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Author Topic:   Carb
Dirt Roller

Total posts: 22
posted October 23, 2000 05:17 PM
Can any one tell me what size Jet. would be a good one two run in my two barrel carb.i run street stock class motor it is a 350/ 30 over.with roller rockers . cam lift is a 486 intake to a 498ext my plugs are the autolites and i am runing the coolest one's they have they are the 303's.runing around 6000 rpm's my plug's are coming out like a lite gray. i Just cant get them to burn brown i was told i have to much jet Just what i thought to. bought the carb from a guy who ran 1/2 mile track's i run 3/8 and 1/3 the carb has 79 jet' wright now.the carb is not a holly but it is tab out for holly Jet' also run's real strong but you can tell that it is loading up at idle. thank you for any and all help.

Dirt Forum Racer

Total posts: 78
posted October 23, 2000 07:04 PM
Contact Earl Parker at
When it comes carbs and tuning them, He's the man. He can answer your questions properly, and get you fixed up.

Earl Parker II
Dirt Roller

Total posts: 17
posted October 23, 2000 07:34 PM
Given that the car seems to be loading up at idle the first thing I would check is the fuel pressure. If you have a stock fuel pump you might be OK, but if you have a high-performance mechanical or electric pump you very likely have too much pressure.

If the fuel pressure is too high the float will be unable to shut off the needle & seat when the fuel level rises to the preset height. In extreme cases the fuel will actually come out of the vent tube. In most cases it just rises too high in the bowl. If this happens there isn't a system in the carburetor that will meter fuel correctly.

Put a fuel pressure gauge in your car (if you don't have one already) and check the pressure at idle. If it's more than 6 psi, get a Holley P/N12-704 fuel pressure regulator, or equivalent, and install it in the fuel line as the last thing before the carburetor. Set the pressure at 6 psi and you'll be surprised at how much better the engine will perform.

I should add here that 6 psi is the best pressure for a Holley with a .110" Viton-tipped needle & seat. If you have a Rochester I think about 4 1/2 to 5 psi is about right and it should be about the same for a Motorcraft- someone that knows for sure may need to correct me here.

The Autolite 303 is equivalent to a Champion RJ6C. You don't mention what compression ratio you have but I'm guessing it isn't too high. A "6" heat range plug might be a little cold for the size tracks you're running on. I'm a big believer in using the warmest heat range an engine will tolerate, especially on a short track. My guess is that you could safely go up a heat range or two and see if performance improves with a warmer plug- I'm betting it will.

If you find that a warmer heat range provides better performance, you may want to try a projected core-nose gap design (often referred to as "turbo action" for some unfathomable reason). Make sure you have room in your combustion chamber for the projected firing end before you try them. Very often, especially in a wedge-type combustion chamber, the projected core-nose gap design will offer superior throttle response and acceleration.

Once you have the above squared away, it's time to find the correct jet. Do not start out rich and gradually go leaner- go to a jet that you're fairly sure is a little on the lean side and gradually go richer. If you're worried about being a little lean to begin with, just take a degree or so of timing out. I'd be surprised, though, if being a little lean on such a short track would result in engine damage.

Run a few laps- no more than 5- then go up two jet sizes and run a few more laps. If it accelerates better, go up a couple more sizes and repeat the procedure until performance starts to fall off. Go back to the previous jet size and you should be pretty close to optimum.

Don't even worry about trying to read plugs to determine the correct air/fuel ratio. There are about 10 to 12 different parameters that you really need to have a handle on before you can look at a set of spark plugs and say anything authoritative about air/fuel ratio (determining the correct timing and heat range is simple by comparison). Just go through the jetting procedure until you find the size that is right for your combination.

[This message has been edited by Earl Parker II (edited October 23, 2000).]

Dirt Freak

Total posts: 239
posted October 23, 2000 08:46 PM UIN: 25419780
When I ran my 2 barrel carb. I had to run 86 jets and the plugs came out fine. Anything less than that they were too lean.

Dirt Roller

Total posts: 22
posted October 24, 2000 04:13 PM
Thank you for the help.

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