I have a Merc 650, 1960s era and it runs fine with no coughing or missing but it doesnt go any faster than crawling speed, any ideas why it doesnt have any thrust
cheers
OK There are several symptoms/causes.
If the motor revs up and doessn't move => prop hub slippage
If the motor won't rev up it could be incorrect throttle cable adjustment, incorrect carb adjustment, incorrect timing settings, low compression, water intrusion, bad spark plug cables, or incorrect prop pitch.
Take a compression test, inspect the spark plugs,and test for spark on all cylinders and post results.
How many RPM at full throttle in the water?
I have one of these on a 16 ft aluminum semi v runnabout.
It was a real dog no better than 15 mph with a 17 pitch prop and I kept stepping down the pitch to 13 and the boat will now do 32 mph by GPS at around 5500 rpm
If you do basic diagnostics and find no issue. (pay attention to compression and ignition) and if you have no water getting into a combustion chamber. If you are running a bit heavier boat I'd bet you are pitched too high on your prop.
About water getting into the combustion chamber. I have 3 mid 60s merc 650s and two of them had this issue. One had a crack in the case and the other had a hole through the exhaust cover water jacket. Both were injecting water right into the exhaust port of the #4 cyl.
the problem seems to be the butterflys in the carbs arent opening far enough, at full throttle they have moved about 5 degrees from fully shut, by hand I can open them to 90 degrees, have adjusted throttle cable to get maybe 30 degrees but now it wont go in reverse so had to adjust back to original position, obviously its some sort of adjustment needed here but not sure what.
Thanks
Dung, It is real important to get the carbs synched with the ignition timing. Do a search on this forum for "Link and Sync of Inlines", and you will find the proper procedure.
There are two synch points. Idle pickup is when the carbs just start to open, and the ign timing is 4-6 degrees BTDC. Max spark advance is the second setting(usually 21 degrees BTDC). it is important to get these right. In additon, the carbs need to open all the way. There is a Throttle stop adjustment that you need to make to achieve this.
