85 750 compression - Suzuki GSX-R Motorcycle Forums Gixxer.com
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post #1 of 11 (permalink) Old 07-23-2018, 03:41 AM Thread Starter
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85 750 compression

Sorry but I have been trying to post for some weeks but now I am in the story so far.
On a weekly run the bike decided not to go anymore. After stopping for a while on the side of the road away it went again.....long story but several times it happened again but managed to get home. The bike went as per normal after restart only die like it had run out of fuel. Took the tank off and drained to fuel via the tap and feed hose. Emptied the tank and inspected the tap and found no contamination at all.Pulled the carburettors down , again clean, reset float levels reinstalled but it still didnít run as before. New plugs but decided to do a compression test and found they were all down to 50 psi ....all cylinders. So now the motor is on the bench, cams look good, valves look good and well seated. Havenít got the block off the pistons yet but from what I can see the bore is not scored.
Any body care to venture a guess? Seeing as the compression is supposed to be 150psi something must be wrong!
Col
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post #2 of 11 (permalink) Old 07-23-2018, 06:59 AM
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Re: 85 750 compression

50 psi isn't good....
I take it the valve clearances have been done ? and that you opened the throttle wide open when you cranked the engine over for the comp. test ?
It maybe that your carb settings are too rich, this can reduce ring sealing ability - if it's 'way' too rich - also an overly rich mixture will make your engine feel worse as it gets hot - once it cools down, it'll be fine again.
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post #3 of 11 (permalink) Old 07-23-2018, 04:14 PM Thread Starter
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Re: 85 750 compression

Did compression test with the carbs off then put them on just to make sure. Was back firing during rapid deceleration for the last couple of rides.
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post #4 of 11 (permalink) Old 07-23-2018, 04:20 PM Thread Starter
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Re: 85 750 compression

Compression tests were done with carbs off and back on,full throttle. Was back firing on rapid deceleration on the last couple of rides which I put down to exhaust leak. I was pulling like a train until last week!
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post #5 of 11 (permalink) Old 07-23-2018, 08:11 PM Thread Starter
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Re: 85 750 compression

Looks like this is going to be a long tread!! Has anyone ever removed the cylinder block successfully? Mine , as everything else, is stuck fast. At the moment it is hanging on ropes through the bridges next to 5,7 and 6,8. Not the total weight but a fair bit. Light hammering with rubber mallet has not even opened a crack along the joint. Anybody able to help.
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post #6 of 11 (permalink) Old 07-23-2018, 11:05 PM
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Re: 85 750 compression

it should come off with some light convincing, seeing as it was a running motor. remember that the block is held down at the front by a 10mm nut
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post #7 of 11 (permalink) Old 07-25-2018, 12:08 AM Thread Starter
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85 750 compression

Ok barrel is off. Canít see any damage to pistons or rings. Bore is a bit shiny so a light hone is called for? A few measurements up next, probably new rings and see what else is called for. Still donít understand the low compression, might find out more when the valves come out.
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post #8 of 11 (permalink) Old 07-25-2018, 12:28 PM
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Re: 85 750 compression

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Originally Posted by Testie View Post
Ok barrel is off. Canít see any damage to pistons or rings. Bore is a bit shiny so a light hone is called for? A few measurements up next, probably new rings and see what else is called for. Still donít understand the low compression, might find out more when the valves come out.
bad head gasket could be a possibility or some crud built up on the valve seats, have a machine shop check piston to cylinder wall clearances after a light hone and using the same pistons. Check your piston ring gaps inside the cylinders. also have them check the cylinder head and block mating surfaces for warping, I've had heads / blocks warped nearly 0.040"
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post #9 of 11 (permalink) Old 07-25-2018, 04:49 PM Thread Starter
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Re: 85 750 compression

[QUOTE=Gixxerider86;9514172]bad head gasket could be a possibility or some crud built up on the valve seats, have a machine shop check piston to cylinder wall clearances after a light hone and using the same pistons. Check your piston ring gaps inside the cylinders. also have them check the cylinder head and block mating surfaces for warping, I've had heads / blocks warped nearly 0.040"[/QUOTE
Thanks , looks like today is measuring and hone. Then the valves. Sleepless nights....thinking how do you get the piston rings back into the barrel? Might have bitten off more than I can chew! Anyway one step at a time.
Col
Ne
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post #10 of 11 (permalink) Old 07-25-2018, 05:06 PM
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Re: 85 750 compression

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Originally Posted by Testie View Post
Thanks , looks like today is measuring and hone. Then the valves. Sleepless nights....thinking how do you get the piston rings back into the barrel? Might have bitten off more than I can chew! Anyway one step at a time.
Col
Ne
put rings in correct positions on pistons, onto conrods, then the assemblies into the block / sleeves. then put the upper case into the block, then lastly the crank. If you only took the top end off, then use smooth clamps with the appropriate diameter and use lots of assembly lube when you use them to compress the rings and slide the pistons into the block. Ive done both methods, use some safety wire to wrap around the cam chain so you can pull it up through the block
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post #11 of 11 (permalink) Old 07-28-2018, 05:43 AM Thread Starter
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Re: 85 750 compression

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Originally Posted by Gixxerider86 View Post
put rings in correct positions on pistons, onto conrods, then the assemblies into the block / sleeves. then put the upper case into the block, then lastly the crank. If you only took the top end off, then use smooth clamps with the appropriate diameter and use lots of assembly lube when you use them to compress the rings and slide the pistons into the block. Ive done both methods, use some safety wire to wrap around the cam chain so you can pull it up through the block
Thanks for the tip on slipping the pistons into the barrel, never thought to use clamps. Valve grind tomorrow. Everything measured within tolerances so just valve grind and new rings to be sure. New gaskets all round and hopefully it will be a goer again.
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