Low rpms at top speed.

[notloc91]

Hi I have a Johnson GT 150 with a stainless 14 1/4x21 prop on a 17 foot fish and ski. The motor has quick responce and turns plenty of rpms when it's not in gear but as soon as I put it in drive it knda bogs down it doesn't cut out or anything when I throttle down just seems to take off slow. The boat planes out good and runs about 40mph at top speed but it's only turning between 2500 and 3000 rpms and I know that the wot is supposed to be around 4500 to 5500. Do I have a problem or is this right any info is greatly appreciated. Thanks

 

[steelespike]

Re: Low rpms at top speed.

See if the choke helps,Check that it is running on all cylinders.Be sure the throttle plates in the carb are opening all the way.Don't assume because it may be freshly tuned that it is on all cylinders.Could be as simple as a plug wire off or on the wrong cylinder
which would make 2 cylinders dead.Do a compression check.

 

[hwsiii]

Re: Low rpms at top speed.

Notloc, your tachometer is reading the wrong numbers, even at 3,000 RPM with a 1.86:1 gear ratio and a 21" prop your prop slip is a negative -25% and your effective prop pith is 26.2" and that is impossible. Replace your tachometer and give us some new numbers and lets see where you are at then.



H