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PostPosted: Sun Apr 09, 2017 7:16 pm 
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Joined: Mon Sep 03, 2012 11:39 am
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Country: US
Bikes owned: GT550, SV650S, RM370
I recently picked up a GT750M in reasonable condition, and wanted to get it cleaned up, and rideable. I have brake repair kits, and carb kits. I was thinking about how to check for really bad crank seals, so I set up a tapered rubber stopper with a vacuum gauge. I pushed it tightly into the right manifold boot, (carbs have been removed), and kicked it over by hand. The plugs are still in, and the compression feels good, and equal. I got 7" or so of vacuum on the up stroke. I repeated this procedure on the center, and left cylinder. Nothing, not hardly a blip of the needle. The oil pimp area, (vent) is dry, but then it hasn't been in service since 2003.

My question is has anyone ever checked there crank seals this way? I know for minor leakage, a proper pressure leak test, with sealed exhaust ports is necessary.


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PostPosted: Sun Apr 09, 2017 11:59 pm 
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Joined: Thu Feb 16, 2012 11:50 am
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Location: The Republic of South Yorkshire
Country: England
Bikes owned: 2 GT550s, 1 resto, 1 GT750 and lots of spares
Never checked the seals on my 550 resto some time ago, but took the top off and filled all three crankcases with diesel, then kept turning the engine over steadily over a period of a few days. Drained and repeated. Logic being that the diesel would flush the bearings, soften the seals and also soften any rusting on the seal faces. 3000 miles on the bike still starts easy and runs well, so it seems my logic was sound. This also worked on a second 550 that had been stood 30 years. All bearing etc. were very well oiled before starting needless to say, and a good dose of easystart got things fired up.


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PostPosted: Tue Apr 11, 2017 8:01 am 
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Location: Manchester, UK
Alan H wrote:
Never checked the seals on my 550 resto some time ago, but took the top off and filled all three crankcases with diesel, then kept turning the engine over steadily over a period of a few days. Drained and repeated. Logic being that the diesel would flush the bearings, soften the seals and also soften any rusting on the seal faces. 3000 miles on the bike still starts easy and runs well, so it seems my logic was sound. This also worked on a second 550 that had been stood 30 years. All bearing etc. were very well oiled before starting needless to say, and a good dose of easystart got things fired up.


I'd like to have seen the smokescreen when she fired up and burned all that diesel off :wink: :lol:

A bloke I've bought parts from over the years is a serial 380 restorer, he's done over a dozen bikes. I once asked him about crank seals and he said he'd never had a problem with failed seals on a 380, even when the bikes had stood for over 10 years without being run.

He did do a similar trick to you, with the difference that he used a 50/50 petroil mix (1 pint petrol, 1 pint cheapo 2-stroke oil) which he'd fill the cases with and leave for a day or two before turning the motor gently by hand.

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2006 SV650S. It's got cams, and valves, and stuff


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PostPosted: Wed Apr 12, 2017 4:48 pm 
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Location: New Hampshire
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lazy leak down

With carbs off, a piston at TDC at a time fill lower case of that piston with a known amount of your favorite gas mix, diesel (your choice), First remove the SRIS valves at bottom of engine case to ensure all cases section are clear of fluid. put a short bolts back in place of SRIS valve for test. Looking into intake you will see the fluid level when full for example...

It should hold that level of fluid for quite a while, If your seal is bad it most likely will leak very fast. It may leak to a neighboring cylinder, into gear oil section or out one of the crankshaft ends.


The other test is more subjective. Carbs off, (or at least air box), spark plugs out. Spin engine over fast with E-start and put your hand over intake port inlet to seal. It should have good suction as noted by the feel in your hand. sometimes at least one as good for relative comparison. I do this first to see how bad, then fill test above.

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Current Bikes
74 GT250 (T350 upgrade),
76 GT250 (T350 upgrade),
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72 Yamaha DS7 (R5 upgrade),
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PostPosted: Wed Apr 19, 2017 10:51 pm 
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Joined: Mon Sep 03, 2012 11:39 am
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Country: US
Bikes owned: GT550, SV650S, RM370
Vintageman wrote:
lazy leak down

With carbs off, a piston at TDC at a time fill lower case of that piston with a known amount of your favorite gas mix, diesel (your choice), First remove the SRIS valves at bottom of engine case to ensure all cases section are clear of fluid. put a short bolts back in place of SRIS valve for test. Looking into intake you will see the fluid level when full for example...

It should hold that level of fluid for quite a while, If your seal is bad it most likely will leak very fast. It may leak to a neighboring cylinder, into gear oil section or out one of the crankshaft ends.


How about this one.....I had some diesel, so I first poured into the right cylinder, (at TDC). This was the cylinder that made some vacuum on the kick starter. Did not take more than a couple of minutes, and there was a nice little stream of diesel coming out of the bottom alternator cover at the harness. :(. I am not even going to try and start it now. Crankshaft/seal time !!


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PostPosted: Thu Apr 20, 2017 7:16 am 
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Joined: Wed May 11, 2016 9:44 am
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Location: Saskatchewan
Country: Canada
Bikes owned: '72 gt750
before you go tearing stuff apart try to pinpoint exactly where it is leaking. I did the same test and found the diesel dripping from a bolt under the motor/trans and thought it was leaking from there. after looking closer i found the diesel was coming out of the oil injection ports, running into the starter cavity and through its drain hole and down to the bottom of the engine from there. I obviously had the oil injection pipes off. Just check to be sure it isn't something goofy like that. It's not much fun paying new parts and tearing something apart only to find things don't look too bad, and the problem was somewhere else anyways.

good luck!

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PostPosted: Thu Apr 20, 2017 9:24 am 
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Joined: Mon Sep 03, 2012 11:39 am
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Country: US
Bikes owned: GT550, SV650S, RM370
dgoodsy wrote:
before you go tearing stuff apart try to pinpoint exactly where it is leaking.


I will take a closer look, and also pull the right side cover,(alternator), to varify. The center, and left cylinder did not produce any vacuum at all, when kicking over, making the crank seal between those cylinders suspect.


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PostPosted: Thu Apr 20, 2017 10:38 am 
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Joined: Wed May 11, 2016 9:44 am
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Location: Saskatchewan
Country: Canada
Bikes owned: '72 gt750
Yes, I think you probably are right. I just know an extra half hour of diagnosing now could save a few hours and a few pennies rebuilding later. I try to be as thorough as i can before i take anything apart.

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