Carb Sync Mystery

[Guest]

Odd thing here.

When I bought the bike at 33k miles the PO of my GS850GX said that he had the carbs synced and the valves adjusted at 24k miles (and I really think he believed that it had been done), but the factory metal seals were still on the adjustment screws.

Naturally, I had to remove the seals when I synced after bringing it home.

You think he got taken for a ride, so to speak?

[Steve]

Not at all. The metal "seals" do not have to be removed to do a sync, only to do proper idle mixture adjustment.


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[Roostabunny]

Not at all. The metal "seals" do not have to be removed to do a sync, only to do proper idle mixture adjustment.


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Gotcha - well that makes sense. And the sync mainly involves the throttle linkages? I kind of assumed that adjusting the idle mixture was part of syncing, but I guess they're separate but related. My wrench buddy explained it to me while we were syncing and adjusting mixtures, but he talks like I'm a mechanic, too, and only about half of what he says sinks in.

[Steve]

The "sync" is just a synchronization of the throttle linkages so they all open at the same time. In theory, this assures that each cylinder will be pulling the same amount of mixture and producing the same amount of power. This is what makes the bike so smooth after a good sync job.

The mixture screws under the metal caps have the ability to adjust the mixture (primarily at idle and low throttle opening) that goes into the cylinders. The quality of the mixture will also affect how much power is produced by each cylinder, so logic would dictate that mixture adjustment should be part of a tune-up. However, our government, in its infinite wisdom, and through the authority of the EPA, dictated that the mixture should be set on the lean side to reduce emissions. It did not matter to them that the bikes ran better and had better fuel economy with a slightly richer mixture. Motorcycle shops were forbidden to tamper with the factory-set mixture screws. In fact, there was a rather hefty penaly if they were caught.

In practice, you need to adjust each mixture screw very slowly until you find the point where it produces the highest engine speed. Adjust the idle speed with the master idle adjuster, then repeat with the other carbs. When I do a tune-up, I will do a mixture adjust, sync the carbs, then repeat the mixture adjust and double-check the sync. Checking it the second time only adds about 5 minutes while the hoses are still connected.


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[TheCafeKid]

And when adjusting mix screws for highest idle, an 1/8 of a turn makes a big difference sometimes, and give it a couple of seconds after adjustment to compensate. You'll have to listen carefully, because it may not neccessarily be a huge change in idle speed.

[Roostabunny]

And when adjusting mix screws for highest idle, an 1/8 of a turn makes a big difference sometimes, and give it a couple of seconds after adjustment to compensate. You'll have to listen carefully, because it may not neccessarily be a huge change in idle speed.

So you're adjusting mixture for fastest idle (because that indicates the best mixture for the bike?), and then using the idle knob to bring the idle speed into spec after you've set the mixture?

[psyguy]

Not at all. The metal "seals" do not have to be removed to do a sync, only to do proper idle mixture adjustment.


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yup, as steve said

[TheCafeKid]

So you're adjusting mixture for fastest idle (because that indicates the best mixture for the bike?), and then using the idle knob to bring the idle speed into spec after you've set the mixture?

Correct. You'll be doin just fine

[psyguy]

So you're adjusting mixture for fastest idle (because that indicates the best mixture for the bike?), and then using the idle knob to bring the idle speed into spec after you've set the mixture?

yup, like that. i like to start "low" with a screw down to two turns or less where the engine clearly performs poorly and work my way up until the highest rpm (but really just listening to the sound)

[Roostabunny]

yup, like that. i like to start "low" with a screw down to two turns or less where the engine clearly performs poorly and work my way up until the highest rpm (but really just listening to the sound)

OK, perfect - well, this is really good prep for my carb rebuild. I'm eventually moving to Huntsville, AL and have to leave my pro wrench friend here in Phoenix, so I gotta learn this stuff!

BTW, I've already replaced the intake boot o-rings (and those screws) with Robert Barr's parts, but my airbox boots don't fit the best either. Do I need to replace those before another sync will make sense, or is that not as critical as the cylinder side?

[TheCafeKid]

Well again as Steve has said, the sync will not be affected by mixture...the sync simply assures that all the carb's slides are adjusted to allow the same amount of vaccum pull. However, those faulty airbox boots WILL effect your mixture, and tuning them will be a nightmarish process, which you will find both very annoying and very unsuccessful until you replace those boots. Any excess air getting in ANYWHERE will mess with the touchy mixture. Assure that your boots on both sides are sealed and tight and in good repair, and that your airbox itself is sealed tight, or you will chase your tail till you go insane...Been there, done that, got the t-shirt...lol

[Steve]

... my airbox boots don't fit the best either. Do I need to replace those before another sync will make sense, or is that not as critical as the cylinder side?

Not quite as critical, but replacement is cheaper than the intake boots. Item #6 on this diagram. Just under $10 each from Bike Bandit.'

If you are going to remove the carbs or airbox anyway, replacing the boots will greatly help with re-assembly, let alone the sealing of the airway. New boots are so much more pliable and will let you move the carb rack around easier. After experiencing new boots, you will wonder why you waited so long. 8-[


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[Roostabunny]

Not quite as critical, but replacement is cheaper than the intake boots. Item #6 on this diagram. Just under $10 each from Bike Bandit.'

If you are going to remove the carbs or airbox anyway, replacing the boots will greatly help with re-assembly, let alone the sealing of the airway. New boots are so much more pliable and will let you move the carb rack around easier. After experiencing new boots, you will wonder why you waited so long. 8-[


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OK, fair enough, I guess I'll add those to my carb re-build parts list. They are a pain in the rear - one side then the other then the other, then squeezing off when you tighten the band. Maybe between now and then I'll make sure the filter and airbox are shipshape.

Any tips on those fiddly foam airbox cover seals?

[TheCafeKid]

Dont have an airbox myself. BUT, i have read where people have suggested weatherstripping as a good seal.

[Roostabunny]

Not quite as critical, but replacement is cheaper than the intake boots. Item #6 on this diagram. Just under $10 each from Bike Bandit.'

If you are going to remove the carbs or airbox anyway, replacing the boots will greatly help with re-assembly, let alone the sealing of the airway. New boots are so much more pliable and will let you move the carb rack around easier. After experiencing new boots, you will wonder why you waited so long. 8-[


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Speaking of those fiddly foam airox cover seals, they don't seem to be in the parts diagram on Bikebandit. I think mine are in fine shape, but hard to get and keep in place. Maybe some adhesive. I like The CafeKid's weather stripping idea. I assume you just want it airtight around that cover?