Mikuni vents and starting issues

[Old Geezer]

Some of you know that I've been battling a starting issue with my '83 500XC. One by one narrowing down possibilities, but the other day I noticed where a usually plugged vent nipple on my VM44 carb is missing the "stock" rubber plug. See the picture I took of my bro's 44mm carb off his CR....same exact carb as on my bike.

See the vent tube which vents to the float bowl, and see the nipple next to it that's a rubber plug. That vent goes straight into the piston barrel about half way up. It's purpose....who knows, which is why I'm asking here. Certainly if it isn't plugged it could allow a small amount of air to be drawn in between the carb slide and the barrel, perhaps reducing vacuum and adding air and maybe compounding cold starting by not allowing enough vacuum to pull fuel up through the starting circuits.

Any thoughts on this? Either way, I think I'll be plugging it as I think it was designed for in the first place.

View attachment 21966

 

[2premo]

if you look where the hose is and behind an below it you se a bulge ine carb body it is a vent to the float bowl area
if you are having a starting problem and it runs otherwise check your air screw as it is the circuit most used in the starting cycle
screw it in for richer
should be at about 1 to 1 1/2 for lower altitudes or really cold days

 

[Picklito]

I don't know off the top of my head where the nipple with the rubber goes to. But I have the same carb and can take a look.

There are several other starting issues on these bikes:

1. The kicker doesn't engage until almost half a kick is gone. I think the factory did that on purpose to put the lever in a position that puts the least amount of stress on the cases. The dog-leg levers, while super "husky-cool," are the worst at stressing the cases because the foot end is waaaaaaay out there. On the 250 you can still start it in tennis shoes with a half-@$$ kick. Not so on the half-litre. You can do the mods outlined in the other threads here, to make the kicker engage sooner... but personally, on the 430/500, I'd only do this along with a swap to a straight kick lever. If not, you're really stressing the cases.

2. Both the SEM and the Motoplat use the frame as ground. The contact points get old and crusty-rusty, and it seems that the first to go is low RPM spark... as in kicking RPM. It's worth the time to go through and clean EVERY contact point from stator to engine, engine to frame, coil to frame, etc. Plug wire, too. Make sure you're connect to bright copper strands. We even had this issue on a fresh rebuild... owner bolted everything to painted surfaces. Had to clean them off and the spark came back! Use a NON-resistor cap and a NON-resistor plug gapped at .022-.024". The fine wire electrodes allow the spark to jump a bit easier, so try something like B8EG, or EV.

3. If the ignition IS dying... again, it's often the low RPM spark that goes first if it doesn't just quit completely due to an open circuit.

4. The kicker gear ratios are not favorable on the early 81/82/83 430 and 500 engines. You get about 1.5 spins per kick. Sometimes just not enough. It's a bit of work to get it right, but a swap to the later ratios helps a lot.

5. The bikes came very rich down low. Naturally we lean them out. They need it. But too small of a pilot jet on a big bore can create a starting issue, as in 2premo's post.

6. I use a ritual. Probably the only superstitious thing left in my life. When cold: Turn on gas. Turn on choke. Put bike in second. Rock back and forth about 4 times. Put bike in neutral. Kick with throttle closed. Starts first kick almost every time. When warm: Crack throttle about 1/8 and HOLD IT STEADY. Don't be one of those guys who yanks the grip with every kick! Hope this helps.

 

[2premo]

good tips
my brother came up with a great solution on the grounding issue
feed a wire into the loom and ground the ignition to the coil directly
the starting ritual is a pretty common way to start these, we jokingly call it the Husky shuffle

 

[Old Geezer]

Thanks for the input! If you have the same carb, pull out the slide and you'll see that the vent in question goes straight through to the slide bore....it does not connect to the casting on the side of the carb that leads down to the float bowl to vent it.

It's been suggested that it's function is for dual carb setups, as a balancer or enrichening circuit. I could see how that would work, since the vent "hole" is above the top of the carb slide when it's at rest, and if a vacuum were to be applied to that vent it would affectively pull the piston up - to a point - thus opening up the needle jet/jet needle and drawing fuel. Whatever the reason, consider it now plugged for good.

As for starting these big bore beasties, believe me I've researched the crap out of it and done almost everything in the book, including all the tips that Picklito just offered....with the exception of freshening-up the coil wire and plug boot (as in hacking the end off, cleaning and refitting). The B8EG plug is a good idea with the finer electrode...will try that. If I can run across a straight kicker that will clear the rear shock, I'll go there (seems an early 80's WR will work from what I've heard, but I'd have to take out a second mortgage to get one). Also, I've installed a used but excellent condition stator/rotor/coil (external, correct for my bike vs. the internal setup it came with). Spark looks ok, not blinding, but seems sufficient. Initially I set the timing to 2.2mm btdc, to ease starting. No dice - I've since set to 1.6mm but have yet to try it. I'm running with a 50 pilot and air screw out 1 1/2 turns, should be sufficient for starting.

As to kicker mods, the gearing ratio swap is something to consider. But right now I'm able to get a fairly decent kick, because awhile back I had installed a compression release in the head. Much less effort to kick her through.

I totally dismantled and cleaned the carb....all jets and passages seem clear but you never know; I had a carb once on my Yamaha 360MX that just would not run right despite being clean as a whistle. A new carb fixed that one; I'll be borrowing the VM44 of my brothers CR to see if mine will run with it - to at least put the carburation question to bed.

I appreciate all the input. Any more ideas, I'm all ears!

