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weber 51mm side draft question

Old 03-12-06, 07:36 AM
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weber 51mm side draft question

i am running the same set up that the formula mazda cars run. the motor has a mild street port that i did last year. i also have direct fire ignition on the leading plugs. all this is in a 85 gsl-se that i drive on the street once or twice a week on the street. i don't expect this to be a perfect driver by any means. the problem i am having may be normal for the weber 51mm dcoe.

when you are crusing at a constant speed with hardly no load on the engine the car does not run smoothly at all. with just a little more throttle it runs smooth again. it does perfect every where else in decel and wot. just going up a slight hill will smooth the running out. everything i got came for a freind that races formula mazda. he says he is always wot or no throttle. i hope it is just jetting but if this is just how it is going to be its not a big deal.
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Old 03-13-06, 10:25 PM
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Originally Posted by werd
i am running the same set up that the formula mazda cars run. the motor has a mild street port that i did last year. i also have direct fire ignition on the leading plugs. all this is in a 85 gsl-se that i drive on the street once or twice a week on the street. i don't expect this to be a perfect driver by any means. the problem i am having may be normal for the weber 51mm dcoe.

when you are crusing at a constant speed with hardly no load on the engine the car does not run smoothly at all. with just a little more throttle it runs smooth again. it does perfect every where else in decel and wot. just going up a slight hill will smooth the running out. everything i got came for a freind that races formula mazda. he says he is always wot or no throttle. i hope it is just jetting but if this is just how it is going to be its not a big deal.
It can be a combination of different things. Without knowing the exact details of the set up one can only guess.
You have to figure if it's lean or rich. Most likely it's a slight lean condition. For the size carb you're running on a street ported motor I'll guess it's a little lean just off idle. The position of the butterfly might just be in the wrong position for the carb circuits to operate correctly. If the idle jets are two small the idle circuit might being going lean just before the idle transfer slot gets un-covered by the butterfly.
That's just a guess.
How far out is the idle mixture screw adjusted? If it's a lean condition sometimes turning the screw out making the mixture richer helps to determined if it's lean but it will also make the idle mixture rich though. That works sometimes. Would it also run rough with no load for example if you just free reving the motor standing still?
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Old 03-15-06, 10:20 AM
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Everything Crispeed said above is correct. I'd like to add a little more:

The transistion region is a common problem with two big venturies trying to operate at low rpm and part throttle. There just isn't enough air flow thru the venturies for them to function really well at low rpm/part throttle. The carb is made for racing, not street cruising, so it is not the best thing for street cruising. That is why a 4 barrel Nikki carb, with small primaries, will run smooth at part throttle cruising conditions; the small venturies operate very well in those conditions.

I have been running the big 2 barrels on rotaries for almost 20 years now.

Last edited by speedturn; 03-15-06 at 10:24 AM.
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Old 03-15-06, 10:37 AM
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For cruising, an oversized idle jet can cause problem like it wants to go. Try using a smaller idle jet than your current.
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