Sticky Mikuni

I know this topic has been brought up here before but I haven't seen a satisfactory answer so I'm bringing it up again. I've had this on and off problem in the past with the slide on my single mikuni hanging up. While the bike was off the road for the past year I put in a new spring and an angled cable adapter. When I took old rainmaker out for a test drive today it was worse than ever. It was like I was power shifting all the time. When the bike isn't running you can hear the slide snap back with a positive click. I know this is a common issue as I have seen questions about it on several occasions. I have emailed Sudco about it and they chose to bombard me with total silence. Possibly someone out there with lots of mikuni experience has an answer.

I've heard you add an Amal spring.

Hi Zotz,
I can't see it from where I live, but if I could I'd be looking at the slide for burrs along the guide slot. You also want to check the pin in the side of the carb body that engages that slot as they have been known to come loose, which would mean that the slide could slightly rotate and not return onto the idle speed screw correctly.

GB

Disconnect from the throttle on the handlebars and see if it works. Lube the cable. Lube the throttle. Pull the cable from the angled connector on top of the carb and see if the slide retracts nicely. Re-route the cable.

Perhaps someone should threaten them with legal action to get the to produce a decent spring because it is bloody dangerous.Engine vacum has alot to do with them sticking.

Cookie said:

I've heard you add an Amal spring.

Click to expand...

I put a new Mikuni 34 on my bike and about the third day I was full throttle and it stuck open.
Pretty exiting for 2 seconds until I hit the cut button.
Added the Ama; spring withing existing Mikuni spring and has been fine since.
Little stiffer throttle pull but not a problem.
I think I got the spring from OldBritts.

Bob

Do you have a fiberglass tank? the sticking can come from the resin being dissolved by ethanol in most gasolines.

Jean

I had this when I fitted a 34 Mikuni to my 1972 750, just before I went camping across Europe. It was worst in light drizzle and on early mornings when it was damp, but not raining. Taking the carb apart and polishing everything, rerouting the cable, replacing the cable etc had no effect. I think it's to do with engine vacuum, as I learnt that if you changed gear the change in pressure made the slide drop. I fitted the Amal concentric spring inside the Mikuni spring and also got a spring from a Guzzi that fitted between the two ; It scared me that much. Since then I've haven't had a problem.

I tend to agree with the vacuum idea. I have a steel tank and have done all of the obvious fixes. You can't get it to do it at idle and you can hear the slide returning no matter how gently you return the throttle when it's turned off. I will try doubling up the spring and check for a loose guide pin.

\hi Lads,

Just sticking (get it :mrgreen: ) my nose in here.

I presume all this discussion is about the round slide Mikuni carb?

Just curious as I'm thinking of getting the flat slide version & was wondering if anybody had similar issues with the flat slide carb?

Kevin

I had that same frustrating problem with an AMAL MK2. The carb worked fine, you could hear the click as it contacted the idle screw and it worked smoothly through the entire throttle range - when cold. On the road, without any warning, it would suddenly stick. It would stick at various levels and once, it stuck wide open...truly an exciting experience on a busy highway. Like you, I did all the obvious fixes including the extra springs inside, the curved cable guide at the top, and a new chrome plated slide. Nothing worked. Off the bike, I heated the carb and found that as the carb warmed, the throat warped ever so slightly and caused the slide to stick. It was absolutely undetectable when cold, no odd wear marks on the carb throat or any other indications of a problem. It couldn't have been anything else but a manufacturing flaw. I replaced the carb body and not one incident of sticking since.

Pulled the carb today and had a look at it. Nothing obvious. I doubled up my old spring with the new one and took it around the block. No evidence whatsoever of sticking but time will be the judge of that. Throttle is stiffer but I don't think it's going to be an issue.

Curious about the settings of your single Mikuni.

On my Norton 850, I have a single 34 Mikuni with a 230 main jet , 35 pilot and clip in the middle. Are these the usual settings?

Well, you're in the ball park. I have two different sources for settings and they are both different which means you need to fine tune for your particular situation. Settings are, as per Norton Tech digest and Victory Library Mikuni Tuning respectively-

Main Jet-----------------230/260
Needle Jet--------------159P2/159 P4,P6
Needle------------------6DH3/6DH3,4 (start off in middle setting)
Idle Jet-----------------40/30-35
Slide#-------------------2.5/3.0
Idle air Jet-------------2.0/1.0