Needle Jet Removal VM20

[Champ]

I'm new to the group. So my attempts to search for this subject has failed. How do you correctly remove and replace the worn needle jet from the VM20 carb that's on the 100? Thanks!

[Zyx]

Remove carb, dump gas, invert, remove four screws holding float bowl in place. Remove bowl, then remove the little o-ring on the needle jet. using crescent or the right, but tiny, wrench, gently unscrew the jet.

Why do you think the jet is work, as opposed, say, to the needle. These parts rarely wear out if you run with a good air filter.

[Pep]

Hey Champ, in case it's not clear...once the main jet is unscrewed from the needle jet, the needle jet should slide out the top of the carb (where the slide exits). Sometimes it needs a gentle tap to move along.
If the finish on the needle taper looks worn (changing colors), it's a good idea to pull the needle jet and inspect the inner bore for roundness. The needle and the needle jet wear together - usually wise to replace both at the same time.
Relatively cheap replacement, so if in doubt, swap it out. Wear at the needle jet can cause pesky uneven running in the mid-range.
-Laurie

[Champ]

The engine is running rich and fouling plugs about every 15 minutes. The plug is carbon fouled and I have re-jetted the main LEAN to the point of cutting off at wide open throttle but still no difference in fouling. The idle mixture is also as lean as I can get it. It cranks first kick and idles fine. The jet needle was worn but replacing it did not make any difference nor running a hotter plug. The air filter is new and clean, and the spark arestor assembly is clean. The float level is set correctly. So the only thing left is the needle jet. I really need it out to compare it to the new one I have ordered. This is my daughter's stock '71 Ace 100 B+ except I fitted a Dirt Squirt air box which significantly improved performance. It ran great 2 years ago but has gradually degraded. I have consistently run pure gas 93 octane mixed 40:1 with WP2 oil. We put new bearings and seals in about 40 engine hours ago. Any other ideas as well as the current responses will be greatly appreciated!

[Dale]

If it ran fine 2 years ago and now it is too rich with that same jetting, then I suspect you are leaning it out to compensate for a seal that has failed. In this case (too rich and fouling plugs) the likely culprit would be the clutch side crank seal. There is something other than carburetor issues here. A leak down test is pretty easy to set up for and will identify any seal or gasket issues.
Dale

[Champ]

Good point Dale. I figure if the needle jet is not worn, that's my next direction. I suppose I can search here for the leak down test?

[Dale]

Yes, there are several posts that discuss air leakdown testing. Here is a link to one where I posted some pictures. It also points out where a good article is located in the Technical Tips section at Strictly Hodaka. I would also suggest doing a search for "2 stroke leakdown testing" on YouTube. There are some good ones there too. Keep in mind that this is a cheap and easy test to identify a problem that will wreck your motor if not resolved.
Dale

viewtopic.php?f=1&t=604&p=2571&hilit=ca ... buff#p2571

[Champ]

Thanks Dale! I'll post results. It's hard to believe that the new seals will go bad after 40 or so hours but I guess it can happen.
Arizona - I think mine has to be tapped out as Laurie describes because the needle jet body is flush with the hole in the bottom of the body. Therefore, there is nothing to hold on to after the main jet is removed. I have a spare body with a needle jet in it that I'm using for practice.
Laurie - does it re-install from the top only?
--Champ

[RHall1972]

Hi Champ,
I'm sure Laurie can explain it better, but a wooden dowel works great for gently tapping out the needle jet. Then, to reinstall it, I use one of those cheap "retriever/grabber" tools to hold it and reach into the carb body. http://www.homedepot.com/p/GrabEasy-Gra ... /202505170

--Russell

[Champ]

Russell - Thanks for the tips! Do you remove the needle jet by tapping it from the bottom (bowl side) so that it exits through the top? Or the other direction?

