RPM Stays High After Riding for a While

[TurcoLoco]

OK, I finally got the Max running again and he does sound much stronger and powerful than it ever did however I am facing an odd issue that I cannot quite tell what could it be caused by: After the bike warms up, it idles and runs really well but sometimes, the RPM will hover around 2K-3K range even in neutral. Sometimes, it comes down by itself after several seconds, sometimes doesn't. The throttle cable is properly adjusted (it snaps when let go), fully closed, the cable is a tad loose so there is no tension on it when fully closed as it rests on the idle screw which is almost at the bottom.
Choke is fully closed and idle screw is at its usual spot which is pretty close to the bottom. If I lower the screw even further than the idle will drop causing the bike to die. It also makes starting without choke almost impossible.
There is no air leak that I can hear, no smoke. Brand new oil, filter, diaphragms. Carbs recently cleaned, new gaskets, rings, etc. I will be doing the carb sync as it might be slightly off but I know that won't cause my current issue so any ideas? :confused2:

Please let me know if I am missing any info.

Thanks guys! unk:

 

[texas-ss-tornado]

Sounds like a vaccuum leak of some kind, or maybe the carbs are out of sync? I just fought a similar issue with one of my VMAX's, and it turned out to be a clogged slow jet. Run a can of "start your engines" through it and see if that helps!

 

[tmosel]

Sounds like a vaccuum leak of some kind, or maybe the carbs are out of sync? I just fought a similar issue with one of my VMAX's, and it turned out to be a clogged slow jet. Run a can of "start your engines" through it and see if that helps!

+1

I'd start with looking for a leak around the carbs. If you haven't synced the carbs in awhile it can cause that same issue.

 

[Diablotin]

I had that issue and it was an air leak with the carb boots.
I would check them and look for cracks. If nothing, make sure the carbs are sitting properly.

Sent from my GT-I9100 using Tapatalk 2

 

[Rusty McNeil]

+1 on a vacuum leak check. Spraying ether at the boots and joints wil check this.

Would also verify, while its high idling, pushing on the linkage to see if it's for sure resting on the throttle stop screw.

Also verify that the slides aren't hanging up you can do this by poking a screwdriver down in the carb and pushing them open and then releasing, they Should be free. Also the needle should have some slight free wiggle so it can self center when it goes into its hole. The needle not aligning correctly can be caused by incorrect assembly of the slide/needle assembly. if its stiffly mounted in the slide it will drag Uand hang up.

Your synch would have to be way the Hell off to cause this but it is possible

 

[wildweasel_pt]

Stuck choke linkage?

 

[TurcoLoco]

Thanks for the ideas guys. To my best knowledge:
- Choke works properly, I checked it multiple times it was closing fully.
- Air leak of some kind is what I was thinking but the boots are really in good condition and seated all the way down, flush and tight on the carb side and I do believe same for the air box but I am not 100%. I will check for a leak with a can of compressed air when I get home.
- Carbs probably do need to be synced but I don't believe they are way off.
- Clogged slow (pilot?) jet? Well, I cleaned and rebuilt all the carbs, new gaskets, rings and even diaphragms and everything looked clean and good.
It did sit about a month since the clean up due to spark issue it was having but I had stabil in the fresh gas, also I did put couple of more gallons of fresh gas at the end of first test ride day where this problem never surfaced!

I actually noticed the slight looseness of the slide needles and I do believe when I put them back together, I did leave a similar slack but I will check again.

It is really hard to do some of the troubleshooting because it is really random.
It surfaces after riding for 10+ minutes while I am on the bike and away from home and most of the time, after 5-10 seconds, the idle goes down by itself so even if I can manage to pull over the RPM is back to normal by then. :damn angry:

This morning though, I rode it to work and it actually stayed on but I didn't have time to play with it. If it happens on the way back then I might be able to see what the culprit is (hopefully).

Again, thank you all. If there are more suggestions you can come up with after the additional info, please do share. I will check on the possibilities that you mentioned to see if I can zero in on it.

unk:

Kudos to Sean for his awesome muscle seat, it feels good and looks even better!
Also, Gannon's COP mod is really nice. The bike does run like a bull on crack now, it kinda scares me half the time.

 

[ninjaneer]

what do you have the idle set at? 1K? after it warms up, with no choke, set the idle so that the top of the indicator needle barely touches the bottom edge of the 1K mark. give the throttle a couple of blips and adjust if necessary. run around town and see what happens.

 

[TurcoLoco]

what do you have the idle set at? 1K? after it warms up, with no choke, set the idle so that the top of the indicator needle barely touches the bottom edge of the 1K mark. give the throttle a couple of blips and adjust if necessary. run around town and see what happens.

