GT750 Failed Rebuild?

[The1970s]

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?

[teazer]

Drain the coolant and pull the head.  If you used a good Cometic head gasket, it will have stuck like shit to a blanket, but cheap ones sometimes come off without too much mess.

You can see the transfer and exhaust ports to check for chamfer and any broken ring issues will probably be visible as scores.

You could set the #1 piston say 20mm down from TDC and pour in some premix and see how fast it drains down.  Remove the SRIS valve on that cylinder to drain out the excess.

The barrels will come off while the motor is still in the frame but you may have to remove that stud sticking out of the barrels using a couple of nuts.

Make sure you catch any broken bits of ring as the barrels rise. If rings drop into the cases, you will be stripping the motor down again and might as well send the crank to Bill Bune for new seals.

The carbs need air correcting jets http://pinkpossum.com/GT750/carb/BS40carbs.htm for details. and check the FUEL level while they are off to be sure they are all the same at 2-3mm below the gasket surface.  Ignore float height.  Fuel height is what matters.
When you strip and clean the carbs, be sure to spray WD40 (safer that carb cleaner) through one circuit on one carb and repeat on the other two to compare "flow" rates.  Do that to all the drillings including the many on the float bowls and don't forget the choke jets.

Repeat one circuit at a time.  DOn't do all circuits in one carb and then the next carb.   

BTW, did you check that the baffles on that left pipe are clean?  A blocked exhaust will screw you up every time.

Good luck

[The1970s]

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

[Old Brit]

Before you start tearing into it, did you set the carb's up on the bench, reason for asking is you mention trying to "balance them" with a vac gauge. After a thorough ultrasonic clean and ensuring all of the airways are clear, needle jets checked, shaft seals replaced, floats aren't punctured, float heights are correct etc. etc.
Float height should be 23-24mm from the gasket surface with the carb fully inverted and with the float weight fully loaded onto the needle valve.
So after all the usual stuff with carb refurbs, you then need to set the butterfly's by eye using a magnifying glass (or use an air flow meter whilst on the bike but with the airbox off), once they're correct you don't touch them again. Once the bike is running hang the vac gauges (assuming it's a multi set) upside down as 2T vac is weak, and then use the small pilot screws on the side of each carb to "balance" them.
Key differences between L and MAB models are:
L models : Pilot screw 1/4 turn out (starting point), needle circlip #3, throttle butterfly #120 (12 degree closing angle)
MAB models, the needle circlip should be #4, pilot screw 3/4 turn out (starting point), throttle butterfly #110 (11 degree closing angle)

Surging is the bane of any Kettle owner life but it can be virtually eliminated if everything is set up right and (usually) the air correction jet mod is done. This is an extract of a video made by a Guy in the UK who knows his 5hit when it comes to Kettles - https://www.youtube.com/watch?v=Cukul7TqLsE

Hope that helps and gives you a starting point before ripping in to the motor.

[rodneya]

I would get the correct parts to do a proper leakdown before taking the motor apart.

[m in sc]

id do as mentioned with the carbs it almost sounds like a fuel level is off. Id def go thru the carbs again. secondly, id run a compression check before tearing into it.  IF the compression on 1 is within 10% of 2&3 its prob not a ring issue. Just my opinion

[Evans Ward]

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.

[The1970s]

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?

[m in sc]

did you actually do a  compression check? I see what your saying yes, id suspect a stuck ring or collapsed ring land, but id also thow a compression gauge on 1st.

[The1970s]

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.

[m in sc]

yup. ok. bet it's a ring then. bummer

[The1970s]

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

[m in sc]

ohhh shit. sorry thats bad.

[jetta90]

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.

I don't know much about Suzuki 2 strokes so just thinking out loud here....

[The1970s]

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