Help troubleshooting throttle sticking @5000 rpm

[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.

[Nessism]

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.

That's exactly what I'm saying; the amount of O-ring compression is set by the depth of the groove in the manifold. Once a nominal amount of torque is applied to the bolts, the aluminum manifold insert will be in full contact with the head and the O-ring will be in full compression. Continuing to torque the bolts will stress the threads in the head, and stress the manifold, but will not result in any more O-ring compression.

Regarding fastners in general, when compressable elements are involved (such as a rubber washer for example) special precautions, such as thread locking agents and/or mechanical saftey locks (cotter pins), are required or the joint wil loosen as the compressable element creaps with time. What i'm trying to say is that we WANT that O-ring to be fully compressed within the manifold groove such that it is not affecting the clamp load on the screws. Otherwise, the screws will loosen with time as the rubber compresses - not good.

Don't mean to go against you here Keith, just want to clarify. Keep up the good work with all your great carb threads. \\/

[Jagir]

I think Mr.Krause wins a prize again. Thank you for not letting me give up on the air leak idea.

I replaced the screw hose with one that has a more positive seal. Took it for a short ride (just past where the throttle hang usually happens) and it ran beautifully. Just need to really test it to be sure.

In this case, I cannot blame the PO. I was away for the weekend when my father-in-law and the mysterious petcock fairies visited my bike. I found a shiny new manual petcock and a hose going nowhere with a screw hanging out. I was reassured that it was fine and that hose just needed to be plugged.

Just to get to the troubleshooting phase, I did enough work on the bike to feel confident and able to diagnose (with GSR help, of course) a host of issues that I hopefully won't have.

The o-rings are pretty new, they replaced the squished flat 25 year old ones. I bought a second set so they can be swapped any old time.

thanks again
I'm going riding. \\/
j

[MelodicMetalGod]

Sounds like an air leak was at issue and that you've cleared the issue. Glad to hear it.

My bike has been doing the same (high idle after warm up). I expect to have the new boots and o-rings on by this weekend. Hopefully a few days from now we're both busy riding without worrying about 4-5000 RPM idling at stop lights. [-o<

Enjoy the road!

[KEITH KRAUSE]

I think Mr.Krause wins a prize again. Thank you for not letting me give up on the air leak idea.

I replaced the screw hose with one that has a more positive seal. Took it for a short ride (just past where the throttle hang usually happens) and it ran beautifully. Just need to really test it to be sure.

In this case, I cannot blame the PO. I was away for the weekend when my father-in-law and the mysterious petcock fairies visited my bike. I found a shiny new manual petcock and a hose going nowhere with a screw hanging out. I was reassured that it was fine and that hose just needed to be plugged.

Just to get to the troubleshooting phase, I did enough work on the bike to feel confident and able to diagnose (with GSR help, of course) a host of issues that I hopefully won't have.

The o-rings are pretty new, they replaced the squished flat 25 year old ones. I bought a second set so they can be swapped any old time.

thanks again
I'm going riding. \\/
j

Glad it's running better.
Just for the record, Nessism first suggested an intake leak, before I even read this thread. Maybe I helped you some but give credit to whom it's due.

[KEITH KRAUSE]

Nessism, we all have our opinions so I don't consider it "going against me".
I'm not exactly sure what you describe in saying "aluminum manifold insert". The manifold-to-head surface is flat, at least the manifold I have in my mind, and it very often has a thin rubber film extending into the outer portions of this flat surface also. Some have more rubber on the face than others, some have none. They are commonly different. If your design in mind is different, then we're not talking about the same thing and everything changes.
Remember, it's just my belief that you can shorten the service life of the o-ring by flattening it more than what's needed to make a good seal. I can't actually see the o-ring compress and see what happens. I think approx' 6 ft/lb is a little beyond the needed torque but then the o-ring will continue to seal even with some "settling" of the parts resulting in a little less torque over the years.
I've seen many o-rings over the years and some removed ones are obviously flatter than others. Most people over-tighten hardware in general, especially hardware that should be torqued at under 10 ft/lb. It seems to me that if the o-ring stops compressing before the manifold bolts are adequately torqued, then every used o-ring would be equally flattened regardless of how much torque was applied. I really doubt that any difference is from under-torquing. An o-ring with a "built in compression limit" would always flatten the same. So I believe that the o-rings that are flatter (and always cracked more) are torqued more than necessary.
Also, I believe the largest manifold bolts are 6mm and they are rated "standard" bolts. The factory recommended torque for 6 mm standard bolts is actually 5 ft/lb max'. If I'm wrong and the largest bolts go up to 8mm, then the factory torque range is 6-10 ft/lb for 8 mm bolts only. In many manuals, they don't specifically say how much to torque these manifold bolts so you have to read the chart.
I actually didn't come up with the "6 ft/lb" torque recommendation myself. It was many years ago that I read an article that specifically mentioned how to torque manifolds and head gaskets. On our style of manifold, it recommended 6 ft/lb. That doesn't mean it has to be exactly 6 but 6 was considered "just right". I no longer remember where I read the article and I don't always believe what I read, but I think it was right and I've always used the method and had no problems.
It's just my opinion. I'm not always right either. I only suggest things if they've worked for me. If someone wants to go beyond the torque I suggest, or the torque the bolt is rated for, then it's their bike.