Mcm140 running rough. No power. Two black plugs two lean plugs

[thebvkid]

I've been searching on this site and on the net since I got the boat( about 3 months ago) and have found several posts similar to mine but always lacking in answers
I have an 86 mach 1 Eclipse with an mcm 140 engine. I can't get it past 2000 rpm's without it missing, backfiring and lose power. If I continue giving it throttle it will die.
I have rebuilt the carb, replaced all fuel filters, fuel lines, checked tank for debris, checked anti siphon valve. Fuel pump seems to be working fine, no fuel in the secondary hose. Replaced cap, rotor, points, condenser, plugs, wires, coil. Timing is at 6* btdc @ 500 rpm. Dist advance weights are free and clear.
Compression readings are cyl 1-100psi, cyl 2-115psi, cyl 3-135psi, cyl 4120 psi. Cylinders 3 and 4 have very black sooty dry spark plugs while cylinders 1 and 2 appear to have normal or possibly lean plugs. These were fresh plugs with only one outing on them and only a few hours run time
What would cause my two healthiest cylinders to have black sooty plugs and also cause the weaker cylinders to have leaner plugs? I know some will say the carb. But the carb doesn't discriminate between cylinders and I believe a carb condition would read across all 4 plugs. Can it be a valve problem? Or head gasket? I'll be adjusting the valves today.
Like so many other posts I've read, I too am at a loss and look g forward to hearing your suggestions. Thanks

 

[thebvkid]

Oh and how do you rotate the crankshaft by hand? There is no nut and even if there was it's directly behind the motor mount bolt. There would be no access anyways

 

[Grub54891]

I may be wrong, but the weak cylinders could be caused by a bad headgasket between the two. When one gets on the intake stroke, it may be pulling air from the other cylinder, and not enough fuel, thus dry plugs. Hopefully others chime in and we can get their opinion. I suppose a cylinder leak down test would tell us more!

 

[flipbro]

Intake leak?

 

[Alumarine]

Oh and how do you rotate the crankshaft by hand? There is no nut and even if there was it's directly behind the motor mount bolt. There would be no access anyways

I usually use the alternator nut. You have to put pressure on the belt so it doesn't slip.

 

[thebvkid]

Ya I think a leakdown test would be a good idea too. I've been watching videos on how to do one. I think I'll do the valve adjustment first then do a leakdown. When doing the valves should I run the engine for a while to get it warmed up or adjust them cold?

 

[Grub54891]

You can do them cold, there should Bo a procedure on the stickies .

 

[philipp10]

those compression numbers seem low.....

 

[thebvkid]

Ok I just did a valve adjustment and then a leakdown test using my compression guage hose. I have a lot of air leaking out the intake on cyl 1 and out the exhaust on cyl 2 and air leaking to cylinder 1 from cyl 3 as well as cyl 3 exhaust. Cyl 4 sounded pretty solid. I dont have a real leakdown tool to get actual numbers. But it was pretty obvious when there was a leak. So looks like head needs to come off and get to the shop.

 

[Grub54891]

Did you remove the sparkplugs? You should be able to hear air from one cylinder to the next of it is a head gasket between two cylinders. Hope it's just a valve job!

 

[thebvkid]

Yes all plugs were removed. Does it make sense to hear air from cylinder 1 coming from cyl 3? I would think if there was a gasket issue it would leak from an adjacent cyl not 2 cylinders away. Hmmm

 

[achris]

Taking the head off is a good start when you know that there is a leak somewhere, and not passed the rings and into the crankcase.

As for a leak from 1 to 3, depending on where the crankshaft was at the time, yes a leak could conceivably go from 1 to 3 if both cylinders had the intake valve open at the same time.... Which give the firing order (1 3 4 2), is very possible.

Chris....