77 Gs 750 carb woes

[lwhite]

Ok, so here's the scenario, several years ago I did total nut & bolt renovations on X7 250, GS 750, GS1000E, GS1000G, & a GSX1100ES, I still have them , but don't use them as I use a new GSX1400. Every year they all get pulled from under the covers at the back of the garage, fired up & ran around the block a few times. All went well exept for the GS750, would't have it, opened up the tank, it stank, old fuel, replaced with new, no go. it had an old MK11 Bransden-Boyer electronic ignition system, removed it & replaced with a brand-new MK111 digital, still no go!!
bought myself an ultrasonic cleaning tank, the carbs were fully stripped, all seals were good, all the brass jets were perfect, each carb & associated bits were emersed in the tank 1 at a time for half a dozen 'cycles'. Although the carbs seemed clean to begin with, a lot of crud ended up on the floor of the tank, one strand of copper electrical wire easily passed through the brass jets, & WD40 came from all orifices when sprayed into the carb body, So you see, I'm of the opinion that everything is ok.
Now here's the rub, Brand-NEW battery, brand-NEW ignition (inc coils)
NEW plugs, NEW leads. Pull the choke, engine immidiately fires & runs fast, too fast, about 3-4000 rpm, push choke to anything less than halfway, it stops, with choke out, I cannot get it to run below 3-4000, choke in, wont run at all, even when its really hot. My new genuine pipes are beginng to blue, running weak? then why the high speed? Ok, so i'm running without the airbox, but all my other GS's will run ok with them off!I've had the carbs off many times & double checked everything, I think!
Everything on the bike is standard (exept the ign) & new, rebored, new genuine pistons, valves, guides, etc.
I've experimented with the vertical fuel screws underneith, air screws, float level, & it has 105 main jets. All my ajustments did make a difference, but nothing I do will make it run with the choke in!!
The throttle slides just clear the inlet bore when fully open, & there is about 1mm gap when in the closed position.
They do not look in bad condition, in fact, they look very good!
I'm convinced its a carb setup problem, but I've now run out of ideas.

[earlfor]

Sounds like the carbs are badly out of synch. That will cause all of those problems.

Earl

[lwhite]

Thanks for that, but I don't think they can be far out, if at all, I have the gauges,but cant get it to run slow enough to set 'em up!! Also, my other bikes had them miles out, but they ran ok enough for me to set them up.

[KEITH KRAUSE]

As Earl said, a poor bench synch will cause a similar problem. Be sure you've set them as closely as you can. If you've set them the same way you always have in the past and it's been good enough for starting purposes, then it's probably not the synch. This assumes you do know how to bench synch VM carbs well.
Anyway, you have a fuel flow problem or an intake leak. I suspect the intake leak.
If it was a fuel flow problem, you SHOULD have found it by now with all the cleaning you've done. The parts that could cause your problem, if blocked, would be the pilot jet and its passage and the pilot fuel screw underneath and its passage. It would be easy to check all of these for being clean.
The reason I believe it's an intake leak is the choke operation and how it effects the motor.
You say it needs choke, hot or cold, to run. You would not need choke for a hot motor. The bike should idle well off the pilot circuit, which you've cleaned well. If it's not drawing fuel correctly, an intake leak (lack of vauum) would explain this lack of fuel.
The bike is running off the choke because the choke draws fuel from the bottom of the bowl. The choke SHOULD operate so you can gradually raise the rpm's at will. But with an intake leak, the choke can operate "all or nothing".
I suggest checking the manifold clamps for tight and the carbs "popped in" correctly. If tight, I'd replace the manifold o-rings and be sure the manifolds are good too. Make sure the copper washers are on the small bolts used for the vacuum tool and be sure the bolts are tight.

[duaneage]

Spray some of your WD-40 around the intake boots and see if the engine speed changes. Leaks will throw everything off.

How is the petcock diaprham? Is it hooked up?

When you were fixing the ignition did you have to touch the advance weights where the sensors are? Check timing with the engine off to make sure the timing is not advanced all the time.

