Motor won't pull under load

[GregPage]

1993 Yamaha 250 carb motors.

As usual, ran fine when I put it up for the winter. Started right up for the spring, just primed the float bowls (I leave them empty over the winter). Went out and fille the tanks with fresh fuel, and then found the port motor won't rev to get on plane.

It won't pull under load to get on plane, but will rev to 5k on the lift in gear with the prop half buried (enough to keep the water pickups under water and put some load on the motor).

New fuel filters last wekend, fresh fuel, oil tanks are full, engine temp right on normal, pulled and cleaned electrical connections. Replaced primer bulbs last year, and all the spark plug caps. Replaced all 6 fuel pumps the year before (I only get about 50 hrs per year to use the boat). I have no way to test the electronics and havn't had any luck getting anyone who can.

I tried to test the fuel pumps/system to see if I could find something good on the good motor and bad on the bad one. Put a vacuume/pressure gauge on both motors. Neither motor shows any vacuume on the inlet side of the water seperator, nor any pressure on the outlet side of the fuel pumps. Based on the readings I wouldn't expect either motor to run, but both motors do run (long enough they would suck the bowls dry if they wern't pulling fuel) and the stbd one will pull to 4k RPM trying to get on plane while the port won't go over 2k. Guess outboard fuel systems just don't run high enough pressure to register/measure.

?? - Greg

 

[GregPage]

Re: Motor won't pull under load

Ok, here's my attempt at checking timing advance. Even though I have a Xenon, battery powered timing light, it's still very hard to see the timing marks in the sunshine.

Both motors: I thought I'd use the inductive pickup to check that the coils are firing first. The port side cyls 1&3 (can't get my pickup down between the cowling to try the bottom cyl) will trigger the timing light. The stbd cyls 2&4 won't trigger the light at all (??). Motors were unloaded at 2000 RPM. Seems strange, but again the stbd behaves fine and it shows the same as the port, and the stbd is running fine. The motors do not sound like they are missing, so I can't figure out why the stbd cyls won't at least trigger the timing light.

at 2000 RPM the timing is at TDC. If I slowly raise the RPM to about 3000 the timing still is at TDC +/- a degree (it's really hard to see the marks). If I just blip the throttle I can see the timing move to 2-4 deg off TDC to the (-) timing mark side (but that is the short side of the scale and only goes to 10, it is the opposite direction that I would expect since the (+) side of the scale goes past 20). for a second or two, then it returns to TDC. The stbd motor movement is pretty smooth, the port motor is very erratic and jumps around a lot.

Maybe one of the sensors to the timing compter is bad or has a bad connection? I think there are 3 thermostat switches, a crank trigger, and according to my manual a knock sensor. I'll see if I can find any test specs for any of them.

-Greg

 

[mattttt25]

Re: Motor won't pull under load

is it possible that it's something simple like the anti-siphon valve at the fuel tank? i had one go bad before that would set off the alarm. finally had it pulled and just didn't replace it, problem solved. also, was told some of the aftermarket bulbs are real crap and that you should always try to replace with oem.

 

[GregPage]

Re: Motor won't pull under load

I thought of the anti-siphon valve. Unfortunately I would need to cut new access hatches into the floor to get at the tank outlets (and hope I guess where they are the first time) as there are no access panels, and to get at the tanks by pulling the floor plate you have to unwire the whole boat and remove the console.

I thought if it was something like that I would be able to see a high suction on the upstream side of the fuel pumps, but when I put a gauge on, it reads zero. Either there is no obstruction, or the fuel pumps don't pull hard enough to get the gauge to read (?).

-Greg