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1991 Yamaha 9.9 4 stroke wont keep running

BIG FOOT

New member
I just took my boat out of storage and tried to test run my motors. My 1991 9.9 4 stroke High Output motor will only start and run fine if i partially block the air intake to the carb with my hand.
I have cleaned the carb, started with new fuel with carb cleaner added.
It seams to be getting to much air but am not sure why, it ran fine last year before storing for the winter.

Any suggestions out there on how to fix this issue?????
 
I have a different 9.9 - a 2002 Honda. But I did have similar problems. So take this with a grain of salt.
1. The problem is fuel related. when you partially block the air intake - that increases the richness of the mixture. Without your hand - the mix is too lean and won't burn.
2. the carb needs to be COMPLETELY cleaned. The idle mixture screw needs to come out as well. Some of these motors have that idle mixture screw sealed off. The Honda had a dial setting on that screw. It was a plastic piece glued to the end of a normal needle valve - but it limited the adjust-ability for emissions reasons. This also prevented me from being able to fully remove that screw. I took the rest of the carb apart - blew carb cleaner through every hole I could find - but it never fixed the problem at idle. More dramatic steps had to be taken.
3. I pried that off that glued on dial with a screwdriver. Then I took a dremel disk to the flat head of the screw so I could get a screwdriver to engage the screw. (made a slot)
4. back that mixture screw out...guess what - it was loaded with carbon and varnish. Clean up that screw, then shoot carb cleaner into the screw hole. Put it back together and adjust the idle.

Adjusting the mixture: The only adjustment is the idle mix. The main jets on the carb are fixed.
1. Start by screwing the mixture screw all the way in.
2. Back out 1.5 turns.
3. Try and start it.
4. If it doesn't start, back it out by half turn increments and repeat till it starts.
5. Let it warm up
6. Once warm, turn the mixture screw back in till the RPM drops, back it out till it is smooth.

The factory tune on these things is so lean - they barely run. I found that a slightly richer idle makes a world of difference in the performance of the engine - no misfires, no stumble from idle to higher throttle settings. It didn't work this good when new.

Good luck!
 
I have a different 9.9 - a 2002 Honda. But I did have similar problems. So take this with a grain of salt.
1. The problem is fuel related. when you partially block the air intake - that increases the richness of the mixture. Without your hand - the mix is too lean and won't burn.
2. the carb needs to be COMPLETELY cleaned. The idle mixture screw needs to come out as well. Some of these motors have that idle mixture screw sealed off. The Honda had a dial setting on that screw. It was a plastic piece glued to the end of a normal needle valve - but it limited the adjust-ability for emissions reasons. This also prevented me from being able to fully remove that screw. I took the rest of the carb apart - blew carb cleaner through every hole I could find - but it never fixed the problem at idle. More dramatic steps had to be taken.
3. I pried that off that glued on dial with a screwdriver. Then I took a dremel disk to the flat head of the screw so I could get a screwdriver to engage the screw. (made a slot)
4. back that mixture screw out...guess what - it was loaded with carbon and varnish. Clean up that screw, then shoot carb cleaner into the screw hole. Put it back together and adjust the idle.

Adjusting the mixture: The only adjustment is the idle mix. The main jets on the carb are fixed.
1. Start by screwing the mixture screw all the way in.
2. Back out 1.5 turns.
3. Try and start it.
4. If it doesn't start, back it out by half turn increments and repeat till it starts.
5. Let it warm up
6. Once warm, turn the mixture screw back in till the RPM drops, back it out till it is smooth.

The factory tune on these things is so lean - they barely run. I found that a slightly richer idle makes a world of difference in the performance of the engine - no misfires, no stumble from idle to higher throttle settings. It didn't work this good when new.

Good luck![
All good info and than you
 
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