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90 2 stroke missing at WOT

N

NZpackhorse

New member
I have a 90hp 2 stroke 2005 ( 130 hours)that runs fine until I really load it up.
It seems to run fine under little load but when I load it up it will run for 10+ seconds then start to miss. If I load it up a bit more before hand then the go WOT it cuts out sooner.

I have rebuilt the fuel pump but no change.
There seems to be plenty of fuel getting to the carbs.
I have disconnect one spark plug lead at a time and then run at WOT. There was no difference regardless of what cylinder I disconnected ( except all 3 times it only ran on 2 cylinders and would not rev above 4500rpm.

At this stage I am guessing....
A. One carb is faulty, perhaps the fuel bowl is not filling up fast enough at WOT.
B. A spark plug is failing under high load/ temp
C A Coil is failing under high load.


Any suggestions?

I would rather not remove and dismantle each carb unless I have too.
 
My best guess: The enrichening circuit used to cold start the motor is not shutting off properly and is flooding it out. You can test this theory by disconnecting the hose coming from the solenoid valve and plugging it. (Be careful not to allow a fuel leak!) Be sure to warm the motor up first since it won't start well cold without the circuit.

Note: This valve, operated by the "choke" button, allows gas going to the carbs to be squirted directly into the motor. Great for cold starts, but not good if the valve leaks when warmed up.

Jeff

PS: If that's the problem, replace the solenoid valve.
 
I would have thought that would be more of a problem at low revs when the ration of extra fuel added by the solenoid would be higher. Under high load wouldnt that extra fuel make little difference?
When at idle I activated the choke and it stalled the engine showing that at least then it is working as expected. Not sure why it would activate under load?
It really does feel like too little fuel not to much.
Still its something to consider.
Thanks
 
Extra fuel is extra fuel, no matter where it comes from. Your test proved the valve will open, but didn't prove it wouldf shut tightly.

Jeff
 
What is the compression on this motor ??-----------------Run with a timing light and see how spark behaves on all cylinders.---------------Be careful with extended running / testing ---------If it is a fuel issue the no gas going into a cylinder means no oil either.---------------Carburetors must be absolutely clean !!
 
have you tried pumping up the primer bulb to see if its soft,or when sqeezed it alters the motor?reed valves broken,or loose,?advance mechanism loose?
 
When it was missing I stopped the engine and checked the bulb and it was hard. Its a little difficult for me to check it while its faulting.
 
Dont have a timing light..... I do have a scope meter..... 800kms away :(

Just changed the plugs and took it for another run. No difference but upon inspecting the new plugs after I found they were quite oily. Not sure how much oil to expect though.

Didnt manage to isolate the fuel solenoid yet. Need to get a plug for the hose.
 
Thermostat ( if equipped ) stuck open ??----------------Oily plugs may mean engine is not producing enough heat ( poor compression ) or weak spark , will spark jump a gap of 3/8" with a snap you can hear on all cylinders ??.
 
After the fault started I rebuilt the fuel pump and changed the thermostat from a 120F to a 143F on the recommendation of a Mercury service tech. Neither solved the issue.

He said the pump could be the cause of the issue and he generally rebuilds them every 100 hour service while the 143deg F thermostat is better suited to local conditions.

I did notice I was not getting any hot water out of the tailtell when running in the ocean but when running with the hose and muffs it was warm??
 
OK I removed the lower port cowling and gained access to the lower carb bowl drain screw.
Once removed the fuel in there drained freely but when I pumped the primer bulb only a little came out. I removed the 2nd carb bowl drain screw and petrol flowed relatively freely when I squeezed the primer bulb.
My original theory of a carb blockage is looking more likely.
So I have now removed the carb and taken the bowl off. I put a hose on the internal outlet for the fuel inlet and gave a blow. It may have just been my imagination but I am sure then was a restriction ... perhaps something flying out then less of a restriction!!
Tomorrow will tell. I will reassemble and test run it again.
 
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