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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.

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

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

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.

Is that pitting on top of cylinders or is that head gasket remnants? 
Coolant level remained constant? 
Motor wouldn't have been sucking in coolant maybe?  Specifically on the suspect cylinder.

No that's just head gasket remnants.I had the block and head surfaced so they're still in good shape. I thought about burning coolant but I think I would have smelled it and seen the steam out the exhaust. I took a good look at the head gasket and it looks like it was making good contact and sealing around all the cylinders. At this point the oil is my prime suspect but I would have thought the piston would be scored pretty badly in that case. It's not. In fact it looks like someone sandblasted the skirts with some fine abrasive

Well I pulled the head this morning and didn't really find what I hoped for. The crosshatching on the left cylinder was all but gone so I decided to remove the cylinders and investigate. I didn't find a broken ring, but the left cylinder is severely worn. See the attached photo of the crosshatch on the left vs center cylinder. That black spot on the cylinder wall is just a shadow from my camera. What's weird to me is that I don't see any obvious scoring or anything, it just seems like that cylinder has done 40k miles while the other 2 look new still.

After I got it apart I started looking around and noticed that the crankcase for the left cylinder seemed to have less oil than the other 2. I had spun the engine several times in my troubleshooting without disconnecting the oil pump, so the crank webs of the other 2 cylinders had a generous coating of oil on them but the left didn't have much at all. I quickly checked for blockages in the oil passages and lines but it seemed clear. I ran out of time to work on it but the oil delivery seems like it could be a cause for this mess. Come to think of it I always thought that mine didn't smoke as much as some others I had seen. I always chocked it up to the CCI system being economical in oil burn 

I also quickly checked the ring end gap and noted that the left cylinder was massive...something like .060" or more while the other two were right around .016". Still on the high side but much better. Along with that the left piston feels much looser in it's bore than the other two. I don't have a dial-bore gauge, but I do have a set of micrometers and a telescoping gauge so maybe i'll get around to measuring it a bit later. Right now it's clear that i'll at least need a new piston and rings. If i'm lucky the bore won't have worn that much and I can get away with a single new piston and set of rings. If I look into the oil system and can't find a definitive fault I may just pull the bottom end apart and send the crank out so I know it's good

Yes the left cylinder showed 88 PSI and the others came back with ~110 +/- 2 PSI. That is a cold reading but i'd expect the same with it hot. Also my tester seems to read consistently low on small engines so that 110 reading is probably more like 130-150 PSI.

I went through the carbs when I did the top end. I ensured all the passageways were clean and installed new bowl gaskets, checked the diaphragms for leaks, set the float height (not fuel height) and made sure the floats were still airtight. Then I bench synced the butterflies using a thin feeler gauge to get them roughly equal. I installed them on the bike and then used the vacuum gauges to sync them further which is where my original post starts.

Maybe the carbs still aren't right but between the low compression, low intake vacuum, and piston rattle coming from that cylinder it seems the engine has a mechanical problem. Those of you asking about the carburetors, would you not agree?

Right. Good info teazer. This is my first 750 and I've heard that a lot of them don't surge once they are synced and running properly. Obviously mine is not. So I'll probably save that for later. I did use an OEM Suzuki gasket so I'll probably need a new one.

I've blocked off the SRIS valves on this one. My plan is eventually to build it into a 70's racer. But for now it's just a stock rider. I have had the baffles out of the exhausts. They're clear and the packing is removed but i'm going to switch to chambers soon anyway. I think where I'm a little unclear is how the ring sealing effects the crankcase pressures. If the rings aren't sealing well, will that cause the poor signal i'm feeling at the inlet? That would make me feel better about ruling out the crank seals.

With the pipe off I can see some marks on the cylinder wall above the transfer port. But it's hard to say if they're scores or just oil streaks. Can't get in there to wipe it off through the exhaust. I'll get to pull the head off tomorrow to know for sure. Just getting some ideas for now

Hi all,

It's been a long while since i've visited the site but this time i'm here hoping for some expertise. I've just recently rebuilt the top end on a new-to-me GT750M and it's got a few problems after only 100-200 miles.

The first thing I noticed is that it had a surge on deceleration, which is typical for GT750's with the CV carbs. In an effort to get rid of this I went to sync the carbs and noticed that if I synced the carbs with vacuum gauges the left cylinder would quit firing. To get the #1 cylinder to contribute equally it required the sync screw to be almost all the way in (more throttle opening). While working on the engine I noticed a bit of piston rattle from the left side as well. Now that I'm thinking about it I don't know if the rattle was present when I first put it together, but I don't think it was. I parked it for a couple weeks and just today dug it back out and started looking at the problem. Suspecting that maybe I didn't do a good enough job checking the crank seals I started with the quick test by pulling the carbs off and cranking the engine with my hand over each inlet to feel how much each of them "sucked" my hand in. the #1 cylinder again seemed to be weaker. I removed the pipe and sealed the outlet before applying ~5 PSI of air to the intake side. I don't have the correct setup for a proper leakdown test but I did my best and wasn't able to find any air leakage past the seals. I even pulled the left cover and sprayed soapy water on the seal, the cylinder base gasket, and the case seam but never found any leaks. Since I found no leaks I figured i'd do a compression test and found that the left cylinder was 20 PSI lower than the right. So that's where I stopped for the night.

