piston inspection

[james otte]

I have an 88 evinrude v6 that runs rough and surges at wot. my spark plugs end up slightly oily and dark leading me to believe the rings and or pistons are worn. I took off the head cover and find the walls to be pristine, pistons have some build op on the face of them but rings look good from the view i have.. when i try to rattle the piston with my fingers, i get a tiny bit of wobble but between the piston and the ring, not the ring and the sleeve as i can see the ring is not moving. is this normal? Also, the gasket looked fine, eliminating it from concern.

 

[Joe Reeves]

Re: piston inspection

The first thing you should have done is to take a compression check. This would have told you if you had a piston blow by problem or not. If you did do this check and simply haven't mentioned it.... what were the readings of each cylinder.

Weak ignition would also result in that type plug appearance. Rig a spark tester whereas you can set a 7/16" gap for the spark to jump. The spark should jump that gap with a strong blue lightning like flame... a real SNAP! Does it? Note that testing the spark with the spark plugs is a waste of time! The 7/16" gap is important.

Yes, it is normal for the piston rings to stay expanded to the cylinder walls and for the piston to move about somewhat. This proves that the rings are not stuck within the piston grooves.

 

[camalot]

Re: piston inspection

Something else to look at are you stator outputs over 3k same as the coil inputs over 3k, doing a test off the start isn't in the rev range your saying your problem is.

 

[boobie]

Re: piston inspection

Put the heads back on it and do a compression test. That way you'll know what you have to start with.

 

[emdsapmgr]

Re: piston inspection

At WOT you need maximum fuel flow. Possible you have a restriction of some sort. Next time out and at WOT, have someone constantly prime the fuel hose. That will force extra fuel into the carbs. If you engine runs fine while you constantly prime the bulb, it's a fuel issue. Could be a weak pump. I've also seen debris clogging the anti-siphon valve and that caused surging at WOT. You can clean carbon off the piston skirts and piston crowns with a can of Bombardier Engine Tuner. If you've never done that procedure, it may be time. Pretty easy with the schrader valve on the side of the fuel primer solenoid.

 

[F_R]

Re: piston inspection

I have an 88 evinrude v6 that runs rough and surges at wot. my spark plugs end up slightly oily and dark leading me to believe the rings and or pistons are worn. I took off the head cover and find the walls to be pristine, pistons have some build op on the face of them but rings look good from the view i have.. when i try to rattle the piston with my fingers, i get a tiny bit of wobble but between the piston and the ring, not the ring and the sleeve as i can see the ring is not moving. is this normal? Also, the gasket looked fine, eliminating it from concern.

In the first place, you are confusing a two-stroke with a four-stroke. You have a two-stroke, and every drop of that oil that you put in it goes through the combustion chamber, so of course the plugs are oily. That's just the way it is, and it is not broke.

Along the same line of thought, the rings in a four-stroke seal the compression, but another ring seals oil from going from the crankcase to the combustion chamber. Two-strokes have no oil control ring because the oil goes through with the gas.

Put the heads back on, check compression, and look for some other cause of your running problem.

 

[daselbee]

Re: piston inspection

You are trying to apply automobile knowledge to this two-stroke. Shouldn't do it.
They are two different animals.

It is normal for the pistons to be "loose". It is OK for carbon to be on top of the piston.

You are looking for the problem in unlikely areas; look elsewhere (ignition, carbs) for the issue, as suggested above.

 

[james otte]

Re: piston inspection

WOW! First off, thank a ton for all the input. a little more history: the fool before me had the ignition wired incorrectly and blew the rectifier, stator and power pack. i have replaced all but coils, spark is good and ohm readings on coils are all good (i know its not a load test, but since spark is good, i haven"t gone down that road). i had done a compression check and all cylinders read at 60 psi( my guage reads about 20-30 low as i have tested it on my compressor). after reading your replies, i thought back to when i got the boat and remembered that the previous owner had installed an electric fuel pump. after i fixed all the air leaks in the fuel line, i took it off because it would then flood the engine at start up. so today, i took off the fuel pump for some inspection. it was a vro that had been bypassed. i found a tear in one of the diaphragms that may have been the vro failure, i now need to research that. the tear was on the smaller brown diaphragm not the large black one, but none the less, i thought this may be affecting things. i have already rebuilt all the carbs, the lower unit, and the prop. they really don't kid when they talk about the expense of a boat, this is my first and i have had quite a learning experience. as far as the spark plugs go, i thought that even on a two stroke , the oil would still burn off leaving a tan insulator whereas mine is black after a couple of outings. again, thanks for all the response.

 

[emdsapmgr]

Re: piston inspection

If you idle the engine, then pull the plugs, they will appear oily, perhaps maybe slightly dark. If you do a high rpm throttle chop and then pull the plugs you will find they are light tan-no sign of oil. That's normal on a 2 stroke.