intermittant rough running

[ShalloWateReds]

Merc 30hp 4-stroke. Will intermittantly run very rough with little power. Then will run perfectly. Have new gas, plugs are clean. What else can I check without taking it in?

 

[medburd]

Re: intermittant rough running

try a different fuel tank (is there any debris in the tank you currently have), check your fuel filters, look for spark plug wires arching (easily seen in low light conditions), check distributor cap if equiped... we need more symptoms...at what rpm ranges...is it just after start up....does it happen after WOT runs...does it happen when you are trying to accelerate..that sort of common theme conditions..Good luck

 

[ShalloWateReds]

Re: intermittant rough running

Fuel filter is brand new and installed correctly. The rough running was at take off and thru acceleration. Ran like it was firing on only one or two cylinders. Let it sit an hour while we ate dinner, then it ran beautifully. Next day it started out running great, then would, from time to time, sputter. Can't really pin down a pattern. Was just wondering what the obvious things to check would be. How about plug gaps? I have never checked those.

 

[ShalloWateReds]

Re: intermittant rough running

Took it out Saturday. Motor was hard to start, hard to keep running. Got out of the channel & opened it up, motor died. Was very hard to start. Starting required 1/3 throttle. If I very gently gave it gas, I could get up on plane, but barely. If at any time & let go of the throttle, it would die. Was always hard to start. Motor smelled "gassy" when it died. Fuel filter is full of fuel, bulb is firm, gas is fresh, carb was cleaned that morning with Gumout. <br /><br />Any suggestions? I'm thinking a plug isn't firing, but don't know how to check.

 

[Chris1956]

Re: intermittant rough running

SWR - It sounds like a carb problem. I once cleaned my Merc's carbs with gumout by filling the bowls with the stuff and letting it sit. Turned out it made the fuel inlet needles stick in their seats. I had to pull the bowl covers and clean and reset the needles and floats.<br /><br />To check spark on all cylinders, put a timing light on each spark plug lead. Watch for consistant spark. If that test passes, remove the spark plugs and compare them. If one is cleaner than the others it may be fouled. Clean them up or replace and reinstall.

 

[ShalloWateReds]

Re: intermittant rough running

I've looked at my carbs and don't see bowls anywhere. This is a 4-stroke with 3 stacked carbs, one for each cylinder. There is no apparent bowl anywhere.

 

[ShalloWateReds]

Re: intermittant rough running

Well, I think I fixed it.<br /><br />I replaced my plugs last weekend. While doing so, I noticed a blue wire that had come unplugged from itself. I plugged it back in, figuring it couldn't hurt. Bought a 6gal portable tank and started it off the fresh gas. It is now running perfectly. I don't know what it was, and that blue wire had me perplexed, but it seems to be better.