[lwhite]

Petcock & diaphram fine & working ok.
Everything to do with the 'old' ignition removed, weights, springs etc. the new digital system takes care of advance/retard electronically, the 2 reluctor coils are mounted on a base-plate the same size as the original timing plate & has a fair + & - ajustment, as I said earlier, I don't think it's far out & the engine starts as soon as the button is pressed. Rotating this timing-plate DOES indeed alter the engines running, but in all cases, the moment the throttle is opened, there is an enormouse induction 'shlssss' & she dies.
You know how the fours 'die' if the throttle is opened when the choke is fully 'on', well thats whats happening, only I cant keep mine running without choke, i'm convinced its fuel, the induction rubbers are not new, but I think ok, & I applied a hylomar sealant between them & the head as the 'o' rings looked a bit flat. BTW, are they 'o' rings? 'cos when I tried to remove them, I found they would'nt come out, & I got the impression they where a 'moulded' section of the unit, & only LOOKED like an 'o' ring.

[earlfor]

Oh!, they can be that far out easily. :-) Been there. My 79 750E had exactly those symptoms if the vacuum levels were too high. They may all be the same, but they are too high. (yeah, I know there is the "it doesnt matter what level you set the vacuum, it only that they all be the same?, debate)
Not true with VM carbs.

Ear

Thanks for that, but I don't think they can be far out, if at all, I have the gauges,but cant get it to run slow enough to set 'em up!! Also, my other bikes had them miles out, but they ran ok enough for me to set them up.

[lwhite]

Thanks for swift reply Earl & others, Ok, so i'll go along with what you're saying, yours was the same symptoms, so what do I do now? What would you do?
How do I slow the rpm's to do ajustments, the throttle-stop screw on the carb-bank is fully off with a gap, should I lower the 4 slides (there's about a 1mm gap at the slide bottom at present) or, how else do I 'lower' the vaccum?
I feel really embarassed about this, I've rebuild dozens of all types of engines & carbs over the last 40 years, but this ones got me by the short n curlies!!

[earlfor]

MY solution was to connect the four tubes to the carb stix, then loosen the four jamb nuts above the slides so each one is adjustable quickly. Use choke as a throttle control to keep the rpm down as much as possible. Start the bike up, let it run for about two seconds and note the vacuum levels highest to lowest on the four carbs. Shut bike off. For first adjustment to get the rpm down, dont bother setting the jamb nuts, just leave the adjustment screws loose. Have screwdriver ready and start bike, quickly turn adjuster screws on all four carbs to decrease vacuum level. Once the rpm is down, you wont need the choke. Once you have the vacuum levels down, then you can balance. It should take less than 30 seconds to get all four carbs down to a normal vacuum level.
Ballpark is about 10 in hg @ 1000 rpm. Anything in the 10-15 hg between 1000 and 1400 rpm should put you within range of doing a normal balance. Depending on where the adjustment screws are now set, the same number of turns on the screw on different carbs may not have the same effect on vacuum level for each carb. Usually, if you find a carb that is particularly sensitive to adjustment, it will be a controlling carb in that its setting is "pulling" the other carbs to the higher rpm.

Raising the slide and increasing throughflow area lowers the vacuum level.
Lowering the slides increases vacuum level.

For a starting point mechanical synch after reassembling a carb rack, I set the slides to 3/32" or about a 2.5mm gap when in the fully closed position.

Earl

Thanks for swift reply Earl & others, Ok, so i'll go along with what you're saying, yours was the same symptoms, so what do I do now? What would you do?
How do I slow the rpm's to do ajustments, the throttle-stop screw on the carb-bank is fully off with a gap, should I lower the 4 slides (there's about a 1mm gap at the slide bottom at present) or, how else do I 'lower' the vaccum?
I feel really embarassed about this, I've rebuild dozens of all types of engines & carbs over the last 40 years, but this ones got me by the short n curlies!!

