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High rpm not the same when running

Floatation Device

Contributing Member
Rebuilt both weber carb's on twin 454 merc cruise engines. Run great. can get 4600 rpm on one engine and 4200 on the other. Weber carb's are very simple. don't know what the fuel pump injector housing is. Looks like two jets to me that sit in the center of the back two ports above the injectors below the butter fly. Not a big issue since I run at 3200 rpm but would like to solve.
 
Are the props the same pitch? Did it ever run OK? Are the tachs OK? Are the throttle cables properly adjusted, i.e. with engine off, do the butterflys in the carb throats open fully on both engines? Is the timing correct?
 
Tach is good, throttle is good, the timing is set by an internal timing senser. One itme I do see is the fuel pump housing that looks like jets in the carb, the lower rpm motor has fuel spraying from this and the higher rpm motor does not when the rpm's reach around 3000+. This fuel pump housing sits in the center of the two back barrels of the carb. One port feeds the two jets. There is a spring and ball in the port under the housing. The effects only impact when running higher rpm's. The engine does not run rough or stall.
 
How do you know the tach is correct ?

Did you test the motors with a shop tach ?

Did you switch the tach's at least to see if the problem follows the tach ?
 
re: " I do see is the fuel pump housing that looks like jets in the carb, the lower rpm motor has fuel spraying from this and the higher rpm motor does not when the rpm's reach around 3000+. This fuel pump housing sits in the center of the two back barrels of the carb. One port feeds the two jets. There is a spring and ball in the port under the housing."

What you are looking at are the jets for the accelerator pump. The only time there should be any fuel coming out is while the throttle lever is being moved , i.e., speed increased. Fix that problem first and then see what is what. The accelerator pump is a fairly simple piston style pump with the piston directly actuated by the throttle lever.
 
Bob - you are a better detective than me...i wouldn't have reached that conclusion.

If it is the accelerator pump leaking and the carbs were recently overhauled, I'd bet on someone leaving out the check ball and the weight.
 
There " MAYBE " 2 problems here.

In my own inexpert opinion, if the problem does not follow the tach wire change then, we can safly assume it's something with the motor.

So then we go to the basics first, before anything fancy is attempted.

Compression check on the motor. Vacuum test of the motor. Lets see if we can get a clue from those 2 tests, and as a bonus we get to see if the plugs will talk to us.
 
Bob - you are a better detective than me...i wouldn't have reached that conclusion.

Thank you...Coming from you that is the biggest compliment I've ever gotten!


If it is the accelerator pump leaking and the carbs were recently overhauled, I'd bet on someone leaving out the check ball and the weight.

Check this first before looking at other stuff. Other possibility, there is crap in the seat, or the valve assembly not tight or missing the seal often under the head of the valve assembly.
 
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