dgoodsy's '72 gt750 revival/log thread

[tz375]

One of the crankcase bolts is open at the top end of teh thread and needs a special sealing washer. It might be #13, but I don't remember for sure

[dgoodsy]

That could be the problem. If i am looking at things correctly the bolt i thought was leaking corresponds with a bolt requiring a gasket according to the partzilla diagram (link below, see bottom view, RH side, item #5 and 16). I will have to look closer and see if there is a missing washer/gasket or what the deal is.

would it make sense for all cylinders to drain from that bolt? I think the left and center cylinders drained out there, not sure about the right hand cylinder.


http://www.partzilla.com/parts/search/S ... parts.html" onclick="window.open(this.href);return false;

[Suzukidave]

I believe the bolt with the copper sealing washer opens into the transmission , a leak from the crank area could get out from the seal on the alternator side .

[dgoodsy]

Hmm. I suppose that bolt is pretty far to the rear of the engine case to be a crankcase drain. Maybe that isn't it.

I had 5 minutes tonight so I went out to the garage and tried the impact on that bolt back and forth and I couldn't get it to budge. 3/8 electric impact. I will have to get after it with a larger ratchet or bar of I'm really curious.

I will have to get the bike out where I can have a better look at where it is leaking from and try again. Currently it is right up against a workbench so I couldn't really get a good look under there.

[Suzukidave]

Here is a site with lots of great info https://www.oldjapanesebikes.com//index.html" onclick="window.open(this.href);return false;

[dgoodsy]

Yes, I have spent a few hours reading on that site! It is actually where i found a picture of the bottom of a gt750 engine to see exactly which bolt i suspected to be leaking, based on the picture i posted (obvoiously not thoroughly familiar with the engine on this bike). Then once i knew what bolt it was i looked at the partzilla diagram and confirmed that a washer/gasket is required at that bolt.

Thank you very much though, Ian's collection of knowledge and pictures is great, something every buffalo owner (especially new guys like me) should be aware of.

[tz375]

Dave is probably right. If it's diesel that's leaking out of the crankcases, it must be passing a seal and either filling the transmission and coming out of a trans seal or it's leaking out behind the alternator.

But you said it's leaking out of the left side which is more of a problem. It would most likely be coming from the outer crank seal into the left side cover, and into the transmission. It could be leaking from any of the trans seals but could also be from around the gear switch.

Remove the alternator cover first to see if it's dry in there and then remove the gear shift lever and the two left rear covers around the sprocket. Pour some more diesel in and see where it leaks out of.

[dgoodsy]

I *think* it is leaking out of the left hand side. The bike is pushed up with its left side against a workbench and a car parked up to the right side of the bike so i couldn't really get in a position to get a good look underneath to see exactly where the diesel was coming out of. What i should say is I am fairly confident that the diesel was dripping off of that transmission bolt (whatever that is for anyways, it's not the drain is it?). When i put the diesel in initially it was more than a drip of diesel coming out though. It was a steady stream for a bit until most of the diesel was drained out. So i am inclined to believe it is more than a slight leak in a seal. It was more than i would expect to leak though the threads of a bolt missing a sealing washer (as in the case of the trans bolt mentioned above) so I feel it is less likely coming from that bolt now. But enough with the thinking, guessing and surmising, I aim to put a few hours together this weekend to make some space to pull the bike out in the open and try again with the alternator cover and left side covers removed as you say tz375. thanks for the advice. Hopefully the diesels path to the atmosphere will be evident.

Also i should note that i tried to pour a bit of diesel into the right hand cylinder as well, but that piston was covering the intake port. As you might assume the diesel quickly filled the intake port and backflowed out of the carb boot intrance on to the engine. Shortly after this spill there was a drip from the bottom right side of the engine as well, but after spilling diesel on the engine itself I can't really assume whether the drip from under the engine was the diesel leaking out of the right hand crank case or just running down from above. For all i know it could have been running along the bottom of the engine from the left hand side leak(s) as well.

[tz375]

It's incredibly hard to find leaks at times. Fluid finds its way along frame rails and other parts and then drips out somewhere different just to keep you guessing. When you pull it and pop the covers off, it should be much easier to find the left leak.

The right and center cylinders can leak into the trans if their inner seals are shot. For the left one, the path is potentially more convoluted. I suppose it is possible for the diesel to leak out of the left case into the center and out of that into the trans. If so the crank needs to be split for new seals.

Fortunately those motors are not very complicated. Big and heavy, for sure but no cams and chains and valves etc.

[dgoodsy]

Yes, i have chased leaks on other vehicles that have been tough to track and ended up originating somewhere not obvious at first glance. They can be tough, but like i said i will be in a much better position to find it once I can access both sides of the bike and crawl around to look underneath, and taking the covers off will help lots too.

I fear I will need a crank rebuild, fingers crossed though. Hopefully this leak is just a bone head error on my part or something simple to fix.

[tz375]

I had a GT550 a couple of years back that supposedly ran perfectly but needed the carbs cleaned - One on the three great lies.

Compression test was OK on center and right and ZERO on the left. I popped the pipes and carbs off to do a leakdown test but when I took the plugs out, I could see a hole in the piston. So off came the head and just for sh!ts and giggles I filled all three pots one at a time with kerosene. It came straight out of the left side and all over the driveway. Off came the alternator cover and I could see it pouring out behind the rotor, so off that came and I could then see what looked like a seal torn in two. The center of the seal had stuck to the crank and torn free from the outer part. It was all rotted.

I have had two GT750 motors with "rebuilt" cranks that also leaked badly with a leakdown test because they had been lying around getting rusty and the person who did the cranks was not a competent engineer.

So I have come to expect to rebuild a crank with any GT I get and I always send them to Bill Bune in MN.

It would be nice if yours turns out to be fine.

[dgoodsy]

That is exactly what I am afraid has happened to the seal(s) on the crank on this bike, that they have torn exactly as you have described. Before i purchased it a friend told me about the bike and since I was a few hours away he went to look at it for me. In his inspection he kicked the motor over to ensure it was not seized. I can't blame him, I would have done the same thing. In fact it was one of the first thing I did when i saw the bike for myself. Since the engine had likely not run for the last decade I have to assume the seals didn't want to let go of the crankshaft.

Anyways i should just get out to the garage and do some dismantling and testing rather than guessing and assuming the worst.

Any links on how to leakdown test a 2 stroke motor? I have a leakdown tester and have done leakdown tests on 4 stroke motors...

[tz375]

4 stroke leakdown involves 100 psi air and a couple of pressure gauges.

2 stroke is the opposite end of the spectrum. Rule of thumb is 6psi for 6 minutes. The methodology is simple: Block off intake port, exhaust port and spark plug and fir one of those three with a connection to a simple bulb and low pressure gauge. Pump up the air pressure to 6 psi and watch the gauge. Ideally it won't move at all in a reasonable time.

Don't go above 10 psi for risk of blowing a crank seal.

It takes a while to build pressure so be patient.

http://www.diymotofix.com/blog/how-to-d ... -down-test" onclick="window.open(this.href);return false;

https://www.youtube.com/watch?v=zdq7NRcUIp8" onclick="window.open(this.href);return false;

https://www.duncanracing.com/TechCenter ... sstest.pdf" onclick="window.open(this.href);return false;

[dgoodsy]

Thanks for the description and links, that's great information. I will need to pick up a few things to be able to leak test this motor. It's not nearly as simple as a 4 stroke leakdown test!

[tz375]

Absolutely. On a 4 stroke we need high pressure to test the valve seats and rings. On a two stroke, we are really testing the crank seals.