New Mikuni VM or Keihin PWK for 420AE?

My 1981 420 auto is running pretty well but the idle speed hunts up and down a bit and sounds lean as the idle speed increases. Off idle, mid range and WOT are all OK. Plug is a nice chocolate color and does not foul. Crack the throttle open, the rear end squats, the front wheel lifts a few inches and the bike lunges off. Great fun!

After starting, the idle speed is greater than you want for shifting into gear. After starting and warming it up I need to reduce the idle speed via the knurled knob on the carb and then shift into gear at a reduced idle speed so the auto tranny doesn't grind. At this lower idle speed, while still in neutral, the tranny sounds really rattly and noisy. I suspect that is the first gear clutch springs and shoes rattling around but don't really know. This noise goes away when you increase the idle speed or open the throttle. (Do I need new 1st gear clutch springs?)

I replaced the intake spigot and crank seals and pressure tested OK.

It has a 30 year old Mikuni 38mm round slide carb and I suspect it is a bit worn so I am going to replace it.

My question is:
Is the Keihin 38mm Air Striker worth $250 bucks (plus jetting) or should I go with a replacement new $50 Mikuni VM round bore (that I already have jets for)?

I figure since it is an automatic maybe I would not feel a $200 benefit of improved off idle and low rpm response of the Air Striker. Will it reduce the rattling tranny noise at low rpms? Will be easier to start?

I can usually get the bike started in 5 or 6 kicks cold (sometimes 10 or 12?) or 1 to 2 kicks hot assuming the sprag clutch doesn't slip while kicking.

Please advise. Thanks Steve

Why not use the kill button to get the rpm down below the klunk threshold?

Of course the rattling in the engine is dependent on rpm, once rpm gets high enough the shoes can't rattle as they are pushing against the drum. Can't see how carb would effect that.

From a previous automatic related post I started here I gather that that one way device on the crank or first gear clutch can get dirt in it. There are bronze bushings on either side of it and dirt can get stuck in there. I have quite a few damaged bushings but some of them come out without damage. I haven't tried to take mine apart on my useable bike since I learned this.

Can you really get a new round bore and a new slide for that price? The carbs on those dual shock ones are longer than the new stuff and longer than the vintage left kicker mono shock ones. At least the Micuni main jets are the same for the newer flat slide and round slide ones.

The 360 had a brake for that first gear drum complete with a cable and lever on the handlebar. The 420 only has some evidence of where that stuff once was. I assume that brake was to eliminate the issue you started this thread about.

Fran

There is a store on ebay selling the snowmobile version of the Mikuni VM38 for $50. See below. But the top cap is different, the choke takes a cable and the float bowl looks deeper than on the motorcycle version. I suppose I could swap those parts over from the old motorcycle version of the carb since I don't think they are worn out?

http://cgi.ebay.com/ebaymotors/NEW-...wmobile_Parts_Accessories&hash=item19c00b5688

You might be able to mix and match on the choke and float bowl bottom and cap. It appears to me you would need the rubber boot (airbox) for the larger 40/44mm carb however that is without holding actual carbs and looking at the pictures at the same time.

From the pictures of this carb and the correct carbs I sould see that the outside diameter of the spigot was even with the float bowl flange, so it looks the correct diamater. Well, I bought one of those snowmobile carbs. I'll let you know how it turns out.

I wonder if the intake flange on this carb unscrews or if the groove is there for another reason.

Your not going to hurt yourself for that price. I would atleast try it and see if it's usable for your application before spending $250 on the Keihin.

Swapped in the VM38 Snowmobile Carb Body

Well I finally got around to swapping in the new snow mobile carb body I got off ebay. The body is the same as the normal motorcycle version of the Mikuni VM38, but the float bowl is deeper (too deep to fit on the husky) and it has a pipe through the bottom of the bowl that is open (maybe for a fuel drain?). It has a cable actuated choke, the top cap is too tall and the #3.0 slide doesn't have the normal slotted hole for the throttle cable. You have to fasten the throttle cable to a plug-like adapter and then attach the adapter on the top of the slide. That makes the throttle cable too loose so you have to turn the throttle a quarter turn before it takes up the slack in the cable and opens the slide. So I went back to the old worn slide. It is stamped 2.0 from the factory, but some one used a vibrator pen thingy to scrawl 2.5 on the slide, so maybe they modified it or ????

