GS850G 1981 with runaway throttle... scary.. whats wrong with it?

[Guest]

I went for a 3 hour ride yesterday, bike was running great, I was a block from home, and did a hard 1-2-3 9000rpm shift.. and then it felt like it had cruise control! Pull in the clutch and rpm climbs fast up 7500, was were I hit the kill switch. Had to ride home hitting the kill switch off, and on to control engine...runaway

Throttle grip isn't sticking, and blades return to idle stops. It will start-up and just scream to kingdom come! Its like the choke is out all the way? Yet its pushed in!

Carb related? vacuum leak?

[Nessism]

Most likely carb related. Vacuum leak is very possible, especially if you haven't changed the intake boots and O-rings, but I've never seen one race to 7500 rpm before. It's hard to say. You might want to check the Newbie Mistakes thread linked in my signature to see if there is anything you missed while performing maintenance.

[tom203]

Check "choke" connection at carbs to make sure that it plungers are retracking properly.

[04colyZQ8]

Most likely carb related. Vacuum leak is very possible, especially if you haven't changed the intake boots and O-rings, but I've never seen one race to 7500 rpm before. It's hard to say. You might want to check the Newbie Mistakes thread linked in my signature to see if there is anything you missed while performing maintenance.

Your talking about the hard rubber boots that are molded over aluminum, with oring that goes to the head I used silicone between the flange and head, doubt they are leaking.? they are a nightmare to get on and off the carbs, I put soap , on them and lean into it for all I'm worth!!!!!!!!!!! horrible design. The rubbers between the air box, and carbs are new last year.

I guess I'll pull the carbs off, I have a spare rebuilt set.

[JTGS850GL]

How do you know the carbs are returning to the throttle stops? There is no way the engine could rev that high if the throttle plates were all closed. No leaky boot would cause it either. Do you hear the carbs slam shut when you let go of the throttle? Do you feel the spring tension at the throttle grip? Sounds to me like the throttle return springs either broke or one of the carbs is jammed open keeping the others open as well. Take the carbs off and inspect what's going on.

[jsandidge]

I agree with JT. No way would an air leak or a stuck choke lead to rpm's that high. The butterflies have to be stuck open. Your carb to head boots and o-rings really need to be replaced though. No way your silicone solution is going to work.

[hillsy]

Your talking about the hard rubber boots that are molded over aluminum, with oring that goes to the head I used silicone between the flange and head, doubt they are leaking.? they are a nightmare to get on and off the carbs, I put soap , on them and lean into it for all I'm worth!!!!!!!!!!! horrible design. The rubbers between the air box, and carbs are new last year.

I guess I'll pull the carbs off, I have a spare rebuilt set.

Sounds like your intake rubbers are hardened (IE: old)

[Steve]

Check the clutch cable between carbs 1 and 2. If you don't have something to keep it out of the throttle linkage, it can get stuck.

Been there, done that.

.

[04colyZQ8]

Ok update, I checked over everything you all recommend! Thanks for all your help and fast responses so far!
linkage good, not jambing, returned to stops, boots are hard old, yet do seal, they have no silicone just a dab between the head and boots, not leaking there either.

Rechecked when cold idled at 3500rpm choke pushed in!! I didn't let it run hot!

Tore the carbs off, ungagnged them, pulled all jets, air bleeds, diaframs out, throughly cleaned each part, it was dirty especially the idle bleed/mix screws, filthy! Replaced all orings, and float bowl gaskets. Put carb on, idles great, 1200rpm!

now I have a bog, lack of power from 5000-8000 rpm, and bad miss from 8-9000!!!

I suspect the mixture screws idk, how to adjust them? I've tired 2 turns out, from lightly seated, 2-1/4 turns, and 2-1/2 turns, doesn't make much difference! And I hate talking the freaking tank off three times!! Any better way to adjust these? What do I need to do? I think it's to rich? Idles low, normally as the bike gets got idle rpm climbs up around 1700, and ran great like that before the high speed problem. Now idle is 1000, I truned the thumb screw up and it idled at 1100 cold, 1500 hot. I think i need to lean it out? So rpms climb when hot? And I get my power back?

Which way do I turn those ##%^^*! Air bleeds/mixture???

Mikini 32mm 81 gs850

thanks guys!

[robertbarr]

Carbs that are out of sync (balance) can cause a runaway idle as the engine warms up. So will intake leaks. I'm guessing you have both.

You don't mention if you replaced the intake O-rings; you keep mentioning some 'dab of silicone'. The solution is dirt cheap and eliminates about 3,000 different variables that can cause these problems and many more. If you're going to apply barnyard solutions to intake sealing, you can expect carb 'problems' for the remainder of eternity.

[04colyZQ8]

It no longer has a run away idle, it's the opposite it doesn't want to rev now. I've never had all the boots off, only took one off to remove the chain tensioner. Looked like there was dab of silicone from the factory so I did the same thing. The other three have never been off. The screws are so tight, I can't get them loose, prob have to use heat. Why mess with something that's not broken? Idling I soaked it with carb clean no rpm change, so I shouldn't have any vaccuum leaks?

I think I'm rich now, so I need to know what those mixture screws do and how to set them up!! The boot orings are fine, bike was runing good untill last week. I think the carb just got dirty, it's. clean now, that fixed the high rpm idle, but created a rich high rpm bog?

[04colyZQ8]

I guess I really messed up! Clymer says not to touch the mixture screws, and that they are covered with a plug? For EPA reasons, and doesn't say how to adjust them? I have a Canadian bike, with lower emissions standards there wasn't any plugs above the screws, I assumed that they could be removed and cleaned? I googled how to adjust them can't find anything?

Anyone have a set of gs850 carbs for sale that they haven't been adjusted? I have a spare set of carbs, I think I messed with screws on it? I'll go to my storage and try that carb I guess?

No pint synching the carbs when mix is way off! Synching has to do with adjusting the individual idle stops of each carb right?

[Nessism]

The mixture screw setting is relatively unimportant. Just set them to 2.5 turns open from lightly seated and call it done. They only affect mixture at idle and tip in.

Problem you have is you are short cutting. Those boots should come off and at minimum get new O-rings. You need to vacuum sync the carbs. Valve adjust. Etc. Go through the Newbie Mistakes thread like suggested earlier.

[04colyZQ8]

The mixture screw setting is relatively unimportant. Just set them to 2.5 turns open from lightly seated and call it done. They only affect mixture at idle and tip in.

Problem you have is you are short cutting. Those boots should come off and at minimum get new O-rings. You need to vacuum sync the carbs. Valve adjust. Etc. Go through the Newbie Mistakes thread like suggested earlier.

Its not $$&&&! Leaking there but ok I'll replace the orings they are so important to you!!! No ones open today, cycle shops open Tuesday to Saturday

Anyone no the generic oring part number? N70 or N90 derometer? I can't find a carb sync tool to save my sole!!

Some threads say the mixture screws do effect the overal running? Maybe it's a different model bike?

Valves were recently shimmed, plug wires are good. I can through in new plugs and try the coils from my other bike that I installed new plug wires into, and new boot ends

[AMK]

Carbs that are out of sync (balance) can cause a runaway idle as the engine warms up. So will intake leaks. I'm guessing you have both.

You don't mention if you replaced the intake O-rings; you keep mentioning some 'dab of silicone'. The solution is dirt cheap and eliminates about 3,000 different variables that can cause these problems and many more. If you're going to apply barnyard solutions to intake sealing, you can expect carb 'problems' for the remainder of eternity.

I like this.