GT750-B crankshaft

[jonathanblair]

Is it time for a crankshaft rebuild?

I have a 1977 GT750-B with 24,000 miles on it. The bike is extremely clean - definitely a keeper. I am the third owner - bought the bike in 1991 with about 13k on the clock. In 1994 I had the cases apart for some repairs and I replaced quite a bit while I had the engine apart:

New gaskets all around, new seals (except inner crank seals) new pistons and rings (1st over), new wrist bearings and pins, new water pump, new oil lines, new clutch bearings, new hoses, rebuilt carbs. Everything else inside checked out ok. I put the bike back together and it ran great for the next 10 years.

Over the past 5-6 years, the bike has been running steadily worse. Gas mileage is down, and power is way down. The engine starts up reliably, but it will not run without breaking up under load or acceleration. Sounds like I am missing a cylilder much of the time. I have gone over the inigtion system several times - checking points, plugs, coils, wires, etc. As near as I can tell, the ignition is spot on. I have gone over carburetors several times. I rebuilt them again, and checked every orifice. As near as I can tell, carbs are working fine. As I was synching the carbs with a manometer, the engine got hot and started to diesel. [That scared the #$*& out of me. Ever have that happen on your bike? I wasn't sure whether the bike would blow up first, or my house would catch on fire first, or I would die first. Not fun...] I have checked over the SRIS lines. Currently, they are pulled off the engine and I have plugs on all the inlets and outlets. Carb boots are in great shape with no cracks or leaks. Stock exhaust is clean and tight.

I spoke with Paul Miller on the phone a couple weeks ago. Paul thinks it's time to rebuild the crank. I tend to think he is correct, but before I yank the engine apart again, I thought I would get some feedback from a few other veterans. Do any of you have any thoughts to offer? If I do rebuild the crank, what else should I replace while I'm in there? Can anyone recommend a good rebuilding service? I live near Philadelphia.

Thanks,

Jonathan

[oldjapanesebikes]

I know there are other better qualified folks on this board who will jump in, but as pulling the crank out for a rebuild is a major job (which I have done a few times now ) I'd be inclined to check a few other things first:

- is your exhaust clear ? You can lose a heck of a lot of performance with a plugged up exhaust so that's always in the top 5 of my check list for the symptoms you describe
- have you done a compression test ? Maybe its just sticky rings.
- have you considered changing the high tension caps and leads ? HT leads and caps do perish/fail over time
- are the inlet boots on the carbs in good shape (soft and pliable) and not leaking ?
- have you changed the fuel lines and vacuum line recently ? Is the petcock working correctly ? Have you changed the in-line filters (if you have these installed), or pulled the petcock and checked/cleaned the two filters inside the tank ?
- what is the voltage across your battery when the engine is running at about 3000 rpm ?

Normally I would have said that for a well cared for Buffalo with the mileage you list, the seals generally would be fine - if the crank indeed has failed, then there should be other signs of trouble (oil pumping out of the breather, very heavy smoking etc.) which you haven't mentioned. To me - what you describe doesn't sound like crank - it sounds more like fuel or electrical.

As I mentioned at the start - I'm sure others will jump in ...

[markj]

What about fuel smell in the tranny oil? Would be a sign of bad seals?

[jonathanblair]

Thanks for the replies so far!

I will sniff the tranny oil for fuel smell. Hadn't thought of that one. Although, if memory serves, there is only one bearing on the right of the crankshaft that is lubed by tranny oil. Aren't the left and center bearings lubed by CCI oil only? If the seal is bad between left and center, it wouldn't allow exhaust gases into the tranny oil - only the gasses from left and center pots could mix and this wouldn't touch any tranny oil yet. Still worth a sniff though...

I'm not sure I know how to check for a clear exhaust as you say. When I began having troubles, I pulled off the pipes and pulled the baffles. I actually removed all of the packing on the baffles, and I cleaned as much carbon as I could from them. At this point, they were reinstalled, but without new packing. Will that lean out my mixture significantly? I also shook the pipes and turned them upside down to see if I could dislodge any carbon or rust. The pipes look very solid and clean. Is there something else I should do to them? I do get a good clean run at idle. Even after the engine is warm, it idles cleanly. It is only under load that I get problems - backfiring, missing, loss of power, etc.

When troubles started I did a compression check - 140 per pot, +/- 2psi. I ran the check both cold and hot and it came out similarly both times. Compressions seem good. Rings seem good.

