GS650 jetting and correcting vibration

[adamdaz]

Hi all -

I thought I had my GS650GL running well, but couldn't understand why bolts were loosening up with some items getting lost altogether. Took a while for me to realize (after reading about vibration on here as GS resources) that it was vibrating too much. I bought this bike in August and got it running fairly well. I went through several things, but didn't go through carbs because PO said that was done. He also said they rebuilt carbs from kits, but didn't change the main jet. I suspect now that he meant main needle.

So, I took the carbs apart to see what was really done. To my surprise (not really) - the carbs had a few problems. One air mixture screw was broken (tip jammed into the carb body and broken off) and another was missing an o-ring entirely. Other than that, the carbs looked decent (clean and intact). What was not good was there were 2 items that were not configured as stock. The main jets were 112.5 instead of 110 and the pilot air jets were 170 instead of 160. These two jets seem counter-productive to me. The main jet is slightly larger, potentially reducing the stock lean condition of these motorcycles - to me not necessarily a bad thing. But the larger pilot air jets seem to work against that.

The PO bought this as a stock GL with the intention of making a cafe racer. I saved it from that fate because he couldn't get it to run. I was told that the carbs were rebuilt from kits, and am sure they no longer have original jets anywhere.

My question to those with experience, is there a reason why this motorcycle would have been jetted this way? Is this a typical step up for this carb? In searching bikecliff's website and carb specs, this setup for BS32SS carbs matches those for a gs750ez/tz. Could it have been simply a mistake or (as I tell my kids) an on purpose?

Should I strive to get replacement Mikuni jets for everything or just those out of spec?

Thanks for any help you can provide.

[phydeauxmutt]

The 112.5 mains are probably not necessary, but won't really hurt anything. These bikes were set up to run lean from the factory, and the quality of the gas has not improved over the years. The addition of ethanol would certainly warrant the up-sized jets, if nothing else.

The 170 air jets are for the PILOT circuit, not the main. They would just provide a little more air in the PILOT mixture, which could be compensated by turning the pilot screw out another 1/4 turn or so.

Good luck getting the jammed tip out. The only direction it can go is UP, and it's pretty darned hard to poke it out through that tiny hole. If you can feel the tip by dragging your finger past it, you might be able to roll over it with a screwdriver shank to dislodge it.

The vibration would have been caused by the imbalanced power output of the engine becaue of one cylinder not firing due to a blocked pilot port. It probably ran much better and smoother above half-throttle, but you can't keep it there very long.

[Nessism]

Did you vacuum sync the carbs and adjust the valves?

[adamdaz]

Did you vacuum sync the carbs and adjust the valves?

Yes, the valves are all in spec and I synced the carbs using a carb carbtune.

The vibration got worse at higher rpms, not smoother. Could that have been from bad intake seals?

[Iceman2]

My1981 gs650gl had carbs done by PO, however I found that the throttle shaft seals were leaking. PO justified that by saying the carb kits didn't include throttle shaft seals. To little is included about replacing these seals.

[adamdaz]

My1981 gs650gl had carbs done by PO, however I found that the throttle shaft seals were leaking. PO justified that by saying the carb kits didn't include throttle shaft seals. To little is included about replacing these seals.

Thanks. Those are not included in the typical o-rings sold by cycleorings.com. I was fortunate that I ordered a set for my bike before he closed down earlier this year.

[adamdaz]

I have a question for the carb experts. I read over and over that its best to keep the original jets, but the PO trashed them when rebuilding the carbs. Is it worth the money to buy all the Mikuni jets that I can for these carbs? The replaced with cheap parts that I have include the main jets, pilot jet, pilot air jet and needle valve seat (for float).

Thanks.
Adam

[tom203]

At a minimum, I’d look for a set of genuine 110 mikuni mains. The PO’s carb kit mains might be real poor quality .

[Nessism]

Mikuni jets are best but if the bike runs good the bike runs good.

Vibration can't come from the carbs unless they are out of sync. Most likely the vibration is just a characteristic of the bike. This assumes no engine bolts are loose of course.

[adamdaz]

Mikuni jets are best but if the bike runs good the bike runs good.

Vibration can't come from the carbs unless they are out of sync. Most likely the vibration is just a characteristic of the bike. This assumes no engine bolts are loose of course.

Thanks Ed. This is helpful. I checked all of the engine bolts, so that was not the issue. While riding, the vibration was never enough to make my hands numb, but was enough so that my mirrors are useless when the engine is over 6000 RPM (Interstate speeds in 5th gear). Sounds like my going through the carbs will help the tune a bit, but may not solve the vibration. I have found multiple problems with the carbs that were not addressed by the PO.