'70 Rude Big Twin 40hp carb questions Model 40052

[New_Era]

Motor starts and idles OK. Will run in gear at idle speed but dies when ANY throttle applied. Have kit and plan to rebuild carb.

Question 1: OMC service manual tells me that at idle speed there should be 8/1 air fuel ratio and 12/1 at high speed. What is considered high speed?

Question 2: Does the low speed jet shut off/close at a certain point and you are only using the high speed jet or at a cetain point the high speed jet "kicks" in along with the low speed? Trying to understand what is killing the engine when throttle applied.

Question 3: Is there a consensus that the gallon of solvent to soak caburetor is better than the aeresol cans and if so can you save the gallon of solvent and use it repeatedly or is it one and done?

Thanks,

Ron

 

[charleso]

Re: '70 Rude Big Twin 40hp carb questions Model 40052

I can't really help with the jet questions, but my suggestion is get both solvents. You should not need the whole gallon, just make sure to seal it up air tight or it will evaporate. The aerosol can is nice to get into all the small holes and blast them clean. Compressed air works, but the solvent will loosen any stuck bits. I would not recommend using the solvent after soaking the carbs.

 

[charleso]

Re: '70 Rude Big Twin 40hp carb questions Model 40052

Excuse me, I just read your questions again. If the motor dies like you describe, the high speed jets are probably blocked. They start supplying fuel as you increase the throttle. They are larger, hence the difference in the fuel/air mix ratio.

 

[F_R]

Re: '70 Rude Big Twin 40hp carb questions Model 40052

Without lab test equipment, how would anybody know the fuel/air ratio? Don't complicate things. You just set the slow speed needle where it idles the best, and the high speed is fixed jet, not adjustable.

Yes, there are two fuel circuits, idle and high speed. The high speed fuel is metered through the fixed orfice jet and is sucked up through the main nozzle by venturi action. If you look closely at the area where the throttle plate closes, you will see several tiny holes. The idle fuel is pulled through those holes when the plate is closed. Actually, there are two sets of holes, idle and intermediate, for when the plate is closed or just cracking open.

Again, it is nice to understand it, but don't make a big deal out of it. Your main orfice jet is probably gummed up or has trash in it. Clean it all properly and use new gaskets to reassemble and it will probably work just fine.

The carb cleaner bulk cans with a dip basket are used over and over again. It would be totally unreasonable to toss it each job...to much $$$$ invested. Main thing to consider is how big a can is required to submerge the carb parts. There is a layer of water on top of the active ingredient and the parts have to be down into the ingredient to do any good. The layer of water prevents evaporation of your precious stuff. Fortunatly, the float bowl is where all the old gas lays and turns to gum and more often than not, that is the only parts that need to be soaked.