RPM holding high, Can't figure out what is wrong for the life of me.

[DrMengleYa]

Whatsup guys,
I'm really new to the XS (i usually work on Harleys) community and hoping you guys will be of some help. My buddy just bought a 77' XS bobber with a 1 into 2 Mikuni carburetor swap and i cant get the damn thing to run right for him. upon riding the bike it holds the revs super high and will keep revving until I blip the throttle super hard and it calms down. when the choke is in the on position it seems to run okay, but tumbles at low rpm with load on the engine and has an issue idling. When i have the choke in the off position it idles beautifully but when riding or opening the throttle, the engine will get caught at high rpms until i blip the throttle hard or shut it off. we have a bunch of different jets that we have been swapping out for the past 8 hours trying to rectify it (the guys that sold us the bike said it would probably need to be rejetted), but nothing i do seems to get it right. we have tried dozens of combinations (main, pilot and needle jet) and to no avail it will still hold the rpms. one combo did seem to make it not as bad but still wasnt running perfectly. I thought I was a good mechanic but today has proven otherwise. for the love of God help me.

 

[fredintoon]

Hi Doc and welcome,
the carb jets most likely ain't the right ones but that won't make the carb stay open.
Look at the throttle linkage, the throttle return spring and the diaphragm instead.
And if all else fails put an old-fashioned non-diaphragm carb on it.

 

[TwoManyXS1Bs]

There's so many variations in the customs that it would take a book of guesses without knowing the specifics of the bike's setup.

But, my first guesses would involve sticking carb slide and ignition timing. Perhaps too advanced with worn advancer and weak springs....

 

[chizler62]

A hung side whether from linkage or cable springs to mind as the others suggest. But, being you're experienced I'd assume you're past that.

Beyond that. I'd pop the nozzle out. Depending on model carb it maybe installed backwards. Yup, even with the tit. Make sure its not worn and seals carb well. I seen an 87 GSXR 1100 with it done and RPM would hang.

Also I've seen where the o'ring on the choke plunger was worn or rotten and fuel would get thru.

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[gggGary]

If it still has the mechanical advance, I'd be looking there for sure.
Weak, broken advance springs, advance rod not turning freely inside the cam.

When one of the nylon points fingers isn't on the upslope of the points cam you should be able to pullout on the advance weight and it should smartly return to the retracted position.

Yes pics please, tell us what exhaust, air filters, and carb model, we can probably give you some ideas on baseline jets.

Please follow up and let us know what the problem was.

 

[gggGary]

If it still has the mechanical advance, I'd be looking there for sure.
Weak, broken advance springs, advance rod not turning freely inside the cam.

When one of the nylon points fingers isn't on the upslope of the points cam you should be able to pullout on the advance weight and it should smartly return to the retracted position.
Slap on the timing light and watch for correct advance/retard action.

If it is the advance, (or not) seriously consider going pamco with electronic advance.

Yes pics please, tell us what exhaust, air filters, and carb model, we can probably give you some ideas on baseline jets.

Please follow up and let us know what the problem was.

 

[TwoManyXS1Bs]

Some threads covering the advance unit:

http://www.xs650.com/forum/showthread.php?t=24080
http://www.xs650.com/forum/showthread.php?t=23028
http://www.xs650.com/forum/showpost.php?p=243539&postcount=2

 

[retiredgentleman]

High hovering idle causes:

-ATU sticking, not free to rotate,springs bad, etc.
-Air leaks around carb intake area
-carb butterflys not sychronized (only if you have 2 carbs)
-carb butterfly(s) not closing fully
-air/fuel mixture screws not set correctly
-BS38 carbs needle jet O-ring worn out, allowing extra unmetered fuel

or maybe a combination of all the above

 

[westonboege]

I am quite familiar with the 1into2 setups. I make 1into2 manifolds set up to run with the Mikuni Roundslide carbs.

If you put a roundslide on a 1into2 manifold, i have found that the return spring for the slide is not strong enough to make the slide close. You can get a stiffer spring from a bigger mm carb, or you can get a replacement spring and double up the springs on the carb.

I ran into this same problem when i first switched over to 1-2 manifold. Popped another spring in and it ran and idled perfect.

Let us know what it is... if you are running a cv carb then disregard this... haha

 

[DrMengleYa]

I am quite familiar with the 1into2 setups. I make 1into2 manifolds set up to run with the Mikuni Roundslide carbs.

If you put a roundslide on a 1into2 manifold, i have found that the return spring for the slide is not strong enough to make the slide close. You can get a stiffer spring from a bigger mm carb, or you can get a replacement spring and double up the springs on the carb.

I ran into this same problem when i first switched over to 1-2 manifold. Popped another spring in and it ran and idled perfect.

Let us know what it is... if you are running a cv carb then disregard this... haha

I do have a 1into2 setup, what I've found on my own research as well as with your input I believe the springs are part of if not all of the issue, we will be looking into that soon unfortunately one of the screws on the spring cover is badly stripped, will post updated info as soon as we get in there, for anyone who may have other ideas here are a few pictures of the carb, I'm not sure of which model or what the dimensions are at this point but I am the party in this project who doesn't know much, the person in the original post was the one who knows much more, just on my account. thanks for the replies and ideas so quickly and will keep you posted with any other progress or issues. If any other pictures may help just ask.

 

[I am Carbon]

There are a couple things brought up.
(-ATU sticking, not free to rotate,springs bad, etc.) has that been checked?
Do you even know what the ATU is?
It is advanced timing unit. here is how you test it. That being sad the springs could be weak.

(-Air leaks around carb intake area) has that been checked?
That's not one of the MikesXS carbs that has problems is it?
Good luck on figuring out the problem.

 

[chizler62]

That mukuni is subject to a poor fiting cap. loosen the cap a bit and give her a whirl

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