Mikuni VM26 stock needle/seat size ?????

[DHXrider]

Hey guys,

Anyone know what the stock needle/seat size if for the VM26?? I have a stuck
float needle/seat that I want to replace but just don't know what size it is,
I notice on-line they have different sizes, 1, 1.5, 2, 2.5, 2.8 etc.

 

[my67xr]

It's a genuine one? VM26 - 606 uses a 2.5
I'm pretty sure you can use a 3.0 or 3.5 as well but it shouldn't need that big if it's on a pit bike.

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[DHXrider]

cheers man, ya actually sorta jumped the gun on that one. It's marked right on it, 2.5. I've ordered one in.
Dunno though I have a sneaking suspicion it might not be the seat.... here's the issue:

engine YX160, few years old, good compression,
no air leaks that I know of. Engine has worked last year, sat over winter, stripped the plug threads so put a helicoil in this year. Had a weird issue with it not running right, took the carb off, cleaned it, didn't work. Took carb off again, fixed a taught throttle cable that had it revving on idle, got it running perfectly (thought I had fixed the issue, thought it was the throttle cable, and maybe that was part of it) But now the issue is back.

I'll get the bike just about to start and idle then it dies then I kick and kick and no gas get's to the chamber. dry as a bone.
I'll take off the tank and try to blow down the gas line into the carb but it's completely blocked. If I take the carb off, tip it sideways it unblocks, sometimes tapping it
will unblock it. I've cleaned the needle and seat with metal cleaner and carb cleaner, it appears to be in fairly decent shape, some very very slight stiction but nothing that should get
it as stuck as it gets. When it get's stuck, it's really stuck there until you tip it or blow it out with a compressor.

Also keep in mind, probably not a jetting issue, because I did get it running great in between all this, and obviously it worked great on the current jets and settings the previous two years so can sort of rule that out.

So guess I'll try a new seat and float but I'm not sure if that's the issue. Seems like it get's really stuck, almost like a pressure issue, like it's being sucked up and binding, someone mentioned that it might be the float getting stuck also, but when I have the carb off, turn it upside and blow down the gasline, it seals fine, then opens when I tip it at about a 45 angle, nothing out of the ordinary. It's a mystery issue. Pretty sure it's nothing to do with the head reinstall either because I had the bike running good for a while, it worked amazing all last year also....seems and feels like a carb issue. Somehow the valve needle is getting stuck upon starts.

For what it's worth, I actually upgraded from a molkt carb to a Vm26 because I was having the same issue with the molkt carbs on this same engine a few years ago, I tried 3 molkt carbs and couldn't solve the issue of the float sticking. I could unstick it, ride for the day and it would be fine, I'd leave it sit for the night and next day, it would be stuck again.
When I put the VM26 on there and figured out the right jets, worked like a champ for two years, now seems like it's going way of the molkt.

anyway, any suggestions or helpful comments are welcomed. More knowledge and troubleshooting options, the better.

Also could be a case of possessed engine syndrome, pretty prevelant in the pit bike engine species.

 

[my67xr]

For the needle and seat,
to check it can you take the carby off, hold the carby upright and pull the fuel bowl off and turn the fuel tap on.
Then carefully lift the float to check the needle is not sticking, and that it is sealing closed and not leaking.
You could carefully file a little of the edge's of the needle and seat's needle, would only take a couple of passes with a fine jeweller's file.
Check the tank filler cap vent is not blocked up too.


But with the spark plug being dry it could also be a pilot jet/pilot circuit problem.
I would check the pilot jet is still open, and that the hole's across it are open too.
And where it screw's into the bottom of the carby, look down at the bottom of the thread's, check you can see light through into the bore of the carby through the small hole there.
Even poke a fine strand of copper wire through the hole's in the jet 's and base of the carby incase there is dried gum/varnish restricting the holes.

Might even pay to remove the mixture screw and spring, and blow compressed air in through the 2 port's/hole's from the air filter side of the carby to make sure there's nothing blocking those circuit's too.
When you blow the air in to one hole, you should feel air coming out the mixture screw hole, then refit the mixture screw and spring, then blow the compressed air in again and the air should come out the pilot jet hole.
The other hole on the filter side is for the Main jet mixture/emulsion tube
If there's a 3rd port/hole on the air filter side of the carby that's for the choke enrichment circuit.

 

[DHXrider]

lol, Ya I hear you. There's only so many things that it can be. I've done most of those things already, pretty much the first go to troubleshooting steps right Clean carb, check holes etc. etc. but I'm going to try them again just for kicks. I did the dip overnight in carb cleaner then the air blow but not quite as meticulous as what you're describing so ya, I'll try it again.

