1988 90 HP Johnson - Runs Rough When NOT Under Load

[InMotion]

Hey Guys,

I have a 90 HP Johnson that runs rough only when not under load and only when at speed! So lemme give you the good and bad:

The Good:
- Starts great
- Idles great
- Low speed operation is great
- Acceleration is great
- Runs great when off plane (or off the "Step") --- will run flawless like this for extended periods of time
- New powerpack, 4 new coils, 4 new wires, newish stator, newish trigger, VRO disconnected, good compression in all 4 cylinders, newish to new looking hoses.

The Bad:
- Accelerate to speed and it will run on all 4 for a bit and then just start running rough. If I choke it, it bogs. If I back off or accerate it will run well again for a bit and then at some point run rough again.
- Trim positions don't seam to make a difference.
- With more people in the boat it, will run better --- meaning it will run on all 4 for a much longer period of time.
- Turning corners doesn't seam to impact it.
- A number of fuel lines were connected using zip ties --- they are loose enough I can turn them and could pull them off their respective fittings

Guys ... just can't figure this one out. A boat mechanic suggested that the 2 check valves connecting the top and bottom carbs could be failing and leaking fuel back into the lower carb and thus flooding the lower carb and knocking out the two lower cylinders. The check valves were next on my list to replace... but thought I would post this to all the experts out their to see what you guys think is the solution!!

Picture of the Motor (and boat):

Thanks,
IM

 

[AEROCOOK]

Re: 1988 90 HP Johnson - Runs Rough When NOT Under Load

I've read your post a number of times and to be honest with you I was hoping someone would reply with an idea. Since nobody has piped in yet I though I would offer my thoughts and bump you back to the top of the list so here goes...I'm wondering if the float in one of the carbs is out of adjustment? It almost sounds like under heavy and prolonged fuel demand, the bowl may be going dry.
I'm not buying into the "fuel leaking back into the carb from the fuel hose" theory. Isn't the fuel line supposed to supply the carb with fuel? I dont understand what he was trying to say there.
If in fact the carb is running dry it is no longer lubricating the respective cylinders which obviously isn't good.
BTW thats a pretty boat you have, I'm just up hwy 11 in Innisfil. Maybe Ill see you out on Simcoe one day. I will definitely remember the boat if I see it.

 

[InMotion]

Re: 1988 90 HP Johnson - Runs Rough When NOT Under Load

I've read your post a number of times and to be honest with you I was hoping someone would reply with an idea. Since nobody has piped in yet I though I would offer my thoughts and bump you back to the top of the list so here goes...I'm wondering if the float in one of the carbs is out of adjustment? It almost sounds like under heavy and prolonged fuel demand, the bowl may be going dry.
I'm not buying into the "fuel leaking back into the carb from the fuel hose" theory. Isn't the fuel line supposed to supply the carb with fuel? I dont understand what he was trying to say there.
If in fact the carb is running dry it is no longer lubricating the respective cylinders which obviously isn't good.
BTW thats a pretty boat you have, I'm just up hwy 11 in Innisfil. Maybe Ill see you out on Simcoe one day. I will definitely remember the boat if I see it.

Thanks AC! --- for the bump and the compliment. It was a fun boat to rebuild!

What he was saying was that if the check valve or valves are leaking, then the fuel will just drain back into the bottom Carb and cause a flooding situation in the bottom two cylinders. However, in chatting last night with a friend of mine who is works on bit motors, he feels that 99% it is an ignition related issue as the engine runs rough vs bogging and when it runs rough if I choke it it bogs... meaning there is plenty of fuel there to begin with.

He felt the roughness is most likely ignition related as a fuel delivery issue usually results in a bogging not a roughness issue. If too much fuel was going to the bottom cylinders then there would most likely be a bogging not a shuddering roughness to the engine. Also, since this is load related --- meaning it only occurs when the engine is not under load, the stator most likely is producing too much current or intermittent current --- and it the excess current has no where to go so to speak when the engine is not under load.

Hope that makes sense --- the way it was explained sounded reasonable.

I was hoping others here would be able to chime in with thoughts on whether the logic sounds right or not!!!

Cheers fellas!!

 

[AEROCOOK]

Re: 1988 90 HP Johnson - Runs Rough When NOT Under Load

If you suspect ignition, take an inductive timing light out with you and see if the spark quits or becomes erratic when the motor is running rough. Obviously you will have to check each of the 4 plug wires individually.