Setting up single VM36 - revs stay high after closing throttle

[Olly]

Hi,

I have been reading xs650.com for a while, great forum, but this is my first post. I have an '81 XS650 engine in my Wasp sidecar outfit, and have just replaced the stock carbs with a single Mikuni VM36 and a 2-into-1 manifold.
The engine runs nicely at idle and at 1/2 to full throttle. The problem is that if I gently rev it (1/4 to 1/2 throttle) the engine revs high and stays at high revs even after closing the throttle. The revs only drop after a quick blip of the throttle, and then it returns to a nice steady tick over.
I think I've got the jetting about right, and it has got a good throttle return spring, and a new cable so definitely not sticking.
What else could be causing this?

 

[littlebill31]

Make sure your throttle cable is clean and not bent all around as it makes it's path to the carbs. Also the throttle itself isn't dirty.
Watch the slide when this happens to see if it's staying open.
Does sound like something is dragging a bit and the blip slaps the spring into action.

Could also be a lean condition, if the throttle is returning, but the engine rpm stays high.

...

 

[retiredgentleman]

Most likely you have air leakage around the carbs or carb manifolds.

 

[Olly]

Thanks both for the ideas, I'll check the throttle slide first, as I read on a Norton forum that they can be sticky on the VM36 carb, and then check for air leakage. and will let you know what I find...

 

[plowboy1]

I had the same problem with my 36mm VM and 2-1 intake. Turned out the return spring was weak. I wound another mikuni 36mm spring to the original and been fine ever since. Seems like I read somewhere that an Amal return spring is stiffer and takes care of the problem.

 

[littlebill31]

There are also 2 types of springs. A light one and a stiffer one.
The Magura throttle requires a lighter spring, as an example.
I think I have a set of stiff springs if you want them.

 

[Olly]

Thanks littlebill31 and ploughboy1 for the expert advice, you were right. The throttle slide isn't closing completely so I need a stronger or second spring, I'll try Allen's Performance here in the UK tomorrow and see if they have one.

 

[TwoManyXS1Bs]

Two cylinders together will pull a little more vacuum on the single carb, trying to pull the slide forward, and jamming it in its bore. You could try polishing the forward (engine side) sliding surfaces of the slide and carb bore, check for and lightly chamfer sharp edges at bottom of slide, see if that stops the slide from sticking...

 

[littlebill31]

Make sure the carb cable adjuster isn't causing drag. I had a 90 degree angle adjuster (pictured below) and the slide stuck, so I switched to a straight one.

 

[DIRTY DOG]

I have done this same set up on 3 bikes now and the only proble i have found is cables sticking in the 90* elbow at the carb top and at the throttle assy. don't need heavy springs just a vey good smoth cable assy