Yamaha vx76 250hp

[crm4887]

Looking for some wisdom:
Yamaha VX 76
1995 TXRT 250HP 2 Stroke Outboard on 24' Pursuit CC

Engine was running great till about a month ago+/-?
Began having fuel / ignition issues.
-Resistance on ignition coils was out of spec and replaced all 6 along with plug boots
-Replaced all fuel filters and then 3 fuel pumps as 2 diaphragms had leaks. Bypassed check valves and anti-siphon to insure healthy fuel flow.

Now when idling or operating under 800-900 rpms it will cough regularly as though possibly not firing on a cylinder.
In gear above 800-900 it seems to operate smoothly. When I go from cruise 4200 rpm to WOT the engine reaches 5200rpm for a split-second and drops immediately to 4800rpm.
Even with the engine trimmed all the way up. Engine used to easily hit 5200-5400.
Runs smooth at 4800 rpm but does not go any more.
When I race the engine in neutral it will peak around 5700rpm.

Compression test on cylinders within 1-2psi of each other. 105psi
-Stator and Pulser coil resistance is within spec
-Throttle sensor within spec
-Crankshaft sensor within spec
-Spark plugs new in the spring; likely only 60hrs on plugs. In decent shape

Still believe its an ignition issue but am unsure at this point.
Any thoughts?

Thanks for any insight!
Chris

 

[rejesterd]

One thing that jumps out at me is that it's rough at idle in neutral, but it smooths out when you put it in gear. Seems like there could be an issue with the linkage. But I would definitely clean/rebuild the carburetors. That's one thing you didn't mention, and screwy carburetors can definitely cause issues like you described.

105 psi is a little low, but if it was running great until a month ago, I doubt that's related to your issue.

Do a spark test on all the plug wires. You just need a small spark tester, which is cheap and can be found at most auto parts stores. That will confirm if the coils are producing enough spark to jump the gap specified in the service manual (which you should get if you don't have it). If any of them can't jump the gap, then you might need to check the output peak voltage on one of the ignition coils, as it's possible that your CDI needs to be replaced.

 

[crm4887]

Thank you for the info!
Figure on carb overhaul this winter.

Will check with spark tester. Plugs are spec'ed at .038"

Whats interesting is it used to top out at about 5200-5400. Now when I go from cruise to WOT it hits 5200 for split second and instantly is retarded to 4800. Just have to thing its something digital that is causing an immediate retard to 4800. With no load in neutral can race the engine to 5700rpm
This is what stumps me...

 

[99yam40]

a motor does not need much fuel to run in neutral, and probably could run above 5700 in neutral with out some of the cylinders firing properly.
doing this can hurt the motor, so do not do that anymore.

My guess is a cylinder is not getting enough fuel into it at high RPM or the plug is not firing as it should.
I would try new plugs.
simple and not expensive to try

a timing light hooked to the individual plug wires should show if that one is getting spark while the problem shows up.
might be interesting job with the cowling off and running WOT on the water with a timing light in hand