MIKUNI STICKING OPEN

Don Tovey · Jun 27, 2010

Hi all,
Ive got a VM36 on a 750 Commando & once the bike has warmed up the revs are slow at going back down to tickover.
If I travel at speed (70mph) the revs stick at 3500 to 4000 for a while before going back to 1000 rpm.
Ive fitted a new cable & the slide can be heard re seating properly at its lowest point.
Ive lowered the pilot jet to 30 instead of 35 in case the mixture was too rich causing the carb to freeze & jam the slide in the open position.
Ive lowered the main jet to the next size down.
Ive re routed the new cable so it feels smooth & re seats the slide properly.
I have not yet put an amal spring in along side the mikuni spring to help the slide go down quicker. (this is my next attempt to get it right)
I dont think I have an air leak in the system as ive sealed everything & put a new connecting rubber between the carb & the manifold & sealed the outside with tape.
ANY SUGGESTIONS PLEASE FROM PEOPLE WHO HAVE SORTED OUT THE SAME PROBLEM.
My underwear is getting worn out after shutting off the throttle in tight bends but not slowong down.
Thanks in advance
Don

zotz · Jun 27, 2010

There are previous posts regarding this.

sticky-mikuni-t6402.html

In my case the double springs seem to have worked.

Hobot · Jun 27, 2010

MUST be solved, hope extra spring with throttle effort does it
since you've fettered the rest of the list.
Riding stuck throttle is upsetting but controllable by clutch
in or kill button
SO
Real danger is what happened to my Ms Peel Combat,
it stuck open just starting cold in shed and over rev'd
to damage engine too bad to be worth putting a blower
on it as first intended.

Get a rev limitered as extra insurance at some point.
Boyer's have extra cost unit to upgrade. Your call.

hobot

highdesert · Jun 27, 2010

Hi Don, I had the same problem with my Mikuni a few years back.

The problem in my case was that the settings were too lean, and not too rich.

And it was not the main jef size or needle clip position, it was the pilot jet size and air screw setting.

An example would be when you set the idle you set it so turning the air screw out making the motor RPM increase to their fastest, then turn the screw in, or richen the mixture, just a little until the revs decrease.
This setting usually the recommended one and one half turns out from fully seated in.
This extra richness makes the motor shut down to your idle rpm quickly.

So, you might try, since you are running a larger than usual Mikuni, going to a 40 pilot jet and then use the slide screw to set the idle at 1000, then set the air screw at one and a quarter turns or so out from full seated.
This should enrich your idle circuit so it shuts the revs down quicker. Too lean makes it want to hunt higher rpm.

highdesert · Jun 27, 2010

Hi Don, I had the same problem with my Mikuni a few years back.

The problem in my case was that the settings were too lean, and not too rich.

And it was not the main jef size or needle clip position, it was the pilot jet size and air screw setting.

An example would be when you set the idle you set it so turning the air screw out making the motor RPM increase to their fastest, then turn the screw in, or richen the mixture, just a little until the revs decrease.
This setting usually the recommended one and one half turns out from fully seated in.
This extra richness makes the motor shut down to your idle rpm quickly.

So, you might try, since you are running a larger than usual Mikuni, going to a 40 pilot jet and then use the slide screw to set the idle at 1000, then set the air screw at one and a quarter turns or so out from full seated.
This should enrich your idle circuit so it shuts the revs down quicker. Too lean makes it want to hunt higher rpm.
I can't find my tech manual, but it may even recommend a 45 pilot jet for a 36mm.
Worked for me, worth a try?

Don Tovey · Jun 29, 2010

Thanks for the replies everyone.
I ended up putting in the extra spring which sorted the problem straight away.
I may put the pilot jet back to 35 & the bigger main jet back in as these changes made no difference.
I took my bike around to my mates house & had a rather fast trip around some bends when the revs would not slow down, but on the way back with the extra spring the revs slowed down as they should. Problem sorted.
Evidently this problem only seems to happen on 750 engines & it appears to have something to do with vacuum holding open the slide once you have shut off the throttle.
This was mentioned on older threads on this site. Maybe its the higher compression on 750 engines.
Anyway, I can once again enjoy riding the old beast at the speed I want to.
Cheers Don

splatt · Aug 12, 2010

Just drag this back up, a MK11 amal spring, it was in a 26mm carb, fits in the mikuni, the wire is 0.010'' thicker so it appears to have a little more tension, so I now have 2 amal springs in the japanese carb. Mine is a shocker in the rain sticking frequently.

rbt11548 · Aug 31, 2010

splatt said:
Just drag this back up, a MK11 amal spring, it was in a 26mm carb, fits in the mikuni, the wire is 0.010'' thicker so it appears to have a little more tension, so I now have 2 amal springs in the japanese carb. Mine is a shocker in the rain sticking frequently.

I was heading off about 10 days ago to where my family are from on the West Coast of sunny Scotland for a party, just got back today, (was great party!!!) I loaded up the Commando, set off in atrocious conditions, the rain was bouncing off the road, got about 5 miles down the motorway and for the first time since getting the Mikuni the bloody thing stuck open, which did, to say the least, scare me a bit. Due to the fact I was going to be riding on single track roads probably packed with tourists further on in my journey , discretion was the better part of valour and I turned back and took the BMW.
As I said this is the first time it had happened to me, anybody any idea why it does it in the rain? Also will a Mk1 Amal spring be the best extra spring to go for?

