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Discussion Starter #1
hello everyone
im in a bit of a quandry as to why my motor bike runs at all after doing a compression test?
2 are reading 40 psi 1 reads 42 and the other reads 38?
it says in the book that it should be about 180 psi?
ive put new jets in carbs (standered exhausts and air filter)
ive balanced carbs and done a color tune on mixture jets.
it throttles through the range well and starts first turn of the key on choke.
but when it is under load ,like when i pull off ,it bogs.thats why i thought it may be compression issue as all else seems fine.
i just dont know how it can start on the button if i only have that much compression?any one please.
 

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1983 GPz 750, green
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Compression can and will be affected if the valves are not seated properly (check the shims/valve adjusters for proper tolerence). If those are okay, Then rings and cylinder maybe the issue. You may need to remove the head and check the valve seats valves for pitting and burnout. And, how exactly are you checking the compression? What tools? Do you crank it over with the starter or turn it by hand? Put some oil in the cylinder and re-check the compression.
 

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Discussion Starter #3
hi alex.
im cranking it by starter about 5 revolutions.
i understand what your saying about valves and rings etc but i cant understand why it starts so easy if only 40 psi compression reading?
 

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Alex's suggestions are all good, but I would also suggest you check your compression gage on any engine that you know is good. If the values are really that low, I have my doubts that it would start, but I have had the pleasure of being wrong before. 😁

I would also say 5 revolutions is not enough and you did not mention holding the throttle wide open while doing the check.
 

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Another trick I know of is, set the engine at TDC with #1 cylinder valves will be closed (do eachcylinder in turn) and put compressed air in through the spark plug hole with a good seal. Then see where the air comes out, exhaust or intake.
 

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This can work but if pressure is too high it will move the piston and open valves. I bought myself a leakdown tester which not only allows an airtight seal at the spark plug hole but also regulates the pressure used. These are not too expensive and will work on any engine for motorcycles, cars, ATVs etc.

In either case, the user must listen for leaks at the airbox, at the muffler and at the crankcase by listening at the oil filler. The crankcase breather should be blocked off or disconnected from the airbox, otherwise when listening at the airbox you might be hearing crankcase leakage that made its way to airbox.
 

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Very good points there WFO. Where did you buy it and how much? And yes of course you need to hold the crank from turning. I forgot to mention that, but you did so that's good.
 

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Thanks Alex,
It was a while ago that I bought my tester but I had a quick look and I think I found it. It cost $63 CDN and free shipping.
I was very impressed with how easy it made it to diagnose engine leaks.

42590
 

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Yes looks like a good product. And so far we forgot to mention the HEAD GASKET!! Head bolts and gasket too.
 

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Discussion Starter #11
Alex's suggestions are all good, but I would also suggest you check your compression gage on any engine that you know is good. If the values are really that low, I have my doubts that it would start, but I have had the pleasure of being wrong before. 😁

I would also say 5 revolutions is not enough and you did not mention holding the throttle wide open while doing the check.
No I didnt open the throttle.what would that do to my compression test.cheers.
 

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Discussion Starter #12
thanks all.
just been watching a video on compression testing and as wfo as said to open throttle fully.
i never did this and i didnt remove other spark plugs also.(thick).
ill see tomorow what the result is when i do it propperly.sorry folks my f*** up.
 

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No worries quango. It's an easy mistake to make since the gage is so simple. But people forget if the engine cannot get air, you won't get compression. So it is essential to open the throttle wide open during the test and I forgot to mention that you should remove all plugs so the engine can spin faster. But I see you have already discovered that on your own!

Please post back here with your new compression test results. Hopefully you will see a large improvement.
 

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Discussion Starter #14
No worries quango. It's an easy mistake to make since the gage is so simple. But people forget if the engine cannot get air, you won't get compression. So it is essential to open the throttle wide open during the test and I forgot to mention that you should remove all plugs so the engine can spin faster. But I see you have already discovered that on your own!

Please post back here with your new compression test results. Hopefully you will see a large improvement.
Couldnt wait did it at 7am as described.got it to 60 psi added oil got it to 80 psi still far to low. Gauge maybe crap but near enough same readings across the board .bit the bullet ordered new piston ring set new gasket set .will order shims once ive ground my valves.will let you know when its done thanks again
 

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Ok, but while you wait for your parts to arrive I would suggest you follow what Alex had suggested earlier and that is to check your valve clearances. I recently had a low cylinder (110 psi) and when I checked that valve I found it had no clearance. That means the valve was not totally closing and thus leaking. When I fitted the correct shim and got the correct valve clearance I gained 60 psi on my next compression check which put that cylinder up to 170.

I would also check your valve timing. If it has jumped by one tooth that also could be holding all of your valves open.
If you do all of the above you just might save yourself the rather large task of removing cylinder head and cylinders and installing new rings.
 

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Discussion Starter #16
Ok, but while you wait for your parts to arrive I would suggest you follow what Alex had suggested earlier and that is to check your valve clearances. I recently had a low cylinder (110 psi) and when I checked that valve I found it had no clearance. That means the valve was not totally closing and thus leaking. When I fitted the correct shim and got the correct valve clearance I gained 60 psi on my next compression check which put that cylinder up to 170.

I would also check your valve timing. If it has jumped by one tooth that also could be holding all of your valves open.
If you do all of the above you just might save yourself the rather large task of removing cylinder head and cylinders and installing new rings.
Appreciatd what youve said but ive paid out now so its getting done id hate to spend 200 pounds on parts and not use them lol.im in it for the long haul now,so may aswell sort it all out as I said before compression increased on all cylinders with oil added.so rings are an issue anyway.while the heads off ill grind me valves then do a clearence test on valves.new shims new oil seals .job done .lol.
 
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