[KEITH KRAUSE]

Earl, it could be a poor carb synch, you never know, but he said he takes the bike out after storage (like the others) and it suddenly had this problem. The bike ran fine before. I doubt the bike would lose its synch or change enough from sitting to cause his problem.
First thought to me would be fuel starvation but he claims he's cleaned the carbs.
So I suggested the intake leak. Now he says he's got some sealant around the manifolds. We both know sealant is a poor fix. An intake leak made perfect sense considering how it runs with choke only, hot or cold. He's also unfamiliar with the o-rings in the manifolds. I bet from just sitting, the sealant job has failed.
lwhite, if you re-synch the carbs, that's fine, but you need to replace the manifold o-rings for sure. You also must be sure the manifolds are in good condition. No cracks allowed. Replace the stock Phillips screws with Allens so you can torque them to about 6 ft/lb. Also, apply a coat of hi-temp grease to the new o-rings. Buy quality o-rings from Suzuki or similar. No cheap ones.

[lwhite]

Thanks again guys, i've had an intensive week-end on this, work will slow due to work!
The inlet rubbers DO have polished stainless capheads, as ive replaced all the fasteners on the bike with them (dos'nt make it run though)!
I think new inlet rubbers will be ordered today, I'll try that & also take on board all the manual sync info as well, although the air-gap on mine is already down to 1mm, i'll take 'em down lower & see what happens.
Is there a world record for the distance a set of Micuni's can be thrown??

[lwhite]

Thanks again guys, i've had an intensive week-end on this, work will slow due to work!
The inlet rubbers DO have polished stainless capheads, as ive replaced all the fasteners on the bike with them (dos'nt make it run though)!
I think new inlet rubbers will be ordered today, I'll try that & also take on board all the manual sync info as well, although the air-gap on mine is already down to 1mm, i'll take 'em down lower & see what happens.
Is there a world record for the distance a set of Micuni's can be thrown??

[earlfor]

I think Keith is right about the induction boot "O" rings. If you have not replaced them, and they are years old, its a sure thing they are brittle.

You said. "although the air-gap on mine is already down to 1mm, i'll take 'em down lower & see what happens." You dont want to decrease the gap. To lower the vacuum levels, you want to increase the gap. :-) Try
2.5mm.

Replace the "O" rings before you do anything more though. If they are broken and leaking, Aint nuttin you can do that will make it run right. :-)
As Keith said, forget using sealant as an alternative. Been there, tried that. Its a waste of time.

Earl

Thanks again guys, i've had an intensive week-end on this, work will slow due to work!
The inlet rubbers DO have polished stainless capheads, as ive replaced all the fasteners on the bike with them (dos'nt make it run though)!
I think new inlet rubbers will be ordered today, I'll try that & also take on board all the manual sync info as well, although the air-gap on mine is already down to 1mm, i'll take 'em down lower & see what happens.
Is there a world record for the distance a set of Micuni's can be thrown??

[lwhite]

I have just again removed the inlet rubbers, as I said earlier, I did'nt think that mine HAD 'O'rings, well, I just dug & prodded until they were well damaged, NO 'O' ring! I then whackked 'em through the bandsaw & sacrificed one (just ordered 4 new ones) sanded them clean on the linisher, again NO 'O' ring, there is a sort of raised annular ring which resembles an 'O' ring, but it aint, its part of the moulding! I took a digi-pic to show, but I can't download it. I do a lot of graphics & cad for my work, but I have no idea how to put pics on this site, the other bike forums i'm on, we just 'attatch' 'em!

[KEITH KRAUSE]

lwhite, I'm still not convinced you're familiar with the manifold o-rings and their location. Replacing the manifolds for new is fine but don't forget the o-rings. The o-ring rests in a groove cut into the flat metal surface of the manifold. It seals between the manifold and head. A PO may have mickey-moused things. Be sure the groove is cleaned out. Don't gouge the groove. Use a toothpick or fine steel wool to clean up. Buy Suzuki quality o-rings.
If your bench synch is all screwed up, the bike won't start either. You can follow Earls advice and see if it starts (after all intake leaks are repaired). If you still have a problem, I post detailed info at another topic that explains how to bench synch the VM carbs. You may have better luck just starting the bench synch over from the beginning.
I don't know how to send a link yet but if you want to see the bench synch info: go to "statistics", go to "most active topics", click "78 GS 1K carb re-jet, bad fuel economy, KK", go to page 3, then 6 replies down.