Shipyard Isl. Marine Engine Parts - Yamaha F90 WOT low

Hello Andy, I have had my 2003 F90 Yamaha and boat a 2003 Scout 175 Sportfish now for 6 years. This season started out good. Last weekend noticed could not get all rpms I am used to . Same 17 pitch prop wot now at 5500 rpm and was at 5900-6000. Also noticed a small spitback at times when in low rpm ranges. From Idle to 2000 while on a flusher hose. I took off and inspected the on engine filter it was clean. I have an onboard Racor canister S3240 filter inline from the boat tank. Next I hooked up an external tank with fresh fuel . Same results. Valve noise is some but minimal. oil is clean. No exhaust smoke . Spark plugs all look the same. This boat motor has only 127 hours total with 26 hours since early this years oil and filter change. I change it once a year with the low hours I put on. I am a ring free user and also use ethanol treatment. My next ring free purchase will be the new stuff for ethanol gas. So cranks up good , small skips and spit backs and high end rpm loss has me worried. Any suggestions.

Assuming your boat bottom is clean, it sounds like internal carb problems, or maybe a weak low pressure fuel pump. You can install a small PSI pressure gauge on the output side of the fuel pump to see how much pressure it is maintaining at WOT. I am guessing in the 5-6 psi range is normal.

Or, get the old spray bottle of gas out and shoot some in each carb while under load at those rpms. If the rpms pickup with the added gas to any of the cylinders, that particular carb I would be taking off and cleaning it. Actually, you would be removing all 4, so you would clean them all. A Ultra-sonic cleaner works really well for doing that.

Or....take it into a qualified dealer.

Andy , Thanks for the info. Qualified dealer sounds the best. If it weren't so far from middle ga :thumbsup:

I am going to try the concentrated ring free treatment first. Would the fuel pump cause low rpm miss / spitback ? Or just be a possible cause of the rpm loss? Pilot valves seem to be sealed with a plug. Could I drill these plugs to get to the low rpm air mixture pilots. Last, are the 4 small fuel filters something that could cause the symptoms I have?

Those caps cover the pilot screws. They are required to be capped. EPA stuff.

Spitback can be caused from a lean running condition. If its lean, figuring out why is the problem. Not enough fuel being delivered to the carbs(weak low pressure pump) will cause a lean running condition. Restrictions in the carbs will cause it too.

If it ran fine, and the screws were not touched, adjusting those is not the problem. They are low rpm mixture screws.

I wouldn't bother with the shock treatment of Ring Free.

If those filters are restricted, that might cause it too.

Andy

Andy , Ball park , what is a service like what I need cost in parts and in labor? That is not having to buy complete kits. thanks