Could be a leak in intakes. Squirt some carb fluid and see if rpm goes up.

Posted by: @odontologist

Could be a leak in intakes. Squirt some carb fluid and see if rpm goes up.

• True, and that leak could be at multiple places. Old rubber intake hoses will have microscopic cracks, or may be so hard they don't seal when clamped. Some one could have left the test vacuum ports open, or if your bike has the "balance hose" between vacuum ports, the hose could be unplugged or old/ cracked. Message: replace all your rubber parts on and in the carbs, if you haven't already.

• Loose cyl head spigot. The spigots the intake hose clamps to is a short, screw-in part and they will loosen, allowing an air leak to develop. They have a cutout you can fit a bar (or screwdriver shank) inside and use to tighten. Removing the spigot and adding a gasoline-proof sealer to the threads before tightening always helps. 

• Tight throttle cables. At idle there must be at least 1mm of slack in ALL the outer cable sheaths. (This so as to insure that the idle speed is being set by the Idle Speed Screw and draw down spring, and nothing else.) Since you probably have the 1-into-2 throttle cables, you'll need to check this at the grip and both carbs. Older 2-cable systems could be more easily checked at the grip only.

• Air leak at the Idle Mixture Screw(s). Replace both the #60 o-rings on the Idle Mixture Screws. This is very common and should probably be done once a year, just on general principle. All it takes is a tiny nick, scrape or cut to render the o-ring useless.

Lets see how these first steps go.

Hope this helps.

Thanks for the pointers- did the carb cleaner check, and no increase in idle- I'll replace the idle mixture screw o rings and see if that helps.  When I got the bike, I replaced throttle cables and Bowden cable, then adjusted idle mixture and balanced carbs.  I am reasonably sure the carbs have never been rebuilt, so that is next on the list.  Planning on full overhaul, as well as a rejet (bike came from sea level and now lives at almost 5,000 feet).

I think you'll most likely find that your new throttle cables are installed incorrectly.

Either there is no slack for 1 or more of the 3 outer sheaths, or that (in an effort to tidy things up) plastic cable ties were used to "dress" the throttle cables under the tank or around the handlebars. In no situation should cable ties be used to dress cables, unless the ties are loose enough that a finger can also be placed through the formed loop when finished.

Tightly tied plastic cable ties is a big no-no, especially on throttle cables.

No cable ties used (I digested Snowbum's thoughts on the subject before I did the swap), and have ~2mm of slack on both cables.  GSPD has 2 cables that go all the way to the throttle grip.  Swapping out the idle mixture screw o ring tomorrow to see if that helps.

Two down !

I have gone through and done the above:

- Valves

- Carb sync

- Confirm no ties on throttle cables

- Replace idle adj screw O ring

- Confirm cylinder head spigots tight

- 1-2 mm end play on accelerator cables.

I have also:

- Cleaned float bowls

- Removed car tops and confirmed sliders are clean and needles not grooved.  I am going to take back earlier statement that Bings have never been rebuilt.  Almost no varnish, so someone has been in there before.  Diaphrams in good shape.

- Canted carbs in toward motor.  They were at right angles to ground which made cables kink a bit at carb.

Still have same issue- it so feels like an ignition issue, but with an Alpha system, that seems impossible (confirmed by Rick- Alphas work or they don't).  Idle set to about 1150, which bike reliably returns to until it gets hot.  They ~1500-1700.  But I can turn it off, and then turn it right back on, and it will idle at 1150.  May also be RPM related, as if it's hot and I don't go over 3250 or so, idle is ok.  Over 4k, and it will idle high.

Have full carb rebuild kit OTW.  Could this be altitude related?  Bike was at sea level and now resides at a bit less than 5k feet.

Any insights appreciatesd

Gary -

Sorry to hear that your issue persists. More thoughts...

• You never commented on the vacuum ports.

• Did you replace the inlet hoses ?

• Not sure why 'clean carbs' need a rebuild kit, unless it's to replace the throttle valve o-rings. Your thoughts, please.

• The reason the carb needles may need replacing can't be seen. It's a size issue, not a color or cleanliness issue.

