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General Category >> Rubber Side Down! >> Why does Idle fall when braking?
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Message started by sailorcolin on 05/29/14 at 07:38:49

Title: Why does Idle fall when braking?
Post by sailorcolin on 05/29/14 at 07:38:49

Why does the idling fall under braking? Is it because of the float in the carb?

Title: Re: Why does Idle fall when braking?
Post by Dave on 05/29/14 at 07:43:57

Most likely it is because you have closed the throttle quickly, and the mixture has gone lean as the engine slows down with a closed throttle.

Raising the idle speed can help to reduce this a bit.

Title: Re: Why does Idle fall when braking?
Post by justin_o_guy2 on 05/29/14 at 08:36:50

Id "Lean" that way,,, Mine used to shut off when Id be runnin 70, pull the clutch & let the gas slap shut & grab the brakes.. jacked the idle speed up & it stopped shutting off,.,It did drop a little,, but you may be a little slow in the idle rpm,,

Title: Re: Why does Idle fall when braking?
Post by Spamy on 05/29/14 at 08:36:52

Ive noticed this also. I suspected that it was the gas flowing forward and losing the gravity fed gas pressure, or getting air in the line.

But, I also think in the 15 seconds of braking thats not likely to happen. So Im not sure. But I know it happens on mine, and if the tank is lower level and I brake hard it will sputter out also.

Title: Re: Why does Idle fall when braking?
Post by Dave on 05/29/14 at 13:27:22

6F4C5D51453C0 wrote:

Ive noticed this also. I suspected that it was the gas flowing forward and losing the gravity fed gas pressure, or getting air in the line.

Look at ths diagram. The jet location is smack dab in the center of the float bowl. As the level of the gas changes with braking.....it might slosh forward and become high in the front and low in the back of the bowl....but the level in the center will be nearly constant. You can't brake hard enough to get the fuel to starve the jets - unless you do a "stoppie"!

Sudden hard braking and quick closing of the throttle makes a high vacuum in the carb throat and a lot of air if pulled past the slide and butterfly - but the closed throttle position closes off the fuel jets so very little fuel can flow. The engine is running very lean - so lean that sometimes it can stop running.

Dave

Title: Re: Why does Idle fall when braking?
Post by sailorcolin on 05/30/14 at 06:32:24

thanks guys!