[Hemi43]

I know it's shot in the dark, but what would cause this engine to only have 35PSI.
I bought this engine for parts, because I knew it had issues. I have not taken it appart, and don't plan to until I need to scavenge parts off of it.
I've heard of timing chains stretching on 660s, and wondered if the cam gear could jump a tooth and cause this.
I removed the 3 inspection covers from the head, and everything looks fine with not too much wear on the cam. even valve clearance was OK.
The person I bought it from said it just started losing power the last time he rode it.
Any ideas?
Thanks

 

[blueranger660]

How are you checking compression? The compression release will keep the exhaust open as you spin the motor over by hand so I'm not confident that is a productive way to check compression. There are a number if issues that prevent compression from building in a motor. Bent valves, worn seat or valve face, broken or worn rings, hole in the piston, hole in cylinder, blown head gasket.
Good luck.

 

[QuadManiac]

THe de-comp does not keep a valve open during the whole stroke, it just opens it earlier (or is it closes it later?) to limit compression. Bottom line, anything lower than around 80psi with the de-comp working is due to one of several things as blue outlined:

Damaged/burnt/stuck valve or valves or seat/s

Damaged rings/cylinder wall

Damaged head gasket

Crack in cylinder liner or piston

I've probably left something out.

One thing to check, however, that is common with the Rappy engine: As the valves wear (especially intake), the clearance can tighten up and actually go through zero - taking compression with it. So, check and set valve clearance. If any are too tight to get any feeler gauge in, this could be the cause of your low compression.

 

[Mad Dog]

You say the previous owner only mentioned it was losing power, but it still ran yes? Does it run now?

I have a hard time believe it'd even run at 35 psi.

 

[Hemi43]

Thanks, those are good tips !!
The engine is sitting in a box and will probably never run again. Like I said, I bought it for parts, but if there's nothing major wrong with it I may try to fix it.

 

[willykiller]

Thanks, those are good tips !!
The engine is sitting in a box and will probably never run again. Like I said, I bought it for parts, but if there's nothing major wrong with it I may try to fix it.

I've seen a few JE pistons that cracked in the valve relief area of the piston, this will cause compression problems also...........
Check that out............

 

[blueranger660]

I've seen a few JE pistons that cracked in the valve relief area of the piston, this will cause compression problems also...........
Check that out............

No way?...in the valve relief? Did you have them fly cut?

 

[willykiller]

No way?...in the valve relief? Did you have them fly cut?

Nope, it was a stock JE piston, 11 or 12 to 1, cant remember...............

 

[Hemi43]

I had a few hours to blow, so I took the top end off.
Piston and wall was in perfect codition, with the cross hatching still visible.
I put penetrating fluid on the valve heads, and no fluid went past the seats.
I was stumped, so I put it all back together.
Before installing the cam, I pressurized the cylinder with 120 PSI, and basically no air was escaping.
I put the cam and rockers back on and re-pressurized. Air was leaking from the intake valves. Sure enough, there was no valve clearance.
I kick myself for not checking valve clearance BEFORE disassembly:
I tuned it over a few times with a battery charger, and I'm now getting 100 PSI.
To me, this number seems fine, considering the decompression is on.
What do you think ?
I don't think I will part it out now !!
Dan

 

[blueranger660]

I had a few hours to blow, so I took the top end off.
Piston and wall was in perfect codition, with the cross hatching still visible.
I put penetrating fluid on the valve heads, and no fluid went past the seats.
I was stumped, so I put it all back together.
Before installing the cam, I pressurized the cylinder with 120 PSI, and basically no air was escaping.
I put the cam and rockers back on and re-pressurized. Air was leaking from the intake valves. Sure enough, there was no valve clearance.
I kick myself for not checking valve clearance BEFORE disassembly:
I tuned it over a few times with a battery charger, and I'm now getting 100 PSI.
To me, this number seems fine, considering the decompression is on.
What do you think ?
I don't think I will part it out now !!
Dan

Well, Quad pitched out the 80psi for a lower limit but I thought most 660s produce around 130 with the decomp. I'm guessing the valve adjustment remedied your problem so all that's left now is the butt dyno test.

 

[QuadManiac]

You didn't listen, did you? Checking valve clearance was my first suggestion... Glad you found and fixed the problem.

130 is common for a good engine with decomp, but will vary depending on valve settings and decomp pin wear... 80, as I said, is just about the lower limit for it to run.

 

[Hemi43]

You didn't listen, did you? Checking valve clearance was my first suggestion... Glad you found and fixed the problem.

130 is common for a good engine with decomp, but will vary depending on valve settings and decomp pin wear... 80, as I said, is just about the lower limit for it to run.

You're right, and I didn't listen !! This is a good learning experience for me.