fritz3000g
Recruit
- Joined
- Oct 22, 2017
- Messages
- 1
Hi there. Great forum! First time poster, with a motor issue that I haven't been able to diagnose this year.
I have a Yamaha 9.9 4-stroke from 1989, which is new to me as of June.
The problem
When hot, the motor sputters above 1/4 throttle. I get 5 minutes of good running, then have to be content with low-throttle operation.
What works
1. Idles fine, both cold and hot.
2. Runs at low throttle fine, both cold and hot
3. Runs at high throttle when cold
4. Compression is good
5. Has spark
6. Starts fine, both hot and cold.
7. Doesn't ever stall unless the gas tank is turned sideways or something..
What I've checked and tried
1. Fuel System
a. Rebuilt carburetor and adjusted screws to specification. This cured the idle problems I'd been having, but didn't help with this issue.
b. Checked fuel pump by spraying it with copious amounts of water (new models are water cooled). No effect.
c. Bypassed the pump while elevating gas tank. This resulted in a couple misfires and a little black smoke (suggesting plenty of fuel) but no reduction in sputtering.
d. Checked gas tank. Works great.
e. Pumped gas ball when sputtering. Sometimes I convince myself this has a tiny effect, but not really. Ball is soft but not collapsed when running at high throttle.
2. Ignition system
a. Replaced spark plugs with proper models.
Other theories
I think I've ruled out the fuel system and engine compression issues, leaving either oxygen or electrical, so...
1. Something is making the engine run rich when hot. Faulty thermostat or air intake screw positioning?
2. Ignition timing is off in such a way that it only matters when hot and at high throttle (sounds weird, but who knows).
I've already got one shop repair bill, and would love to avoid another (and learn more about my engine). Any ideas what you'd try next?
I have a Yamaha 9.9 4-stroke from 1989, which is new to me as of June.
The problem
When hot, the motor sputters above 1/4 throttle. I get 5 minutes of good running, then have to be content with low-throttle operation.
What works
1. Idles fine, both cold and hot.
2. Runs at low throttle fine, both cold and hot
3. Runs at high throttle when cold
4. Compression is good
5. Has spark
6. Starts fine, both hot and cold.
7. Doesn't ever stall unless the gas tank is turned sideways or something..
What I've checked and tried
1. Fuel System
a. Rebuilt carburetor and adjusted screws to specification. This cured the idle problems I'd been having, but didn't help with this issue.
b. Checked fuel pump by spraying it with copious amounts of water (new models are water cooled). No effect.
c. Bypassed the pump while elevating gas tank. This resulted in a couple misfires and a little black smoke (suggesting plenty of fuel) but no reduction in sputtering.
d. Checked gas tank. Works great.
e. Pumped gas ball when sputtering. Sometimes I convince myself this has a tiny effect, but not really. Ball is soft but not collapsed when running at high throttle.
2. Ignition system
a. Replaced spark plugs with proper models.
Other theories
I think I've ruled out the fuel system and engine compression issues, leaving either oxygen or electrical, so...
1. Something is making the engine run rich when hot. Faulty thermostat or air intake screw positioning?
2. Ignition timing is off in such a way that it only matters when hot and at high throttle (sounds weird, but who knows).
I've already got one shop repair bill, and would love to avoid another (and learn more about my engine). Any ideas what you'd try next?