1989 115hp L6 carb no response to idle screw adjustments

[tomconnolly]

This motor is on a 1989 winner fiberglass bass boat. It struggles greatly to get on plane and wot is 4100 trimmed up. I just rebuilt the motor and timed it (and it didn't seem to change the struggle either) and now I'm attempting to adjust idle mix. The top 2 carbs are responding as expected to the idle screw adjustments, but the bottom carb doesn't respond at all the the idle screw adjustments. I can turn the idle screw all the way from 1.75 turns out to lightly seated with no change to the motor rpm or sound. Sound like I need to take apart the carb and check for clogged idle jet? Could clogged idle jet be one of the reasons it struggles so much?

 

[Faztbullet]

Check carb for debris and that it is getting fuel. You still running that 23p prop?

 

[JDusza]

Timing done with a light? Confirm ignition on all cylinders. Are the bottom 2 leaking spark? Check at night in the dark.
Clogged idle jet could hurt idle to midrange but high speed should run.
Could be a stuck needle valve in lower carb.
Does the carb have a drain plug you can open to see if the bowl is getting fuel?
J

 

[Chris1956]

If the above checks don't fix it, check the color of the spark plugs. if the bottom ones are real clean, you have water infiltration putting out your spark.

 

[tomconnolly]

Timing done with a light? Confirm ignition on all cylinders. Are the bottom 2 leaking spark? Check at night in the dark.
Clogged idle jet could hurt idle to midrange but high speed should run.
Could be a stuck needle valve in lower carb.
Does the carb have a drain plug you can open to see if the bowl is getting fuel?
J

Timing done with a light. Used gauge to find Piston 1 TDC and set timing pointer to 0 deg.

All other thoughts above I'll get done tonight. Thank you!

 

[tomconnolly]

Check carb for debris and that it is getting fuel. You still running that 23p prop?

Yep, 23p prop. I think that will be my next change, after I get the motor running decent. Surely a 23p can't be good for that motor. 4000 rpm WOT, no slip, huge struggle to get on plane...

 

[Faztbullet]

Need a 17-19p on that 18ft boat

 

[tomconnolly]

How critical is it to set timing marker based on .464 mark in service manual? Is setting based on 0 TDC with piston at TDC sufficient instead?

 

[Chris1956]

If you think about it, the piston is at TDC for several degrees, as the crankshaft connecting rod journal turns and the piston reaches TDC. You can see this on any motor. Simply put a screwdriver in the spark plug hole and set the motor to TDC. Now rock the flywheel both ways and see that the piston does not move immediately, as the flywheel moves.

So your timing can be off by several degrees.

The .464 offset assures that the TDC timing is only at one point, making timing several degrees more accurate.

 

[tomconnolly]

If you think about it, the piston is at TDC for several degrees, as the crankshaft connecting rod journal turns and the piston reaches TDC. You can see this on any motor. Simply put a screwdriver in the spark plug hole and set the motor to TDC. Now rock the flywheel both ways and see that the piston does not move immediately, as the flywheel moves.

So your timing can be off by several degrees.

The .464 offset assures that the TDC timing is only at one point, making timing several degrees more accurate.

I had a feeling that was the case. Darn it. Time to borrow that gauge again...