motor surges as if running out of gas, runs weak

[Home Cookin']

I wish we had an "any outboard repair" forum since questions like these could pertain to any motor

Anyway, 1986(?) johnson 70--runs fine for a couple hours, then while running at higher speeds, starts surge/drop like running out of gas, then runs at reduced power. It did this a few times over the winter while hunting, where you can't stop and mess with it you just ahve to get back to a safe place. Get home, shut it off, run it again and it's fine.

This time when I made it back to the dock I checked the plugs, all getting spark, but the motor (at idle) would stall out when I unhooked the top plug. But when I took it back out it still ran poorly--or it would hit OK then drop down.

No VRO and water pump tell tale running fine, so I don't suspect an overheat/no oil issue.

Gas is non-ethanol and treated, the filter has maybe 10 hours on it if that. New plugs.

I should have run it from a remote tank but didn't have the time or spare tank with me.

I suspect the carbs need cleaning and link/sync, maybe a stuck float--all I know about what to guess I learned here! But I left it at a shop yesterday afternoon (they were closed) and due to many circumstances including my incompetence I am going to have them work on it. But I know we both benefit from an informed diagnosis.

What's the collective wisdom from all you experts and experienced boaters?

thanks

 

[oldcatamount]

Re: motor surges as if running out of gas, runs weak

It could be a weak ignition coil (or two). Coils will become intermittant when hot and be fine when they cool down. I doubt it's the carbs because you said it died when you pulled the top plug wire. There are dozens of things it could be but, if it were me, I'd check for a bad ground wire or weak coil first.

 

[emdsapmgr]

Re: motor surges as if running out of gas, runs weak

The engine may not be running on all 3 spark plugs all the time. Your action to pull the plug wire off the plug was a good test. If the engine's rpm drops when idling when a plug wire is pulled off, that indicates it is firing. Do that same test with the other two plugs once the engine starts to cause you problems. You should see/hear a noticeable rpm drop when you pull each plugwire off. (a pair of insulated pliers makes this job easier.) It's not unusual to have an ignition problem as you describe, once the engine is warmed up. Weak ingnition components tend to fail once they heat up to normal operating temps. Just how warmed up the engine needs to be before a component breaks down can vary.

 

[Home Cookin']

Re: motor surges as if running out of gas, runs weak

I got a drop in rpm (idle, by sound) as I pulled each plug plus I used a spark tester on them. But the engine quit when I pulled the top one--not suddenly; just died.

thanks for the suggestion on the coils. Makes sense. Is that something that can be tested in a shop? Will a weak coil show up on the test if the motor isn't hot? My problems occured at high speeds after running for a while so my shop can't duplicate that.

 

[emdsapmgr]

Re: motor surges as if running out of gas, runs weak

The coils are generally pretty reliable. You can test them by swapping them to see what happens. That ignition system also has other expensive components which can cause problems: the power pack, timer base and stator. These components are all pretty pricey. Spending some time diagnosing the problem may save you some $ in the long run. You can find some really good troubleshooting tips either in the factory service manual or at this website: cdielectronics.com. Again-do your troubleshooting when the engine is well warmed up.

 

[oldcatamount]

Re: motor surges as if running out of gas, runs weak

How about an induction timing light when running warmed up? It might show you an intermittant cylinder (or two). You'd need a co-pilot to do the steering if you want to test it under a load. Coils are strange creatures, a "bad" coil can test good and a "good coil can test bad, but a timing light will point to which one is the weakest.

 

[Home Cookin']

Re: motor surges as if running out of gas, runs weak

good idea on the timing light; my plug tester is a cheapie and very hard to see the light, and intermittant spark is a good possibility. I have the boat at a good mechanic's and just want to give him the best guidance since he can't run it in the water.