GS450 won't return to idle if revved to 3,000 or above!

[dennco2]

I have to turn off the ignition to kill the engine now if I rev it to 3,000 or above. I rebuilt the carbs 2 years ago and all was fine ... then a starter issue popped up and I let the bike sit for almost 2 years. I have been riding it lately but can't solve the runaway rpm problem. It idles fine. Today I just received a set of Viton high temp rubber O-rings ... in case I have an intake air leak and those are needed to replace the previous set from 2 years ago. I am thinking I need to pull the carbs and clean again because somehow the idle circuits are clogged and engine is running off the main jets at low speed?? Or another guess is the vacuum diaphragm on top of carbs is malfunctioning and the metering needle is not shutting off fuel properly?? I have been messing with variable but totally bamboozled as to what is causing the runaway rpms.

Any sage advice would be greatly appreciated here!

 

[timebombprod]

I believe main reasons would be, the vacuum leak on intake boots, running lean, a torn diaphragm, and cam chain off a tooth. I'd say one of the first three should be the issue ifnyouve never opened the engine.

Its a bit of a stretch but see what voltage you get at your coils, maybe its not getting good power at idle and once you give it some throttle it gets the appropriate voltage to coils. Its a far stretch but if none of the previous suggestions work and nobody else has other suggestions, check it out.

 

[timebombprod]

Throttle cable aswell if you haven't checked

 

[tom203]

If it was ok two years ago, the replaced carb boot/ head interface orings should still be ok. I?d suspect a leaking boot or loose clamp

 

[Gorminrider]

Throttle cable aswell if you haven't checked

Yes My first guess too. Not routing,so much, though often turning handlebars side-to-side is a quick clue for this... but I'd check the cable adjuster near the handlegrip turn it close-up...
Second guess is the throttle adjust KNOB between the carbs....turn it in...

Sometimes these things get adjusted for some particular purpose and get forgotten...so a whole dizzy troubleshooting commences when the simplest thing is the fault.
Nobody needs to ask how i know this..

 

[dennco2]

My son and I worked on this problem yesterday, but didn't get far at all. He works on cars ... not bikes. We pulled off the rubber boots connecting the air box to the carbs. He was going to try to use a VW vacuum sync tool he had ... no luck. We lifted up the tank several inches to play with the 2 idle air screws accessed from above. He closed them and then <tried> to open them 2 + turns. No improvement and we quit for the time being.

Today I started the engine and it idles fine when cold but as usual will stay at 4,000 rpms when revved that high. I backed out the central idle screw to a very low idle speed .... but then the rpms returned to idle smoothly and a bit slowly after each rev up. Yay! Then I completely removed the fuel tank to see what''s what. I checked the idle air screws and found they both were almost closed. My son thought he had opened them a couple turns ... but was working blindly and going by feel. I am surprised the engine even ran with those air settings! So now I don't know if those air screws have been almost closed all along ... or just since yesterday. They are now set to 2 1/2 turns out from closed!

Previously I had shot some WD40 around the intake boots to see if that affected the idle. No change. (I ran out of carb cleaner) All seems good the with the 2 intake boots and O-ring seals. Now I might have to use a lot of choke at idle when the engine is cold because I really backed off the idle screw to solve the runaway engine rev. So anyway, I have made some progress and tomorrow should find out more. Don't want to pull those carbs again ... unless I have to!

 

[timebombprod]

Goodstuff, glad it's going well!

 

[Rob S.]

I might have to use a lot of choke at idle when the engine is cold...

That sounds normal.

 

[cowboyup3371]

Yep, the “choke”, really more of a fuel enrichener than a real choke, is needed

 

[dennco2]

Yep, the “choke”, really more of a fuel enrichener than a real choke, is needed

Update: I partially removed the 2 carbs from the engine ... enuf to pop off the intake boots and inspect the O-rings. One O-ring had a small gap of about 1/2" broken off. I cleaned up the boots and inspected and saw no cracks or pin holes. I installed new Viton O-rings and clamped the boots back to the engine head. The 2 carbs went back on the boots ... but as usual I had trouble getting the rear boots on without mangling. Shoving the stock air box backwards helps but there has to be a better way!

After everything was back together ... I had a hard time starting the engine. I had to hold some throttle to keep it running even with choke. Later I adjusted the idle KNOB IN for higher idle speed and had the same issues as before. Engine would slowly return to idle or not at all when revved up to 3,000 or more. The I backed the KNOB out and the rev return problem went away, but would not idle well and quit soon after. Took it for a ride and all was well in all gears with decent power but would not idle at stop signs or stop lights unless I held some throttle with my hand. The throttle cable seems to operate perfectly ... no hangups.

So, after more effort and finding a split and missing section of the intake O-ring, I was hoping for way better results. Both idle air screws are currently out 2.5 turns. Does the idle mixture get richer as I turn the screw OUT for more turns? Or leaner?

 

[grcamna2]

The last time you opened-up the carburetors,did you set the float height to spec? I think the pilot jets/idle circuit(the smallest jets in the carbs,they get clogged real easy if your tank isn't spotless inside)may need cleaning.