[Zyx]

They are right, I left out the tap it out the top part. Needle jets are pretty simple, and I doubt very much that this is the problem. Look through some of the tech articles on the Strictly Hodaka site, and there is one about hop up tips for the Super Combat (I think it is the second such article, not the first on the list). Now I know that isn't what you have, but one of the modifications shown in that tech article is the drilling out of the needle jet, and shows one in cross section top to bottom, so it is easy to get an idea of what one looks like inside. While the needle jet is a critical part of the total function of a Mikuni, I would not expect that jet to foul out an engine unless you leave the jet out when rebuilding. That "might" foul a plug every 15 minutes, but I wouldn't bet on it, and the needle itself is softer than the jet, so it usually shows sign of torture first. If the needle is not worn out, the jet isn't either.

I'm thinking the other folks here are right, and it is a crank seal (clutch side is exposed to oil, ignition side is not). It is a bit tedious to do a leak down test if you don't already have the parts to do it, but replacing the seal isn't a two minute job, either. The alternative can be to trust the diagnosis, and proceed to replace the clutch side seal, because that is far more likely to be the problem than the needle jet. Maybe I just never beat on my bike hard enough, but I have never seen a worn out needle jet. My poor little 24 Mikuni, which I used on a Webco kitted Rat engine from 1973 to this year, including two years of cross country racing in 73' and 74' shows no particular signs of wear on the needle or the jet, and if the Webco didn't run perfectly any more, it did run, and has gotten me into the woods and back on the same plug I have had in it since 1980. I took the engine out of service this spring because I could not find sources for Webco parts to overhaul the top end, so I bought a Super Combat motor for it, instead. Point is, unless you ran the bike with no air cleaner, it isn't likely to be the needle jet doing this. I would aim for crank seal or seals and just do it, because even if you find it is leaking down, the test doesn't really tell you where it is leaking from unless you are luck enough to get at the leak with soapy water or other tracer. Several places can leak under pressure, but only the crank seal on the clutch side is immersed in oil, and the suction of the intake stroke sucks in a bit of oil every stroke. THAT will foul a plug in a matter of minutes. Most every other kind of seal or gasket leak will make it run lean, not rich.

By the way, if you do decide to tap out the needle jet, memorize the orientation of the half skirt that you can see in the throttle bore, so you can put it back correctly. I don't remember if the jet has an orientation pin or whatever to keep it from being put in incorrectly. Haven't had one out since maybe 1973, and frankly I don't remember.

[Dale]

All good points, Arizona Shorty. Personally, I am a big fan of doing air leakdown testing and I have put together adaptors to fit every model of 2-stroke that I have. A bottle of Windex is a good choice to identify the leaks. I also agree that most leaks lead to lean conditions and engine failure rather than a rich condition and fouling plugs. So your theory to just go after the clutch side seal is solid.

I would still recommend an air leakdown test after the engine has been torn down and then re-assembled. Everytime. It is just cheap insurance to make sure that all is good.
Dale

[BrianZ]

I had a similar problem with a worn needle jet:

viewtopic.php?f=1&t=274&p=501#p501

Brian

[RHall1972]

Hi Champ,

Yes, you do tap it out from the bottom, so that it exits from inside the carb body. Arizona Shorty makes a good point about the orientation, but in the case of the VM20 there is indeed an orientation pin, so you really can't go wrong when reinstalling.

--Russell

[Champ]

Thanks everyone for the great advise. I'm purchasing hardware to perform the leak down test this weekend. I've built the same kind of rig for my turbo Ford for intake pressure testing so I have most of the parts. I like the idea of leak testing after engine rebuilds but never thought of it before.

I did note the orientation of the half skirt and will pay attention to the alignment pin. The needle was significantly worn and suspect the jet is too but can't really see the jet very good while installed. I have already tried replacing just the needle with no change in performance or fowling. I saw the article posted in Brian's response and that got me thinking. I wanted to try the jet replacement first since that's easier than tearing the motor down to replace the clutch-side seal. It's hard for me to understand how the seal will go bad after 40-50 hours on the motor but anything can happen. It's only been used as a pit bike at hare scramble races and about 4 hours of light trail riding.

