Compression Test Results...?

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Hubbskiboy
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Compression Test Results...?

Post by Hubbskiboy » Wed May 31, 2017 9:49 pm

I did a compression test recently and I got some really weird results.
Basically all 4 cylinders except one were giving me 140psi, which is about right.
However the 3rd cylinder from left to right was showing 80psi.
I squirted a bit of oil into the troublesome cylinder and the compression shot back up to about 145psi.
Now I was thinking that this must be a worn piston ring problem, but now I've thought about it, maybe it's just the valve clearance's?
After all, what are the chances of 3 of the cylinders showing near perfect compression and just one showing 80psi?
Answers on a postcard please?
BTW, how long would it take for the oil to disperse and the readings to drop back to what they were originally?
I need to do some more testing!

Ta very much,
Hubbskiboy.

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Variablevalves suck
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Re: Compression Test Results...?

Post by Variablevalves suck » Wed May 31, 2017 9:57 pm

You should always do the valves before a comp test and also worth swapping a plug across and rechecking in case there's a bad seat on the plug.

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Re: Compression Test Results...?

Post by Variablevalves suck » Wed May 31, 2017 9:59 pm

I mean cleaning the plug seat!

Hubbskiboy
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Re: Compression Test Results...?

Post by Hubbskiboy » Wed May 31, 2017 10:18 pm

Thanks Variable,
I've just had another brainwave.
Would it be a completely stupid idea to check the compression of an engine when it's in an engine stand by attaching a variable speed cordless drill to the generator rotor bolt?
i.e the bolt that you would usually turn to check the valve clearances.
I've taken the starter motor out of the engine and it's more trouble to put it back on.
I don't think it would be a good idea to attach a 12v battery directly to the starter motor would it? :o

moforockband
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Re: Compression Test Results...?

Post by moforockband » Wed May 31, 2017 11:14 pm

Nope it's 80 amps! Wire in the starter relay with some wires and just touch the relay wires together. Much saferer.
Make sure you have a good seal on the compression tester.
Piston rings do wear out quite fast on a 15000rpm engine, especially if oil changes neglected. Mine were worn down to 140psi-155 after 40000 hard kilometers. I put some new OEM rings in and lightly honed the bore. Got back up to 160-175 psi.
Strangely doesn't make much more power, but does seem to idle stronger.

Hubbskiboy
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Re: Compression Test Results...?

Post by Hubbskiboy » Thu Jun 01, 2017 8:02 am

Thanks Mofo,
I'll just buy a replacement solenoid off eBay purely for testing purposes rather than messing around with my wiring loom.
Although there is a spare loom on there for £29.99.
After a quick scan, they look quite cheap and I'll only be using it for a few bursts once in a while.
So I just need to connect it to the battery with some decent gauge wires and all will be fine yeah?
I've got some spare battery wire that I was using for an inverter in the shed. That'll do nicely!
I think it would make sense to have a system that I can use to test an engine on it's own so I can work on it and get to parts easier.
I'm hoping that better compression will equate to easier starting especially in the cold winter days.
Not that I'm riding it! Just want to keep the engine turned over.

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Re: Compression Test Results...?

Post by moforockband » Thu Jun 01, 2017 10:06 am

I made up a simple push button with a bullet connector to connect on the starter solenoid relay coil contacts, and wired directly to the battery. Use the same gauge wire for the starter motor as your normal loom and all will be OK.
Just make sure that your pushbutton is on the low current side lol. If you're unsure on the technical side, don't do it , as 80 amps will deffo leave a mark :oops:
75 psi could mean a stuck/damaged/worn ring, carbon sooted valve seats (common) or corrosion/wear in the bore. If you're keeping the bike, and the bore is good, then new rings and a DIY hone job is a good investment.

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Re: Compression Test Results...?

Post by minkathecat » Thu Jun 08, 2017 4:22 pm

best way i have found is drop the exhaust system drop the carbs of and pressurise the cylinder using the plug hole and see if you here hissing at top dead centre on that barrel you will here it out the carb intake or the exhaust intake
then if its the cylinder take out the oilm filler cap and listen You can also if ok check how long it takes the pressurised cylinder on the clock see if the air disipates fast.

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Re: Compression Test Results...?

Post by biofotis » Mon Jul 24, 2017 11:44 pm

mine was like that as the engine was standing for a while. I put some WD40 through the plug hole, let it stand overnight and it went back to 145 the other day when the engine started. Now its always 145


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