Smoking GT750

[garry55]

mate of mine has bought a GT750 M Kettle and it is smoking excessively on the l/h cylinder (#1, or the one on the left when sat on the bike). Have taken the exhaust off and hung it to drain off any oil and checked the SRIS pipes etc. But it still smokes - a lot.
Any ideas as to what's wrong - piston rings - bore ?

Ta

Garry.

[oldjapanesebikes]

Right hand seal perhaps ? The SRIS for the right cylinder dumps to the left cylinder after 1972 - if the inboard seal for the right cylinder is passing, then the left cylinder will smoke. It could also be the right side SRIS valve is stuck open. What I would do is pull the SRIS valves and check them for proper operation. If they are fine, then I regret to say that eventually you will be looking at a seal replacement on the crank.

[Craig380]

It could simply be running richer on the left cylinder too, which would make it smoke more. If your mate has only had the bike a few weeks, it can also take AGES for accumulated oil, crud etc to burn off out of the pipes. By ages, I mean literally months and months.

Another check for a failing crank seal is this - is there oil around the base of the oil pump, leaking from the little hole in the oil pump cover? There a gearbox breather near there. If the crank seals are failing, it will suck oil in and burn it, and will also pressurise the gearbox a little, blowing gear oil out of the gearbox breather vent.

An even cruder test is to remove the gearbox filler plug and sniff the oil. If it smells strongly of petrol, the seals may be dodgy.

Does the bike start easily both hot and cold, and run well? A compression test will quickly tell you if the rings and bores are OK. Hot engine, wide open throttle, kick 10 times in rapid succession (ignition off, obviously ). If it reads 120psi or higher with no more than 10% difference between any of the cylinders, the bores / pistons / rings are good enough. If it's 130psi or higher, they're very good. Less than 110psi, they're tired.

[Vintageman]

Try to discern if it's gear oil or two stroke oil making the smoke

Does it only smoke at startup?

Does it smoke all the time?

Is any gear oil coming out crankcase vent (by oil injection pump) when you rev it up to red line?


GT750 seals do go bad for sure. They can be real bad and still runs (kind of well surprisingly). If you get air box off put your hand over carb bell mouths and if it sucks less than the others this is another go no go way to know if seal is real bad (that is pieces and chunks missing out of it... must be changed).

Now if it seams to sucks just as good and you discern it is gear oil making the smoke there is one thing you can try to fix the seal. Blue Devil seal re-conditioner or ATP-205. This will work if seal is not missing pieces just dry. But again GT750 tend to waste seals versus just being a little dry. But if a little dry it does take much gear seepage to cause heavy smoke

This is what I would do. I don't like to rebuild if I don't have to. I like to ride them ... only if hey run as they should.

[tz375]

Before you condemn the seals, check the oil supply likes aka the spider or Octopus. The check valves on those tend to allow oil to pass and if so, that could be the source of surplus oil. It could be the left side feed or the right that is then passed to the right via the SRIS system.

A good way to start is remove the three SRIS valves and see how much drains out of each crankcase after leaving the bike to sit for a few days/weeks.

[garry55]

thanks for the replies guys

we will have a look at these later this week and I will report our findings.

[Eddie]

my 74 gt750 has smoked more from the left cylinder ever since I bought it ,,,I adjusted carbs a few times and it did seem to help, also timing helped ,,,my compression is low also around 110,109,100 but it runs good and after its warmed up the majority of smoke goes away from that cylinder,,,but it smoked a lot at first and after putting some miles on it and adjusting carbs,timing it seems a lot better so I'm just saying maybe if the bike sat for a while (mine only had 11,000 miles on it ) like mine did , which is bad for the seals, maybe you just need to look at the small stuff 1st,,,,,i finally gave up on figuring mine out and put miles on it and now it seems better so I have decided to just ride it till it wont go anymore ,,also like was mentioned before the oil injection lines (octopus) could be leaking also ,,,so fix the little stuff 1st , and if it still doesn't stop then try and put some miles on it ,,,,it could be the piston rings from sitting to long ,, then if that doesn't work I guess your looking at crank seals , but that for me would be my last option just my opinion,,,,,also if you have the bike on the side stand all the time the left carb seems to flood on mine and it will smoke heavy until its upright and warmed up ,,,,good luck....also just to note I have had my bike for about 6 or 7 years now and it had all your symptoms when I 1st got it but seems better now,,, so I'm sure my seals may not be great and probably would help to replace and maybe new rings but id rather ride it for now , when the smoke gets worse and I know its running shitty then I guess I'm stuck with a rebuild , I honestly didn't think it would get better but it did so either I'm real lucky or its just a matter of time but try everything else 1st unless you really want to rebuild it then that's different...

[garry55]

Hi Eddie
thanks for the info on your bike - this one was in regular use prior to my friend buying it. To be fair, the vendor didn't shy away from telling him about the l/h cylinder issue and did charge him less for the bike to cover a crank rebuild. Hopefully it just needs the top end sorting, but we will work our way through all the suggestions and see where it leads us.

[Vintageman]

determine if the oil burning oil is gear oil or 2 stroke injection oil. That will let you know if seal or not. I can tell by the smell when it ain't feeling well.

figure out our questions if it smoked only at start up after setting a spell or all the time? any signs of gear oil coming out crank case vent if you rev is high.... Is gear oil level dropping as you put miles on it. ... Does it suck in just as hard into carb...etc.

Not too hard to tell if seal or not.

Looking into intake and exhaust port with carb and pipe off tells a lot about how the top end is (piston, rings and liner) without a tear down

Determine if it is burning gear oil or two stroke oil. Gear oil it's the seal.

If it's gear oil and not blowing oil out crankcase vent high revs try a little Blue Devil or ATP-205. Sometime the seal is dry and it doesn't take too much gear oil to cause smoking

Good luck

[garry55]

cheers for that Vintageman - will add to my "to do" list