mk2 carbs

What does an 850 go like with mk2 carbs , do they fit properly, I see there's a set for sale at the moment and I'm sick of the single mic sticking every time the airs a bit cool

Why, what does you Mik do in the cool air ?

Doesn't shut, warm sunny day fine, cool evening air come into a corner and it sticks about 2.5~3k rpm

Make sure your cable is tip top if it is and it still hangs up fit a spring from a mk1 concentric inside your mikuni spring

Sounds like an overly rich idle/pilot setup. The symptoms you describe are often referred to as a "hang throttle", but likely isn't actually hanging up.

Check the enriching valve, if it leaks it can freeze up the carb.

Dave

splatt said:

Doesn't shut, warm sunny day fine, cool evening air come into a corner and it sticks about 2.5~3k rpm

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Stock ignition? AAU's are notorious for holding full advance which will create that symptom.

Nathan

I had a single Mk2 concentric that used to hang up like that, I presumed it was ice, better with an air filter but the single mikuni was a better set-up. I did have a velocity stack on the single Mk2 and it flew but starting and general running was not good. I could have spent more time on it but the sticking was becoming a life threatening issue.

when it sticks the cable still moves , has extra slack, xtra amal springs fitted,boyer ign. Nice warm day all day long without sticking, head home at night and it will stick , terrible in the wet.
Any way I know some of you fitted mk2's and must be able to relay some feed back, I have a brand new set of 32mm mikuni's and manifolds but don't really like the hassel with the air filters so haven't fitted them.
I don't have std manifold or air filters ect to fit a new set of premiers as they are really well priced, but all the extas will start adding up,

Could be ice. Some people wrap the carburettor in rags and say it helps.

Or- as you asked, a random change of carburettor type can stop it happening, if you're lucky.

I admit that I don't know how a mikuni carb works,.... but I had a sticky inner body that I fixed on one of my amals. They had been sleeved previously so I couldn't just try a new inner slide body to cure the sticky fit on one carb.

I took the carb off the bike and disassembled it. Then I took some 800 wet or dry sandpaper and gentley sanded the inside of the carb body to remove the sheen which was glossy from being honed by the carb inner body traveling up and down during use. Then I cleaned the carb and inserted the inner body back into the carb and slide it up and down. When I remove the inner body after a few strokes, all the high spots inside the carb are buffed shiney again and I could see where I needed to remove material to loosen up the slide. I tried valve grinding paste but it didn't seem like the right application to me. I ended up just sanding the high spot pattern that I identified, then washing the carb with brakeclean and re-trying the fit over and over until I had the same tolerance as the carb that didn't stick... It worked fine. I went slow, used fine paper and kept cleaning and checking the fit as I went until I was satisfied that the inner body would no longer stick. I've had no issues with sticking since I did this procedure.

I don't know how the mikuni's inner and outter bodies work, but I assume you could do the same thing if they are similar to the amals.

There is a thing which can happen with Mikuni carbs on a four stroke - the vacuum can cause the slide to hang up particularly if the body is worn. I run two 34mm Mk2 Amals with a dual twist grip and separate cables. I haven't ever looked to see how or if they differ from Mikunis internally, however I would expect they could also hang up for the same reason.

Getting closer do twin mk2 fit and go alright
I believe it's the engine vacuum and body distortion due to the different air temp that causes it to stick, it wad fitted 25 years ago so it's probably well worn by now, and no I didn't own it from new.
One day on a long straight it actually startted acceletating by itself sucking the slide up
But do mk2 fit and go alright

Hello!

Had problem with sticking slides on new Amals.
Problem grew and was dangerous at times.
Solved witch a squirt of 2-T oil in the gastank!

Benton......

With the Seeley there is room for long bell-mouths on the Mk2 Amals. I don't think you would have any other problem fitting them to a standard commando, except for the air-cleaner. If I was doing it, I'd try to construct an air-box.

it could be icing up in the cold air if you stop the engine for a couple of mins does it clear ?

As it is difficult to physically look in side the carburettor quick enough to check for ice before it melts
it may be worth try using FST in your fuel and see if it stops

you can source it from marine suppliers icing is a very common problem on boat outboard motors used in cold climates

Splatt,

Can readily identify with your observations regarding a sticking mikuni carb slide. My experience was with a different marque and carb size but the rest of the tale is identical. It occurred so long ago (circa 1976) it seems in another lifetime, but a friend and I both had well prepared 76" Sportsters with 44 mm Mikuni VMs on them, and they behaved exactly as you describe. By light of day they were a pleasure to operate and easily spanked just about anything we ran across. There was never ever an issue with the slide sticking during sun-up riding. However, as night fell and the air temperature approached the dew point, look out, because the slide would stick at the most inopportune times and result in an operator with some pretty wide eyes (like seeing the white of a horse's eye, which is seldom a good thing)! My friend had a magneto with kill switch on the handle bar, so a tap on the kill switch instantly released his slide. Unfortunately mine was a battery ignition and had a lot more engine than it had brakes and the only way to release the slide was to blip it wide open and release it. Needless to say when the throttle is stuck on and you're becoming concerned (WIDE EYED) the last thing in the world you want to do is open it wide to snap it shut.

We tried brass slides, extra springs, etc but never found a satisfactory solution, other than watch out for certain weather conditions. Perhaps the fuel additive referenced above may have provided a simple and elegant solution.

Regarding the cause, all I can say is that it had to do with atmospheric moisture, perhaps simply condensed water droplets in the vicinity of the slide or possibly frost/ice. Any time it ever occurred the carb was always cold to the touch. I recall running on the freeway one night at sustained high speed (required minimal throttle to run 85 mph), looking down at the carb and seeing the carb body snow white in the vicinity of the venturi/slide, where frost had formed. Also, It always occurred under a high vacuum condition (engine revving freely with very small throttle opening).

WZ507 - That description is exactly the same, may be a proper air box with a hot air intake would fixit.
Anybody got any experiences with mk2 carbs on a road going commando they would like to share

I noticed that comment about adding 2T oil to the fuel to stop the slides from sticking. I suggest it needs to be the right sort of 2T oil if you are going to do that. Back in the 80s Australian rider Ray Quincey crashed during a race in the rain in Europe and became a paraplegic. He did not know that some 2T oils make the slides stick when they are combined with moisture. Ray powered off the start line and the throttles stuck open, so he bounced off the Armco fence. I think all the European riders were using Motul 2T oil, however you would need to check.

cash said:

Check the enriching valve, if it leaks it can freeze up the carb.

Dave

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Funny you should mention that, been riding my bike to work most days lately, carb hasn't stuck once?????,even in the pouring rain ,drizzle mist good as gold, cruising along at 60-65mph, about 1/4 throttle , 7or8 km out of town , both directions, the bike starts missing it will accelerate happily above 1/2 throttle misses and farts at 60, open choke goes better, not perfect but better, 1km later its all cleared up and humming along again.
My assumption is the primary bypass is freezing up, causing the mixture to lean out, if I stop it will still idle on the primary circuit, if I accelerate it takes the primary bypass out of the circuit.
Still haven't fitted the MK2s yet as I'm having trouble finding air filters that will fit, S&B RC98 possibly, just not readily available in NZ