• Do the throttle cables retain their slack clearance when the bike is hot ?

• What happens if you push down on the Idle Speed Screw when it's doing the fast idle thing ?

• What is the condition of the air filter ?

• What happens if you simply readjust the idle speed down to ~1000 RPM when the engine is doing the "high idle" thing ?

• You replaced the Idle Mixture Screw o-ring, but what setting did you install the screw to upon reinsertion ?

Hope this helps.

Richard- appreciate your ongoing help!

- Vacuum ports are still attached to hoses, which I replaced.  I do have the screws and washers OTW to replace.

- Did not replace the inlet hoses as they are very pliable and spraying carb cleaner on/around them does not increase engine speed

- I already had the rebuild kit on the way; not sure if I'll use it right now having seen condition of carbs.  Did have the thought that the one of the choke gaskets might have collapsed.

- I'll get a set of carb needles and seats OTW

- Cables retain slack clearance when hot

- Have not pushed idle speed screw when hot- will check that today

- Air filter is clean and in good condition

- Have not tried to adjust idle screw when doing fast idle.  My rationale is that since bike returns to correct idle up on shutting off then immediately turning back on it would simply idle way too low.  Having said that, it is most assuredly worth a try.

- Installed to all the way in, back out 3/4 turn, then adjusted to leanest clean idle.

Thanks!

Posted by: @grcrowell

- Installed to all the way in, back out 3/4 turn, then adjusted to leanest clean idle.

 Thanks!

I have several manuals, all of which give different settings, but they are all between 1 to 1-1/2 turns out. I set mine at 1-1/2 and leave it. If you want to adjust it from the initial setting, then go for the highest idle speed (as set by only adjusting the mixture screw). Highest idle speed is the most efficient mixture at idle. We don't care if it's lean or rich, what we're after is the best idle... which naturally happens at the most efficient (or "best") setting.

We call it a mixture screw, but I really believe it's a volume of mixture screw (the English call it a "pilot") because the "mixture" itself is set by the #45 idle jet. So you're not "leaning out" anything, you're only starving the engine of fuel and air.

We might be on to something here. Diagnosing engines without sound or pictures is really difficult. Sorry to give you 92 things with each response. Just set both screws to 1-1/2 turns out and go for a ride. It can't hurt.

Hope this helps.

Interesting.  I was following William @ boxer2valves guidance- kinda like I set idle on a Stromberg (long time British car/bike guy)  I’ll try 1 1/2 turns out and see how it goes.

I can post pics/video if this does not help.  First airhead (but not first air cooled engine), so I’m really don’t know how she is supposed to sound/feel.

1 1/2 turns is definitely way too rich, as was 1 turn.  I set both to 3/4 turn and set idle screw to 1050.   Same result- link to bike idling post ride here-> https://vimeo.com/466404786

The fact the bike will immediately idle correctly after a shut off is confounding.

Thanks- Gary

I had exactly the same problem on some mikuni carbs on a honda with a very similar transverse engine but water cooled. 1100rpm normal shooting to 2000 when hot, then came down again just as you describe. Same diagnostics as you have gone through with same results...nothing. Carbs were clean and balanced, no detectable leaks anywhere, but when It was idling high I could feel more pressure from one exhaust than the other on the palm of my hand about 2" away. So I started to adjust pilot on the high pressure side whilst idling high until rpm dropped, then adjusted the idle to set rpm to desired value. It required a bit of tweeking on both sides but I got rid of it. In the end, all I needed was my hand and a tacho. I'm  not knowleadgable on these machines but thought I'd pitch in as you seem to be running out of options.

And of course if the above is bad advice, please disregard.

@odontologist

Disconnect the battery and open your front engine cover. Does this ignition incorporate the OEM mechanical ignition advance mechanism ? If so, then your advance mechanism needs 1) lubrication, and 2) new return springs.

The springs are not strong enough to overcome the friction of the advance mechanism. The ignition timing simply doesn't auto "retard" at low speed. Thus the ignition remains stuck in the "advanced" timing position at low RPM, which results in a higher idle speed.

I'm 98% sure that's it. Thanks for the video. BTW... I think your idle is still too high.