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Author Topic:   I can't get this carb jetted right
CrazyT
Dirt Forum Racer

Total posts: 79
posted June 06, 2004 12:01 PM  
I am running a flat top 355 with a good set of ported and polished double humps. I have steel shim head gaskets for a little more compression. The carb is a rochester 2G and it has been cut out and polished up. I am currently running 81 jets and the motor runs real good of the corner and starts to cut out on the other end...possibly a spark plug issue because that just started last night after i changed the plugs. Anyway i went from a set of 78 jets, with the plugs still as white as new, to a 81 jet and the plugs still look lean. I pulled the intake off expecting to find a big intake leak but everything looked good. Is it common to run a big jet with good flowing heads and carb?

Thanks
Tim

iowa_dirt_track_girl
Dirt Freak

Total posts: 461
posted June 06, 2004 12:23 PM  
The main thing you want to remmember when running a 2gc carb is that you want it rich at idle cause it will lean out on the top end

If your plugs are still white even after 5 laps then go up one jet size go out run your heat and after the checker flag is thrown shut your car off across the finish line under power that is the best way to tell how your jetting is then just let the push truck push you back to your trailer

But remmember go one size at a time cause with a rochester one size can make all the difference in the world

CrazyT
Dirt Forum Racer

Total posts: 79
posted June 06, 2004 05:35 PM  
When we built the carb we bored the throat out and removed the choke tower and polished everything up smooth. We didn't do anything to the venturi or the tubes leading to the venturi. The butterflies are also stock. We drilled the needle and seat out to .130 and i run a new stock Carter fuel pump. Is there something we missed when building the carb?

CrazyT
Dirt Forum Racer

Total posts: 79
posted June 06, 2004 05:56 PM  
Also the power valve is the original one, but the plunger moves up and down freely when you push on it. I am assuming the power valve is the deal in the bottom of the bowl with the spring loaded plunger. I am not a carb man that is why i had a guy help me on it.

KPLugnut
Dirt Forum Champ
Total posts: 1019
posted June 06, 2004 07:09 PM  
The "power valve" system in a 2G consists of both parts, the spring loaded "thing" in the bottom of the bowl and the power piston that is attached to the underside of the top of the carb. The combined spring tensions in both parts are what determing the opening and closing points (based on vacuum signal) of the system.
If you bored and enlarged the top and body of the carb yet left the baseplate alone, then you've effectively killed the much-needed pressure drop across the venturi section of the carb, and have also hurt the signal thru the boosters which will hurt fuel flow as rpms go up. This is why you are seeing the common symptom of leaning out as rpms climb.
You're also likely to have a bowl starvation problem.
To fix a carb like that, a lot needs to be done to the baseplate, booster assembly, accelerator pump ciruit, air bleeds, and idle circuit as well.

The trick with the throat (venturi) area of 2G's is to remember that bigger is not always better...

KP

CrazyT
Dirt Forum Racer

Total posts: 79
posted June 06, 2004 07:46 PM  
Hey KP what would it cost me to get this carb fixed....like everyone else i am on a budget. Or are these things something i can do myself....i can be smart if i can get the information.

Thanks alot
Tim D

KPLugnut
Dirt Forum Champ
Total posts: 1019
posted June 07, 2004 07:15 AM  
Check PM...

monteregal7
Dirt Maniac

Total posts: 126
posted June 07, 2004 07:39 PM  
If you get your carb bored out as much as we have you will need 96 jets for best results.

CrazyT
Dirt Forum Racer

Total posts: 79
posted June 08, 2004 04:57 PM  
I bored mine to 1.382 and cleared the choke out and drilled and tapped an air cleaner mount bolt.

    

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