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Original Message
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| RE: Whenever I read about someone... |
| By Gerry Proctor - 01/29/2003 9:29:33 AM; IP 207.133.188.254 |
This is one of those things that isn't well understood beyond how folks THINK the mixture screws work.
The idle mixture screws control the emulsified mixture amount not the ratio although there is a ratio balance achieved through throttle opening and the idle mixture screws but if your mixture is real fat at idle, you will never make it leaner by twiddling with the mixture screw. The idle A/F ratio is determined by the idle air bleeds in the throttle body and the fuel by the idle fuel bleed passages in the metering plate. The only way to change that ratio is by fiddling with the air and fuel bleeds which is not easy or recommended unless you name is Barry Grant.
Most fat idle mixtures are caused by having the throttle butterfly exposing the idle transition slot beyond .060" which causes fuel to be pulled through the main circut while the engine is at idle. The purpose of this slot is to smooth the transition from the idle circuit to the main circuit. There are some tricks that are fairly common in engines that can't achieve a stable idle without excessive throttle opening which exposes the transfer slot.
One method is to slightly open the secondary butterfly by turning the set screw under the secondary diaphram (you have to flip the carb over to see this screw) to assist the idle speed without introducing additional fuel but this isn't entirely successful since there will be some fuel coming from the secondary well since there will be some air flowing through the venturi. Not quite as much as would be coming though the transfer slot, but still some.
The other method is to drill two small holes in the primary butterfly to allow more air through the throttle body while keeping the throttle plates closed to the transition slot. You have to start small with the holes...under 1/16" and work your way up until you can achieve a stable idle without exposing the transition circuit. This will allow the additional airflow the engine needs to idle without dumping extra fuel into the mixture.
However, the correct solution to multi-carb set ups is to use carburetors that are calibrated in the air and fuel circuits to perform as advertised. I have no idea on Weber tuning but all carburetors work on the same principal so it's only a matter of tuning and testing to get them dialed in. |
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