I *think* this is a fuelling issue...

[nowhere.elysium]

...but I'm not 100% sure.

I've just recently resurrected a GS650GT after a twenty-two year layup, and after having stripped and rebuilt the carbs several times, I know for a fact that they're out of balance still, and the mixture setting is insanely rich. Since Suzuki doesn't mention anything about it in the workshop manual, I operated under the assumption that 2 turns out would be an appropriate starting point for the mixture screw. I've got all the gear to be able to tune it for proper running (colourtune and carbtune), but I've noticed some slightly weird behaviour when cold.

I'm well aware of the fact that being an aircooled four, it'll be noisy and a bit rattly, but there's this weird intermittent 'clonk' noise that happens if the idle gets too low - it'll dip to about 800rpm, make a gentle 'clonk', stutter for a second, and then get back to idling again. For the record - the idle is still not right at the moment - it's OK when it's gotten good and hot on a long run, but for short stints it's still a bit all over the place.

I'm intending to balance the carbs and sort out the mixture, but I was wondering if this is indicative of anything else?

I've put just under 300 miles on the bike since getting it MOT'd last week, and it was perfectly happy on the motorway, so I know that all the higher-end fuelling is fine (well, it survived getting to a forum BBQ and back without problems, at least). If anyone's encountered something similar, I'd love to hear from you. I am also prepared for the possibility that it's just nerves on my part, due to not having had an aircooled bike for a while.

[Nessism]

Please check the carb rebuild tutorial and Newbie Mistakes thread linked in my signature to see if you missed anything. The noise is most likely nothing to be concerned about but it's hard to say for sure.

Oh, and remember that short cuts are long cuts when it comes to maintenance. For example: clean the carbs one time properly and be done with it instead of cleaning them several times taking short cuts.

Good luck

[tc862011]

mine was doing that after the rebuild and i checked and lightly sanding the points and reset the gaps and timing and it stopped. the points were a little rough i dont know why so that is why i sanded them, i am thinking the light clunking noise is a detonation issue sine the spark isnt being delivered at the right time. after i did it to mine it ran like a champ and hasnt had any weird noises after at any rpm range

[Badooka]

Sounds like a good old fashioned vacuum leak- I just chased one down- mine was in the middle of my boots-they looked fine until you squeezed them- cracked really bad-
spray some wd40 or starting fluid around you intake boots and around the airbox connections- if the rpms rise you have a leak. Sometimes tightening the clamps helps but when they are this old they usually need replacing. ALso some GS models have an intake O-ring behind the boots that needs tended too- I don't know your year model or if you do have one of those. You'll never get them adjusted with a vacuum leak- find it, fix it and then tune.

[Steve]

mine was doing that after the rebuild and i checked and lightly sanding the points and reset the gaps and timing and it stopped.

While that might have worked on your '78 bike that has points, his 650 (regardless of year) does not have points.

Still a good thing to keep in mind for others that read this, but won't help in this case.

.

[tc862011]

While that might have worked on your '78 bike that has points, his 650 (regardless of year) does not have points.

Still a good thing to keep in mind for others that read this, but won't help in this case.

.

ok good to know, i didnt realize that the 650 never came with points, do they have the pulse ignition

[Tom R]

A vacuum leak usually results in a lean mixture, but always be on the lookout for them. I'm not sure if your idle mixture screws control fuel or air; but could turn them in a 1/2 turn if it's fuel, and out a 1/2 turn if it's air. This will let you know if you're going in the right direction.
Also, check your floats. Make sure they're set to the right level and that none of them are sunk (usually only a problem with brass floats).

[Nessism]

Sounds like a good old fashioned vacuum leak- I just chased one down- mine was in the middle of my boots-they looked fine until you squeezed them- cracked really bad-
spray some wd40 or starting fluid around you intake boots and around the airbox connections- if the rpms rise you have a leak. Sometimes tightening the clamps helps but when they are this old they usually need replacing. ALso some GS models have an intake O-ring behind the boots that needs tended too- I don't know your year model or if you do have one of those. You'll never get them adjusted with a vacuum leak- find it, fix it and then tune.

The WD-40 vacuum leak check method is unreliable.

There are a multitude of possible problems with old bikes which is why it's imperative to perform maintenance. Change the intake boot O-rings, properly clean the carbs and replace all the O-rings, vacuum sync the carbs and adjust the mixture screws using the "highest idle" method, etc.

First do maintenance, then troubleshoot. Even if the bike is running half way decently, a lack of maintenance could (will) cause problems very soon. Why take chances?

[tom203]

A "gentle clonk".... odd, doublecheck ignition advance springs to make sure they move freely without hanging in full advance.

[BigD_83]

Normally, a sagging idle is related to an overly rich mixture. What method did you use to determine that you are running rich? Your Colourtune?

The idle mixture adjustments were set slightly lean from the factory and were considered "non-adjustable", at least on the North American versions. The preset was 1.5 turns out, but 2 is not out of the realm of possibilities, based on the highest RPM method of setting the adjusters. If you have an air leak, then you'll find that the idle adjusters will have to be turned out further to compensate.

As a point of reference, I have a 4 into 1 exhaust with stock airbox and filter and via the highest rpm method of setting the idle adjust have settled in around 2 turns, without being overly rich, determined by plug chops.

The "clonk" is an odd sound, and not one that I can personally relate to, but it could be a backfire through a carb- the one and only time I have experienced something like a "clonk" on my 650.


As Nessism suggested, I encourage you to get the valve adjustments and air leaks sorted out before you chase your tail on the idle issues.

[Badooka]

One must have a running bike to do maintenance to- Or else it's furniture repair.