8 PSI Difference Between Cylinders?

[SkiDaddy]

Just got done assembling my new engine (790cc Wammer big bore, bought well used:biggrin: ) & decided to do a compression check before it ever started. I noticed 8 PSi difference between cylinders. Odd I thought, but figured it needed to break in.

After about 4 gallons thru it the difference is still there.:bigeyes: It held 7 PSI for 20 minutes w/only 1/4PSI drop, turned over very smoothly by hand w/out the head on too.

This engine has been extensively ported and the domes may have been custom cut. It does run well, even jetted rich for break in.

Any thoughts? Porting? Domes? Something else?

 

[WaveDemon]

did you check the ring end gap when installing it?

 

[SkiDaddy]

Uhhh-no.:banghead: Maybe I'll pop the head tomorrow night & eyeball it. They looked/felt the same during installation.

 

[jacob06]

if its well used , then try a hone and new rings, if there wisecos you might want to get some more, but i'd be cheap and try the rings first.

 

[Speedfreak]

I thought when it varied more than 10% is when it was bad. 8 PSI is only 4.4% at 180 PSI compression. I would ride it like I stole it.

 

[SkiDaddy]

It has new pistons & rings.

The problem was a slightly bent top ring; the last 1/2 inch from the end on one side had a little daylight showing. I tweaked it with my fingers, problem solved.:cool2:

 

[THRUST]

10lbs or 10% is the rull just spend a few bucks and replace those rings and run a hone through it before t cost you a motor .

 

[WaveDemon]

10lbs or 10% is the rull just spend a few bucks and replace those rings and run a hone through it before t cost you a motor .

good advice.

tweeking you can do with your finger would case me to be conserned. don't you think those rings are under a lot more pressure than you finger applied at 7,000 rpm?

rings are cheap and so are hones.

 

[SkiDaddy]

I'm sure they're under tremendous pressure, but travel is limited by the cylinder wall. There was only a few thousandths of an inch of daylight showing, and I only applied maybe 7 pounds (those things are sharp edged!) of pressure.

I'll give Paul a buzz & see what he says though. I plan to rering in 6 months or so anyway with these giant exhaust ports.

 

[D-Roc]

i assembled my new 82mm topend and am getting 10 psi difference between front and rear. 182 front 172 on rear, I have only run 1 tank through it, should i be concerned? It runs strong on the water and turns over smooth. I looked down the plug hole with a fiber optic light and could not see any scoring on the cylinder walls. It did this right after assembly and i thought maybe the assembly lube was giving me false readings so i ran a tank through it and no difference. I ported this 61x jug myself, is it typical to have this much psi difference on a amature ported motor or should i pull the jug and check to make sure the rings are not hanging up?

 

[Matt_E]

I think you are probably fine. You said you ported this yourself. It's very possible that the exhaust ports are not 100% identical, so that would give you a difference.

 

[D-Roc]

yeah that is what i keep telling myself, i did a search and most people where saying 8 psi isn't bad and 10 psi is the limit, i don't want to be at the limit, lol, i will run another tank through to hopefully get the rings to seat proper, the pistons still look new through the plug holes so i can't read the piston wash yet. also i put the front piston in first and then the rear cylinder so it gets difficult to compress the rings to install, that was also in my mind because it was the rear that was reading low.

 

[SkiDaddy]

Try smearing a dab of grease all around the o-ring on the tester. Screw it into the low cylinder & see if any grease gets blown out.

Mine was doing this, still can't see why, the plugs don't leak. If I can use teflon tape & get the low cylinder to seal properly.

 

[D-Roc]

are you still running a low cylinder? its an old post and i thought you fixed the problem. One other thing i may check, when i looked at my ada domes from running the motor last year they where burning totally different, I examined the domes as did friends and everyone opinion is that one dome was bigger. (never cc'd them) they have the same part numbers but i marked the questionable dome and was going not going to reuse it. later i decided that they had to be the same and went with it. I will pull the head and if this marked dome is on the low cylinder i will swap it with a new one.

 

[Boris]

Swap the domes front to rear and check compression.

Maybe it is the dome.

 

[SkiDaddy]

Mine's groovy now.:arms: Just seemed to need some run time, & teflon tape on the gauge threads.

 

[D-Roc]

i have a new set i can use.

 

[D-Roc]

Mine's groovy now.:arms: Just seemed to need some run time, & teflon tape on the gauge threads.

i gotcha now, i will try that first. It is a cheap guage.

 

[Mouthfulloflake]

Ive got an old 650- that has almost 30 psi of difference.

180+ in one, and 150 something in the other.

been running it for 2 years.

no worries.

hell its actually pretty strong for a 650 ( has a head and pipe)

I would NEVER tear an engine down based upon compression numbers alone....
unless it was like 170psi/70 psi OR if there was a noticeble problem running or power output.

 

[D-Roc]

:biggthumpup: i am going to run another tank through it and then check it all over once more.:Banane01: