Mikuni carb and o-ring problem

[Brian4236]

Hi. I hope someone will be familiar with this issue and be able to help.
Bike: 1972 Wombat 125.
Issue: Bike starts fine, but when I put it in gear and give it some throttle, it dies.
Solution: After some troubleshooting with a mechanic friend, we discovered that it’s an o-ring issue in the carb.
Details: The carb is a Mikuni VM22-133 (Pic 1). Pic 2 shows the needle jet setter with one o-ring (as it should have; see red arrow). Pic 3 shows the orifice where the needle fits into the carb bowl (see red arrow). Pic 4 shows how we solved the problem—by adding 2 o-rings (see red arrow). Pic 5 shows the need jet setter without o-rings (see red arrow). It certainly looks like the o-ring should be a much thicker one to fill that space. Pic 6 shows the correct o-rings ordered from Sudco.
Question: Has anyone run into this problem? Is anyone aware of a different o-ring that I can use? It needs to be thicker. The problem seems to be that when we couple the top half of the carb to the bowl, the o-ring is not fitting snug enough in the orifice (shown in Pic 3).
The bike dies every time with just one o-ring. When I add the second, it runs like a champ. Over time, the o-rings stretch and I run into the same problem of the bike not running. When I add two new o-rings, the problem is once again solved. I spoke with the folks at Sudco; they assured me they sent the correct o-ring for this carb.

[Brian4236]

For whatever reason, I am not able to upload the photos. I keep getting a HTTP Error. Any suggestions?

OK. I've uploaded the photos in photobucket.

[Bullfrog]

Check out the final comment in the "?????Posting Photos" thread just a few items down the home page here on the Forum. Your photos have to be available on a public server (Photobucket, etc) so we can link to them.

Sure want to see your photos. Might be able to help then.
Ed

[Brian4236]

Thanks, Ed. I will do this later today.

[Brian4236]

All pics:

http://s291.photobucket.com/user/BCHeck ... 3.jpg.html

[Brian4236]

Pic 2:

http://s291.photobucket.com/user/BCHeck ... g.jpg.html

[Brian4236]

Pic 3:

http://s291.photobucket.com/user/BCHeck ... l.jpg.html

[Brian4236]

Pic 4:

http://s291.photobucket.com/user/BCHeck ... s.jpg.html

[Brian4236]

Pic 5:

http://s291.photobucket.com/user/BCHeck ... g.jpg.html

[Brian4236]

Pic 6:

http://s291.photobucket.com/user/BCHeck ... o.jpg.html

[Bill2001]

Ed, he should be able to upload directly from his local 'puter.

Here is a copy of carb page from the Wbat94 parts book:

Wombat 94

[Bill2001]

The o-ring is #27 and only one is used. It should seal the "annulus" between the needle jet carrier #18 and the main jet cavity in the float bowl. It must seal-- any leakage upsets the fuel flow through the main jet to the needle jet. I don't recall the width of that o-ring groove, but one o-ring will work and two should be fine.

[Brian4236]

Hi, Bill. The o-rings I purchased from Sudco after speaking with a rep told me the ones I purchased--002-212--for this particular carb are correct. However, one o-ring does not work. I've tried it. Two will work, but after a while they need to be replaced. I am convinced these are not the correct size. It looks to me that a "fat," "stubbier" o-ring is needed. What do you think?

[michael_perrett]

I would get a kit of assorted O rings from Harbor Freight. They have many types, even metric and are less than $10.00 per kit.
Mike Perrett

[Bullfrog]

So why do the O-rings need to be replaced? Are they getting "chewed up" by roughness of some surface they touch? Something is not adding up. Once the float bowl is installed, the "joint" being sealed by the O-ring is a "non-moving" joint. Sealing should last until the O-ring hardens and/or shrinks from old age.

Here is a "next step". Measure the OD of the bottom of the groove on the needle jet (record both metric and "standard" dimensions). Measure the ID of the float bowl cup at the point where the O-ring is supposed to seal. The subtract the ID from the OD and divide by two - that gives you the diameter of the rubber which makes up the O-ring. Go to your various local suppliers of O-rings and start your search - take your calipers (? and/or take the needle jet and float bowl with you too?) My local ACE Hardware and the local NAPA parts store would be places I would visit in search of the O-ring.
Ed

[Bill2001]

And if the float bowl is corroded from having water in it the surface will be rough.
On assembly a part with o-rings ought to have a light smear of Vaseline (or even premix oil) to lubricate them.

This is said to be a VM-22 carb. A Wombat94 takes a VM-24. Wonder if the jetting is good?