I haven't done anything with the coils or the tension leads other than to make sure I had a good spark at each cylinder. I have an NOS set of coils still new in the box. It would be pretty easy to put new caps on them and swap them in to see if that makes a difference. I'll give that a go.

I put on a brand new from the factory tank when I did the rebuild in 1994. It was completely new fresh metal inside and out. I have kept the bike indoors and pampered since then, so I would be surprised if I were getting rust, but it's possible. I also rebuilt the petcock and put in new seals and new filters at that time. It's definitely time to check them again, though. I put new fuel line and vacuum line on at the same time. I cleaned and rebuilt the carbs 4-5 years ago and they were still quite clean inside at that time. I so no evidence of rust or gunk so I didn't think that fuel delivery was the problem.

Inlet boots are still soft and pliable. I tried spraying a bit of WD-40 around the edges while the bike idled as per my mechanic's suggestion. The rpm's didn't change so I surmised that none of the spray was getting into the induction tract.

Charging system is excellent. I measure 14.5 or better at the battery at 3000 rpm.

Engine smoking seems quite normal for a 2-stroke. There is a typical mild blue fog at warm-up. Once the enricheners come off and the bike has gone a mile or two, you can barely tell that it's a stroker from the exhaust. You usually can only see the smoke at night in the headlights of another vehicle, or if you really hang wide open on the throttle.

One other anectdote might be worthwhile:

When I pulled the bike out for the first time in the spring of '09, I took it out for a run to see if it was still having troubles. Up to this point, the engine troubles would show up in the first mile or two of highway riding. I was having a good run out in the country for at least 25-30 miles and it was running like new again. I was doing 65-70 mph out on the interstate and the bike was doing great. As I was riding I was thinking that it must have been a piece of crud lodged in a carb somewhere and it had now worked itself out, but after a few more miles, the engine trouble came back as before and it has stayed with me since. That made me think that the trouble is being caused by something that is responding to heat and expansion, like a seal or a coil...

Thanks again!

JBL

[tz375]

After all these years, i think that the crank should be pulled and sent up to Bill Bune in MN. He's probably the best in terms of price & quality.

When the top end is off, check the ring end gaps and see if they are still in spec. Make sure they move easily in and out of the ring grooves too. And clean the carbs while they are off the bike.

[rbond]

My .02 worth, change the coils first. If still no better, do a leakdown test on the cylinders. Despite the mileage, it maybe time has caught up with the crankseals if it fails the leakdown....

[Coyote]

Could be as simple as excess carbon build upon piston crowns and head. What kind of injection oil have you been running? 140 psi compression seems high. That would also point at the same thing.

[jonathanblair]

Thanks for your reply,

I've been using Suzuki CCI oil for the past 5-6 years. I did use a case of Honda 2-stroke oil for a few years before that. And ever before that it was CCI.

As to compression numbers, I may be off a notch or two. I'm writing this at work and my Suzuki notes are all back at the house. Now that you mention it, I think you are correct - the compression numbers were more like 120-125 per cylinder. I also have a Honda CB1000 and the 145-ish numbers were probably what it read last time I checked it... What I remember for sure though, is that I measured the WB's compression right after I rebuilt the top end, and when I started having engine troubles I measured it again and it was the same. (I documented all of this in my WB journal so I know it was the same, I just couldn't remember what number it was...) Point being, I figured that I must not have too much carbon build up if there was no change in compression over an 8-10-year span of operation. Is this a reasonable conclusion?

Thanks

[tz375]

I just re-read the whole thread and that 09 anecdote was interesting. I'd pull the carbs and fuel tap and clean them both thoroughly. It's quite possible that you have crud in there. And modern gas is prone to deterioration so it's time to empty the tank into the lawn mower/snow blower/truck and fill it with fresh gas.

It's also possible that someone nested in the air box.

With the carbs off it would be good to do a leakdown/vacuum test on the bottom end to see how well the crank seals are holding up.

[jonathanblair]

Thanks,

I will definitely go over tank, petcock, fuel lines and carbs. I'm also wondering if I might have a sticky float valve in one of the carbs...

Yes, I should check the airbox for mice. I've had them in my garage before and I know how they love airboxes for nesting spots. I put out D-con for them ever year. One year I went to do the spring start up on my Honda. When it wouldn't fire up, I pulled the cover off the air cleaner and a pint of D-con pellets spilled down onto my lap. Clever mouse stored it rather than ate it.

