Fuel Dripping from Mikuni overflow

I've had an issue with my Mikuni carb for some time now where forgetting to shut off the fuel petcocks almost guarantees that when I return, fuel will be dripping from the carburetor overflow. The problem is intermittent. Sometimes it won't leak at all (rare), sometimes it takes a few hours, and sometimes it will begin within a few minutes of shutting down. Normally I turn the valves off after a ride and sometimes I forget, but lately I've been leaving them on intentionally trying to figure out what's going on.

I inspected the float needle and saw no problem. Blowing air by mouth into the fuel inlet with the float up also reveled no leakage. I moved the bike from shade to full sun, thinking maybe the gas tank was not venting properly, causing it to pressurize with the rising temperatures. It didn't leak.

Thinking perhaps I was missing something, I sent the carb to a reputable and well known Mikuni technician, who overhauled it. In the process he replaced both the float needle and seat.

When I got the carb reinstalled on the bike, I intentionally left the petcocks open. With the bike sitting in the garage, several hours went by with no leakage. Finally, after about five hours, I returned to find a small puddle of fuel under the bike. Damn! It doesn't necessarily take five hours either. This morning, after running the bike, fuel was dripping from the overflow within 10 or 15 minutes of shutting down.

Now I know you're supposed to shut off the fuel petcocks, but I'm beginning to wonder if this behavior is common with the Mikuni. Do any of you with Mikuni's leave your petcocks open? Do you get overflow?

The only thing I've changed with the fuel system on this bike is I've replaced the fuel lines and both petcocks. The old ones had very anemic fuel flow so I replaced them with a modern set that have a 90 degree outlet which allowed for much neater and shorter fuel lines. My fuel flow is most excellent. Could it be too good?

-John

Assuming you're talking about the Mikuni VM: You don't mention your floats. The floats have to move freely up and down on the float guide pins. The float rubber body should be intact/no holes and should feel completely empty when handled. You say you've checked your float needle and needle seat but you don't mention the floats. The brass guide plate that moves the needle down when the floats go up has to close the float needle. You can adjust it if you need to. The Mikuni VM is very easy to work on. My single 34mm Mikuni VM does not leak at all, ever. I often leave the gas on overnight, no leaks. When set up properly a 34mm Mikuni VM does a good job of running a Norton Commando. My Mk3 idles at 350-400rpm and starts first kick every time when warm. I usually use the electric starter and it lights up nearly instantly. There is alot of controversy about twin vs single carburetors on the Commando. My experience is only with a pair of rebuilt Concentrics and a rebuilt 34mm VM Mikuni. The Mikuni is my choice.
>OK
[video]http://www.youtube.com/watch?v=hbVKRLFW2tg[/video]

tylerdntexas said:

Assuming you're talking about the Mikuni VM: You don't mention your floats. The floats have to move freely up and down on the float guide pins. The float rubber body should be intact/no holes and should feel completely empty when handled. You say you've checked your float needle and needle seat but you don't mention the floats. The brass guide plate that moves the needle down when the floats go up has to close the float needle. You can adjust it if you need to. The Mikuni VM is very easy to work on. My single 34mm Mikuni VM does not leak at all, ever. I often leave the gas on overnight, no leaks. When set up properly a 34mm Mikuni VM does a good job of running a Norton Commando. My Mk3 idles at 350-400rpm and starts first kick every time when warm. I usually use the electric starter and it lights up nearly instantly. There is alot of controversy about twin vs single carburetors on the Commando. My experience is only with a pair of rebuilt Concentrics and a rebuilt 34mm VM Mikuni. The Mikuni is my choice.
>OK
[video]http://www.youtube.com/watch?v=hbVKRLFW2tg[/video]

Click to expand...

nice bags- craven?