 

[LAB3]

My bike ran just fine at 2 1/2 turns out. Seems that the only thing that hasn't been addressed is the carb diaphragms, those can easily be removed from the top without having to pull the carbs off. Check to see if they have any holes in them, holding them up to a light is a good method to check them out. The rubber seal at the top that's molded into them is "keyed" to make sure they go back in the carb body the proper way, you probably want to check that out as you disassemble them.

 

[dennco2]

When I rebuilt the 2 carbs about 2 years ago, I followed the carb tutorial in the GS Forum ... and all was well with the performance. Then I had a starter motor issue late in the riding season and just let the bike sit for almost 2 years. That is what has caused these latest carb issues.On my 450, there seems to be a delicate balance when turning the idle speed knob ... turning it clockwise helps the idle speed but hurts the idle return rpms ... or causes a runaway rpms to 4,000 and holds there. I agree, I think the idle jet / idle circuit is partially clogged from sitting? Possibly just remove the float bowls and idle jet from the bottom with carbs on the bike .... for a simple effort to test the suspicions?

Also, when reinstalling the 2 carbs to the rear rubber boots connected to the air cleaner ... I neglected to roll that circular spiral steel flexible retainer back over the plastic housing as it didn't seem to do anything as far as sealing the connection. Could easily be wrong on that.

 

[grcamna2]

No disrespect intended,a compliment.There was an exhaust manufacturer who made 2-stroke exhaust systems back in the 70's with the name of Denco,good quality expansion chambers;they also made some 4-stroke systems.

 

[LAB3]

When I rebuilt the 2 carbs about 2 years ago, I followed the carb tutorial in the GS Forum ... and all was well with the performance. Then I had a starter motor issue late in the riding season and just let the bike sit for almost 2 years. That is what has caused these latest carb issues.On my 450, there seems to be a delicate balance when turning the idle speed knob ... turning it clockwise helps the idle speed but hurts the idle return rpms ... or causes a runaway rpms to 4,000 and holds there. I agree, I think the idle jet / idle circuit is partially clogged from sitting? Possibly just remove the float bowls and idle jet from the bottom with carbs on the bike .... for a simple effort to test the suspicions?

Also, when reinstalling the 2 carbs to the rear rubber boots connected to the air cleaner ... I neglected to roll that circular spiral steel flexible retainer back over the plastic housing as it didn't seem to do anything as far as sealing the connection. Could easily be wrong on that.

One more thing you may want to look at that just came to mind after reading your latest post. On my 82 there's a sleeve inside the air cleaner side of the carb intake, it has a lip on it. When you're wrestling the carbs in and out it can very easily slip back into the airbox. Ended up taking a pair of small needle nosed pliers to grab that lip on the airbox side to coax it over the carb throat after the carbs are installed back into place. Don't know if yours has that or not but it's worth checking out, it made a difference on mine when it was slipped about 1/8" out of the airbox onto the carb to create a better seal.

 

[timebombprod]

You may be running lean, I'd say mess with the fuel mixture screws if you havent found anycogs in the pilots. You have a better seal now between the boots and the head, so it may have been enough to cause it to rub leaner. Turning the mixture screws out does make the mixture richer, I'm not sure how it exactly works but I'm guessing turning the screws out gives a less pressurised air suction causing the airflow to decrease.

I am having the same problem, I got my bike running pretty good today, needs more fine tuning so I dont get any hang at all but it's really not that bad. If you're bike doesnt pop at the exhaust at all you're pretty dang close to where you need to be

 

[dennco2]

One more thing you may want to look at that just came to mind after reading your latest post. On my 82 there's a sleeve inside the air cleaner side of the carb intake, it has a lip on it. When you're wrestling the carbs in and out it can very easily slip back into the airbox. Ended up taking a pair of small needle nosed pliers to grab that lip on the airbox side to coax it over the carb throat after the carbs are installed back into place. Don't know if yours has that or not but it's worth checking out, it made a difference on mine when it was slipped about 1/8" out of the airbox onto the carb to create a better seal.

When installing the carbs 2 years ago, I vaguely remember messing with that rubber sleeve you mention. I did not see it this time ... so it is probably rattling around inside the plastic housing. As it is, the bike is running better with the idle knob settings lower than before .... and controlling the engine urge to hold on to the higher revs. Getting pretty close to saying this bike is running good .... but more work is needed to button it up.

 

[LAB3]

When installing the carbs 2 years ago, I vaguely remember messing with that rubber sleeve you mention. I did not see it this time ... so it is probably rattling around inside the plastic housing. As it is, the bike is running better with the idle knob settings lower than before .... and controlling the engine urge to hold on to the higher revs. Getting pretty close to saying this bike is running good .... but more work is needed to button it up.

Those sleeves not being in place certainly would contribute to the issue you're having, the lack of air resistance the airbox produces would cause the engine to run lean. They don't actually slip INTO the filter housing, that lip I mentioned keeps that from happening. Also I remembered that I needed to unbolt the airbox and slip those sleeves into place then re-bolt it into place to make it work right. Good to hear you're close, too many people don't accept that good enough is good enough!

 

[Guest]

I had the same problem and all it was that the ideal was set too high. I lowered my ideal a little at a time until the problem went away. I think the magic number for me was 800+/-, it almost felt like it was going to stall but actually ran great.

 

[Grimly]

I had the same problem and all it was that the ideal was set too high. I lowered my ideal a little at a time until the problem went away.

I found the same over the years. The lowering of ideals is inevitable.