GT750 Failed Rebuild?

[The1970s]

Thought I would also share these to see if anyone else had any opinions on this. Note the dull finish on the left one. The more I think about it...I think there would have been much more damage had the oil supply been weak for the 150 miles or so it has been ridden.

[Czakky]

Not sure if it's been mentioned but, clearance? If it's a relatively fresh rebuild could the clearance have been too loose? Tapered?
I've had a few oil seizures and they didn't look anything like that. Mine might've been more extreme being an almost total failure.

[m in sc]

id def consider sending the cyls out to get them looked at by another shop for a second opinion. i've never seen an oil failure issue cause that, its a very weird failure indeed. 
def test the oiling system. maybe it was a defective piston? the dull coloring, being uniform,  on the sides of the piston, , is baffling. Ive never ever seen that myself.

[rodneya]

Seen something similar when a motor was put together after honing without washing the grit out of the cylinder.

The middle piston does not look so good either

[The1970s]

It could have been a clearance issue. I had them bored at a local machine shop that also welded a pitted area on the cylinder top and then surfaced it flat again. It looked like they did a good job. I measured them with my telescoping gauges and from what I remember they were on the loose side, but still within the factory spec which is .0018" - .0022". But I am by no means a machinist. I just wanted to check their work and make sure it was close. I cleaned and assembled it myself so i'm sure the engine was clean when it went together. You're right that center one has quite a lot of scuffing given the mileage. But those are just scuffs of the finish and not scoring.

I'm local to Chuck (Supertune), I wonder if I could convince him to take a look at a triple for once?  ;D

Something new i've thought of is that maybe there was something in that side of the crankcase that got into the cylinder when I started it up? I cleaned them out the best I could and I filled them with 2 stroke oil and let it drain out the SRIS valve hole to help get any particulates out. But maybe there were still some in there. My other thought is that I did have some ignition issues when I first put it together. I basically bought 3 "new" coils that were complete junk. I ended up back with the originals but during the first couple rides I rode the last few miles back to my garage with it misfiring. I suppose it's possible that it could have washed out that cylinder from not firing and caused the rings to not seal and wear quickly? I think at this point i'm going to pull the cases apart and go through it top to bottom

[SUPERTUNE]

I said Yes, I could help him out...planned for Thursday at the moment.
Chuck

[Old Brit]

Not sure if it's been mentioned but, clearance? If it's a relatively fresh rebuild could the clearance have been too loose? Tapered?
I've had a few oil seizures and they didn't look anything like that. Mine might've been more extreme being an almost total failure.

Totally agree this theory, I have a feeling we'll all know by Friday  :P

[IR8D8R]

I have seen this before but not a 2-stroke. It may apply if it uses an oil pump. Explains the frosted appearance on the piston.

It was excess fuel washing the oil off the cylinder wall. Since there was a sump in my case there was some fuel in the oil. But you wouldn't find that on a 2T. Piston wear was similar. Mine was from a Weber DCOE with a bad circuit on one side in an Alfa Romeo 1300 Sprint Veloce. Piston looked like it had been blasted and the cylinder was worn through the hone marks but shiny. Also brand new engine. The piston was very worn also. Oil control rings kept the oil out of the top of the piston so it wasn't smoking blue yet but it had been leaving soot clouds. Top rings were way worn. Cast iron rings not chrome.

You said the area under the cylinder was cleaner than the other 2? The fuel provides enough lube to prevent a seize but isn't enough to keep the cylinder from wearing. I would expect to see the piston crown completely clean but maybe didn't happen until there was already carbon built up?

IR8D8R

[The1970s]

I took some more free time to take a look at the oil pump. I set up a test rig in a hand drill and captured the oil from each pair of injector lines into separate cups. Once the lines were primed again it pumped the identical amount into each cup. So oil wasn't the issue. The lack of oil I saw on the crank webs must have been due to me blowing air in for the leakdown test.

Here's my final guess before I let Chuck take a look:

I took a close look into the cylinders and noticed that the lower edge of the exhaust port on the L cylinder had a weird looking chamfer to it. Looking closer it looks like that edge may have not had any chamfer at all. Almost like the machine shop guys did all of them and somehow missed that one. I must have also missed it when I did the assembly. To confirm this I took the rings and measured the width of the ring all the way around. I noticed that the width was almost .010" narrower in the area of that exhaust port. So my final guess is that the sharp edge wore down the ring which then sent those metal shavings into the cylinder and caused the dull finish on that piston. We'll see if Chuck agrees

[teazer]

I sell a fuel height kit for BS40/ CV carbs for the GT750 LMAB. $12 shipped to you if interested. Fuel height is that crucial on these bikes as teaser points out- his site is a wealth of knowledge! The anti surge jets are also a good idea and you can get those from blue smoke baz out of the UK. Comes with the correct tap and instructions. My fuel height kit also comes with instructions.

That is the only way to set fuel levels.  Forget about setting float height.  It is not accurate enough.  The jets you need are just plain old Mikuni air jets as described in that link.  http://pinkpossum.com/GT750/carb/BS40carbs.htm

It is always a good idea to do a leakdown and compression test before stripping a motor. Poor rings will cause lower compression readings but don't change leakdown.

[The1970s]

For anyone wondering I went to see Chuck yesterday and he seems to agree with Rodneya that the cylinder looks like it has had some sand or something run through it. The ports are not chamfered as much as Chuck usually does, so that probably didn't help anything. The bore and piston on that cylinder is totally destroyed...we measured around .006" clearance. The clearance of the other two are pretty close...so the clearance was probably set up correct by the machine shop. We did note that the finish hone was done with a very aggressive grit though. I'm sure the cylinders were clean when it went together but I never split the cases so there's a chance there was some grit still down there that got into the cylinder on startup. That's my best guess for now.

The last bore was already 1.5mm OS so i've ordered a set of 2.0 os pistons which is the biggest i've seen for a GT750. The plan is to bore that left cylinder and hope it cleans up at 2.0 os. Otherwise I may have to find another set of cylinders. I have also split the case and am going to do the crank and water pump while its open.

[Czakky]

That's brutal. Glad your getting it sorted.

Thanks for updating us

[1976RD400C]

I scrub the cylinders real good with hot soapy water and a nylon brush to make sure they are as clean as they can be.

[Old Brit]

You must be soooo pi55ed but at least you have a solution now 👍

[Evans Ward]

Well that sure sucks, but good you are keeping good spirits and a positive attitude to get her back going.