40 yamaha with 70 carbs?

[bassstrat]

I just aquired a 93 40 yamaha that has 70 carbs on it.The last owner told me he thought it got better gas milage than when it had the original carbs on it. Any thoughts on this? Could this be true?

 

[Silvertip]

Re: 40 yamaha with 70 carbs?

Without a fuel flow monitor and GPS how would he know other than running wide open with a stop watch for a full tank of fuel with both setups. In my view, NO! The engine will burn more fuel due to bigger jets, bigger bore and possibly restricted intake and exhaust to flow the amount of air the carbs are capable of. Perhaps a slight power increase, but along with power comes fuel consumption. There ain't no free lunch there mate!

 

[pine island fred]

Re: 40 yamaha with 70 carbs?

MABY the only difference is main jet size??? Install the correct size jets. I am thinking it is running to rich now. FRED

 

[deejaycee_2000]

Re: 40 yamaha with 70 carbs?

your fuel will be heaver not better .....

 

[mphelle8vld]

Re: 40 yamaha with 70 carbs?

Surely the "last owner" didn't hang those carbs on that powerhead for better mileage, he was trying to increase hp. It's possible that it is more thrifty with fuel but only because the 70 carbs were jetted for the displacement and exhaust of the 70hp motor, with your displacement and exhaust, you may well be operating lean (read hot! on a 2 stroke) and therefore getting better mileage at the expense of your motor.
Hopefully the "last owner" provided or still has the original carbs, if you're interested in increasing the hp of your motor to 50hp, you will need a set of carbs from a 50hp motor and an exhaust tube from the same (it has a much larger opening) This will give proper flow for your displacement.
Your other post regarding wrist strain could be related to the return springs on the 70hp carbs, or whatever else the "last owner" had to do to make his silly conversion operate at all.