Fine tuning mixture screws to fix finicky idle?

[sacruickshank]

'81 GS550T with 8k miles, resurrected since Fall'20 ... Doh, longer than usual, got distracted by others projects.

Carbs cleaned & synced (full dis-assemble, new orings, ultrasonic, etc.), stock airbox, valves set, stock exhaust, etc. The carb cleaning wasa while ago, so some re-gumming might have occurred, but i don't think the idle passages are blocked again.

Starts and runs fine, but the idle setting is very finicky. A little too high and it idles at ~2k. A little too low and it will die out.

Mixture screws currently at 2.5 turns out. Could tweaking them in or out make the idle more reliable?

It's ridable as is, I would just like to have a smooth idle at the correct speed.

 

[tom203]

How are those o-rings at carb boot/cylinder head interface….

 

[sacruickshank]

How are those o-rings at carb boot/cylinder head interface….

New orings and boots, but I don't recall if the orings were high quality or HW store. I'll do the spray while running test next chance I get

 

[Gorminrider]

ASSUMING your tach works, Needing to run at 2k rpm sounds too high to idle means you are needing to open the throttle plates and thereby engaging the main throttle fuel circuit to keep the bike running...(oh, and did you bench-synch" the throttle plates?)
My first guess would be Too much air or low float bowl levels or two or more idle circuit restricted. But easy is to first turn the idlemix screws out a bit (within 1 turn) with the main idle setting turned "down" a bit to see if you can encourage the idle circuit and thereafter tweak to get rpm towards 800-1200rpm
.... If no response, it's something else...Me, thinking "too lean",I'd firstly think back and try to remember if I'd removed the bowl gaskets before checking float levels on the bench...

 

[Nessism]

Try 3.5 turns open on the pilot screws. And doublecheck vacuum sync. My old 550T had a hanging idle until I opened the pilot screws full open (any further open than 3.5 turns won't change performance.)

 

[sacruickshank]

Thanks all, I'll give those a shot.
Re: the tach - Unfortunately it gave up the ghost just a few rides ago, so I'm estimating the 2k rpm comment. I can't turn the cable by hand when disconnected at the tach gauge, so I assume the problem is with the gauge rather than the cable.

Re: bench syncing, yes I did bench sync them and then again with a Carbtune while the bike was running. This was back in Nov'21 and the bike was started only periodically between then due to distractions from other projects, so there is a chance of passages are slightly blocked again. Running some Berryman's thru the gas to see if that helps. trying to avoid pulling the carbs again.

Re: the mixture screws, if there's no air leak at the intake boots I'll turn the screws out by 1 turn

 

[gustovh]

I would start with lean best idle adjustment first. To do this, start with the #1carb with the bike at idle…whatever rpm it will idle at…turn the pilot screw in slowly until the idle falls off then back out the screw 1/8-1/4 turn. The idle may increase and need to be readjusted. Do this to all the carbs one at a time until you are satisfied with the leanest setting. Then I would sync the carbs again, I believe this will give you good results.

V

 

[sacruickshank]

Related question ... The connection between carb#3 and the airbox is tenuous. There are no visible gaps, but the rubber boot isn't perfectly clamped either. If air is getting in thru there would it cause a high/finicky idle or would it be irrelevant on the airbox side of the carbs? The other three carbs are securely mounted to their boots, but a slightly deformed section of the airbox's plastic makes connection difficult for carb3.

 

[Gorminrider]

Airbox.... you can stuff/wrap your suspect connection with tape to see for yourself. ( Sometime, you might want to get a heat gun and fix the deformed airbox ...)

Bad tach, eh? These old tachs can get tired anyways....I can accept your estimate that the idle's too high-to most people it's pretty obvious. Having to be careful engaging forward motion would be another way to describe it

 

[gustovh]

I doubt very much that your airbox is the issue, have you looked at the plugs by doing a chop at idle?

V

 

[sacruickshank]

thanks all for the suggestions. Problem is 99% cured with setting the mixture screws to 3.5+ and running a super heavy dose of Berryman's and MMO. Working assumption is the idle circuit clogged enough to affect the idle, but not so much it required full disassembly.

Now to find out why my GS650 is getting such terrible mileage.