GS750ES smoking above 7k or so (yes I searched)

[Guest]

Okay, I've searched and read a ton of threads over the last couple of days and while I'm sure this exact topic has been covered 14 times in the last five years I can't find a thread that points me in the right directly... probably.

Here is the symptom: Hot or cool, when I rev past about 7,000 RPM she starts smoking like a two dollar pistol out the left side exhaust. She also smokes slightly at startup and burns some oil but I can't say how much, it varies from time to time but always at least 1qt in 1,000mi and usually more.

I've done a compression check, all pretty even and high enough at 135,130,130,135 (+/- 2psi or so) and a leakdown test which showed about 6.5% leakdown on 1,3,4 and HUGE loss on #2. I found a tight exhaust valve on #2 and adjusted that one which brought the leakage into line with the rest and then I adjust all the valves as needed (a little tweaking, nothing else tight or loose really) and repeated the leakdown with the good results across the board. I also rode the snot out of the bike, way up into the red, after letting a seafoam/MMO cocktail sit in the bores for 48 hours just to see what it would do. Surprisingly I didn't get a whole lot of extra smoke on startup after that. Odd..

I know this engine sat for at LEAST eight years before I got it so it could well have issues from that. It's always run well since I built up the bike but also always smoked a little and burned some oil. I can't find my log from the first year but I don't think it was as thirsty then and mileage has gone down, that could be just methanol in all the gas these days though.

So what should I look at next? I've got time right now to work on her but I've got to be careful with funds for the same reason I've got time. I have a full gasket set and a spare GS700 engine destined for another project but this bike is more important than a conceptual bike! I'm thinking maybe just doing valve stem seals might help the general smoking and oil consumption but what about the smoking from one side at higher revs? It really puts out a pretty good cloud on the centerstand and I'm told I'm not fun to follow.. If needed I'll hone out the GS700 cylinders and do an upper end job on that head so I can just swap them in but until I've got a paycheck again I'd rather not drop the money on rings and ring compressors. (which I've vowed to buy before I re-ring another one of these engines!)

Right now I'm going to warm her up and synch the carbs since I had them out to replace the gasket on the cam chain tensioner and it feels a lot more buzzy that it did a couple days ago.

Ideas? Thread links?

/\/\ac

[TheCafeKid]

No expert here but it sounds like blowby on the rings. Loaded pretty good at 7K. But you didn't mention: did you drain that nasty cocktail out of your bike yet? With comp numbers and leakdown numbers nearly the same after the cocktail and now no smoke (any MORE smoke) me wonders if you had a sticky ring somehow? Vale seals would be more inclined to be the culprit under no load (cold at idle et ) and insuppose the case could be here too. But it's a slippery slope. If ya have the head off, doing the seals, check it ALL. It's worth it and you're gonna probably have to pull the jugs anyway when you pull the head. Would suck to put it a back together after replacing the seals only to find it still smoking. Believe me I've made that mistake before....

[Macmatic]

No expert here but it sounds like blowby on the rings. Loaded pretty good at 7K. But you didn't mention: did you drain that nasty cocktail out of your bike yet? With comp numbers and leakdown numbers nearly the same after the cocktail and now no smoke (any MORE smoke) me wonders if you had a sticky ring somehow? Vale seals would be more inclined to be the culprit under no load (cold at idle et ) and insuppose the case could be here too. But it's a slippery slope. If ya have the head off, doing the seals, check it ALL. It's worth it and you're gonna probably have to pull the jugs anyway when you pull the head. Would suck to put it a back together after replacing the seals only to find it still smoking. Believe me I've made that mistake before....

No expert?! But I thought you were Mr. Motor God!

I haven't drained the oil, yet. I only put in about an Oz per cylinder so I'm not too worried about diluting the oil down but I'll be changing it out with the filter before I do anything more than local test hops.

I guess it could well be blow by, maybe I have a damaged bore on #1 or #2. I was/am hoping for something simpler but it is what it is I guess. I should have walked the piston down when I had the leakdown tester on I guess.

If I pull the head I'm probably going to go the full shot and put the whole 700 top end onto my 750 lower end but I hadn't planned to pull the head to do the seals. I haven't done them on a bike before but looking at it yesterday I couldn't see why I couldn't just to 'the rope trick" to keep the valves in place while I pulled off the springs and replaced the seals one cylinder at a time.

And thanks for your input, I was hoping to hear from you. You don't always have the answer but your posts are always good ideas.

/\/\ac

[Nessism]

Valve stem seals along with the rings.

How come you didn't adjust the valves earlier?

[TheCafeKid]

Mac the 750 is different fro the 700. Shorter stroke so I've been told so I am not sure a simple swap is going to Be the case. Again, no real world experience here but if the stroke is in fact shorter it seems to me that the conrods are shorter too no? Conversely, the 750 would be longer and while the bores may be the same size wouldn't putting the 700 bores and cylinder head on a 750 without changing the conrods cause interference issues? I think you're better off just breaking down and rebuilding your 750 motor. But I may be completely off base here.

[chef1366]

If it is oil blowby you will have oil in your airbox.

[Macmatic]

Valve stem seals along with the rings.