I used the new carb body and swapped the old slide, top cap, choke lever assembly, float bowl, 430 main jet, 45 pilot jet and 6DH4 needle with clip at the bottom. The bike starts easier now and doesn't rattle as much at idle, but there is a very noticable flat spot in the acceleration of idle. After reading through the '82 430xc Husky jetting thread I discovered that these carbs actually have a Needle Jet! WHO KNEW?

So I pulled the Needle Jet out of the old carb and it is a R8, which is as rich as they get (I think). I have not pulled the new carb body off the bike yet to check, but I bet it has something leaner.

I am going to try jetting I found in the '82 430xc bike jetting thread as follows:
Main 450
Needle Jet R-4
Idle jet 45
Needle pos #2
Slide 2.0
Air jet 2.0
Needle 6DH3
1.5-2.5 from set on air screw.

So I spent $50 to get a new carb body, floats and fuel needle valve assembly. Oh I almost forgot that the snowmobile carb does not have a baffle plate in it so I need to swap that in from the old carb too. Education is never cheap.

When I buy the new slide I wonder if I should go with a 2.0 or a 2.5 cut out? Looking at the Mikuni VM Manual it shows that the 2.0 slide allows more fuel flow at lower rpms so maybe more torque off idle. And it is recommended in the 430xc thread so I just answered my own question. I'll go with the 2.0 slide.

You can file a 2.0 slide to 2.5 but not the other way around.

I just ordered my 2.0 slide, 450 main, 2.0 air jet, R-4 Needle jet and 6DH3 needle from Pats Small Engine.

I took another look at the needle jet in the old carb and it is a P-8 NOT an R-8. So it was wayyyy lean. It sucks when you need reading glasses, and a magnifying glass and you still can't see the little pee pee numbers

Wow this bike is going to be lots more fun with a carb that is jetted properly. The original carb was ruined by a rats nest in the air box so I never got the original slide or Needle Jet out of that carb. I swapped the original 430 main jet and 45 pilot jet into the ebay carb, but was running it with the really lean P-8 Needle Jet.

Maybe it won't rattle so much now.

I've always got my Husky's to start better and had better idle "tuneability" with a 3.0 slide. Smoother idle may lessen intake pulses thus reduce auto rattle

At first my 420 would rev out and change up,made it imposable to cruise or lug up a hill,
I filed a bit of the slide and it made riding a less frantic experience and extended the fuel range.It would hold 2nd or 3rd and not suddenly go blaslist and change up.Its been a while so I should drag it out,clean out the jets and go for a ride,really fun bike when there going well,fingers crossed

Finally Installed new jets in the Mik 38VM

I finally installed the new jets, 2.0 slide and needle. This is the frankenbike carb that I put together using the new eBay Mikuni snowmobile carb body with the transplanted dirtbike float bowl, choke lever, top cap, new slide, needle and jets.

It was after dark so I just rode up and down the street a few times. The bike is much more responsive now and the idle is much more steady. I didn't really warm it up fully and it felt a bit blubbery just off idle into the midrange, but then the plug would clear and she runs great on the main jet. That automatic can really pick up speed. I think I need to surface the rear brake pads. She could use a bit more stopping power now that I have decent jets in the carb.

I am going to make a spring retainer plate for the 1st gear clutch springs. Hopefully that will protect the side case when one of the springs breaks. And maybe it will stop some of the rattling at idle.

I dont remember if I posted this before but make sure you use the right spring for the 1st gear,the 430 spring is a differant length and will brake very quickly.With a road tyre and the right gearing I would be very intersted in the standing quarter mile times they would make,mine is bloody fast.