 

[2premo]

sorry that I misread your vent comments , I see that you were discussing the capped one
of interest when using the starting circuit less is more, with a reed intake the best pulse to load fuel is a slow kick, so no throttle enricher on 2 slow kicks to load fuel crack throttle and typically mine lights right up
try closing the air screw a bit and see if it helps

 

[Motosportz]

The rubber niple needs to be there or it will cause a lean issue, hard starting and let crap into your motor. Also when you do get it running spray some carb cleaner around that intake boot and see if what appears to be cracks int he picture leak and are causing vacuum leaks. That would also be a reason it will not start. Many things to consider. Additionally if the carb is old that might be the issue. I have a KX500 and that thing was real hard to start and the idle would hang. I found out the carb bore was warn out and the slide did not seal well and I was chasing that. Those old carbs are kinda crap but should work Ok if not warn out.

 

[Motosportz]

6. I use a ritual. Probably the only superstitious thing left in my life. When cold: Turn on gas. Turn on choke. Put bike in second. Rock back and forth about 4 times. Put bike in neutral. Kick with throttle closed. Starts first kick almost every time. When warm: Crack throttle about 1/8 and HOLD IT STEADY. Don't be one of those guys who yanks the grip with every kick! Hope this helps.

I also find on these older bikes the lean it over til gas comes out the overflow seems to help a lot with cold starting as well.

 

[Old Geezer]

Thanks again for the new comments! The intake boot in the picture is on my brother's CR, not on my bike, so it's not causing "me" any problems (I don't recall it looking bad, though; just a poor picture shadow I think but will look at it regardless). I just used his carb for illustration purposes. My boot is in good shape.

As to carb condition...yep, she's see a hard life I think. Pulled the slide out last night and the bore does not look good. Might be a large part of the problem. Will be swapping my brother's carb over to my bike to see if that makes a difference.

The old 'tilt and flood' routine will indeed flood gas into the carb throat...as will blowing into the bowl vent tube (pressurizes the bowl and forces gas up through the main and needle jet - pooling in the throat). A lot easier than leaning the bike over for us old geezers!

 

[Motosportz]

Pulled the slide out last night and the bore does not look good. Might be a large part of the problem.

Very well could be as that causes a lean condition which is the opposite of what you want when cold starting.

 

[ronn450]

Your 500 came with a 40mm carb . Not a 44mm those were only on the 82 1/2 CR500 bike and they were a hard starting bike . I am with George Earl and Leftcoast Kicker go to the 38mm carb and the long lever kick starter . You will not need the compression release .

 

[Old Geezer]

Your 500 came with a 40mm carb . Not a 44mm those were only on the 82 1/2 CR500 bike and they were a hard starting bike . I am with George Earl and Leftcoast Kicker go to the 38mm carb and the long lever kick starter . You will not need the compression release .

Now you've done it....I feel like a dunce! Don't ask me where I came up with the extra 4 digits...of course it's a 40mm.

Believe me, I have toyed with going to a 38 and may eventually find that to be the answer.

Thanks for the correction!

 

[Kartwheel68]

There are 4 early 80s straight kick lever assemblies on eBay right now, one looks like its steel and is only $79.

 

[Old Geezer]

Well kids here's a head scratcher. In all the net searching I just did, nowhere can be found a VM40mm carb. They jump from 38 to 44 with nothing in between. Also, the measurements of the body per Mikuni's info show both a VM40 and 44 with exactly the same 'outer' dimensions of the engine and air cleaner sides of the carb (52 and 70mm, respectively) so apparently only the actual throat bore is different by 4mm. So if a person were to buy a VM44, what you may actually end up with is a 40mm bore, since the other mounting dimensions are identical between the two. I challenge you to find a new VM40 anywhere in the world. So is this an error in the Husky tech sheets, or an error in how Mikuni lists their carbs?
My carb measures 52 and 70mm outer dimensions as I mentioned above, and since I can't get far enough past the taper and into the throat with my calipers I'm measuring just 41-something....so I'm assuming the throat is indeed 40mm.

 

[ronn450]

The straight kick lever is the way to go . It makes it a lot easy to start a 500 husky .

 

[Kartwheel68]

Mikuni does not sell the 40 any more, but it is not a misprint, your bike came with a 40. All mikuni carbs are like that, the 30-34 share the same body, the 36-38 share the same body, and the 40-44 shared the same body, its cheaper than making a separate casting for each size. Get a 38 and forget the 40.

 

[Old Geezer]

Mikuni does not sell the 40 any more, but it is not a misprint, your bike came with a 40. All mikuni carbs are like that, the 30-34 share the same body, the 36-38 share the same body, and the 40-44 shared the same body, its cheaper than making a separate casting for each size. Get a 38 and forget the 40.

Thanks for confirming what I've come to realize. Seems Mikuni is still selling the 44's for the snowmobile crowd, but as you said the 40's are history. 38 is where I'll be going. Now to find the intake and airbox boots to match.

 

[Picklito]

To put a 38 on my 82.5 500 I'm using the boots from an 82-83 air cooled 250. Perfect match for the 500 reed block.

 

[Motosportz]

38mm Lectron.

 

[speedyauto]

I've just bought a brand new 38mm carby for my 85 500 (under $150 delivered) on advice from a Husky tuner for easier starting & is better suited for general riding, 40mm was really only good for high speed stuff apparently.
I've got it all hooked up using 430 intake & reeds & it runs great.
I use the lean it over method for cold starts.
Something I also saw is that you are using a 50 pilot, mine is a 40 & I was told the larger you go makes it harder to start.
Hope this helps,
Speedy