How come you didn't adjust the valves earlier?

Yeah, its looking like rings but I'm not 100% sold yet. If I do them of course I'll be doing the stem seals, lapping the valves* and checking for wear. I last checked the clearances roughly 1,500 - 2,000 miles ago before I left GA with the bike and either I screwed up and set that one tight/on the hairy edge or its closing up for some reason. It rode fine all the way up here and until I parked it in the Fall but this year I've only been riding my friend's GS1100 I worked on over the winter until now. It STILL feels fine actually.

*Need to double check here and in the factory manual but I seem to remember reading here a few years ago about not lapping valves on the 16v motors? Stellite faces on the seats or something?

In relation to setting the valve clearance I've got a question on the marks too. On the end of the crank you have the rotor type thing (washer with sticking out bit) that has T | F on it. From the manual it looks like you line up the | with the 1&4 trigger but where? Bottom edge? Midddle? Either way my cams look to be out of spec, when I line up the | with the middle of the 1&4 trigger the intake cam looks to be about 3/4 of a tooth off (retarded) and the exhaust a little less. This is based on the position of the notch and line at the end of the cams in relation to the gasket surface and trying to measure how much rotation was needed to get the notch and line to match the gasket surface.

I should see if I can trade some time at the sand blaster for getting my sprockets slotted.

/\/\ac

[Macmatic]

Mac the 750 is different fro the 700. Shorter stroke so I've been told so I am not sure a simple swap is going to Be the case. Again, no real world experience here but if the stroke is in fact shorter it seems to me that the conrods are shorter too no? Conversely, the 750 would be longer and while the bores may be the same size wouldn't putting the 700 bores and cylinder head on a 750 without changing the conrods cause interference issues? I think you're better off just breaking down and rebuilding your 750 motor. But I may be completely off base here.

I think a fairly common mod for the GS750 (1983 type) is to put in pistons and cams from a 700. As I understand it this boosts the compression a little and the cams are slightly hotter so AFAIK putting a whole 700 top end on should be the same as swapping in the pistons and cams. Leaving the 750 rods. Don't worry, I'll be reading the threads on that before I do any real parts swapping!

Chef, I'll check the airbox right now. I meant to clean the filter today anyway. What about the breather on top of the valve cover? I had that off the other day and it was dry inside and I was expecting to find residue at least. That's why I pulled it off, I wanted to make sure it wasn't plugged up and causing a pressure buildup but instead all I found was something that looks more like black paint over spray.

/\/\ac

[Billy Ricks]

The 700 has a shorter throw crank and longer rods. With the shorter throw the rods have to be longer so the pistons end up at the same spot in the bores as the 750 for compression's sake. The 750 has a longer throw with a shorter rod.

[Macmatic]

The 700 has a shorter throw crank and longer rods. With the shorter throw the rods have to be longer so the pistons end up at the same spot in the bores as the 750 for compression's sake. The 750 has a longer throw with a shorter rod.

My brain knocked off for the day about an hour ago so just to clarify does what you say above mean that the mod is 700 pistons into a 750 with 750 crank and rods or am I still missing something. I know that's been written up over and over on the board though so I'll dig it up.

Update. Carbs all synched up and I'll go out for a spin as soon as the rain cuts out to see how it feels. Yesterday was a little buzzy after I'd had the carbs off an on again. (No bleeding this time!)

Airbox is dry other than a little oil from the K&N directly under it where it rests on the bottom.

How about that breather cover on top of the valve cover, is it normal to have that dry inside? Seems odd to me. Maybe this is all just stem seals and one or more of them really lets it flow by at higher rpms? The plugs all look good too, with blowby I'd think I'd see more carbon buildup on one or more.

/\/\ac

[TheCafeKid]

The 700 has a shorter throw crank and longer rods. With the shorter throw the rods have to be longer so the pistons end up at the same spot in the bores as the 750 for compression's sake. The 750 has a longer throw with a shorter rod.

Ok, well that makes sense then. I wasnt sure how they were set up, just that i was told they shortened up the stroke, and set it up to spin up to get the punch out of it that the 750 had or better.. Wasnt sure if it was crank or rods.. Thanks for the enlightenment!

[Macmatic]

So about two weeks ago now I replaced the valve stem seals with the head on. Roughly 500mi later now and oil consumption is down significantly and it seems to smoke less at higher rpm. Of course I can only see it smoke when I've got it up on the stand so maybe load has something to do with it, I'd think rpm was rpm though.

Anyway I just wanted to post back and say that if you've got a bike with good compression, good leakdown numbers and a dry air filter box and breather cover but the bike is still burning oil at speed try the valve stem seals. Doing them with the head on took about 3 hours the first time, not including valve adjustment or an extra good cleaning of the valve cover gasket channel.

Not sure if I mentioned it earlier in the thread but the bike never smoked much on startup and that little smoke is always more black than blue so I assume it is rich from the choke. I'd discounted stem seals in the past because I've had people watch while I was riding and a long deceleration with engine braking followed by sharp acceleration didn't produce a puff of smoke or heavier smoke. I thought stem seals would suck in more oil under decel, at least that was what I was taught at my pappy's knee.

/\/\ac