50hp johnson ignition...wierd

[Seanmarty93]

Hoping someone can point me in right direction.

Motor will start. While in neutral I throttle up, motor runs rough and sounds like its firing on one cylinder. I have replaced spark plugs, power pack, fuel filter, rebuilt carbs, did compression test (131 in one cylinder 140 in other.) Here's the kicker, it will do this for a few minutes, then suddenly like magic, it runs perfectly and purrs like a kitten and runs flawlessly. I'm at a loss, any help would be greatly appreciated. It is an electric start late 90s model. Thanks!!!
Sean

 

[jakedaawg]

So, runs poorly when cold...runs fine once warmed up. Does trim angle effect this? Do you have any spit back out the front of the carbs while its cold? Is there a lean pop?

While cold an open an open air gap spark test will tell you something. You want a nice blue snap across a 7/16" gap.

I might also try a can of OMC engine tuner.

 

[Seanmarty93]

I bought an inkine spark tester, dont know if that would give same results as test your reccomending, it seemed it was not firing right for just a few seconds, then started running good suddenly, however, i didnt think about it, but you are correct, it did appear to run better after it was warm. Took it for a spin (boat was still on trailer while performing spark test, backed into water of coarse) it ran like a champ driving around lake for 15 or 20 minutes. I just bought boat. Previous owner told me it was acting funny and a mechanic, (who rebuilt carbs) told him it needed a new power pack. I bought the boat put the powerpack on along with the abive mentioned things. So thats kinda of the history of where im at now. Thanks for the quick response, im at a loss.

 

[gm280]

Obviously engines don't atomize fuel very well when cold and that means you need a richer fuel mixture during that warm up time. Hence the choke system on every internal combustion engine. So if your engine runs rough while warming up, maybe you need more fuel to air ratio to run smoother until the engine can be leaned out to the 50:1 ratio. So look at your choke system to verify it is working properly... JMHO!

 

[Seanmarty93]

I should clarify that it was running rough for about 10 minutes, ran to auto parts store down the road and came back to boat ramp to continue trying to diagnose. It was abiut 15 min between the time i left ramp and came back to test with spark tester so i imagine motor was still warm. Put spark tester on and it appearred to not be flashing well and ran rough for less than 30 seconds, then suddenly started running fine and spark tester was flashing properly.

 

[Seanmarty93]

Jake, I forgot to finish answering your post. Trim angle foesnt matter, no lean pop, and not sure what you mean about spitback, or how to check it. Thanks!

 

[Seanmarty93]

Gm280, I'll check into choke adjustment, however what throws me for a loop is the spark tester acting funny while running rough. Thanks!!

 

[Bosunsmate]

might be a stuck open thermostat and its therefore taking ages to warm up

 

[gm280]

Gm280, I'll check into choke adjustment, however what throws me for a loop is the spark tester acting funny while running rough. Thanks!!

Okay new info about your problem. Odd that the spark is erratic during the warm up time. That points to a problem in the ignition system and not fuel delivery. That could be a lot harder to find the problem because if it only happens when cold, you have to either allow it to totally cool off each time, OR buy a can of cool spray and start spraying ignition parts down to see if the spark starts acting up and misfiring. And the real problem doing that is most ignition setups are under the flywheel. So maybe the spark coils can be tested with cool spray. One other thing you can test is see is if the spark is erratic on both cylinders when cold. If so then what ever feed the spark coils is next inline to check out. If it is only one coil, swap them with each other and see if the problem swaps positions as well. If so replace the defective coil and see if that fixes the problem. If that doesn't follow the coil swap, then you verified the coils are good and what feeding the defective position is the next area to look at... JMHO!

 

[Faztbullet]

Could be a bad plug.......

 

[Seanmarty93]

I changed the plugs before I took it out.Thanks for everyones input and suggestions. I'll look more into it this weekend and report back.

 

[jakedaawg]

Plugs can be bad right out of the box...

I think you need an open air gap spark check while cold...There is a thread in the sticky at top about how to build one if you dont want to spend money on one. (If your gonna fix your own outboards your going to need one)

I believe you will need a OEM manual but the CDI troubleshooting guide will probably be enough to get you through this. There are links all over on here to it.

 

[Seanmarty93]

Thats a big 10-4 Jakedawg! Thank you!!!!!

 

[Seanmarty93]

Ok, took boat out this weekend. I replaced coil and wires and spark plugs. Motor fired up just fine. Ran it to a couple different fishing holes. Then at the end of the evening took about a 15 minute boat ride then headed for the ramp. Everything was fine, boat running perfect. I looked at my brother in law smiled and said " looks like we fixed it" and i swear on my grandmothers grave the second i finished that sentence, the thing started acting up again. I'm more confused now than ever. ALTHOUGH...... My brother in law noticed while i was getting truck, if he shut the key off, then started it back up, it would run fine for about ten seconds, then start acting up again. He did this three or four times with the same result. Does that help anyone have any ideas of what the problem could be? Could it be a "computer" issue?