Help troubleshooting throttle sticking @5000 rpm

[Jagir]

Hey all,

The original problem is the throttle hanging around 4000 rpm once the bike wrams up. It did not do this before the first carb rebuild. That sounds like an easy one, but read on...
I've been tinkering on my 80 GS400 all summer trying to get to the point of figuring this rascal out. Here's the work that has been done to try to solve the problem and/or cuz it demanded attention:

• New mufflers
• new manual petcock
• valve clearance (new gasket)
• carb rebuild 3 times (new float bowl gaskets and orings) blown through with compressed air
• rear brakes
• seafoam in the gas tank
• oil change (with seafoam)
• new boots in the airbox
• clean and oil air filter
• many thanks to those of you that have assisted me through all this

Since the last batch of work, it runs like a champ, except for the throttle stick:?, now at 5000 rpm. It didn't happen for the first post-servicing work (1/2 hour warm engine, last night), but it did today.

I am thinking of:

• replacing the throttle cable
• Getting another set of carbs and trying swapology
• cleaning the tank
• new clear fuel lines
• buying a V-strom

Any other suggestions?
thanks
j

 

[bikechump]

Do you mean the revs wont increase past 5k regardless of throttle position? The bike idles fine then jumps in revs?
Or is the throttle physically hanging up on something?

 

[Jagir]

once it warms up, the throttle will not sink below 5000 until I let the clutch out - almost like the choke is pulled out full.

 

[Guest]

The bike is warmed up and shut off. When restarted with no throttle input from you....
(a) Does it idle at 4k immediately after starting?
(b) Do you have to hit the throttle first and then it won't return to idle?
(c) none of the above
(d) huh?

 

[Jagir]

The bike is warmed up and shut off. When restarted with no throttle input from you....
(a) Does it idle at 4k immediately after starting?
(b) Do you have to hit the throttle first and then it won't return to idle?
(c) none of the above
(d) huh?

mostly (b)

the bike has sat for an hour or three. I start it up and everything is perfect. Crank the throttle and it drops to around 1250. I Take off and by the time I am 5 minutes up the road, the revs won't drop.
I am that jerk revving real high at the stop light.8-[
WAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA!
j

 

[Guest]

I had this problem before and I posted something about it a while back, dunno How to link but i can copy and paste...................................................

I experienced this with a couple of old bikes with CV carbs. I think the idle/pilot circuit is plugged. Try taking out the idle mixture screws and spraying some aerosol carburetor cleaner in there maybe blow a little air in there as well. Put the screws back in lightly seat them and back them out a couple of turns. See if that helps. You will have to adjust idle speed as well, as it will be idling too fast if this routine works.
OK, here's my $0.02 theory on the problem. I think the idle jets or whatever get plugged with fuel deposits and crap so the engine can't pull fuel through the idle circuit. To compensate the idle speed gets adjusted with the throttle plate stop screw til the bike idles on the main jet.At this point the port in the carb bore that feeds the passage to the top of the diaphragam is just starting to get exposed to venturi vacuum. While not enough of a vacuum signal is there to overcome the stichon (sic?) of the pistons and start them rising, the pistons are very sensitive and lift at the first hint of throttle application. With the increased air flow there is now enough of a vacuum signal to hold the pistons up at idle allowing more fuel to flow out of the mains and the engine idles really high. If the engine is shut off and restarted it will idle "normally" again until the throttle is cracked and away it goes again.
Carbs may still have to be removed and cleaned as well as the fuel tank if the problem continues.
I hope this makes sense to more than just me. Then again I could be right out to lunch.
dave
................ i realize you probably don't think the idle circuit is plugged but the " dance of the cv carb pistons" maybe happening for another similar reason .. Can you tell if the idle speed screw is turned in further than it was before the carbs were rebuilt?
Another goofy thing I seen done that may have caused this in the past was put the floats in upside down, It easier to do than you think , Kehin carb floats look better upside down , (i'm not sure about mikuni haven't tried yet) but the fuel level ends up so low it is really hard for the engine to pull any at idle.... you can check the fuel level with a clear piece of hose attached to the carb drain and curved up beside the bowl, open the drain and see where the level is............. appologies for the lengthy babble.
good luck
dave

 

[Nessism]

Hanging idle usually means stuck throttle cable or intake air leak. Did you replace the rubber intake runner boots and/or orings?

 

[Jagir]

Hanging idle usually means stuck throttle cable or intake air leak. Did you replace the rubber intake runner boots and/or orings?

Thanks,

Yeah I thought about air leaking too. I thought I fixed any type of possible air leak;
new boots, removed and cleaned the airbox, air filter, new o-rings, etc...
Can you think of anything I missed?

Can't remember inspecting the cable, but I'll pull it and have a good look and feel. If that is fine, then I guess it's the carbs again. At least I'm getting good and removing them.

 

[Jagir]

I had this problem before and I posted something about it a while back, dunno How to link but i can copy and paste...................................................