DavidT · May 14, 2012

Sorry to resurrect this, but did anyone ever come up with a definitive answer? My ’72 750 with a single Mikuni has developed a sticking throttle only when ridden in heavy rain. With the engine stopped I cannot get it to stick. The action is light and smooth, I have replaced and re-routed the cable but still after an hour of splashing about, the revs start to hang up around 4000 rpm. Sometimes this can be cleared by snapping the throttle open and closed but its not something you want to be doing in tricky riding conditions! On other occasions I need to kill the motor but when I check the throttle is snapping down onto its stop perfectly.

I have ordered an Amal MK11 spring to use inside the existing one as advised here, but does anyone actually know what’s going on? Why does it only happen in the rain? Is the wet affecting/increasing the vacuum?

Thanks for the help.

David

concours · May 14, 2012

DavidT said:
Sorry to resurrect this, but did anyone ever come up with a definitive answer? My ’72 750 with a single Mikuni has developed a sticking throttle only when ridden in heavy rain. With the engine stopped I cannot get it to stick. The action is light and smooth, I have replaced and re-routed the cable but still after an hour of splashing about, the revs start to hang up around 4000 rpm. Sometimes this can be cleared by snapping the throttle open and closed but its not something you want to be doing in tricky riding conditions! On other occasions I need to kill the motor but when I check the throttle is snapping down onto its stop perfectly.

I have ordered an Amal MK11 spring to use inside the existing one as advised here, but does anyone actually know what’s going on? Why does it only happen in the rain? Is the wet affecting/increasing the vacuum?

Thanks for the help.

David

THAT sounds like carb icing. "hang idle" is common on Mikuni equipped sleds. high humidity/dew point spread. Also, if the idle circuit is partially/totally plugged, the idle screw takes the slide open enough to run on the pilot circuit. That causes hang idles. FIRST, before you do anything else, CLEAN the passages, VERIFY they are open with spray solvent, INSPECT the main well and needle jet for deposits. If you're unsure, get help from a knowledgeable carb person.

Gino Rondelli · May 14, 2012

David, I've told you before, the solution to your problem is only use it on dry sunny warm days, so I'll meet you somewhere in Scotland on the 29th of June because that'll be it!

tricatcent · May 14, 2012

I had the same problem. I solved it by tilting the carburetor to one side a bit. That way the cable didn't have to make quite as sudden a bend on the top of the carburetor. Once I did that I had no more problem.

Now I have switched to a single Amal 1038 instead. It is a bit lower so it has less clearance problems. I would not have the option of tilting that carb because it is bolted on.

splatt · May 14, 2012

David, as I said I ended up with a mk2 spring and a mk1 spring inside that,mine was identical to yours, when putting along in the wet, shut throttle for corner,no slow down, arse puckered to seat.I don't feel that the mixture could cause the slide to hang up, it may slow the return to idle, but not to sit at 4k for an hour. As gino said it's much more fun on a nicer day

DavidT · May 14, 2012

Gents,

Thanks for your suggestions. I think I like Ginos best, but only riding in dry weather would lead to owning the lowest mileage Commando in the country!

Interestingly I am very familiar with carb icing on my Ducati. But this is quite definitely not mixture related - while hurtling along at 4000 rpm/70mph with no throttle control, I can snap the throttle open and feel the change at the grip when the slide is hanging on the cable again.
The consensus seems to be that the standard Mikuni spring just isn't strong enough to overcome striction caused by the vacuum.
Will report back when I have tried the double spring.

David

concours · May 14, 2012

The cables been lubed, the barrel on bar has a slathering of nice moly grease... ?? :mrgreen:

DavidT · May 14, 2012

Yes, new good quality, nylon lined cable lubed with light cycle oil & greased at the twist grip. Nice smooth pull and a resounding snap when letting go the grip except when splashing through heavy rain. Steel tank and no sign of gum or resin on the slide. I don't like dangly cables so normally I have it run forward over the top of the front brake lever, but to eliminate that from the equation now, have it dangling down ready to snare pheasants.

David

1up3down · May 14, 2012

but to eliminate that from the equation now, have it dangling down ready to snare pheasants.

ha!

But that comment brings up a question: WHY isn't there a way to avoid the dangling throttle cable?

Does anybody make a "modern" type throttle where the cable stays inside or can go right back along the bars?

Is there a reason this cannot be done?

Diablouph · May 14, 2012

But that comment brings up a question: WHY isn't there a way to avoid the dangling throttle cable?

I doubt it. It will work at first, maybe, but as the wire cuts into the lining, I bet it will start to bind. This is why I think a GOOD push/pull throtle cable is the best way to go. In the 70's I had one on my street/dirt Honda and I loved it.

How about comments out there from members who have installed one on their bikes?

DavidT · May 14, 2012

I think the cable running forward or along the bars is just about standard on modern bikes. It doesn't seem to affect the longevity of the cable, although admittedly they do have very little weight on them. I found the best action on my Monster was using only the opening cable and relying on the spring closure. Think I'm possibly on my third cable in over 100000 miles so they seem to last OK!

David

pete.v · May 14, 2012

I have a push pull system but i my carb has a bell crank(seen upper right in avitar) that raises and lowers the slide mechanically. Push pull is just not applicable if the carb is not designed for it. Route the cable cleanly and bump up the spring. Go to a hardware store. There is usually an array of springs of all shapes and sizes. Nothing wrong with getting creative.

MIKUNI STICKING OPEN

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