Old Yamaha 40hp outboard problem

[Berty123]

Hi I wonder if someone can help me with a problem I am having with a old Yamaha 40hp outboard.

The engine starts OK and revs OK in neutral but when I put it in gear it never reaches max revs, it seems to me that the revs under load only reach about half way.

I have stripped the carbs and gave them a good clean, I have replaced the spark plugs, used new fuel.

Can any body help me ?

Regards,

Martin.

[reinier]

What boat is it behind and what prop are you using?

[Berty123]

Hi Thanks for the reply.

The boat is an old inflatable of ancient origions.
The Prop is unknown.
The engine performed very well as it was but one day it just wouldn't rev high enough when placed in gear. I have cheked and the prop is undamaged.

Regards,

Martin.

[Nos4r2]

Have you checked for a spark on both plugs?


Don't rev it in neutral-it'll bugger it and it's no indication of engine health as it'll hit the redline on one cylinder.

[spartacus]

You need to rule out a few things, so you can pin-point the problem. There are a multitude of things it could be. If you've cleaned carbs and used fresh fuel and sparks then - you should have seen some difference. Did you adjust anything on the carbs?

Ruling that out:

Check: fuel filter on engine block. Commonly overlooked but can clog up.

Check: sparking for both cylinders? (also check leads and ignition coils)

Check: Ignition sequence and timing correct?

Check: Do you have good compression on both cylinders?

Check: fuel pump operating normally?

Check: all wiring tight, no corroded terminals?

Last but not least - fuel tank vented properly?