rough running at low rpm

[fisherman363]

My 1998 Yamaha 25ELRW outboard runs rough at RPM below 2200. Above that it runs smooth and has plenty of pep, gets right up on plane. It starts right up but idles rough and shakes until the RPM rises above 2200. The carburetors have been cleaned and the fuel pump was rebuilt. The compression is 90 lbs in both cylinders. Adjusting pilot screws doesn't seem to make any difference. Any ideas?

[99yam40]

I would think that motor should have more compression than that, but it could be the gauge used.
either the carbs pilot jets and passages are not really cleaned properly or maybe the fuel pump is leaking some fuel into the crankcase would be my guess

but then a small vacuum leak or reeds might cause a low RPM problem.
did you check to make sure idle timing was in sec and advancing as it should?

[fisherman363]

Checked flywheel markings for static timing, they lined up as manual shows. Timing light indicated advance when engine running. Also I assume timing is advancing as motor quickly runs with power and runs up to 5,000 RPM when throttle is applied. I couldn't figure out how to read timing marks with timing light.

[fisherman363]

I'm familiar with compression testing and I believe the reading is correct. tested with engine warm and throttle wide open.

[fisherman363]

fuel pump just rebuilt

[99yam40]

Checked flywheel markings for static timing, they lined up as manual shows. Timing light indicated advance when engine running. Also I assume timing is advancing as motor quickly runs with power and runs up to 5,000 RPM when throttle is applied. I couldn't figure out how to read timing marks with timing light.

you point the timing light at the timing marks while the motor is running to see if the timing is at idle timing or advanced where it should not be while throttle is at idling position

[99yam40]

I'm familiar with compression testing and I believe the reading is correct. tested with engine warm and throttle wide open.

have you checked to see that your compression gauge is reading correctly?
have you used more than one gauge to see if they agree?

[fisherman363]

I have checked with two different gauges and the result is the same, I'll re-check timing at idle.

[fisherman363]

Finally got back to trying to solve this problem. The static timing as shown in the shop manual is adjusted as indicated in the book. Also the manual calls for 7 degrees ATDC at idle but my timing light reading shows that it's actually running at 2 degrees BTDC. So I need to retard the spark but there is no adjustment left to make. If I adjust the magneto linkage to its longest length it still doesn't retard the spark enough to get the proper timing light reading of 7 degrees ATDC? Is there another adjustment to retard timing that I'm missing?

[99yam40]

It has been a long time since I played with the timing on my C40, but I think at start up the timing was advance some and then when motor reached a warmed up state (temp switch activated) it would drop back .
how long did you run the motor before checking with light?
does you motor have two temp switches like my C40?
if it does is it functioning properly?

I had to change my CDI to correct the 14 ATDC it was doing instead of 7.
With yours advanced the RPM should be too high,

[fisherman363]

motor was running for a while- 1 temp switch connected to an alarm- looking for a way to retard timing.

[fisherman363]

It appears to act like a car engine where the timing chain jumped or the distributer is one tooth off- of course an outboard doesn't have this.

[fisherman363]

Engine was recently disassembled with flywheel off and linkage disconnected.

[fisherman363]

The rpm is to high- around 1,200 instead of 900- so I'm thinking this is causing my rough running below 2,200 RPM. Once over 2,200 engine runs fine.

[fisherman363]

99yam - Sounds like you had a similar problem where you couldn't adjust the timing to the proper degree and had to replace the CDI.