[Pep]

Sounds like you're on the right path Champ. I'm with you - I'd explore results from a needle jet change before tearing down a recent rebuild. Quick and cheap - and suits my impatient nature.
The jet is not hard to re-install with just your hands (or maybe I have smallish hands ) But cool advice from Russell on the grabbing tool.
Be sure you keep it oriented with the guide pin when sliding it back in.
All of this will be humorously obvious once you have things apart.
Good luck and please keep us posted on your results!
-Laurie

[Zyx]

We all try the simple things first. That's our nature, and frankly it works more than half the time. As to why a seal would go bad, there are two or three possibilities that come to mind:

1) the seal was NOS going on 40 years old, and did not have the vitality it needed to last. If you have to replace the seal, be sure to get a new, new seal, not an old, new seal.

2) the seal was either damaged on installation or installed wrong. These things do happen. A small scratch, or getting the seal lip tucked in on itself during install could set the seal up for failure.

3) the seal failed because the crank bearing was bad, went bad, was incorrectly installed or whatever causing the crank to wobble on its axis, which will wear out a seal in short order. In that case you will be looking at replacing the bearings as well, which is not a bad idea if you have to break the case apart anyway. Also inspect the bearing seat to verify that the bearing did not wander and displace aluminum.

And while you are in there, inspect the seal running surface of the crankshaft for damage. Even good seals, after years of use, wear out, and when they do, they often also wear on the running surface of the crank. If there is a small wear groove on the crank and that groove was not dressed out last time, it could cause a new seal to wear out faster than it should, or make it difficult to seal at all.

I guess I never thought of using Windex to find leaks. Whatever is handy and works. I always used a spray bottle of mostly water with a bit of dish soap or Simple Green, because it holds bubbles really well.

I kinda figured there was an orientation pin or whatever for the needle jet, but didn't clearly remember. but such pins, frequently brass, don't always stop folks from hammering the jet home incorrectly. There are many ways to do it wrong, and only one way to do it right. So don't use excessive force. If it does not go back together smoothly, find out why before you smack it. We all been there at least once.

[Champ]

All really good points!! My confidence level is now 300% better. Looks like I have my work cut out for me this weekend. Just picked up hardware to build leak tester. I have now learned a lot from your posts and greatly appreciate it! I will post everything I find out as soon as I can.
--Champ

[Zyx]

Just to put Mikuni carbs in perspective, my wife once took an adult education course in motorcycle maintenance. I don't remember why, because she didn't know how to ride, and wasn't allowed to work on my bike. Then I visited her class one night, which was being taught by my Hodaka parts supplier and riding buddy who taught shop at the community college. She was sitting on the floor with a Mikuni completely disassembled in front of her, and as I watched, she put it back together in like 5 minutes without help from anyone. She thought it was easy and was miffed that I wouldn't let her work on my bike. Go figure.

[Champ]

My new needle jet should arrive by mid week. I disassembled the carb today and the needle jet doesn't really look worn even though I don't have one to compare yet. So I'm thinking of plan B. I have a few more items to get to make my leak tester. I did find black oil in the exhaust port when I removed the exhaust pipe. Is that more indication of a bad crank seal?

The clutch side seal has to come out through the crank case, right? Am I remembering correctly that the seal can't come out the clutch side due to the crank bearing is shouldered on the outside of the case? Where is the best place to get NEW seals? Will a bearing supply have them? I have one in town and are pretty good at matching odd bearings. Thanks!

[Zyx]

I have to refer to the shop manual for that question, having not done an engine case tear down since the late 70's, but the pictures in the shop manual show the clutch side seal inboard from the bearing, so yes, the cases have to be split to get that one out.

