Got Low Compression? Find Out Why

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ElPiloto
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Got Low Compression? Find Out Why

Post #1 by ElPiloto » Sun Jul 27, 2008 1:36 pm

To find the cause of your low compression for sure, get an old spark plug and beat the porcelein out of it. Remove the ground electrode and you will be left with just the steel shell with threads to screw into the spark plug hole.

Now, braze or weld an air fitting on it so that it will snap into an air line from a compressor.

Now, make sure the cylinder being tested is at top dead center.

Screw this tool into one cylinder at a time then snap on the air line with pressure at least at 100 psi.

Listen through a hose at the intake, take off the air cleaner lid and open the throttle all the way. If you hear hissing, you have a leaky intake valve.

Then listen at the exhaust. If you hear hissing, you have a leaky exhaust valve.

Take off the oil filler and listen inside the crankcase. If you hear the hissing, you have leaky or broken rings.

You can also diagnose a head gasket that is leaking compression into the coolant. Apply the pressure and, with the radiator filler cap off, listen, and watch, for bubbles in the coolant.
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Post #2 by Zacsdaddy » Thu Jul 31, 2008 11:41 pm

Great Tip!

Next time I change plugs I've got a new project!

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Post #3 by hiddenone » Fri Dec 26, 2008 1:53 am

great tip. thanks
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Post #4 by Old Fogey » Fri Dec 26, 2008 7:57 pm

If you check out Alley Kat's thread on the stem seals there is a good pic of the tool that EP is talking about.

http://www.ngwclub.com/forum/viewtopic.php?t=14861
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Re: Got Low Compression? Find Out Why

Post #5 by AXL Stubbs » Wed Mar 31, 2010 11:03 am

Brilliant action1
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Re: Got Low Compression? Find Out Why

Post #6 by AXL Stubbs » Tue Apr 06, 2010 11:50 pm

Good trick, BAD news. I built the tool and filled my Dirty Bird full of of air.

I could not keep any amount of compressed air in. It seemed to be leaking out the intake valves mostly. Decided I'd check and maybe lap the valves and found one #1 and #3 pitted on the faces and #4 bent.

The bike ran (not super smooth) when I started this project and I don't really want to dump $100-150 on new valves. Can I lap these valves, re-install with the new head gaskets to recheck the compression? Will the new head gasket be ruined if I'm unsatisfied with compression?
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Re: Got Low Compression? Find Out Why

Post #7 by Whiskerfish » Sun Apr 11, 2010 8:40 am

Well I certainly would not buy new valves. Try a lap job first. If they do not seal up then look for used heads on Flea Bay. The valves have a coating and should not be ground but lapping is OK. For the one that is bent I would try asking around in the for sale section. I bet someone had some laying around that can be had for cheap.
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Re: Got Low Compression? Find Out Why

Post #8 by mrflamewerks » Sun Apr 11, 2010 8:57 am

AXL Stubbs wrote:Good trick, BAD news. I built the tool and filled my Dirty Bird full of of air.

I could not keep any amount of compressed air in. It seemed to be leaking out the intake valves mostly. Decided I'd check and maybe lap the valves and found one #1 and #3 pitted on the faces and #4 bent.

The bike ran (not super smooth) when I started this project and I don't really want to dump $100-150 on new valves. Can I lap these valves, re-install with the new head gaskets to recheck the compression? Will the new head gasket be ruined if I'm unsatisfied with compression?


What was the compression in each cylinder prior to doing the leak down test EP described?
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Re: Got Low Compression? Find Out Why

Post #9 by mrbswing » Thu Jun 10, 2010 1:00 am

ElPiloto wrote:To find the cause of your low compression for sure, get an old spark plug and beat the porcelein out of it. Remove the ground electrode and you will be left with just the steel shell with threads to screw into the spark plug hole.

Now, braze or weld an air fitting on it so that it will snap into an air line from a compressor.

Now, make sure the cylinder being tested is at top dead center.

Screw this tool into one cylinder at a time then snap on the air line with pressure at least at 100 psi.

Listen through a hose at the intake, take off the air cleaner lid and open the throttle all the way. If you hear hissing, you have a leaky intake valve.

Then listen at the exhaust. If you hear hissing, you have a leaky exhaust valve.

Take off the oil filler and listen inside the crankcase. If you hear the hissing, you have leaky or broken rings.

You can also diagnose a head gasket that is leaking compression into the coolant. Apply the pressure and, with the radiator filler cap off, listen, and watch, for bubbles in the coolant.

I am not trying to be the bad guy here BUT isn't it easier to take a comp test then put some oil in the cylinder to see if your comp comes up" which will tell you its the rings and if it don't come up its the valves then maybe go ahead with the test which would not really be needed because you would know if it was the rings or valves correct? the way I check the head once its off is by taking a piece of plexiglass and drilling a small hole in it to poor a liquid in and see if it leaks and it will run out the exhaust if its an exhaust valve or intake if its an intake valve! but what the heck do I know anyhow! dancr just a thought?
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Re: Got Low Compression? Find Out Why

Post #10 by mtclimited » Thu Jun 10, 2010 1:04 am

The tip as described has been used for years by me on autos.

one other thing. If you see bubbles in the radiator when you do this....Time for a headjob, and don't go there.

Pouring water in your cyl. won't tell you that!
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Re: Got Low Compression? Find Out Why

Post #11 by JBz » Thu Jun 10, 2010 3:09 am

whats the trick to keep the piston at TDC when doing this test.? JB
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Re: Got Low Compression? Find Out Why

Post #12 by mrbswing » Thu Jun 10, 2010 1:13 pm

mtclimited wrote:The tip as described has been used for years by me on autos.

one other thing. If you see bubbles in the radiator when you do this....Time for a headjob, and don't go there.

Pouring water in your cyl. won't tell you that!

yes true on the coolant leak
till then :8)
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Re: Got Low Compression? Find Out Why

Post #13 by mrbswing » Thu Jun 10, 2010 1:16 pm

jbz wrote:whats the trick to keep the piston at TDC when doing this test.? JB

yes good question because the minute you put air it will push the piston down!
till then :8)
Mr Bill
(2)77 wings,two 83 wings 76 CB550 bobber,80 wing,CX500 turbo,700 Nighthawk,2 750 Yamaha triples,80 XS1100 yammy,CB360,600 yammy,CB750 s/s,and a few more? and my main bike(a beautiful 79 XS1100 Yamaha 19,000 miles) and it is very fast! Got another bike a 84 1200 Yamaha I think its a 1200? and more coming. Till then :8)

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Re: Got Low Compression? Find Out Why

Post #14 by evilratwing » Fri Feb 22, 2013 2:18 am

I did this in reverse on my old Toyota... I set the regulator on my Air compressor at 10PSI and hooked up the air to my radiator... I pulled spark plugs out and let it sit for about 30 seconds then cranked it over... coolant all over the bottom of my hood from the middle cylinders.


Probably not the best way, but definitely the most entertaining.

I'll be making one of these tools and testing. If the bike is in gear, the piston shouldn't move. You're only acting on one cylinder, so the rest of the engine is still working against compression on whichever one is on the compression stroke. If it does move, it won't be much.

..that, and if it's truly at TDC it should be mechanically locked anyway. Pushing straight down on the crank at true TDC should produce no movement.
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