Explain carb jet overlap

[jack20]

I have a 1981 johnson 90hp, and curious how the low, intermediate, and hi speed jets work together? Obviously the low does the idle, but when does the intermediate kick in and low shut off. as well as the high? Trying to work thru my issues and realized I didn't know this .

 

[Chris1956]

Re: Explain carb jet overlap

Idle jets function when the air flow thru the carb is too little to cause fuel to be sucked out of the intermediate or high speed jets. The idle jets never shut off, however, they provide so little fuel, they have no influence on the mixture, once the motor revs up past 1000RPM or so. The same applies to the intermediate jets. They start metering fuel, as Revs rise from 1000RPM. Above a certain RPM, they have little effect on the mixture, and hte main jets have all the control.

You need not concern yourself with redesigning the carb. Make sure all the passages are clean and the float height and idle mixture is adjusted (if there is an adjustment on the idle jets), and the intermediate and high speed jets should operate propely.

Does that answer it?

 

[jack20]

Re: Explain carb jet overlap

it does but... i have 4 size smaller low speed jets (thinking my 85 boat had a 85 motor, turned out to be a 81 they have different size low speed) it won't idle with out the choke, but when on it a little (above 1000 rpm) it won't run with the choke(after warm up). New jets coming but was thinking about at what rpms each jet work. nothing major just thirsting for more knowledge, thanks

 

[Dhadley]

Re: Explain carb jet overlap

It's doubtful new jets will solve the problem alone. If the jets are dirty the passages in the carb are dirty. Jets are relatively expensive, clean the carbs and the existing jets. You'll probably be fine. Once in a while you my have to rejet but that's because your motor was designed around 1981 fuel. Running on 2010 fuel is different.