Acceptable Cylinder Pressures?

[Kyler]

Motor: '80 GS750 w/ roughly 14K miles

Synopsis - this is the new motor for my sidecar rig. The motor runs, carbs have been rebuilt, oil changed.

I decided that before I pull the motor to finish other mods and install it in the hack, I should checked cylinder pressures. Did that today.

#1 - 100#
#2 - 97#
#3 - 110#
#4 - 110#

Plugs look decent, slightly black but the carbs aren't tuned yet so that may improve.

I'm going to change the oil again, add Marvel Mystery Oil to the oil and gas, and run it in the shop for 30-45 minutes to get good and hot then change the oil/filter again.

As I recall, #2 is always a problem and that is the cylinder that gave out on my '77 motor. I'm a bit concerned that both left cylinders are lower.

What does the collective think? Should I worry about #2 being lower than the others?

[tkent02]

#2 is the one with the vacuum port for the petcock. Bad petcock, leaks too much fuel in that one and can wash away the oil from the cylinder walls and cause wear. It will likely improve with some miles, (miles without the bad petcock) those numbers are OK for a daily rider. It will run just fine like that, last a long time, no worries, unless you need the max performance.

You are going to be racing it?
In that case new rings would make sense, no?

INstead of running it in the shop before the oil change, I'd take it for a spirited ride.

[Nessism]

Compression should be judged after the engine is warmed up and the throttle held open. Check your valve clearances first too if you haven't already.

It's not uncommon for the compression to be low on an engine that has sat for a while. You need to ride the bike regularly before you can truly judge compression.

If you are still only getting 100 psi after taking care of the above, I'd tear down the engine and go though it. For a racing engine I think you would want at least 140 psi, indicating the engine is well sealed. Blow by heats the pistons, not a good thing for a highly stressed racing rig.

Good luck

[portdave]

Make sure you have a really good gauge. I just went thru this and saw a 30 psi difference. Borrow one and compare.
I picked up a snap-on used on craigslist for 40. Awesome tool.

[Kyler]

thanks for the input guys! I'm going to press on and recheck everything after our annual test/track days this spring. I should have everything hooked up by then and can give some hard numbers on oil pressure/temp, etc.

more to come!

[Kyler]

#2 is the one with the vacuum port for the petcock. Bad petcock, leaks too much fuel in that one and can wash away the oil from the cylinder walls and cause wear. It will likely improve with some miles, (miles without the bad petcock) those numbers are OK for a daily rider. It will run just fine like that, last a long time, no worries, unless you need the max performance.

You are going to be racing it?
In that case new rings would make sense, no?

INstead of running it in the shop before the oil change, I'd take it for a spirited ride.

If the weather improves one weekend, it might yet go out (with my KTM plates attached) for a local ride. The tires are cracked pretty bad so it wouldn't be too spirited.

Yes, I will be racing it but I'm trying to get season on the cheap. I'm expecting the rules in my class to change this year and allow up to 1200cc. If that happens, these two motors will go and I'll get an 1100 motor. That motor would get a 100% overhaul with race quality components.

There aren't many guys in my class right now so I'm not too worried.

[Guest]

Motor: '80 GS750 w/ roughly 14K miles

Synopsis - this is the new motor for my sidecar rig. The motor runs, carbs have been rebuilt, oil changed.

I decided that before I pull the motor to finish other mods and install it in the hack, I should checked cylinder pressures. Did that today.

#1 - 100#
#2 - 97#
#3 - 110#
#4 - 110#

Plugs look decent, slightly black but the carbs aren't tuned yet so that may improve.

I'm going to change the oil again, add Marvel Mystery Oil to the oil and gas, and run it in the shop for 30-45 minutes to get good and hot then change the oil/filter again.

As I recall, #2 is always a problem and that is the cylinder that gave out on my '77 motor. I'm a bit concerned that both left cylinders are lower.

What does the collective think? Should I worry about #2 being lower than the others?

Was the compression test done with the carbs wot or even better with the carbs removed if not you need to re do the test. At those readings (if done correctly) tells me that it needs to be rebuilt. I pop the carbs off to do compression test then after testing a cylinder I do another test with a couple/few drops of oil in the cylinder just a very little amount of oil. If the cylinder pressure goes up less than a 15% or ( 15 psi in your case) I know most of the compression loss is in the head. If it goes way up it is your rings. This does work If you don't have a leakdown tester. I say for what your going to do with it. Tear it down and freshen everything up and be done with it for a couple of seasons. Plus if you go with a 11oo motor later then you could save it for a spare
But I am the kinda guy that put a fresh bore with new rings on a 1216cc motor that was showing 150 psi. on the two outsides and 140 psi. on the two inside cylinders.

[tkent02]

Some folks love to waste other people's money.
If it has low compression, check the valve adjustment.
Run it. if it runs fine, who cares?

[Kyler]

Some folks love to waste other people's money

If it has low compression, check the valve adjustment.

We have a practice weekend coming up in a few months. This motor goes as is until then and after that, I'll run the tests again.

I haven't checked the valves yet shiite... almost forgot! Its on the list - thanks!

Run it. if it runs fine, who cares?

I have a spare '82 motor but haven't touched it yet. Been looking at the mud dauber wasp nests in the exhaust ports and thinking that motor needs to be torn down first.

[tkent02]

If the valves don't close completely there can be no compression. Very common on these GSes for misadjusted valves to be the only cause of a low compression problem.