Throttle Sticking?

[Tornado]

New issue today. Amal mkI single on 850 Commando. Ran fine last week. Today after first kick started up normal. Idles normal. Went to blip throttle and found rpm stay high, slowly dropping well after grip fully returned. Did this over and over. Tried lubing throttle roller. Checked that slide can be felt banging down into idle adjuster screw when grip released. All seems normal. Yet opening throttle from idle seems to run rpm up quite fast, stays there momentarily then falls over 2-3 seconds after grip rolled closed.

What to check next?

 

[acotrel]

If you take the throttle cable off the bike and hold it up vertical, the inner cable should just fall freely to the bottom of it's travel If the outer is kinked, it can cause the inner to drag or stick. You should not have to pull the inner to get it to move. Japanese cables are usually nylon lined and have lots of internal clearance. The other thing to check is the internal bore of the carb, it can wear so there is a ridge low down, also it can get gummy if there is something strange about the petrol or oil you use.
Incidentally, the idle adjustment screw you mentioned is the throttle stop. The idle adjustment is the screw which adjusts the fuel mixture. You should be able to have the slide hit bottom with the stop wound out and still get idle. All the stop does is make the idle faster. Sometimes that is necessary, if the bike refuses to open up cleanly from idling as you wind the throttle on.

 

[Tornado]

THx...yes, got the idle and throttle stop mixed up, it is the the T.stop that I can feel getting hit by the slide when I release the grip...and it is immediate, not delayed...so that tells me the slide is returning to its lowest position normally...yet idle lingers at higher rpm as described.
Is this indicating an air leak somewhere? Since last week, I have replaced the air filter (with an old one of unknown history until I recieve a new one) and did put a little more torque on the carb to manifold nuts to try to help with a rougher than normal idle. I've since loosened them again to solve this new issue but no effect.

 

[acadian]

Commando maintenance lesson #1 - Fix 1 thing at a time. Check. Then move on...

If the last thing you did was remove/refit the carb, then go back and check that the flange is not warped (air leak). After you've checked this, then may try repeating the blip test with the air filter removed to ensure the slide is returning properly.

 

[ashman]

Isn't owning a Norton great always something to do, a well sorted Norton will give you trouble free use for years but when you buy one without experience or not knowing its history things can be frustrating and its normally the little things that give problems to, another thing to look at is to make sure your throttle grip in not hard up against the cable housing, sometimes that causes slow return, so look at grip, cable, slide and if they are find then look inside the carbie, needle, jet etc etc till you find the problem.
I once replaced new carbie cables and when I fired it up the bike kept reving to high, after sometime investergating found my local british bike shop gave me cables for a Triumph and not for the Norton they were a tiny bit shorter, as Al has said make sure your cables are running free in the outer cable and no tight spots or kinks.

Ashley

 

[CanukNortonNut]

It doesn't take much to distort an Amal from over tightening. Some say just enough turn on the nut to see the split lock washer go flat under it.Then go no further.

Cheers,
Thomas

 

[Paul W.]

Have you chucked the o-ring in favor of a gasket on the carb? The o-rings help warp the carb body. I ran into that with my old Bonnie.

 

[Tornado]

It doesn't take much to distort an Amal from over tightening. Some say just enough turn on the nut to see the split lock washer go flat under it.Then go no further.

Cheers,
Thomas

Lock washer you say? Ain't none there (as it came from DPO)!
I'll pull the air cleaner off, check to see needle is still clipped in/moving as expected etc. Will try a test run without filter. If still no joy, carbie coming off for more thorough review....

 

[Tornado]

Have you chucked the o-ring in favor of a gasket on the carb? The o-rings help warp the carb body. I ran into that with my old Bonnie.

Yes actually...I removed o-ring, placed a phenolic carb to mani and two more mani to head last season to solve the carb overheating issue (which did that nicely). Used permatex grey gasket sealant to keep air tight. She'd overheat within 10-15 minutes after shutdown....too hot to touch float bowl and then found the slide jamming at full open when using throttle (engine not running but when still hot). Since this has not been reported by others, I suspect it has something to do with the 1-2 manifold passing more heat than two single manifolds. Anyway, that is not todays issue...carb not hot at all during these fire ups.

