Swapping compression?

[JasonAChastain]

I have a 1990 90 H.P. Nissan motor that was working fine on our first day of a fishing trip then when i started it on the second day it was like it had lost half the power. A compression test showed a weird fluctuation between the 2nd and 3rd cylinder. I've tried a new head gasket, changing the coil packs around, rebuilding the carbs, then a new CDI box but with no help. I've heard of a gasket on the bottom of the cylinder around the crankshaftthat could go bad but my manual only shows some sort of "compression ring" (i guess like a piston ring?)on the crankshaft between each of the cylinders. Any help would be appreciated. <br /> Thanks

 

[TOHATSU GURU]

Re: Swapping compression?

There are rings on the crankshaft, but it would be rare if that was the problem. What's the fluctuation that you are referring to? Is it the test result swapping back in forth with the exact same reading?

 

[JasonAChastain]

Re: Swapping compression?

Yeah between the 2nd and 3rd cylinders there is a fluctuation in compression, its hard to read the exact numbers because the needle is varying about 20-25 p.s.i. sort of a sweeping motion while the number one cylinder is a steady reading. That was leading me to a bad head gasket but it didn't help after a new on was installed. I was told by a 2 cycle mechanic(motorcycle) that there could be a "base" gasket bad, but i didn't see one in the repair manual...

 

[TOHATSU GURU]

Re: Swapping compression?

The engine does have a base gasket...But, I'm still not clear about your compression reading. Is the reading swapping values between 2 and 3 or do those two cylinders have a problem holding a steady value? If it is not holding the values as a constant I would like you to try the following:<br /><br />Start the engine and then remove either the number 2 or number 3 spark plug boot off the sparkplug. Use something that is insulated to do this or you will have a vague idea of what it is like to be hit by lightning. If the RPM goes up when you pull off one of the caps then you probably have a twisted crankshaft. If the RPM goes down then I would look at a scored block and or pistons. Either way...You are going to have to pull the block apart.

 

[TimMattson]

Re: Swapping compression?

Jason,<br /> I sold a motor that came from AK Skiff that looked like a zinc and was thought to be blown.The buyer took off the head to find a crack in the water jacket and then ordered a new head from Tohatsu/Nissan.They sent him,FREE OF CHARGE, a Head upgrade kit with longer bolts,a thicker re-designed Head with all gaskets required.<br /> After fixing it they ran the 90 hp on a Dyno and it put out 112 HP!<br /> This motor had low compression in two adjacent cyl.---if the very same # then it is the same leak.<br /> Tim M