84 Merc 90hp 6cyl sputtering, where to start looking??

[444]

Took the boat out on the lake the other day, for the first hour and a half everything was great. Then when I took it to 3/4 throttle it randomly started sputtering like it was misfiring. So we nursed it back to the boat launch and took it home. Sputters/misfires slightly at idle but gets worse the more throttle I give it. On the way back I realized I forgot to open the gas tank vent, however the tank is old and doesn't seal quite 100%. Got home and fired it up on the muffs with the vent open and still sputtering. I tried pumping the primer bulb while it was misfiring and I could see plenty of fuel in the fuel filter. Did a compression check and all cylinders were in the 130-135psi range. Tested with the motor warmed up, all plugs out and throttle wide open. Looked at the plugs(tried to get a pic but camera batteries dead, will take a pic tomorrow) all the plugs were wet with sort of a white-ish grey-ish paste on them that wipes right off. Since all plugs look the same I suspect a fuel delivery issue, but since pumping the primer bulb didn't help(even though it did increase the amount of fuel in the fuel filter) I'm not sure where to start....?

I will put on a new fuel filter tomorrow and check the old one for flow but I doubt solving this issue will be that easy.

 

[jerryjerry05]

Re: 84 Merc 90hp 6cyl sputtering, where to start looking??

Sounds like water in your system.An old tank is trouble.
Ethanol is messing up a lot of motors.It's delaminating the inside of older hoses causing clogs in the carbs and fuel filter.Check for soft hoses and junk in the filter on the motor.Check the diaphram J

 

[444]

Re: 84 Merc 90hp 6cyl sputtering, where to start looking??

I'm going to try checking for spark on all cylinders today and replacing the fuel filter and checking out the fuel pump. Before I took it out that day I filled up the gas tank from empty with 91 octane non-ethanol gasoline. But you are right, something could have dislodged. I noticed there's a fair amount of crud sitting on the bottom of the fuel filter so I'll change that for sure.

 

[444]

Re: 84 Merc 90hp 6cyl sputtering, where to start looking??

Well fuel filter is fine, tons of flow thru it when I disconnected it from the carb feed and pumped the fuel primer bulb. Here's a pic of what the plugs look like, they were perfectly clean before I put them in and ran it for about 1 minute. I'm checking out the fuel pump diaphragm as per another thread I have going. I'm also going to check each plug/coil for spark.

 

[444]

Re: 84 Merc 90hp 6cyl sputtering, where to start looking??

Hooked my inductive timing light up to each plug wire, hit the starter and the timing light flashed on each one, so I guess each plug/coil is good.

 

[444]

Re: 84 Merc 90hp 6cyl sputtering, where to start looking??

I've determined the fuel pump is not the problem, nor is spark or fuel flow through the filter. At idle it runs basically okay and the more throttle I give it, the worse misfires. It's enough to shake the motor when it does it. Further inspection shows fuel is leaking out of the front of the carb throat/air inlet on the middle carb and dripping onto the middle carb. Perhaps that carb picked up some dirt that made it past the filter...

 

[roncoop75]

Re: 84 Merc 90hp 6cyl sputtering, where to start looking??

Hello again,

Try starting the engine at night, in the dark. Look for wires arcing to ground, should be plain as day... er, night, as it were, if that's the issue.

Good luck.

 

[arsenalpsu]

Re: 84 Merc 90hp 6cyl sputtering, where to start looking??

plugs LOOK fine(do an ohm test to make sure), could be carbs BUT just because you have spark on all 6 doesn't mean it's sparking correctly or at the right time. I'd test your electoral system a bit more thoroughly. Go to the CDI we site for procedures, you'll need a DVA tester. If you're dripping fuel out the carbs at idle or start up that's normal (hence the little fuel bowl to catch it below the last carb) But if it's still dripping after running a bit they are dirty.

and do that test as mentioned above, if you have any arking you'll need new plug wires.

 

[444]

Re: 84 Merc 90hp 6cyl sputtering, where to start looking??

Thank you for referring me to the cdielectronics.com website. The problem is the rectifier that I just replaced LESS THAN ONE WEEK AGO!! I disconnected the yellow wires from the rectifier and gave the motor a run on the muffs and all is well in boat land. Hooked the rectifier back up and the problem reappeared. I never thought the rectifier could cause a misfire like this because I've had bad rectifiers on my 2-stroke sled before and never had a run problem. I had just replaced the rectifier because my tach wasn't working and my old rectifier failed the rectifier continuity test. Tach didn't work with the new rectifier either, I wonder if it died because of me playing around with the tach or if I got a bad rectifier out of the box. Just odd because I had about 1.5hrs use on this rectifier before it crapped out. I think I will just leave that tach disconnected this time. I don't trust it!