 

[CanukNortonNut]

Lock washer you say? Ain't none there (as it came from DPO)!
I'll pull the air cleaner off, check to see needle is still clipped in/moving as expected etc. Will try a test run without filter. If still no joy, carbie coming off for more thorough review....

Tornado,
Ok Sorry to trouble you. I now see you have a single carb setup. I was assuming that you had a twin amal carb. My mistake to interfere.
Best of luck sorting your issues. I will leave others that have this manifold system to help.
Cheers,
Tom

 

[concours]

A “hang throttle” as you describe is not always an actual throttle staying partly open.
Did you change needle position?

 

[Tornado]

A “hang throttle” as you describe is not always an actual throttle staying partly open.
Did you change needle position?

Needle is as it was since last season, middle position.

 

[Tornado]

Tornado,
Ok Sorry to trouble you. I now see you have a single carb setup. I was assuming that you had a twin amal carb. My mistake to interfere.
Best of luck sorting your issues. I will leave others that have this manifold system to help.
Cheers,
Tom

No trouble....I really think the single carb mani should have lock washers where the carb nuts go on...makes perfect sense...but it never had any...so previous owner didn't run them. I've got a bit of locktite to help hold them without using much torque...but will get some lock washers. Thanks for pointing out the omission.

 

[concours]

Re-visit the manifold & carb gaskets.

 

[Torontonian]

Lock washer you say? Ain't none there (as it came from DPO)!
I'll pull the air cleaner off, check to see needle is still clipped in/moving as expected etc. Will try a test run without filter. If still no joy, carbie coming off for more thorough review....

My notes say the carb. to manifold nuts should be tightened up no more than 4 ft. lbs. That's light.

 

[Steves]

Probably over tightened the two screws holding the carb top. Very easy to do, just need snubbing up gently on lock washers. Easy to demonstrate to yourself by playing around with how tight you make them up. Applies to all fastners on or about the carb.

 

[norton73]

A couple things come to mind.

Do you still have the points with the centrifugal advance? Stretched or weak springs can keep the ignition advanced and cause a fast idle.

If you have a fiberglass fuel tank, ethanol in the fuel will dissolve the 'glass and cause the slide to hang. My single Mikuni did that and it was the first indication that my tank was failing years ago.

Vacuum leak?

 

[Tornado]

Wassell Vape EI since early last season. Fires up first kick 90% of time this season. Tank is steel. I cleaned, converted light rust using Metal Rescue over winter.

Before I took carb and tank off, I checked slide was moving fine with throttle twist by feeling directly with finger in the throat. Needle also moving with it. Noted the slide posituon...fully down on stop screw. Turning screw out, by maybe 1 turn, slide no longer in contact. But, stop screw seemed a long way in...holding slide up at a fair gap. And just stayed hanging on cable as screw turned out. Took it out (maybe 10-20 turns to remove). Same slide position. Took carb off mani, and slide now seems full lowered...I think the tight bend of cables as the meet frame might be the culprit. Recall the single carb mani places cable exit almost on centerline, so frame interfence. Is an issue.
I've rerouted cable...running more vertical out of top, so it angles slightly up the side of frame spine...then a more gentle turn forward. Used a zip tie to secure the bend via strapping around battery box bulkhead. Also took cable top of headsteady instead of through it...didn't like the bend and abrasion to cable casing on sharp headsteady edge.

After reconnecting twist grip, I'm still getting a small bit of throttle slide movement as I throw bars lock to lock. Can't seem to get rid of that no matter where I route cable or how much slack I give it. Guess some movement is normal?

Taking break before buttoning up and trying a start.

 

[Tornado]

Welp...no joy! Same issue...blip throttle from 1k...it jumps up 2-3k and hangs there for 2-3 second after grip fully rolled back. Also seems way over sensitive to very slight throttle turn...jumping up 2-3k with only a small slow turn on grip.

Not sure where to look next.

 

[acotrel]

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.