[Hydraulic Jack]

The mathematical difference in diameters between seating surfaces would be the thickness of the o-ring installed, i.e., compressed. In its relaxed state, the o-ring should have a slightly smaller inner diameter and a slightly larger outer diameter to allow for proper compression. If you can install two o-rings in the groove intended for one such ring, they are the wrong rings. The problem is that buying standard range o-rings from most any supplier will get you a difference in diameters to choose from but not a range of thicknesses to choose from. I have found that most over the counter rings are thinner than those used originally, and not just in Mikuni carbs.

You may or may not be able to find a correct o-ring for your application. Even Sudco may not know which o-ring was used 40 years ago. Instead, they would have rings for current production carbs that may have been machined or cast for a different, more readily available ring. So you may need to use two rings, as you have been doing, but only because those are not the right ring.

I would agree, though, that once installed, they should stay sealed for a long time. Don't know why you would have to replace them so rapidly unless they are only marginally sealing to start with, and exposure to fuel is changing their size or elasticity. Not all o-rings are intended for exposure to gasoline. You may need to find rings made from a different material.

You can also change the dimensions of both the needle jet and carb bowl so that the rings fit more tightly. The needle jet is easy to change with solder. The carb bowl harder to do with epoxy. But doable.

[Bill2001]

Below is a photo of my junked VM-24 needle jet holder and o-ring groove. Like Jack said, these carbs are old, obsoleted and have had a rough 40 year life. I decided a year ago to not muck with mine and replaced it with new for about the price of repair parts. Even then, I ended up with an odd jetting problem that kept me jumping for a year. And I'm still tinkering towards perfection.

And the correct o-ring composition is Nitrile. Viton or Buna rubber is affected by gasoline.

Welcome to the world of old scoots.

Mikuni VM24

[olddogs]

Some of the early dt 125 and 175 Yamahas used this same type Mikuni carb. I have been able to walk in and buy the correct O ring over the counter at the local Yamaha dealer.

The bowl should rock slightly when put it on the carb. A little grease on the o ring to keep it from tearing and use the bowl screws to pull the bowl down onto the o ring.

[Hydraulic Jack]

Brian,

Can you provide two sets of measurements?

1. The diameter of the needle jet ring groove where the o-ring seats and the diameter of the float bowl seating surface that the o-ring seals to.

2. The inside diameter, outside diameter, and cross section of the o-rings Sudco provided you as the "correct" ring for your carb.

If you happen to still have an original o-ring that can be reasonably measured, measure that one also for inner and outer diameter, and cross section. Not that a 40 year old ring will be perfect in size, but it could give us an idea of what to search for.

Thanks.

[Brian4236]

Someone asked why the o-rings need to be replaced. They are not getting chewed up by any roughness from the bowl. I believe it's because the o-rings are not the correct size, so over time they shrink or get "stretched" (maybe because they were too small to begin with). I don't have the tool to accurately measure the o-rings or the groove on the needle. But (as you can see from the pictures) it is definitely much wider than one (or even two) of the o-rings that were on the bike when I purchased it and the ones I purchased from Sudco. I was looking at the original Sudco receipt, which idicates this description: BV32/04 O-Ring, Part No. MI 02.212. The measurement I get with a millimeter ruler is 7mm OD and 3.75mm ID. I don't want to take apart the carb now (because the bike is running so well!), but I can say that the OD of the jet is significantly wider than 3.75mm. So the o-ring that's on the jet now does need to be "stretched" more than I would expect to get it on needle jet. Thanks to everyone for their advice. I will visit Harbor Freight or NAPA this week to see what I can find.

[Hydraulic Jack]

If the o-ring changes size when immersed in fuel for a while, it sounds like it isn't fuel compatible. When looking for a ring, choose metric sizes for this application, and use Viton. Don't use rubber or other compounds unless specifically listed as compatible with gasoline.

[Hydraulic Jack]

http://catalog.monroeseals.com/item/vit ... x-1-8-v-75

Here is an example of a metric Viton ring that closely matches the dimensions you provided. The chart doesn't give the outside diameter, but if you add the inside diameter to double the cross section, you get 7.35mm, which is in the ballpark of the number you have. Not saying this exact ring is the one you need, but it does show that there are Viton rings out there that would probably work.

[Brian4236]

Thanks, Hydraulic Jack. I think you hit the nail on the head (or close to the head) regarding size. BTW, in regard to your previous post, the o-rings I purchased from Sudco are fuel compatible.

[vwlifer64]

Brian,
I found your post while researching the exact same problem. My carb is on an old Cushman scooter though. Here is a link to a page that provides 3 different sources for what is supposed to be the correct part. I'm picking mine up from the John Deere dealer today - $1.04 each. Hope these work for me and you.
http://vintagesleds.com/bs/index.php?PH ... ode=mobile;