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One fuel mix screw out of 4 controls all 4 carbs.

  • Thread starter Thread starter Bortasqu
  • Start date Start date
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Bortasqu

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Hello.

So the problem I got with my 1985 GS650 shaft drive is that I can turn all 3 of the 4 fuel mix screws closed and the engine is still going fine no sputter just normal 1200 rpm. Something is clearly wrong here, but I don't know what.
The number 3 carb screw litterally controls the engine rpm when I turn it down. The other 3 screws has little to no effect exept it bangs in the exhaust if I turn them up.

Backstory: I took every piece of the carb down, rinsed everything vigerously with carb cleaner I visually made sure there vasan opening in all of the jets, and I poked the holes of the brass parts to make sure no crud was left in. Everything that is made out of rubber got replaced in the carb a few months ago, there is fresh gas in the tank, and I adjusted the valves.
I'm worried that I may have forgotten to set the choke setting back to factory, but the bike starts easely with choke on, and stays running at 1200rpm when I turn the choke off a few moments later. It is timed corretly, and the cam chain is within spec. All coils are firing, as with spark plugs.

Regards,
 
I've bought several bikes that had been stored a long time over the years and gotten them running by pulling the carbs apart, spraying carb cleaner through them and running wire, etc through the holes. In the case of my current bike, I had to get the cars hot (with boiling water) to free up the slides and used Pine Sol cleaner as a carb dip soaking it for about three hours. The bike ran OK and knowing it needed a few other things done I left it be but had a tough time getting it to rev higher than 7,000 RPM. After getting everything else sorted out but still not getting the revs up I pulled the carbs apart again and let them soak (in Pine Sol) for two full days and it runs like a champ now. Most people swear by a harsher chemical carb dip but I prefer the Pine Sol because it's cheap, I can use it for other degreasing on the bike and don't need to worry about disposing of it. Long and short of the story, let those carbs soak in carb dip!
 
Apparently, you are not getting fuel through three pilot ports. :-k

You will have to remove the carbs from the bike to check. First, determine the current settings of your pilot (mixture) screws. Turn each one IN slowly, counting as you go. Record that number, continue for all four carbs.
Next, remove the screws completely. Be sure to also remove the spring, flat washer and o-ring. Examine the condition of the screw. There should be a nice point on the end. If it is squared-off, it has been broken, and is likely still in the carb body. Shine a light in the outlet of the carb, see if you can see it when looking through the hole where the pilot screw just came out. If you don't see any light, you will need to remove the blockage. Determine if the blockage is general crud or the broken tip of the pilot screw.

.
 
Th e choke is not an issue

More likely the tips, as Steve says, or passages in the carb bodies are clogged. I see no mention that you cleaned the carb bodies. If you replaced all of the rubber bits, does that include all of the O rings?
 
Seeing as you did do a pretty thorough cleanup...
Backstory: I took every piece of the carb down, rinsed everything vigerously with carb cleaner I visually made sure there vasan opening in all of the jets, and I poked the holes of the brass parts to make sure no crud was left in. Everything that is made out of rubber got replaced in the carb a few months ago, there is fresh gas in the tank, and I adjusted the valves.
Click to expand...

first off,I'm not great at adjusting four carbs and I haven't had your problem with my 650 ...but, I find it pretty hard to hear on a four cylinder when I tweak at a single idlemix screw but I'd first suspect that the one that controls the others might be the "trouble" carb.... dragging the others along with it (and especially suspect if it's the one with the petcock vacuum...).. a check of spark plugs can indicate which is rich .

....You really have to take the throttle out of the mix as much as possible. It's going to mask efforts at idle mix..the idle mix is independent of the throttle...so turn the throttle down as much as possible when the bike is started from a close-as-you-can-get-it bench synch....
mikuni cv IDLE circuit.jpg


and,

or there might be a vacuum leak at one or more of the rubber manifold boots ... more possibilities when you scrutinise what the shop manual doesn't say!