Write to either Paul at Strictly Hodaka or Bill Cook at Hodaka Parts, Inc and ask how old their seals are or where they get them. Seals and bearings usually have a reference number on them, and if you have one with a number, run the number on the internet and see if they are still available, which I suspect they are. The parts guys have to be getting them from somewhere, but if seals from the Hodaka parts specialists like Paul are fresh, I would buy from them to support the community.

[Champ]

Finally! I was able to work on the bike this week after being under the weather for weeks and backed up at work. So, I think the plug fouling is solved! The carb parts came but did not change the main needle jet. It looked new compared to the new one I ordered. So I checked the float setting and re-assembled the carb. Before installing it, I pressure tested the motor at 6 psi and it was tight. I installed the carb, etc. and got bronchitis.

Having some thinking time, I remembered that last fall, I changed the gearbox oil to automatic trans fluid to get a better clutch release. So I took the pains of emptying the case completely and putting gear saver oil back in, thinking that the AT fluid was too thin and slick for the crank seal. After one test ride, my daughter lowered the jet needle all the way down (from #3 slot) and it runs perfectly!! We put about a half hour on it today with no running issues and very little blue smoke out the pipe. I pulled the plug and it's burning brown on the ceramic. I think we avoided a tear-down!

In summary, I think the AT fluid was causing the fowled plug issue. I had forgotten this when I started this thread. Thanks for all your help and comments!

[BrianZ]

As for removing the needle jet, what I do is remove the main jet then thread a long metric screw into the needle jet. Gently tapping the screw will drive the main jet up and out of the carb.

For bearings and seals, I usually use a local bearing supply house. They can provide pretty much everything I need. When you get the seals make sure you specify double lip seals as they need to seal the crankcase from both positive and negative pressures.

As you suspect, the clutch side seal can only be accessed by splitting the cases. It will need to be gently pryed out of the cutch side crankcase half. Don't try to remove it by driving the bearing and seal out simultaneously. There is a circlip between the seal and bearing!

Brian

[Champ]

Thanks BrianZ! The needle jet removal trick is a great idea. We have a bearing supplier in town that stocks or can get anything. Next time I tear it down, I'll go that route. I had completely forgotten about the circlip too. I hope that changing back to gearsaver oil fixes my problem. Now, I have parts o tune the carb with.

[2.stroke.smoke]

I recently was gifted a 1969 Ace 100 in near mint condition as one of the most bad ass wedding presents ever. I'm going to love and cherish this beautiful piece of motorcycle history.

I am working through the bike before I take it out, and it has carb problems, some lighting problems, but overall in excellent condition. I had a few questions to ask the group about it's Mikuni VM20 carb as I am getting stumped by a sticky throttle issue.

Initially I lightly cleaned the carb, replaced the throttle cable, emptied the gas tank, and cleaned the airbox, installed a new oiled uni air filter. Since then, it idles okay but it dies at anything above 1/4 throttle or sometimes - revs the ** out when WOT.

I'm going back through the carb, this time actually doing more than clean the float bowl and spray some carb cleaner through it. I cannot get the main jet out; I have the proper Mikuni wrench but when I rotate the main jet, the entire needle jet rotates as well. In addition, the Needle Jet has a C shape when looking through the carb body, and now that the brass is rotating, once I get the main jet out and needle jet / needle replaced with new - what orientation do I want the C facing? a) Engine > Airbox or b) Engine < Airbox . I plan on replacing both the needle and needle jet, and I hope not much more than that for now.

This carb has a weird Idle setting and Carb top that I don't understand. The Idle screw is on the Carb top, right next to the throttle cable. When you pull the throttle, the slide moves and this bar or pin in the center of the idle screw moves up too, but you can put your finger ontop of the pin, pull the throttle, and then it stops moving but the carb is set to always be ~20%open. I am baffled by this thing, and its purpose - I can't find anything about it in any mikuni information I've been able to dig up.


Thanks for the guidance

Mike

Portland OR