Way high idle with Nevada -98 after changing air filter

[s151669]

Changed air filter.

Now the thing idles at 3000rpm. Not good.

Spraying brake cleaner at things suggests that breathing system leaks here and there. But breather system is outside of the carburetor. Not like in vacuum side of the intake manifold. Basically this thing should run fine without any filter at all nevermind leaks in the breather. Or should it ?

Adding choke brings idle down -> lean condition.

Adjusting mixture screws brings idle down to 2000 rpm or so but then the screws are about to fall out.

No leaks at intake manifold.

What did I miss ? What is wrong with this thing ?

[rocker59]

Probably an air leak.

Check the hose connections between the airbox and throttle bodies, and between the throttle bodies and the engine.

In my experience, it has been a problem with the hose connecting the throttle body to the engine.

[s151669]

Probably an air leak.

Check the hose connections between the airbox and throttle bodies, and between the throttle bodies and the engine.

In my experience, it has been a problem with the hose connecting the throttle body to the engine.

Well, that is the most obvious culprit - yet it does not leak with brake cleaner test. Nor was it disturbed while changing the air filter.

And any air leak before carburettor should not cause any issues AFAIK.

[Tkelly]

Throttle cable hanging up?

[Dirk_S]

Throttle cable hanging up?

Related, but not necessarily the same—making sure the butterfly valve has the proper gap for idle?

[pehayes]

Throttle cable outer sheath end has popped out of its end support.

Patrick Hayes
Fremont CA

[s151669]

Throttle cable outer sheath end has popped out of its end support.

Patrick Hayes
Fremont CA

Not that either. It did happen though but no. Difference was that when cable had popped out of support, idle was high and it had power. Now it is just high but no power.

Dismantled carburettors as far as possible on bike: fuel filter, bottom plug, idle jet, main(?) jet. Rust everywhere, plugged holes. Cleaned everything, about to assemble, came here to look where the breather hoses should go. Seems not to matter too much, one of those was disconnected and the fitting was full of wet crud. Grreaat.

[guzzisteve]

Should be the 1st one in this group of manuals if you don't have a manual. The last carb model PHBH 30's, never came to USA.  Crank vent hoses go to back of airbox.

https://guzzitek.org/atelier/pb/750/750Nevada_Atelier_Compil.pdf

[pehayes]

Difference was that when cable had popped out of support, idle was high and it had power. Now it is just high but no power.

Ah, new information.
What kind of ignition does this thing have?  Does it use a mechanical or an electronic spark advancer?  If the former, then the advancer can get dirty and hang up.  Performance really falls off quickly if the spark can't adequately advance or retard.

Patrick Hayes
Fremont CA

[s151669]

And solved.

Usually when a problem like this appears after you do something - it is something that you did.

Now it turns out to be dirty carburettors. Which decided to act up same time. And of course - japanese carbs would not idle when jets are clogged. These Dellortos do the opposite. Quite a surprise :)

Cleaned jets with compressed air. Now it idles as it should - altough idle screws are 3.5 turns out when factory setting is something like 1.5 ... 2. Apparently there still might be some crud somewhere. No point fixing that before I fix the rusty tank. Previous owner had apparently stored it with tank half full...