I experienced this with a couple of old bikes with CV carbs. I think the idle/pilot circuit is plugged. Try taking out the idle mixture screws and spraying some aerosol carburetor cleaner in there maybe blow a little air in there as well. Put the screws back in lightly seat them and back them out a couple of turns. See if that helps. You will have to adjust idle speed as well, as it will be idling too fast if this routine works.
OK, here's my $0.02 theory on the problem. I think the idle jets or whatever get plugged with fuel deposits and crap so the engine can't pull fuel through the idle circuit. To compensate the idle speed gets adjusted with the throttle plate stop screw til the bike idles on the main jet.At this point the port in the carb bore that feeds the passage to the top of the diaphragam is just starting to get exposed to venturi vacuum. While not enough of a vacuum signal is there to overcome the stichon (sic?) of the pistons and start them rising, the pistons are very sensitive and lift at the first hint of throttle application. With the increased air flow there is now enough of a vacuum signal to hold the pistons up at idle allowing more fuel to flow out of the mains and the engine idles really high. If the engine is shut off and restarted it will idle "normally" again until the throttle is cracked and away it goes again.
Carbs may still have to be removed and cleaned as well as the fuel tank if the problem continues.
I hope this makes sense to more than just me. Then again I could be right out to lunch.
dave
................ i realize you probably don't think the idle circuit is plugged but the " dance of the cv carb pistons" maybe happening for another similar reason .. Can you tell if the idle speed screw is turned in further than it was before the carbs were rebuilt?
Another goofy thing I seen done that may have caused this in the past was put the floats in upside down, It easier to do than you think , Kehin carb floats look better upside down , (i'm not sure about mikuni haven't tried yet) but the fuel level ends up so low it is really hard for the engine to pull any at idle.... you can check the fuel level with a clear piece of hose attached to the carb drain and curved up beside the bowl, open the drain and see where the level is............. appologies for the lengthy babble.
good luck
dave

Good theory!

I'll pull em off again soon and let you know how it goes. I may still try and get a second set of carbs, just to try to isolate the issue

thanks,

j

 

[waterman]

I had trouble like this once, I found that the previous owner had wrongly installed the slides in the carbs and thus one was hanging open. When the PO put the slides in they didn't line up the little tab into the slot on the top. It sounds like you have been through the carbs already but I would check to make sure the slides function properly. Just my .02.

 

[Jagir]

I had trouble like this once, I found that the previous owner had wrongly installed the slides in the carbs and thus one was hanging open. When the PO put the slides in they didn't line up the little tab into the slot on the top. It sounds like you have been through the carbs already but I would check to make sure the slides function properly. Just my .02.

I like this train of thought...
I don't think the problem existed before the first carb clean, but that was last summer and lots has happened in between.
I did find some ?varnish? or something (looked like a thin dribble of nail polish) on the side of the slide needle which I knocked off with my fingernail. At that point, I thought I had this issue diagnosed and licked.

The tabs at the top of the slides are lined up.

It's starting to sound like I'll be going into the carbs again,:-s

thanks for the hunch,
j

 

[Nessism]

Sorry to sound redundant but did you replace parts #19, 21, 22 as shown below? Very important that the engine is not sucking air that bypasses the carbs.

 

[Jagir]

Sorry to sound redundant but did you replace parts #19, 21, 22 as shown below? Very important that the engine is not sucking air that bypasses the carbs.

Thanks for asking, that's troubleshooting.

My bike (1980 GSX400E) doesn't have #19 or 20, but I did replace #22 and the airbox boots.

I know it sounds like an air leak, that's why I overhauled the whole airbox.
At the same time, I seafoamed the gas, did the valves, oil and some other tweaking and now the hang-up is at 5000 instead of 4000. That tells me that if it IS an airleak, then I have made it worse. Not likely because the old airbox boots were damaged (my bad8-[) and most likely leaking. I took great care in fitting the airbox to the carbs to the engine.
More likely, the higher rpm is a result of better performance now that everything is properly dialed in.

Unless I overlooked something obvious, I think air leak can be ruled out, but please keep asking the questions and thanks again for the hints.

j

 

[Jagir]

How important are "atomiser rings"?
I don't have those either.

 

[KEITH KRAUSE]

Well, the hanging rpm's sound like either a mechanical problem or an intake leak.
On your CV carbs, 5 minutes of warming up and the kind of rising operating temps involved shouldn't effect the throttle plates or the throttle cable so if it's heat related it doesn't seem like a mechanical/cable routing problem. Sounds like an intake leak which does effect rpms after 5 minutes of warming up.
If the bike idles about 1100 rpm after starting/choke off, but "idles" much higher after full warm up and you believe the cable/throttle plates aren't sticking, then you have an intake leak somewhere.

 

[Jagir]

Well, the hanging rpm's sound like either a mechanical problem or an intake leak.
On your CV carbs, 5 minutes of warming up and the kind of rising operating temps involved shouldn't effect the throttle plates or the throttle cable so if it's heat related it doesn't seem like a mechanical/cable routing problem. Sounds like an intake leak which does effect rpms after 5 minutes of warming up.
If the bike idles about 1100 rpm after starting/choke off, but "idles" much higher after full warm up and you believe the cable/throttle plates aren't sticking, then you have an intake leak somewhere.

