Difference of spark plugs pulled from engine

bart98stang · Mar 17, 2011

Greetings all. I have attached pictures of two different spark plugs I pulled from my 1988 Mercury 135. The plug that has more carbon/oil is the only one that looks like that and the other 5 look clean like the other. These are brand new plugs with about 1 hour of run time. Can some one tell me what's going on with these? My engine idles rough and when it's in the water I get very little power. Going to do compression test next. Thank you in advance.

saumon · Mar 17, 2011

Re: Difference of spark plugs pulled from engine

That cylinder (with the dirtier plug) isn't probably firing at all.

Next after the compression tester is the spark gap tester...

bart98stang · Mar 17, 2011

Re: Difference of spark plugs pulled from engine

Thanks saumon. I failed to mention that I did check for spark and all 6 cylinders and there was spark.

CharlieB · Mar 18, 2011

Re: Difference of spark plugs pulled from engine

Time to properly clean the carbs.

The first plug has been run overly rich and is accumulating carbon. I suspect you will find the float set higher than the others OR the needle is leaking.

Confirm that compression is good and that you do not have low compression on that cyl.

Verify that spark on all cyls is at least 7/16 and bright.

Then order those carb kits.

Chris1956 · Mar 18, 2011

Re: Difference of spark plugs pulled from engine

Check the compression. What cylinder was the dirty plug from? That carb barrell may have an issue.

capslock118 · Mar 18, 2011

Re: Difference of spark plugs pulled from engine

Time to properly clean the carbs.

The first plug has been run overly rich and is accumulating carbon. I suspect you will find the float set higher than the others OR the needle is leaking.

If it's a carb problem, why isn't he seeing 2 dirty plugs out of 6 and not just 1? I thought a 135 that old has 3 carbs - 2 plugs for each.

Chris1956 · Mar 18, 2011

Re: Difference of spark plugs pulled from engine

V6 Mercs have 3 2bbl carbs. Some have dual float chambers in each carb as well.

bart98stang · Mar 18, 2011

Re: Difference of spark plugs pulled from engine

I actually took my carbs apart to clean them yesterday. If any of the passages were dirty/clogged they are clear now. I'll clean the plug and put it back in. My carbs have the dual float chambers.

I checked the compression today but hopefully the gauge I was using didn't have a good seal. All six cylinders were in between 40-45lbs. I would believe my motor was shot if a cylinder or two was that low but all six were low and almost the same pressure. I'm going to have my neighbor come over tomorrow to check it with his.

The dirty plug came out of what I believe is the #2 cylinder. Middle starboard.

Chris1956 · Mar 19, 2011

Re: Difference of spark plugs pulled from engine

Yes, I think your compression gauge is bad. The motor wouldn't run with 45lbs compression. As long as they are all even, it is likely OK, although I do recommend a new compression gauge.

j_martin · Mar 19, 2011

Re: Difference of spark plugs pulled from engine

That's a vertical reed motor, so the carburetor barrel directly in front of the dirty plug is the offender. It looks like way rich, either sunk float or more likely a leaking needle valve, or defective gasket under the needle valve seat.

Any time you open up a WH carb, replace the gaskets. They don't go a second time very well, and a leaking center bowl gasket can give you just that kind of trouble.

hope it helps
John

bart98stang · Mar 19, 2011

Re: Difference of spark plugs pulled from engine

Chris. Thanks, I was thinking the same thing. I'm not a mechanic by any means but common sense tells me the compression gauge is bad. Fortunately, I rented it from Auto Zone...not mine! John, I can't thank you enough. I'll buy new gaskets and check everything you suggested.

bart98stang · Mar 20, 2011

Re: Difference of spark plugs pulled from engine

UPDATE: Cleaning the carb(s) did the trick. Took the boat out today for a good run and ran it wot a few times. Came home, took the plug out, and it was clean like the others. The engine ran great. Thank you to all!

Difference of spark plugs pulled from engine

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