Lack of power 9.9hp

[coke39294]

Hi again from England,

My Yamaha 4 stroke 9.9hp does not like to go passed the 2 thirds mark on the control lever. It starts "hunting" and loses power as though something is slipping, the engine revs drop. If I put it back to 2 thirds or below, the engine returns to normal and away we go. It doesn't seem to have a lot of power anyway, I know it is a 9.9 but is it too small for our GRP 25 footer?

Many thanks in anticipation.

[Livinlinelife]

Not shore what a GRP is but I have a 12 ft john boat with a 7.5 Honda 4-stroke and it is way under powered! But it's juts my giggin boat so I needed something small. It moves it but getting on a plain is not going to happen.

[TownsendsFJR1300]

Hi again from England,

My Yamaha 4 stroke 9.9hp does not like to go passed the 2 thirds mark on the control lever. It starts "hunting" and loses power as though something is slipping, the engine revs drop. If I put it back to 2 thirds or below, the engine returns to normal and away we go. It doesn't seem to have a lot of power anyway, I know it is a 9.9 but is it too small for our GRP 25 footer?

Many thanks in anticipation.

What is the max RPM your getting out of the engine when trimmed/loaded, etc?

You should be seeing between 5,000-5,500 RPM's at WOT (in gear, underway).

Has this just started or?


And yes, what is GRP?? I can't find anything related to boating with "GRP"...

[coke39294]

Hi, GRP is a term for Glassfibre build, I don't know what the revs are but it doesn't like to go into full power mode, engine seems to "hunt" if you understand and revs drop as though something isn't quite engaging, i.e. when throttle is fully open the revs then drop and power is lost, I then put it back to approx. 2/3 rds on the throttle and it regains the boats drive, it has only recently started ,but it has never really had any real power, I have also looked at the prop which is slightly damaged on one fin, would that make a difference ?, the engine was put with the boat in 1986, perhaps I just need an upgrade, the boat is only used as a cruiser on our canal and river system.

[TownsendsFJR1300]

Ok, thanks for the additional info.

The damaged prop won't help anything but won't make the engine hunt at a certain RPM, just hurt OVERALL performance, especially at higher RPMs.

When was the last time the carbs were cleaned?

Specifically the main and idle jets?

Yo want to check your fuel pick up screen for any debris/varnish. It may be allowing just enough fuel for 2/3 throttle and not anymore to go above that(RPM wise)..

If you list the exact year or at least the model #, the carb parts fisch is likely here under the red section Yamaha parts. You can see exactly where the jets are. With the crappy fuel(at least here in the states), untreated fuel can go bad in as quickly as a month and varnish shut jets.

Running Yamaha's "Ringfree" is an EXCELENT FUEL system/ top end cleaner.

An aftermarket (or Yamaha) tach IMO, would be a good investment. You'll know if your operating at 100% of what it should, increasing speed, miles per gallon, etc..