Thanks. Those are Craven Golden Arrows with a Craven rack. The rack is from a 1970's vintage BMW. I adapted it to the Norton by using the head or muffler Lord mount rubbers on grab rail mounts on the frame loop. Most of the load on the rack is taken by a single stay on each side which attaches to the Z plate, so the frame loop doesn't take much load. Many Commandos with luggage racks have bent frame loops because that loop isn't designed for a rack.
>OK

I had the same problem with my VM. It wasn't as extreme as yours but I was getting fuel from the overflow tube. I installed a new float needle kit and set the level to 24mm, (without compressing the spring). Didn't help. So I dropped the float level around 2-3mm. The weeping gas problem went away, but more importantly, it transformed the throttle response at under 1/8 opening. The Mikuni Super tuning Manual is very good, it shows how to set the float level. This is a link to it.

http://home.planet.nl/~roelwijn/files/MIKUNI.PDF

A more exact method is the one below. It's from the XS650 forum. I'm too lazy myself 8)


"When you're setting up new carbs, Step One is a thorough cleaning; there's often manufacturing debris left in carburetors. While you're ordering parts, order a spare float bowl plug, drill and tap it for a fuel barb, and attach a short piece of fuel line and a piece of rigid plastic tubing for a sight tube. Clamp a carb on the bench, level it carefully on both axes, attach a remote fuel source, and observe the fuel level. It should rise no higher than the bottom of the carb body flange over the float bowl."

rpatton said:

I had the same problem with my VM. It wasn't as extreme as yours but I was getting fuel from the overflow tube. I installed a new float needle kit and set the level to 24mm, (without compressing the spring). Didn't help. So I dropped the float level around 2-3mm. The weeping gas problem went away, but more importantly, it transformed the throttle response at under 1/8 opening. The Mikuni Super tuning Manual is very good, it shows how to set the float level. This is a link to it.

http://home.planet.nl/~roelwijn/files/MIKUNI.PDF

A more exact method is the one below. It's from the XS650 forum. I'm too lazy myself 8)


"When you're setting up new carbs, Step One is a thorough cleaning; there's often manufacturing debris left in carburetors. While you're ordering parts, order a spare float bowl plug, drill and tap it for a fuel barb, and attach a short piece of fuel line and a piece of rigid plastic tubing for a sight tube. Clamp a carb on the bench, level it carefully on both axes, attach a remote fuel source, and observe the fuel level. It should rise no higher than the bottom of the carb body flange over the float bowl."

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That reads just like the process for an Amal. Manufacturing debris? What kind of crap is that? :mrgreen:

Sorry, couldn't help myself...

As I stated, the carbs were sent out for inspection and rebuild, which included new float needle and seat. Also, the float level was set at that time. I'm wondering if there could be anything else going on here.

Bonwit said:

When I got the carb reinstalled on the bike, I intentionally left the petcocks open. With the bike sitting in the garage, several hours went by with no leakage. Finally, after about five hours, I returned to find a small puddle of fuel under the bike. Damn! It doesn't necessarily take five hours either. This morning, after running the bike, fuel was dripping from the overflow within 10 or 15 minutes of shutting down.

Click to expand...

Bonwit said:

As I stated, the carbs were sent out for inspection and rebuild, which included new float needle and seat. Also, the float level was set at that time. I'm wondering if there could be anything else going on here.

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John,

IMHO, the answer would be no, there is nothing else going on. I did everything your carb guy did and got the same result you got. The spec's on the Mikuni float is 22mm-24mm. It's obvious that whatever he set it at, and what I set it at was too high. Take the carb off and recheck the height of the float arms. If it is set at 22mm, then drop the arms by bending the tang at the float valve till you get something closer to 24mm. (When I say height, raising, and lowering the float, I'm referring to the carb being right side up.) Keep it in mind that you're not measuring the actual fuel level, which is why Amal measures the fuel level with a sight tube when they set up carbs. It's entirely possible that the particular floats on your carb require a lower setting than what's specified in the manual. So, recheck it and see what it's set at now and go from there.

rpatton said:

IMHO, the answer would be no, there is nothing else going on. I did everything your carb guy did and got the same result you got. . .

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Thank you Bob. What you say makes sense. . . that there is something going on with the float, and that even though it is set to factory specs, the characteristics of my float is somehow nonstandard. Is it possible too that maybe the float is somehow hanging up near the top of the guide pins and that it's just not shutting off completely, but when the engine is running, the vibration keeps this from happening. Does this make sense?

So, in your case, making this adjustment to the float height solved the problem? Did yours always leak right away, or was the problem intermittent?

Thanks for your input.