I'd bench sync it carefully, set all the mix screws at factory settings and if it runs nicely, when warmed up, lower the idle to just as low as the bike will run before tweaking any mixture screw gently..in fact to be honest- I don't bother much at all if the bike runs very well from bench settings and then a close effort with a synch tool...a later look at the spark plugs might indicate a too- rich idle in one cylinder.
 
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LAB3 said:
I've bought several bikes that had been stored a long time over the years and gotten them running by pulling the carbs apart, spraying carb cleaner through them and running wire, etc through the holes. In the case of my current bike, I had to get the cars hot (with boiling water) to free up the slides and used Pine Sol cleaner as a carb dip soaking it for about three hours. The bike ran OK and knowing it needed a few other things done I left it be but had a tough time getting it to rev higher than 7,000 RPM. After getting everything else sorted out but still not getting the revs up I pulled the carbs apart again and let them soak (in Pine Sol) for two full days and it runs like a champ now. Most people swear by a harsher chemical carb dip but I prefer the Pine Sol because it's cheap, I can use it for other degreasing on the bike and don't need to worry about disposing of it. Long and short of the story, let those carbs soak in carb dip!
Click to expand...

Sorry for the hijack. In your experience Does pine sol affect or damage rubber? Carb dip appears to.
 
Yes carb dip does but you aren't supposed to put any of the rubber pieces in it. In other words you are supposed to completely disassemble them and if needed put new ones on that you can get from www.cycleorings.com whose owner is a member here.
 
cowboyup3371 said:
Yes carb dip does but you aren't supposed to put any of the rubber pieces in it. In other words you are supposed to completely disassemble them and if needed put new ones on that you can get from www.cycleorings.com whose owner is a member here.
Click to expand...

i do disassemble the carbs to dip in the carb dip stuff. Been to cycleorings and book marked a few things.

Just wondering if the pinesol affects rubber. I?ve got a set of core carbs for my cavalcade. I could always experiment I guess
 
I don't think PineSol affects the rubber parts, but I am also not so convinced that it cleans the metal parts properly, either. I have never tried it on carbs, but have had less than stellar results on other stuff.
 
Word has it that Pine Sol doesn't hurt the rubber, keep in mind it will darken the metal a bit. Yes, you need to strip everything down before soaking so you know that you're getting everything as clean as possible THEN give it a blast with the aerosol carb cleaner.
 
Simple Green D will not react with aluminum. I did some test soaks on some old scooter carbs and found that in the ultrasonic cleaner my best result was at 50C in Simple Green D for 30 minutes.
 
I'll throw in an update here.
So I dis-mounted the carb once again, I removed all of the piot screws, checked everything maticulously. I then proceeded to clean the passageways for the fuel screws, and made sure there waslight comming trhough everything.
So to rule out any air leak intrusion, I laid a very thin bead of rtv where the o-rings are faced to the engine side. You may read this as discust in your eyes, but I got burned from an employer last year so now I'm more broke than you can imagine.

The bike now runs and idles with the choke off, and is very responsive to the throttle input. It doersn't hang anymoore, and the exhaust doesn't bang from too much fuel, and the #3 carb has been taken down as the dictator so the other carbs are independent and relevant. Very easy to start when hot. I'm going to borrow an idle adjusting tool from a friend this week so we can dial in the carbs and temperatures from the exhaust to be consistent across all ranges again, as when it came from factory. I was debating to put in a slightly larger pilot jet, but that will have to be later.
 
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Sounds like you really need to replace the o-rings. Go to cycleorings.com, order a set for your bike. Not sure how it will translate into your currency, but the price is very reasonable.

Please consider the RTV as a diagnostic tool, rather than a fix. Clean the RTV off of there just as soon as you can and don't run the engine any more than necessary. Fuel and RTV do not play well together.

.
 
The RTV is going to stay there for a while, read: I'm very broke.
Also since my last reply, the bike starts easely. It now only bogs down and stalls sometimes at full throttle, low and medium throttle input is normal. The oil fill glass shows it's sligtly over-filled. So I'm going to drain it tomorrow, start it up and see if anything changes.
I got the vacuum synch. tool, but my friend forgot to bring the adapters for the vacuum hoses, so the sync will happen soon this week.
 
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