For some reason I have this false concept that there is vaccum pressue pulling on the needle to keep it closed. But there is isn't there? like if your tank vents are plugged.... I'm wondering if it has something to do with that, not exactly a tank venting issue (checked for that, actually had the cap on too tight, again, thought that was going to fix it, but no)

but somehow vaccum pressue is effecting it from some other source or would I be off on that?

 

[DHXrider]

Just did a better inspection of the pilot circuit. Pilot circuit looks ok but the pilot jet might need changing after all. Getting some sketchiness when I blow through the circuit with the pilot installed, pull the pilot out and clean and clear. I'll change all those parts, basically a light rebuild and post back the results...

 

[my67xr]

Nah there isn't anything that could pull the needle and seat shut/lift the float
The vent tube's for the carby could be blocked up, or the hoses that fit on them
That would be the same as the tank cap vent being blocked

I'm pretty sure it'll be a problem in the pilot circuit though


While the carbys' off, check that the float level is correct too.
Hold the carby upright and so the manifold side is lower than the filter side, now tilt it so the float tab in the centre of the float arm just touch's the pin in the end of the needle (needle/seat)
The float should look square to the base of the carby.

If it's not you'll need to bend the centre tab on the float arm up or down to correct it
That picture above also list's the float height as 26mm,
so measure from the base of the carby down to the bottom of the float and check it's 26mm while the tab on the float is just touching the pin in the needle



edit just saw your reply.

Take the mixture screw out too and blow compressed air through the port on the air filter side like i mentioned above
If there's something in there blocking it there wont be much air flowing through the pilot circuit and will hardly draw up any fuel, if any

 

[DHXrider]

Yep. I followed the steps you've mentioned. There was definitely something sketchy going on in the pilot circuit, looks to be the pilot jet itself and maybe a little bit of gunk in the area just after the pilot jet, where that very small hole opens up into the carb. When I take the pilot out, everything is as clear as day. I can see a clear hole in the pilot itself but if I spray carb cleaner in, it's almost like it dislodges material and then it get's smaller and tighter, dunno, it's a really small jet #20, can be finicky. They're only like 5$ so I'm going to order a few tomorrow along with seat valve.

From all of what I seen and tested, especially the sketchy airflow when testing the pilot, that makes ALOT of sense because the bike would run sometimes, then just recently it would run, but idle was all over the place, would wad up then down to normal, just all over the place, very consistant with a sketchy pilot.

As for the stuck needle valve, dunno, just hoping somehow it's related to the pilot issue, clogged pilot creates a vaccum scenario in the carb? would that make sense?

 

[my67xr]

Nah i doesn't work like that with the vacuum problem


The pilot circuit supply's the fuel/air mixture for the idle and up to about 1/4 throttle.
If it's partly blocked or fully blocked then it'll cause symptom's like you have.

To clean the pilot jet just get a piece of automotive wiring, strip the insulation back and cut off a few strands's
Use them to poke through the hole's in the jet and in the base of the pilot jet thread's in the carby
The hole in the carby will be a bit bigger (about 50%) than the hole in the pilot jet

The Mikuni #20 jet's have about a .3mm hole through the middle
Do you have any fractional drill and a pin vise ?
Or a welding tip cleaner ?

 

[DHXrider]

Guys,

First of all, I appreciate all the help and feedback, having a community like this is pretty awesome right. Like instant troubleshooting. Hopefully I'll make it down under from up top someday .

Ok, so here's the update. My new pilots and seat valve arrived. I replaced them both and guess what, still the same problem. I'd go to start it and bam, kick and kick and nothing, no gas getting to plug. Take the tank off and blow through the fuel hose, stuck again. So I take the carb off, tilt it 45 and bam, free's up. So there really is no other explanation other than the float is getting stuck shut.....BUT HOW??? well I take the bowl apart and I bend the two floats just slightly outwards and I bend that brass fuel overflow pipe that comes up and in between the two floats slightly inwards away from one of the plastic floats (I suspected it was getting hung up on that). Another thing I did was when re-installing into the boot, I noticed my clamp was basically broke and not grabbing but it the SCREW would tighten up so it gave the illusion that it was getting tight and clamping the circumfrance, even though it wasn't (I suspect the seal around the intake wasn't good making a lean situation), I also moves the carb back a little bit in the boot.

Anyway, after all these things, I re-installed the carb and she fired up like new. Ripped around for a few hours and it ran like it always used to. So as it sits, I think I have this one wrapped up. Basically it was a combination of a lot of small little unusual things. Bottom line, listen to what the engine is telling you, post up your issues on the forums and they will get fixed, there's no issue that can't be solved. Thanks for all the quick replies.