Throttle Sticking?

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acotrel said:
If you have a blockage in the slow running jets, you might have the throttle stop wound up too high in order to get the motor to idle. The slide should hit bottom with all the slow running jets working. If it doesn't do that the idle might be irregular or unreliable. I don't think your problem has anything to do with the needle position. That usually only does it's thing above a quarter throttle. Below that, it is the slow running jets which work. When you wind the idle mixture screw in, if it is a fuel adjustment, you should be able to cause the motor to miss - then back it off slightly so it idles smoothly. If the slow running jets are blocked, you probably won't get that response.
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Mixture adjustment does have the effect you describe...starting out as 1.5 turns from fully in, I can wind it out (leaner) and get changes in idle until it gets lumpy. Can also wind it in 1/2-3/4 turn from 1.5 out and get rougher idle. So seems like idle circuit is flowing.

So to summarize, engine rpm stays high from immediate throttle slide closing for 2-3 seconds. Confirmed slide is moving freely & fully closing with the grip twist...not a cable hang up.
For rpm to remain high, does that mean too much air being sucked in? Mani to head bolts are good and tight (were locktited in last season). Carb to mani sealed with fresh permatex grey today. Could there be leakage around the slide, like with a worn out bore? Could the choke slide have a role there? What about possible timing issue...say the EI stator plate has become loose and rotated to incorrect position...would that account for what's happening?
 
this is brain tease, if it was good before , should be repeatable ,right ? .... have you made certain ends of cables are seated completely as they were , can you mount cable exactly the same route as previous when it was working ? ..... got to be a simple solution
 
Had not purposely changed cable routing since last season before today. Between the last run last season, all working fine, and first few runs of this season (also running fine, maybe on the rough side; I'd refreshed the valve springs with head still on), carb was never opened/adjusted. Grip not apart any time. Was running pretty much the same last week...a bit rough but not sticking at high revs when closing grip. That just happened this week as I was prepping to go for a ride. The day before, I was checking for air leaks at mani & carb to see if the rough running cause could be identified. As reported in another thread, I got a bit heavy with the spray I was using and the air filter caught on fire briefly. Replaced it with an older one I'd received with some other used parts...unknown history but looks OK. The very next day, 1st start after filter replaced and found the high revving issue. Thought maybe the cables had been jostled into a poor position during the hamcan R&R. But with todays revamping of the cable routing, less severe curve right out of the carb top etc, all seems freely moving.
 
With cables proven good, I guess a closer look at the replacement filter for air leaks would be good, given the timing of when the . On another less likely track, if you are fortunate enough to have access to a strobe light it might point to weak or partly hung up advance unit return springs. Thanks for continuing to post the adventure!
 
Atlas Commando said:
With cables proven good, I guess a closer look at the replacement filter for air leaks would be good, given the timing of when the . On another less likely track, if you are fortunate enough to have access to a strobe light it might point to weak or partly hung up advance unit return springs. Thanks for continuing to post the adventure!
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Could try it with filter removed...I'm waiting on a replacement (K&N) but not here until next week.
Bike has Electronic Ignition, so no pesky AA unit springs to fuss on. No timing light handy...would like to avoid having to go through the static timing steps again until other things eliminated.
 
Sounds like you have not found the sweet spot for idle. Need to start at about 1500rp with idle screw, then adjust air screw for optimum. Adjust idle down to 1200 adjust air screw for optimum, down to 900 and repeat. Final adjustment is about 1/8th turn in with air screw, (I have seen recommended to slightly richen idle circuit).You could leave this until after a quick ride to see if any popping on the overrun. All on warm engine of course. Once dialled in should be responsive to throttle and idle steady. If you have done this and I am stating the bleeding obvious please ignore.
 
What air filter?
I’ve had a clogged air passage, (hay seed) causing a grossly off mixture adjustment trying to compensate, & hang throttle ensued. Just a possibility.
By back blasting the air passage, (with an aerosol can of WD40 (all I had in the field), I found the offending impurity.
 