The way the idle screw is currently set up, if the bike is cold, I do have to open the choke a bit, like 1/4 to 1/3 for about a minute or so, then I can close the choke completely and it idles right around 1K. I let it run for about 3-5 minutes to make sure it is totally warmed up and the temp gauge confirms. Then, I rev it a few times and it goes back down 1K as it should. I get on the bike and start riding and it runs strong, RPM defaulting back to 1K at neutral or stops. Then about 10+ minutes of riding, the issue surfaces where if I hold in the clutch lever or let the throttle off, the RPM remains high like the throttle is still rolled on even though it is not. I actually rolled the throttle on a few times after it occurred to see if it would somehow "snap" out of it but it didn't help in both times I tried it. :ummm:

I will sync the carbs tonight but I don't believe that is the cause.

 

[Dr.n2o]

I'm with Rusty. Air leak or stuck needle

 

[leroycp]

Drain the bowls again and run about 1/3 can of seafoam.

 

[TurcoLoco]

I actually did put 1/4 or may be 1/3 can of seafoam (good stuff) in it the last time I filled the tank.

Anyhow, despite being under the weather a bit, I bust out the CarbTune (I love this awesome tool) and checked the sync and they were off, I mean really off.
Not sure how it became that bad since I synced them like 6 months ago but lesson learned:

If you worked on the carbs and disassembled them, sync the carbs afterward, period. :bang head:

Still not 100% certain if that was the real cause but I will test ride it later this week to see if RPM is still acting up.

Muchas Gracias Amigos! :clapping: unk:

 

[Rusty McNeil]

Glad you may have found it. Keep us updated. we could all learn from it.

If you want to check for leaks I'm not sure compressed air will do much, never tried it myself. I though something flammable was required.

An air leak upstream from the carbs isn't going to display much symptoms I don't believe.

 

[naughtyG]

Yup - sync is very important on Max. I believe the Service Manual (or Owner's?) recommends checking it every 3 months. Whenever you disassemble the carbs though, you must sync afterwards. I always did it regularly on mine, just as an added precaution. It's so quick and easy to do, and always well worth it!

 

[TurcoLoco]

Man, I was so hopeful that the carb sync was the fix but no dice. I rode the Max to work today and again, after few miles of city riding, the dreaded high RPM demon came back! :damn angry: :bang head:

Since I replaced the handle bar, I was thinking there might be a cable binding issue so at a red light, while the RPM was kicking, I reached down and looked at the throttle cable and it appeared to be sitting on the idle screw so it didn't seem like a cable binding issue. Playing with the idle screw seems to help a bit but I know the idle screw was set correctly after the bike was already warmed up. I also adjusted the throttle free play after adjusting the idle screw as the service manual advised so I am a bit lost as to what to check next. :ummm:

Slides or ....?

 

[gamorg02]

can u check the synch while its high rpm?

 

[TurcoLoco]

can u check the synch while its high rpm?

Isn't the carb sync supposed to be done on idle? The problem is the high RPM issue doesn't kick in unless I have been riding (hitting at least the 3rd gear) then decelerating.
If this occurred on the way hone (and it likely will), I was going to pop the faux tank cover, then open the air box cover to check the slides but I'd hate to let the bike run at 3K-4K RPM while I am doing all that.

Other than this annoying thing, the bike runs really strong.

Why do I always get the weird sh.t? :bang head:

 

[sdt354]

Like Fred said, make sure all Choke/fuel enrichers are shutting off. This would be less noticable
when cold, if they're the problem.
Steve

 

[TurcoLoco]

Like Fred said, make sure all Choke/fuel enrichers are shutting off. This would be less noticable
when cold, if they're the problem.
Steve

I actually stepped out and started the bike and after it warmed up quite a bit, I checked the choke, enrichers (I guess they are those little piston looking thing they get pulled out when choke is operated?) and throttle linkage and all appeared to function as designed. Also, I do shut off the choke completely by the time I am ready to ride. If a leak of some kind is the culprit then it is not detectable or visible on from the outside.

I guess I am going to ride after taking the screws from the top of the air box so if it happens again, checking the slides will be possible.
This thing is going to drive me nuts if I cannot find the cause soon.

Thanks again for all the help guys, much appreciated!
unk:

 

[Fire-medic]

So, did you use some starting fluid and check for a vacuum leak by spraying around the boots above & below the carbs, & the VBoost manifolds to the heads? The high idle has to be happening when you spray. If the high idle goes down where you're spraying, you found one of your leaks. Keep spraying until you find them all. Maybe it's only one you have to fix. Did you check the rubber vacuum plugs on the intake manifolds, the synch ones?