Limited RPM's

[aboatman]

I have a 1989 90 HP 2 Stroke motor on a 17 Ft Boston Whaler.
90 ETLG 6H1 465195 with a 17-K prop, 800 hours with a new power head at 500 hours. I can only get up to 4000 rpm's then it falls off to 3600 rpm's and holds steady for as long as needed. This vessel was originally at altitude
(3500 ft.) and is now at sea level. I use Sea Foam in the fuel as per directions.
Is this indicative of dirty carbs, a bad fuel pump or a tired motor?
Is there a carb rebuild kit ? If so what's the part number ?
If I remove the carbs to clean, should I replace the seals ?
Any input would be appreaciated.

[jonp]

Hello Aboatman,

I have the same set up, and with a 17 inch Yamaha Black stainless I can see 5700 RPM's, 5200 with a 19.

I assume it idles fine? Spark plugs are in good shape and set properly?

Before tearing any thing down, I would go through all of your fuel connections and lines, check for leaks or poor connections. Make sure your tank and water separator is clear. Drain the carbs by unscrewing the brass nut on each of the bowls, may be junk and water in the bottom of the bowls which will prevent it from running properly.

Also, if you have not replaced the fuel pump in a few years it may be time to do so. Relativly inexpensive $40.

Then see where you are at.

[jonp]

90 HP 2 stroke

I would also add that you do a compression check to rule out a more serious situation.

[aboatman]

Limited RPM's

Thanks for the input. Idle is fine. What should the plug gap be? (These have about 75 hours on them, but even new I could not get 4000+) What should the compression be? I have a 3 and 5 gal tank so there is no external fuel/water separator. The fuel is usually less than a week old.

[jonp]

The gap on the plugs should be .035. Probably time to replace after 75 hours...

As far a compression, that is good question... I am not positive what they should be. But they should even, within 7% of each reading and no more than 10%. Last time I checked my 90, 2 seasons ago I was at 125-123-125 stone cold. Numbers may rise when engine is hot.

Numbers below 100 could mean some significant wear and a tired engine although it would still run.

Is your fuel line and bulb newer? I had a problem with a primer bulb once, it restricted flow and would not run properly at high speed. Since no water separator, have you looked and cleaned the internal filter? small clear plastic screw on container with filter inside, just above the fuel pump.

Also does you primer bulb stay hard... or do you need to pump it? If you regularly pumping that means a weak fuel pump. I never pump the primer bulb always stays firm.

Feel free to email me.

Jon

[Timbo down under]

Hey Guys I am seeing something similar with my 130hp 2 stroke.
I bought it with a blown piston (oil starvation) and rebuilt it for a bit of fun. Once together it ran fine and was seeing 5500rpm (after the intitial running in period). Then max rpm dropped to 4700. I took it to a new Yamaha dealer in Perth and found one of the carby jets blocked and the mixtures all set up wrong.
Once cleaned and set up properly I saw 5600rpm and 36+knots, however on the next run I was back to 4800rpm maximum.
The boat has new fuel lines, new water seperator and filter as well as a new primer bulb. The throttle linkage is all working fine and the plugs are also new.
Its back in the shop this week, I'll let you know what they have found, though in the meantime does this sound familiar to anyone?
Cheers
Timbo
PS My compression ratios are not as consistent as you indicate Jon however the engine sounds good through out the rev range - it just seems to be limited at the top end. What effect could the poor(ish) CR have?

[jonp]

Timbo,

Low compression readings could result in weak power out put. I can't be sure but generally speaking readings should be 100 or better.