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Last to responses are useful. will check them. Air filter looks very much like the Emgo I had on previously, but element looks a bit darker/dingier. Could be stained or just different brand.

If no joy after the above checks, will try running without the filter, then recheck valve lash, check torque on headbolts, check static timing. All acts of desperation, no?
 
concours said:
What air filter?
I’ve had a clogged air passage, (hay seed) causing a grossly off mixture adjustment trying to compensate, & hang throttle ensued. Just a possibility.
By back blasting the air passage, (with an aerosol can of WD40 (all I had in the field), I found the offending impurity.
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What HE said !
 
There is another possibility. With a single carb, you have much more vacuum acting in the slide. Your spring might not be strong enough. And if you have been using the single carb for a long time, there might be a ridge or step worn into the bore in which the slide operates.
 
acotrel said:
There is another possibility. With a single carb, you have much more vacuum acting in the slide. Your spring might not be strong enough. And if you have been using the single carb for a long time, there might be a ridge or step worn into the bore in which the slide operates.
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Ditto. I've heard of this vacuum theory as well. In fact I have the same issue and cured it by either running a stronger spring or doubling up the current one. Can't remember off hand. Also mine is a 750 with a single Mikuni.
 
Yes I think I may have doubled up on the spring for my single VM34 as well ... kinda forgot that , was a couple of whiles ago ....
 
She's Back in Business Folks!
Took carb fully apart and went at the passages and jets. Found a fair amount of brown tar build up along slide bore, slide sides and down in the milled groove where slide edge seats down at fully closed. Took a good poke with bendy wire on the fuel pickup opening bottom side of body...have heard that's a very problematic passage...not sure I made much difference. Did get tiny jets spraying together and could get reverse spray into those tiny holes to come out the pickup hole.
Reinstalled and tried a run without airfilter...fired up and no blip hang. Put filter back on...still no blip hanging. Seems to be cured!
I'm out on a test ride now.

Thanks for all the tips and suggestions.
 
And test ride was pretty good. No stalls nor repeat of the dreaded rpm hanging. It was a little hesistant when coming just off idle if I was being very slow with the twisting. Likely needs more dialing in.

Question for the masses: On the Amal Premiers, does cleaning that fuel pickup passge get any easier? I know the idle jet can be taken apart, but what about that passage with the 90 degree bend for fuel deliver to idle circuit? On the MK!'s would need to pop out the blanking plug at the carb face to do an effective job..then figure a way to re-seal.
 
Good result - I hope my comments helped you. Many years ago, we fitted Mikuni carbs to an XS650 Yamaha speedway sidecar outfit. It did not matter what we did, we could not stop the throttles from hanging, due to the vacuum and wear in the carbs. In the end we had dual springs which made the throttle operation far too heavy. I've never had problems with the slow-running jets because I use methanol fuel with an aluminium tank. But even with that, I always need to take care that I don't leave the bike standing with fuel in the carbs.
When I had my Suzuki T250 racer, I forgot to drain the carbs after using methanol fuel. The next time I rode it, it starved and seized . That is how I found out how much more power methanol fuel gives when you run it lean. The problem with jetting two-strokes for methanol is you cannot drill the holes in the main jets smaller - only bigger. So you take a stab in the dark and drill them big enough to avoid seizing the motor. And getting the needle jets right is almost impossible, you keep getting new ones and drilling them, until the motor starts.
Petrol is twice as difficult. With a Commando, Norton give you a starting point for the jetting. Without that, you would be in real trouble.
 
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I did check slide movement while pushing against it with a finger...it took a fair pressure before slide closing via spring as affected. So concluded it was likely good.

If I choose to pull out blanking plug on idle circuit pickup passage, what's a good way to seal it again? Thread a bolt in? Epoxy or sealant an unthreadwd bolt?
 
Throttle Sticking?

Gotta be one of these.
You might be able to find something in a small engine shop if you can find the right size and shape.
 
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