Logo
FULL SITE MENU

96 Yamaha 40 2 Stroke cant reach full rpm

J

jbizub5192

New member
Hi Everyone,

Im working on a friends boat, this is my first Yamaha as Im used to Mercury. Its a 96 40hp 2 stroke. I believe the rpm range should be between 4500 & 5500? Ideally I would like to be around 5k. Right now its maxing out at 3900. This engine is on a 18ft sweetwater pontoon boat. The boat itself is pretty light. It has a current prop of 10 5/8 x 12 which looks to be an oem Yamaha prop. At first I was thinking the prop was too big. However upon looking, this already seems to be a small prop and shouldn't have any issues getting it up to max rpm. Can someone give their opinion on that? If that assumption is correct then my next step is trying to trace down the issue. Will probably start with a carb rebuild and fuel pump rebuild. The engine is currently running absolutely fine other then not achieving max rpm. I know the previous owner put a new lower unit on as it was damaged by a stump I believe. I know on inboard outboards the lower units have different gearing that can be changed. I don't believe that is the case with most outboards. That was by only other thought is if the gearing was different.

Thanks for the assistance,
Justin
 
Simple.-----Test the motor on a MARINE DYNAMOMETER or a test propeller.---- See if it is putting out the rated power.-----If it is then it is a prop issue or boat issue.----Local shops might assist you.-----But if you disagree then rebuild the carburetors and fuel pump.-----But you have stated that motor is " running absolutely fine "-----Perhap check if it has strong spark on all 3 as it might be running on 2 cylinders.
 
Last edited:
racerone said:
Simple.-----Test the motor on a MARINE DYNAMOMETER or a test propeller.---- See if it is putting out the rated power.-----If it is then it is a prop issue or boat issue.----Local shops might assist you.-----But if you disagree then rebuild the carburetors and fuel pump.-----But you have stated that motor is " running absolutely fine "-----Perhap check if it has strong spark on all 3 as it might be running on 2 cylinders.
Click to expand...

Thanks for the advice, no shops in my area will assist with that without paying for a whole service. Looking at all prop charts it seems I'm already running the smallest or close to smallest prop for that engine. This engine only has 2 cylinders and not three. When I clean the fuel filter there was junk in there. So the carbs should be done to rule that out. Ill check the ignition also. I was just wondering if there was any other common failure points on these engines.
 
No model # in post #1 , so me bad for assuming it was one of the popular 3 cylinder ones.-----Good luck with it.------Keep posting with the outcome.
 
Back
Top