WOT Surges/Stumbles on a 1975 70 hp

[jdaghir]

I have a 1975 70 hp, long shaft, mechanical shift, model 70573E.

It starts easily and runs great at everything except WOT of 4900 rpm (I am slightly over-propped). At WOT I am experiencing surges/stumbles from 4900 to 3300 rpm and back again. The pattern will be something like this:

15 minutes @ 4900, 3 sec @ 3300, 45 sec @ 4900, 2 min @ 3300, 10 sec @ 4900, 5 sec @ 3300, 20 min @4900....and so on.

It only happens at WOT. I can run at 3/4 throttle (4000 rpm) all day long with no issues.

The probelm first appeared last season, but only happend very occasionally and went away quickly so I assumed it was water in the fuel or some other containiment. But this season it is happening almost anytime I try to go to WOT.

Sometimes going to neutral and then back to WOT will cause it to go away, but not always. I have two tanks, both with fresh fuel treated with Stabil and it occurs with either tank so I don't think it's a contamination issues. I removed the fuel tank camp to see if it was a venting issues but that didn't help either.

The fuel pump on the motor is actually from a 2004-2009 40-90 hp, but since pumping the primer bulb does not seem to help I don't think it is a fuel starvation issue.

Last summer I completely rebuilt all three carbs including new high speed orifices, set the WOT timing and did a link & sync. At the end of the season I drained the fuel bowls and blew out the orifices with carb cleaner so I don't think it's a carb or sync issue.

Last season all three plugs could jump a 7/16" gap and since it runs well at anything other than WOT I don't think it's a high voltage ignition coil problem.

I do know the alternator circuit is not working properly. The battery doesn't charge and the resistance of the alternator coil in the stator measures 0.6 ohms while the specifications call for 1.0 +- 0.2 ohms. I actually have a replacment stator but I haven't installed it because I didn't feel like dealing with pulling the flywheel. It's easy to just put the battery on a charger after taking the boat out.

Before I go through the effort of pulling the flywheel and replacing the stator, is there anything else I should check first or other suggestions to try?

If I do pull the flywheel, is there any other routine checks/adjustments/cleaning/lubircation I should do while the flyweel is removed?

Thanks in advance for sharing your knowledge!

-Jeff

 

[emdsapmgr]

A couple of comments from your testing. 1. Possible you have the wrong fuel pump and it could be causing the fuel bowls in the carb to run out of fuel. Not sure just how a 2004-2009 fuel pump would fit on your engine. If you take the original factory fuel pump to a Bombardier dealer (385784) they will give you the latest superceded part, a 438559 fuel pump. That is one of the two pumps put on the old 235 V6 engines. That one pump will have the correct volume capacity and fuel fitting (hose barb) size openings to provide plenty of fuel volume and pressure for your 70. The bowls should never run low on fuel with that pump. 2. Could be a weak ignition component. Weak components are affected by heat. (may fun fine when cold, then crap out when the engine warms to normal operating temps. Powerhead temps will vary, depending on whether the pressure relief valve in the thermo housing is open or not. When on plane, the pressure relief valve will open and the powerhead temps can go down as much as 20 degrees. If that nylon pressure relief valve is heat-distorted-it may not be opening properly and the engine could be running warmer than normal when planing. So, you need to check for spark after the engine is well warmed up and it's bogged to 3300. Intermittent spark from a weak stator or power pack could be just enough to cause random misfires and the low rpm's you noted at times. 3. Make sure none of the spark plug leads are jumping spark to ground. Run the engine till heated up to normal temps, at night. In the dark- remove the cowling and visually check to see that none of the spark plug wires are shorting to ground. 4 That engine should run at least 5500 rpm's, so you correct, you are at least a couple of prop sizes too high.

 

[jdaghir]

Additional info I should have included in my original post:

- It happens both when the engine is cold and well-warmed up so it's not heat related, but still could be a weak electrical component.

- It's on a pontoon so it never really planes out much.

But thank you for the suggestions. I'll check into them.

- Jeff