 76 secondary jets, 9.5 Power valve, 31 squirters, yellow secondary vacuum spring with a pink cam. 30cc accelorator pump. Any help or advice would be great. I have been reading several different tuning manuals, as well as the manual for the carb. Some sections are writen poorly and don't explain enough to make a good tuning decision. Thanks again for the help .
Burning Brakes
Joined: Jul 2001
Posts: 1,160
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From: Ajax Ontario
Re: Carb adjustments , need some help please. (IRON WILL)
I can't help....but suggest you get in touch with Lars....he has a number of papers in the tech section and he's very knowledgeable and ready to help. You may want to send him an email. Good luck!
Le Mans Master
Joined: Aug 2000
Posts: 6,362
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From: Brazos TX
Cruise-In IV Veteran
Cruise-In V Veteran
Re: Carb adjustments , need some help please. (IRON WILL)
I'm running FI on the Vette and a 780cfm Holley on my Nova so that wont help you much (especially since my cam is way bigger). However, let me suggest that you want to jet down 2 steps at a time so try 66's. Also, be certain that the idle adjustment hasn't been turned so far that the throttle blades are open too far and thus exposing the idle transition slots. No more than .030" of the transition slot should be exposed below the throttle blades when the blades are fully closed. If more is exposed you will have a rough idling engine and it will run rich at idle. you will have to remove the carb to check this.
Also, did you set your idle mixture with the vacuum gage to get the highest vacuum reading? Did you set your float level?
Also, did you set your idle mixture with the vacuum gage to get the highest vacuum reading? Did you set your float level?
Thread Starter
Instructor
Joined: Mar 2003
Posts: 235
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Re: Carb adjustments , need some help please. (burners)
The floats are adjusted to the center of the sight glass where they are supposed to be and also the Idle mixture screws were set with a vacuum guage.
Intermediate
Joined: Nov 2000
Posts: 36
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From: Spartanburg SC
Re: Carb adjustments , need some help please. (IRON WILL)
Sounds like you've got a pretty hot cam by those vacuum numbers. With a hot cam you can expect a rich idle mixture. Most hot cams don't start working until you reach 2000 RPM so idle at 700-900 RPM it is not working well at all. Sounds like to me by your plug readings your carb is pretty close. Be careful not to lean it out too far as this would be worse than what you have now!!! I think you will find it hard not to have some richness at idle iwth those vacuum numbers. How do I know this??? because my car runs just like yours!!!
Steve
73 454/4psd roadster, Edlebrock 850 Qjet, Lars dist with MSD 6AL,
Comp XR274 roller cam... and it is Rich at Idle but runs fine everywhere else!! :cheers:
Steve
73 454/4psd roadster, Edlebrock 850 Qjet, Lars dist with MSD 6AL,
Comp XR274 roller cam... and it is Rich at Idle but runs fine everywhere else!! :cheers:
