1990 Yamaha 40 ETLD high speed miss and low power at WOT

[Sdlloyd2003]

Hi all, Second post here and first one was answered with great results. Got this 1990 40 ETLD and it has a high speed miss. Just had it in the shop and had the carbs torn down, cleaned, rebuilt and sync'd. Timing set and on ears it runs pretty good (will take out at 4000+. It also runs good at low speed and idles well, little miss here and there but its a two stroke.

The problem is when I get it up to full throttle it only hits 3000 RPM (10 mph, its on a 20' pontoon) and has a miss/stutter/sputter. Drop the throttle down to about 3/4 and its fine and runs along without issues. Has a 11 3/4x12 prop (thought it might be over prop'd). Run StarTron and Seafom in the fuel, new fuel, clean filter and all the usual suspects. Plugs are new. Tested the coils but havent heard from the mechanic yet if they are OK. Could this be the CDI or Stator? Any other ideas or things I should look at? This thing is driving me nuts. Would love to figure it out before the end of the season here in Ohio so I can go into the next year by only putting fuel in it.. Help or suggestions are appreciated!

[99yam40]

40 Hp motor pushing a 20 foot boat with that prop does not sound right to me.
Have you talked to any others with a setup like yours?

but then you can find all kinds of strange things sometimes.
I found my C40 with drilled out main jets that took me a long time to figure out

[TownsendsFJR1300]

It shouldn't miss, stutter or spit at all. If over propped, RPMs will just be limited at WOT.

A compression test wouldn't hurt. I'M assuming the carbs were fully cleaned correctly (hopefully)..

A "test wheel" (at your dealer) in place of the propeller should show if it's an engine issue or prop issue.

[Sdlloyd2003]

It shouldn't miss, stutter or spit at all. If over propped, RPMs will just be limited at WOT.

A compression test wouldn't hurt. I'M assuming the carbs were fully cleaned correctly (hopefully)..

A "test wheel" (at your dealer) in place of the propeller should show if it's an engine issue or prop issue.

Carbs were fully disassembled and cleaned and runs 100% better than it did before I took it in. Starts right up, no choke, runs great at low end and mid range, just not WOT. I believe he checked compression but not 100% sure or what the pressures were. Could this be a CDI or Stator?

[99yam40]

test everything you can (compression, spark, timing)
check voltages into and out of the CDI
if all is good you should be left with fuel to air mixture problem
too high and too low and the cylinders have a problem burning properly

the prime start system could be not shutting off, a jet could be plugging from something in line or fuel pump that got in there after the carbs were cleaned, fuel pump could be leaking into #3 crankcase, the rubbed plug in one of the carbs over the pilot jet could have fallen out, crankshaft seals bad,
and probably a lot more things could cause problems

has this motor ever run properly on you boat?

[Sdlloyd2003]

Thanks!

When I bought it it ran good at top end but like crap at bottom and wouldnt idle and died at low end. Finally bit the bullet and took it to a shop to have the carbs rebuilt as the person before me did little to winterize the motor.(told me he didnt have any issues with it (yeh, right...LOL) it good at the low end and mid I cant get it to max out at top end. Put a new fuel pump on it (cracked the barb off trying to get the hose off of it. New plugs, new gas and all the usual. I have the service manual and will have to do a couple of the electrical tests on the CDI and other electrical items. This thing is making me crazy as it shouldnt be this hard!

[TownsendsFJR1300]

Did the fuel tank get drained and cleaned???

With the carbs cleaned, if the tank had water / crap in it / OLD un-stabilized fuel (and not removed), that junk got re-introduced into the carbs and re-clogged them.

Adding new fuel to bad fuel = more bad fuel..

[99yam40]

you will need a peak reading voltmeter made for testing these ignition system or a DVA adapter with a regular meter.

If it was my motor and I had paid someone to clean the carbs and it ran bad after at high RPM ( it ran good before he worked on it) I would be after them to go back into it to see what they did to make the problem

[Sdlloyd2003]

you will need a peak reading voltmeter made for testing these ignition system or a DVA adapter with a regular meter.

If it was my motor and I had paid someone to clean the carbs and it ran bad after at high RPM ( it ran good before he worked on it) I would be after them to go back into it to see what they did to make the problem

Working on it with the mechanic right now. Just looking for experience and hoping that someone else had a similar condition that was a head slapping easy fix. If we get it figured out I will let everyone know. Thanks for the help and suggestions..

[walleye1]

Did the tech clean the carbs in a ultrasonic cleaner?

[Sdlloyd2003]

Did the tech clean the carbs in a ultrasonic cleaner?

Not that I know of.