-John

Setting the float level to specs only works with a brand new float. As the float gets older it gets heavier-especially with ethanol fuels. With a used float you will have to set it a couple MM lower.
Sometimes it is necessary even with a new float to get it to run well. Floats don't control the fuel very well when they are shaking so setting them lower is common on a Norton. Jim

A petrol tap might be leaking too. It wont cause the overflow but will certainly feed it and give other problems later.

Cash

Bon, this is going to sound like a stupid question, but depending on your answer, I'll tell you why I am asking. When your carb is leaking, is the bike on the side stand or the center stand?

chapmanmd said:

Bon, this is going to sound like a stupid question, but depending on your answer, I'll tell you why I am asking. When your carb is leaking, is the bike on the side stand or the center stand?

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Nope, always on the center stand. I hardly ever use the side stand.

Thanks,

Bonwit said:

So, in your case, making this adjustment to the float height solved the problem? Did yours always leak right away, or was the problem intermittent?

Click to expand...

Hi John,
The problem was constant but the only time I ever saw a puddle was when I left the petcock open for a few days. What i did see regularly was a small amount of gas hanging on the end of the overflow tube. The thing that actually sold me on the idea that something wasn't right was that I was going to a 25 pilot jet just to get any kind of response from air screw adjustments.

Bonwit said:

Is it possible too that maybe the float is somehow hanging up near the top of the guide pins and that it's just not shutting off completely, but when the engine is running, the vibration keeps this from happening. Does this make sense?

Click to expand...

I know that the Amal's floats can hang up on the gasket but I've never heard of it happening with the Mikuni. The floats could hang up if the rods are crooked or the inside of the floats cruded up. The lever arm could bind on the pin too? I like the idea that the floats might get soaked with gas. I think the floats are solid, cast in some kind of plastic foam.

I think anyone who does not shut the petcocks when a bike is parked inside a building unattended is making a big mistake. Even in good shape and properly set up you never know if the vibration from a ride or a piece of dirt is going to change things. Opening up the door to a garage in the morning to the smell of heavy gasoline fumes too many times is literally playing with fire. Even a spark from a light switch when it is flipped by whoever is entering may be enough to set it off.

One guy local to me burned his house to the ground from fuel leaking from his motorcycle being ignited by a water heater. A neighbor of my brother died when he had a natural gas leak in his home which was ignited when he got home and flipped on the lights. The second one I know had nothing to do with a motorcycle, but it makes you think.
A local bike shop/dealership owner here left the petcock on one of the bikes in the shop on overnight. The next morning a large amount of it's fuel was on the floor and the shop smelled really bad, I was there. The hot water tank was on the same floor in a separate room. Makes me wonder what might have happened with a little more time or if some door was ajar or not well sealed.....

The risk of loss it too great as far as I am concerned to not get into the habit of turning the petcock off whenever I am not sitting on the bike.

Point is really good to take to heart. Not to bore you all with a sim story( well I allways do anyway) but my freind who restores Indians had a bike start leaking from a petcock & the gas slowly flowed across the floor to the lowest point. The wash room area wheat the water heater was sitting not up off the floor per code but three inches off the slab & if not for his needing to go work on a Sunday morning it would have blown. The gas was six feet from the heater so about three mour hours & K-BOOM.

You are not alone, if I don't turn my taps off there is a small puddle on the floor overnight, leave the taps on while standing around jaw flapping and it may flood causing much embaresment

comnoz said:

Setting the float level to specs only works with a brand new float. As the float gets older it gets heavier-especially with ethanol fuels. With a used float you will have to set it a couple MM lower.
Sometimes it is necessary even with a new float to get it to run well. Floats don't control the fuel very well when they are shaking so setting them lower is common on a Norton. Jim

Click to expand...

Jim, I think we have a winner. New floats are in the mail. I'll report back how they work out. Thanks for your help!
-John

I know on my 73' with the 36VM Mikuni all the shaken going on while riding gas will spit out of the over flow tubes. Especially on a hot day. Now on my old 74' with a VM38 it didn't leak at all.......


Tim_S

Well, I've replaced both floats with new units, and the carb still chugs fuel from the overflow when parked w/ the petcocks open. Next step is to tweak the float height.