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Discussion Starter · #1 ·
Ok so I have a CX500 with low compression. 90 PSI in left cylinder, 60 in right (both measured dry). If I put a little oil down the spark plug hole, both cylinders get ~150 PSI compression measurements.

Does this FOR SURE mean worn piston rings?

And if so, is there a good breakdown guide on how to replace? I've done car pistons and scooter pistons before, but this looks more complicated (possibly have to split the case).
 

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Discussion Starter · #3 ·
How long since the bike was last in use?
Long time. Probably at least a decade. Was given to me by a friend, "ran when parked". I changed the oil, cleaned the carbs, put a little oil in the spark plug holes to lube the piston rings, and fired it up. I've had nothing but problems since (mostly with the right side cylinder not firing at low RPM), hard starting, etc...
 

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Most likely it's OK. The rings get gummy and jam in their grooves. Note the compressions and see if it'll start. Maybe first let it sit a day or two with a good dose of marvel mystery oil on the pistons. A more aggressive option would be 50/50 acetone and transmission fluid. Disconnect the ignition system if you use this.

If you can get it to run it often sorts itself out.

At 60/90 you will be lucky if you can get it to consistently run.

It may take some time. Soak it.

The bad firing on the 60 cylinder at low revs is probably the compression but may be an ignition pickup or a dirty idle or low jet. The faster an engine revs the less time there is in a cycle to lose compression so they're effected less at higher RPM.

Anyhow, keep soaking it. Put a set of plugs in that you are prepared to lose as the oil saturation will likely ruin them and crank it with the plugs in. It won't hurt to try to start it but it will likely saturate the plugs with the oil in the cylinder.

If you can pressurise the cylinder this will help. I'd use my leakdown tester. Check the compression here and there to see if you're making any progress.

If it becomes a manual job we can talk you through it and I have a thread showing one or two rebuilds.
 

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Discussion Starter · #5 ·
Most likely it's OK. The rings get gummy and jam in their grooves. Note the compressions and see if it'll start. Maybe first let it sit a day or two with a good dose of marvel mystery oil on the pistons. A more aggressive option would be 50/50 acetone and transmission fluid. Disconnect the ignition system if you use this.

If you can get it to run it often sorts itself out.

At 60/90 you will be lucky if you can get it to consistently run.

It may take some time. Soak it.

The bad firing on the 60 cylinder at low revs is probably the compression but may be an ignition pickup or a dirty idle or low jet. The faster an engine revs the less time there is in a cycle to lose compression so they're effected less at higher RPM.

Anyhow, keep soaking it. Put a set of plugs in that you are prepared to lose as the oil saturation will likely ruin them and crank it with the plugs in. It won't hurt to try to start it but it will likely saturate the plugs with the oil in the cylinder.

If you can pressurise the cylinder this will help. I'd use my leakdown tester. Check the compression here and there to see if you're making any progress.

If it becomes a manual job we can talk you through it and I have a thread showing one or two rebuilds.
Cool I'll give the soaking it a shot.

I HAVE gotten it to run, it will sort of idle, and I buzzed it a couple hundred feet down the street. Still low compression tho which makes me think the rings are pretty shot.

I'll put some marvel mystery oil down the spark plug holes tonight and let it sit for a bit.

When you say "pressurizing the cylinder will help", what exactly does that help? I have an air compressor so that would be pretty easy to do, just curious what it helps with.

Thanks.
 

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When you say "pressurizing the cylinder will help", what exactly does that help? I have an air compressor so that would be pretty easy to do, just curious what it helps with.

Set the motor to TDC on compression. Pressurise the cylinder with the compreesor using a fitting made from a spark plug, steel tube and a suitable compressor fitting.

Pressure from the top helps to push the rings out of the grooves. It will also help to get the oil in .

Being a CX500 it is unlikely to have bore or ring wear unless it has extremely high milage.

If a bore problem it is more likely to be bore rust than wear but I don't think it's bore rust based on your oil test.. It's crapulence of the ring.

One of mine went from 100 PSI to 170 over a few years use.
 

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Discussion Starter · #7 ·
When you say "pressurizing the cylinder will help", what exactly does that help? I have an air compressor so that would be pretty easy to do, just curious what it helps with.

Set the motor to TDC on compression. Pressurise the cylinder with the compreesor using a fitting made from a spark plug, steel tube and a suitable compressor fitting.

Pressure from the top helps to push the rings out of the grooves. It will also help to get the oil in .

Being a CX500 it is unlikely to have bore or ring wear unless it has extremely high milage.

If a bore problem it is more likely to be bore rust than wear but I don't think it's bore rust based on your oil test.. It's crapulence of the ring.

One of mine went from 100 PSI to 170 over a few years use.
You rock. I'll give that a shot. So I assume its best to let the oil soak for a bit BEFORE i pressurize the cylinder?
 

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After you have soaked the pistons a good long run on a highway for a couple hundred miles usually does wonders for these motors. And Cxphreak is correct bring some cheap sparkplugs with you and of course some tools to change them with.
Another note hows the gas tank? Any rust and you will never get it to stay running right.
 

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Discussion Starter · #9 ·
After you have soaked the pistons a good long run on a highway for a couple hundred miles usually does wonders for these motors. And Cxphreak is correct bring some cheap sparkplugs with you and of course some tools to change them with.
Another note hows the gas tank? Any rust and you will never get it to stay running right.
Good to know. It's not ready for a long highway run (tire manufacture date is from the mid-90s!!!), but it will be there soon if I can get the compression sorted. So far have only putted it down my street to make sure it shifted fine.

Gas tank is actually pristine! Got lucky, the previous owner stored it with a full tank of gas so no rust formed. Just needed to drain it and flush it a few times and its good as new!
 

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Gerard is right about a few hundred miles on the road being the best thing to release the rings.

I'm just not sure the bike is ready for this with the compression it has - but if you can get the bike to run well enough, why not?

Be sure when checking the compression that you have the throttles wide open and the choke off for a true reading
 

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Discussion Starter · #11 ·
Gerard is right about a few hundred miles on the road being the best thing to release the rings.

I'm just not sure the bike is ready for this with the compression it has - but if you can get the bike to run well enough, why not?

Be sure when checking the compression that you have the throttles wide open and the choke off for a true reading
Will do. Marvel mystery oil sat in the cylinders over night, planning to set the right side piston to TDC and then pressurize with my air compressor tonight. Will report back with results., Thanks again for all the help!
 

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Nah, if you have a ball valve at your outlet open the regulator all the way and turn the ball valve on and off.

If no ball valve play with the nitto fitting. Pull the collar back and let it blow pressure and stuff it back together repeatedly.

Take off a valve cover. The blowby past the pistons may blow seals and gaskets if the pressure has nowhere to go.
 

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Discussion Starter · #15 ·
Nah, if you have a ball valve at your outlet open the regulator all the way and turn the ball valve on and off.

If no ball valve play with the nitto fitting. Pull the collar back and let it blow pressure and stuff it back together repeatedly.

Take off a valve cover. The blowby past the pistons may blow seals and gaskets if the pressure has nowhere to go.
Good thought. Thanks!
 

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Take off a valve cover. The blowby past the pistons may blow seals and gaskets if the pressure has nowhere to go.
Or open the oil filler/dipstick.

BTW: If you decide to use an acetone mixture to free them up I would advise at very least smelling the dipstick afterward and replacing the oil if there is any smell of the acetone in the oil on it before you start the engine.
 
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