I have never done a leak down test. Can anyone recommend a source for leak down test instructions? I have a Clymer manual, a Haynes Manual, and the factory manual. Is it described in any of them?

Thanks,

JBL

[pezcat19]

Hi all,

I have a 77 GT750B (known as a "Water Bottle" here in Australia) that is showing very similar symptoms to jonathanblair's bike. It is driving me mad!

It starts easily and runs like a rocket-ship for a while, but then will rapidly loose power and go bad.... This problem has been around for a while I think, but it only ever used to do it occasionally, very randomly. Now, in the last couple of weeks, it is doing it all the time, every time! For example, I can ride about 3 - 4KM to get to the highway with no problems, then when I hit the Highway and sit on 100Km/h it will begin playing up very quickly (within a minute or two). It feels like I am dropping cylinders, and it struggles to maintain 100kmh. If I slow down and pull over, it used to come good until I took off again, but now it is worse, and often wont come out of this "mode" untill it has cooled down / been rested for a while...

To give you all some history on what has been done so far:

- Motor is fresh - had a rebuild around 6 years ago and has done very little work since (It has been mostly sitting in the shed until a couple of months ago.)
- Has had carbs put back to original jetting, new needle/seats etc.
- New plugs
- All 3 coils test OK with a multimeter
- Fuel flow / petcock appears to be working correctly - setting to "prime" does not change the results
- New Battery
- New Points
- Cleaned all electrical connectors

Does anyone have any ideas? I am running out if things to try....

jonathanblair - Did you have any luck with yours?

Thanks in advance guys!


Here is a pic:




Cheers,

Pez

[jabcb]

Others on this forum know a lot more about this than I do. But based on my experience I'd look into the coils.

My GT750 ran pretty good when I got it. I upgraded the electricals shortly after getting it. Replaced the coils, voltage regulator, rectifier & headlight with current aftermarket parts.

New voltage regulator & rectifier didn't cause noticeable changes. Headlight upgrade was to convert to halogen H4.
Bike ran much better when I changed the coils. Got a big reduction in vibration.

I got the replacement coils from Rex Caunt Racing. Coils fit on the existing bracket. He sells 4 ohm & 2.5 ohm coils. The GT/T series bikes use the 4 ohm coils. He might have them in stock now -- I checked a few weeks ago & he said they were backordered. Coils came with new wires & I used NGK resistor caps.
http://rexcauntracing.com/

[uzidzit]

I would go for the coil swap very first thing, it is pretty common for these old coils to crack and lose the spark under heat and compression. (they should jump 8-10mm in air), One other thought that is ignored often which causes really odd running (I mean bozo what in the world kind of odd running) is if the thermostat is screwing up. I fought with the symptoms you describe, on a buff i bought (that was to turn out to be a superb runner). and after fiddling I noticed that the temp gauge would swing more than my other buffs. Go cold on the highway, hot in the city, so the mix was all over the place because the engine temp was all over the place. too lean when I was on the pin, and sloppy rich plodding around town.
If your temp gauge is doing anything except sitting at about 1 needle width below 1/2, you have a thermostat problem. The Golf Tango really loves to run best at a very even engine temp. If the thermo is out, the temp swings so wide it will flood and die in town and run out of fuel and die after a long burn, (ask me how I know) I installed a old one out of a donor motor and cut a gasket and all has been well for the last year.

[GreenMachine]

Pez - Your problem sounds like it could be vacuum building up in the tank causing fuel starvation... Try opening the tank next time it stalls and see if it sucks air. Make sure the breather etc is clear..

This happens all the time on older cars.

[tz375]

The other thing to check is fuel. Was the tank left full or almost full for the last couple of years without doing a lot of miles? In that case the gas is stale. Drain it and put it in your car or truck.

Nice bike Pez, That house looks almost like my old place in Geelong. Where are you located?

Seal test is fairly easy on a Golf Tango 750, but not quick. Remove the carbs and exhausts and then remove one plug and replace it with an old plug that you have knocked the center out of and connect that to a Mityvac. Add 6psi positive and see if each cylinder in turn holds a positive 6psi and negative 6psi pressure for 6 minutes. that's the old 6 for 6 approach.

If it won't hold pressure, listen to where it's coming out of. Oops almost forgot. You have to pinch all 3 SRIS tubes or else pressure will leak from one pot to another.