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How to adjust carburetor?
I have a brand new Ford Racing 392ci with a Holley 4150 dual feed, double pump carb in an ERA FIA car. I've put less than 1000 miles on the car and have already fouled the plugs (black and sooty). The car is clearly running rich. How do I adjust the air / fuel mixture, and how do I know when I've "got it right"? How do I know if it is an idle mixture problem or requires adjustment to the jets? Help!!
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Here's a good place to start, but a good chassis dyno will tell the whole story.
http://www.bob2000.com/carb.htm |
Are the plugs wet? If yes, the air fuel ratio is probably too rich and the primary jets may also be too rich.
If you have never worked on a Holley, you are not going to get enough info from posts on this or any other forum. Go out and buy a Holley book. The best book (IMHO) is "Super Tuning and Modifying Holley Carburetors" by Dave Emanuel. |
Thank you for the replies - the link in the first reply is very helpful.
The plugs are not wet. They are dry and covered with black soot. I get the same soot if I rub my finger on the inside of the exhause. Definitely seems to be running too rich. In the link, they recommend using an O2 sensor to get a precise reading on the air / fuel mixture. Is there any way to use this without actually welding a fitting to the exhaust? |
This is why you should go to a good shop that can do a dyno tune. They will hook up a wide band exhaust sniffer that will give better info than a O2 sensor. The price is not all that bad from what I was told. Seems like it was about $175 last I was told, but my memory cannot be trusted.
You don't have to run the engine beyond where your comfortable to get a good tune. |
Not that you want to run out and buy one but I have one of those LM-1testers from innovate motorsports with the exhaust clamp.. Handy handy handy.. I have used it with all of my cars and my neighbors supercharged Miata.
http://www.innovatemotorsports.com/products/lm1.php |
Check this site.Find your carb list no and it will give all the correct jets etc.Great Info here!!!
http://www.mortec.com/carbs.htm |
And this formula,
There is a formula that will get you very close to the proper cfm for your engine. The formula goes like this: cubic displacement divided by 2 (ex: 351=175.5) maximum rpm divided by 1728 (ex: 6000 rpm=3.47) multiply those two figuires (ex: 175.5 x 3.47=608.98 cfm) Hold on we are not done. You now have to factor in volumetric efficiency. A Nascar race engine runs at 85%-90% volumetric efficiency. A normal street car wil be 75-80%, depending on what you have done with your heads and exhaust. Ex: 608.98 x 75%=456.74cfm |
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