It's clear to me the engine is going to have to come back apart.My guess is one of three different things:
1. The ports did not have enough chamfer and broke one of the rings.
2. Some sort of blockage in the oil passages to that cylinder.
3. A leaking crank seal that I couldn't find.

So that's where i'm at now. Is there anything else I should be looking at before tearing it down? Or something I should specifically be looking for while doing so? My main concern is that I never found a crank seal leaking, so if I take it apart and don't find a piston/ring problem, then what? I'd hate to put it back together and have the same issue. I'd also like to avoid doing the crank seals if it doesn't need it. The outer seals that I am able to see look brand new. Whats your take on it?

Another quick but unrelated question to those with GT750's:

Does anyone know if the inner fork tubes are interchangeable between the 1974 'L' model and the 1975 'M' model? I know they changed from the threaded chrome caps to the spring seat/circlip design, but are the lower portions of the tubes the same? Mine are not it great shape and I see that the L version is reproduced and sold by a few different vendors but not the later style. I would actually prefer the threaded cap style if they are compatible with the later fork lowers.

I would really like to buy one of the Accent electronic ignitions but I have been told that they are produced in batches and are currently out of stock. They expect to have them available again in 6 weeks. So until then the points will have to do. If I have any more problems I'll likely just wire in an automotive condenser since they are readily available.

Now I'll have some time to start planning my porting changes for then next rebuild  ;D I put it back to stock specs while I get all the frame and suspension dialed in but I know I will be looking for more power in the not so distant future

Well it certainly seems like the condensers where at fault. I pulled the new ones and found that they where all essentially acting like a direct short to ground. I put in the old ones, re-cleaned the points and reset the timing. So far its been running great. I still notice some light arcing at the points, maybe I need to get another new set of condensers, or maybe that's just the freshly cleaned points "wearing in". I guess time will tell

I always had it in the back of my mind that the condensers could be the only thing causing the arcing, but for some reason couldn't believe that all three of the new (not NOS) ones I received didn't work.

After posting I had some more time to work on the bike and tried a few more things. First I'll note that during the rebuild I installed a new battery. It sits at 12.58v after leaving overnight. In addition the charging system is working correctly, it puts out ~13.5 volts at 2000 rpm. I just made up a short little harness that connected the 3 coils directly to the positive post of the battery with an external switch. That let me run the bike with no other electrical load but I noticed no real change in how it runs.

I did notice that the ignition points seem to be arcing more than what is normal. I've had many bikes with points in the past and noticed that they do arc occasionally and very lightly. But these are bright blue arcs across the points. Could it be because they are new? It has all 3 new condensers so I'm unsure of what else would cause this.

Hello all, I've been a long time reader of the forum but just made an account to seek some help with a strange problem that has recently come up on my GT750. I know this forum is primarily Yamaha's but it seems the registration is no longer functional with the Suzuki 2 stroke forum, I have met Chuck (supertune) in the past and am local to his area. I figure my question is general enough that I can at least get some ideas of what to look for from the wealth of knowledge here, so here goes:

I bought my 1975 GT750 as a project about 6 months ago now. It was very tired when I bought it but still seemed to run fairly well all things considered. I took some time to check out the bottom end i.e. checked crank seals, gearbox leaks, oil injection, etc and it all checked out okay. So I decided on just a top end rebuild. I went 1.5mm overbore and had both surfaces of the block and head decked. I just got it back up and running after the top end this past week and at first everything went well. The same day I got it started I heat cycled and re-torqued, did about 50 miles of break in riding with the oil injection set to a higher setting, and kept away from the last 2000 rpm of the powerband. The next day I take it out and decide to take it to redline once to see how it feels and it starts to misfire and pop out the exhaust in the upper RPM's. From there it only got worse. I tried to take it out for a ride with my friends RD350 and after about 10 miles the right cylinder stopped firing and eventually ended up backfiring through the carburetor, blowing my uni filter apart.

Since then I've installed new points and aftermarket coils and have had trouble getting it to run right. I can test each point/coil system independently and they all produce a nice blue spark. When I put it together and go for a test ride it runs great...for about 500 yards, then starts to misfire. Even at idle it will misfire and run poorly. If I clean the points and put in a fresh set of plugs it will run great again for another short period, then back to the same situation. Here's the strange part: it's always a different cylinder. Sometimes the left will misfire, i'll clean the plugs, and then it will move over to the right. Sometimes the misfire will follow specific spark plug, other times it wont. The only thing I can think of is that the wiring to the coils may carry enough current to support one coil when I'm testing, but not enough to run all 3 coils at the same time causing the misfire. I'm at a loss now and figured I would elicit some suggestions before I go tearing the harness apart.

Sorry for the long post, but any suggestions are appreciated