Seized my GT750A/ tip for owners

[Evans Ward]

Well…. took my GT750A to the big Deal’s Gap 2 Stroke Meet last month and it had been running fine. On the first day there, a group of about 6 of us were having a spirited ride back down Hwy 129 back from the Joyce Kilmer Forest where we had visited the renovated overlook there. As soon as I passed the Tapaco Lodge on 129, the bike started slowing/ bogging and I suspected a seize so I pulled in the clutch and spotted a clear pull off to the left just before the bridge at Fugitive Dam. With clutch pulled in, the engine cut off on its on and I pulled off to the left. As I stopped with clutch pulled in, the bike started up on its on running again on all 3 cylinders. Back on 129, rounding the sharp right turn past the bridge, I spot a trooper off the road to the right running radar so the last 2 miles back to Deals Gap Motorcycle Lodge were easy going. After getting back, I checked my oil injection tank which looked almost full even after the 50 mile ride! I had installed an in-line shut off valve from the oil tank to pump which was in the ON position but pulled the line after it to verify it was working, which it was. I decided to pull the oil tank out and check it and found sludge material in the bottom near the exit nipple. I’ve always used Honda GN2 injection oil in this bike (since 2016 purchase) as well as my 72 Kawasaki H2 750 (last 15 years). When I purchased this A model in 2016, the previous owner gave me all the receipts and paperwork which showed quality work including a Bune built crankshaft, cylinder reboring and fresh new pistons. I did not see or ask him what type of injection oil he was using and felt no need to empty and clean the injection tank. At Deals Gap, the sludge that came out of the tank was both large and small pieces one as large as a cock roach! I flushed and shook the tank (off the bike) with half filled water X 3 and then did the same with mineral spirits the last time allowing the inside to dry before reinstalling and refilling it with GN2. After that, rode it sparingly enjoying my RZ350 for the bulk of the rides through the week. On the last day of the meet, decided to ride the GT750 along with a friend down Hwy 28 to have lunch at Fontana Village. At about 6 miles into the ride, I was leading with my friend behind and caught up with some modern bikes. I was in the process of passing a modern bike on a straight section running about 75 mph when without warning, the engine fully locked up/ seized and I immediately pulled the clutch lever which did nothing to stop a violently left to right long rear tire locked skid. My friend Tom said he doesn’t know how I didn’t go down and said it was epic how I saved it. Tom rode back to the resort to get the truck/ trailer and had two friends volunteer to come back with him to rescue me and push/ load the GT on the trailer. After returning home, I received a call from Allen Wood from FL who had heard of a GT750 seizure there and was able to get my # from a mutual friend. He suspects the sludge had already traveled from the oil pump to the lines into the cylinders causing the restriction and seizure. He also made a gracious generous offer of helping me rebuild the engine at his place stating he has a low mile A cylinder bank! I did pull the plug on the left cylinder positions the piston all the way down with gentle movements of the kick starter and then took at look inside with my boroscope. My My! The cylinder bore is pretty well trashed. Also, there has never been a time where my oil tank cap was off and not screwed down! Part of my annual maintainence on my H2 has always been to drain the oil injection tank and clean the nipple screen that screws into the exit line. This is an area where I will be doing the same for my other bikes as well. IMHO, Suzuki missed the mark by not designing a similar nipple/ screen like the Kawasaki H2 has. And… the nipple I.D. on the GT is so small! Soon, the bike will be trailed from GA to FL (about a 5 hr one way drive) to Allen’s place for needed repair. He just rebuilt his A model’s engine about 3 months ago so his skill set here is well above mine. I’ve rebuilt my H2 and RZ350 both a couple of times but have never split the cases on a GT750. Hope to learn a lot from Allen who will be compensated well too. Have decided that from this point on, I will be using nothing but Yamalube 2S injection oil in all my bikes as it has color whereas the Honda GN2 is colorless. Will use the GN2 as premix in my 2 cycle walk behind lawnmowers. It will be back running again soon but I wanted to share with all especially about flushing and cleaning the GT750 oil injection tank. Any ideas how the sludge formed?

[tz375]

Probably the prior owner decided to add some Castrol R or similar bean oil for the smell.

It starts to coagulate fairly quickly and from there it's all downhill. I always empty the oil tank on any two stroke and flush it out. Now I think about it, i didn't do that to my RZ. I might have a look at that before I ride it again....

[Alan H]

Bad luck with the motor and oil, but at least you survived the seize with nothing worst that slight 'clench marks' on the bike seat.
I had a GT750 nip up at an indicated 90mph about 4 years ago while (almost) completely run in (700 miles from a full rebuild) but felt it start to nip so whipped the clutch in and saved the motor. Found the cause of that was actually barrels that had been decked (bottom) so the compression was 160psi instead of 125psi and had run slightly hotter. Eventually changed the barrels and it's run ok since - 8000 miles on now.