Ok then, when the petcock was replaced, the vacuum pipe was simply plugged with a long screw. Could this be the trouble? If so, what should i do with this hose?

Or, are there any o-rings in the carbs that could be causing this issue?

thanks,
j

 

[Jagir]

Carbs are cleaned again. I did find the tiniest piece of crud in one of the pilot jets. I'll try to get new clear hoses, reinstall the carbs tomorrow and report back.

j

 

[KEITH KRAUSE]

Ok then, when the petcock was replaced, the vacuum pipe was simply plugged with a long screw. Could this be the trouble? If so, what should i do with this hose?

Or, are there any o-rings in the carbs that could be causing this issue?

thanks,
j

A screw isn't a positive stop. It could still leak and yes, any intake leak will effect idling.
If you have a non-vacuum fuel valve now, then you need to positively plug that hose. Actually, if my bike, I'd remove the hose and buy a rubber cap for the vacuum nipple at the carb body. Many hardware stores have a selection of caps. Some are made for capping parts such as on carbs, some are made to go over sharp bolt ends and protect you from scratching yourself, etc. Some people would just snip the cracked end off that hose and plug it with whatever. A golf tee taped over, works. If you fold the end over and tie it, it will eventually crack and you'll get another leak.
Only o-ring in the carb itself that could cause a lean mixture would be the mixture screw o-rings.
In your pic, the manifold o-rings, I always suggest should be changed. They're cheap. Most intake leaks are because of these o-rings. I suggest using a coat of hi temp bearing grease on the manifold o-rings. Buy some hex head manifold bolts so you can torque the manifolds to about 6ft/lb. Over-tightening just squishes the o-rings and shortens service life.

 

[Nessism]

In your pic, the manifold o-rings, I always suggest should be changed. They're cheap. Most intake leaks are because of these o-rings. I suggest using a coat of hi temp bearing grease on the manifold o-rings. Buy some hex head manifold bolts so you can torque the manifolds to about 6ft/lb. Over-tightening just squishes the o-rings and shortens service life.

Keith is "the man" regarding carbs around here so don't take this wrong...but the idea of squishing the O-ring by over tightening the screws doesn't seem possible to me. The intake runner has an aluminum base molded into the rubber which the O-ring fits into. When tightening the screws the aluminum insert is in full contact with the head thus compressing the O-ring. Over tightening the screws shouldn't make any difference in terms of how much O-ring compression that occurs since the aluminum is not compressible. Am I missing something here?

 

[KEITH KRAUSE]

Keith is "the man" regarding carbs around here so don't take this wrong...but the idea of squishing the O-ring by over tightening the screws doesn't seem possible to me. The intake runner has an aluminum base molded into the rubber which the O-ring fits into. When tightening the screws the aluminum insert is in full contact with the head thus compressing the O-ring. Over tightening the screws shouldn't make any difference in terms of how much O-ring compression that occurs since the aluminum is not compressible. Am I missing something here?

I don't pretend to be an expert on manifolds or how parts tighten down, but despite the manifold making full contact, it's still the thin rubber o-ring that does the sealing and that's why it's there.
I don't know about the manifold being truly compressible or not, but like any part, it can be over-torqued. It's expected to expand and contract quite a bit, just like the valve cover or cylinder head. Over-tightening forces the metal to fatigue instead of simply expanding and contracting.
If you place a rubber o-ring in this same situation, its service life will shorten. If the o-ring is under correct torque, it will stay much "rounder" and retaining the shape it was molded into will help the o-ring against cracking. If tightened down too hard, the round o-ring will flatten out and take on an excessive oval shape instead. Just as metal can be fatigued by over-tightening, rubber will develop cracks much sooner if forced out of its original molded shape. In this case, the cracks will appear on the inner and outer edge of the o-rings.
Besides compression, the rubber also deals with heat. This naturally hardens the rubber over time. The sooner the rubber loses its ability to expand and contract, the sooner it will leak. Over-tightening and excessive deforming of the rubber just shortens its ability to expand and contract and promotes cracking.
I don't know just how precision the o-ring grooves are cut but I've seen a lot of used o-rings and it's the flatter ones that hardened faster and developed cracks sooner and more severely than o-rings that weren't so flattened. How flat they were is a direct result of how much they were tightened.
I think you're saying that at some point the o-rings no longer continue to flatten and it's the manifold that takes all the torque. At some point that is probably true but when you reach that point, you've gone too far in my opinion. It's not necessary to make a good seal.
I suppose anyone who wants to tighten the manifolds harder is free to do so. I'm not exactly sure of the recommended torque rating but the bolts aren't that big and they aren't designed to handle the extra torquing that many "#7" bolts are made for. They are just standard bolts. I'm sure that 6 ft/lb is adequate to make a good seal without causing the parts to wear out sooner.