VM 26 tuning
 

Yes, I'm full of issues today. So, I put in my new Mikuni VM26-606 and it fired right up with the choke off. Will die instantly if I put the choke on. Motor is a 4V 155cc GY6 I'm at sea level running 20 pilot, 175 main, 60 degrees F, 110 octane leaded race fuel... This is an indication of too big a pilot, correct? Couldn't mess with it too much due to electrical problem detailed in another post :)

Not sure why you're worried about the choke. If it starts up without it, why apply it?

How is acceleration? Would be my question.

Yeah, just going by the tuning guide. Won't be able to test it until I solve my weird electrical problem.. Something shorting out when I crank the motor and melting the ground wire that goes from the harness to the CVT bolt..Like, full on smoke and stuff..not good..

Check your negative battery cable, connector, and connection.

Sounds like alot of amps are trying to travel through your small gauge wire causing it to melt down.

Hmm.. ok.. the cable was new (as the old one melted), but I'll clean all the connections.. I did a test crank when I got home and it fired right up without a issue.. I'll take a look at the connections..

Silly question, but I have to ask....

Are the wires you're replacing the same gauge (size) or bigger than OEM?

I'm sure you already did, but also look for any chafed/pinched wires, or burn marks from an elec arc.

If it was water that seems to have started this issue, maybe disconnect all your elec connections, make sure it's dry and add some dielectric grease to them to keep water out.

By doing that, I'm hoping you will inspect all of your elec system and maybe find the source of your problem.

You may be right.. Negative terminal was a bit loose. Tightened it up and seems to have corrected the problem..

Yup..I think you were right on.. Probably wasn't even the water the first time...

Just waiting on the correct throttle cable to arrive....

If it starts cold without the choke, it is rich.

I don't know if you have found this yet.
http://www.mikunioz.com/vm_tuning_manual.htm

Nice find 351.

You're prob right about the choke thing, but mine starts up without the choke (TM 28) and takes off great. So I don't mess with it

So, the theory would be that my pilot jet is too big, correct? I'm still waiting for a throttle cable so I can't work on issues past idle.. Yes, I have the tuning guide, but thank you!!! :)

threw in a 17.5 pilot.. I'll try it out in a little while.. The Pats are on.. :)

Hi Folks,
Back from some army stuff. still boggy with a 17.5 pilot. fires right up without the choke, no-go with choke on.

Sounds like then it's to big of a pilot jet or maybe try turning the fuel mixture screw in ?

I have the same carb. On yd gy6. 155cc. 1500 feet roughly. Only half to use the choke when air temp. Gets in 50's or below. Even then only need to choke it once on full choke. Will die shortly. Then take choke off and fires up. If try to choke after its running. Will die. I haven't changed the jets. Using stock ones it came with. That being said I believe at lower throttle I think I'm slightly rich. But mid, and WOT. spark plug check looks good. It does bog slightly on take off. I've read that down jetting would be best for current setup. (Even with A12 cam..high flow intake and exhaust. Which I have.) The stock jets are good once you get into the 175 Bb. Kits. But I don't have 4V. So not sure how that applies in your case. But sounds like your setup with that carb. Reacts much like mine does. Plan on getting a 175 bb kit. Its not running so rich that I think its a big issue. Since do want to up grade. But if I wasn't going to upgrade, I'd go ahead and down.

hey guys I am currently changing my carb also, I have a hammerhead and I am going from a cvk 24 to a oko flat slide 26mm. I am just wondering as my carb came with two extra sets of jets and pilots yet still none of them (the two extra sets and the ones installed) have any numbers on them. on my 24mm I had to go up on the pilot from a 35 to a 38 and up on the jet from a 111 stock to a 130... do you guys have any tips on carb adjusting?

You need a base line to tune from. Being that you have un numbered jets you don't have a baseline. The slide carbs tune differently than a CV carb in that not only will you be changing out the jets but also needle, needle seat and in some instances the barrel slide. When a slide carb is used on say a Honda dirt bike the factory will tune that carb for the bike. Unlike a cv carb If you have the same size carb on say a Honda and KTM bike of same cc engine they could be tuned totally different since a slide carb is more of a custom tuned carb as compared to a cv carb which is universal. Not only do the jets need to be numbered also does the barrel slide, needle, and needle seat. If not you will be shooting in the dark. Also a flat slide is more difficult to tune than a round slide. I would return that carb to get one that is numbered properly. Or you can spend the extra money for a Mikuni VM26 series 606. I would also pick up a book which instructs how to tune. They require more to tune but when tuned properly, performance is unmatched as compared to a cv carb. Also a flat slide is more temperamental to elevation, temperature and humidity as compared to a round slide.

O ok thanks, man sounds like a alot of work but i want to learn how to tune carbs and all, i want to learn these bad boys inside and out, thanks! I think i'll order me some jets so i ca have a good starting point

Just to cap this thread off, I'd mis-measured the float level on the new carb. It was off right out of the box. Got it back in spec and it seems to run pretty good. I've just ripped up and down the street a few times, but I should be able to do a real plug check this weekend and get it dialed in. Currently #15 Pilot, #145 main, clip one down on the needle, sea level, running 110 leaded fuel.. CHECK YER FLOATS!!! I ended up making my own harness to correct my electrical woes.

Unless you have a high compression motor, the 110 is probably hurting performance. The higher the octane the slower the burn.

Good to know. I get it for free and it's pre-stabilized though..

Free is good..lol

If you mixed equal parts of the 110 with some non ethanol 87 you would have a manageable 98.5 octane. Also keep in mind that if using avgas it is blended to burn at high altitude(less O2) so at sea-level(more O2) it will run lean.

I have heard stories of guys blowing up their motors using aviation fuel. I wonder if jetting would fix that problem?

I'm using sunoco standard race fuel, maybe a bit of a blend would work nice. Just don't blend my scotch!!

Wish I had access to free Sunoco 110 octane fuel. Last time I come back from Chicago my Son noticed when we stopped to fuel that they had Sunoco 110 @$7.90 / gallon. I had a 5 gal can in the truck and purchased another at the WalMart next door and got 10 gallons. Down here all I have access to is either VP or Renegade for $15 plus at the speed shops.

Yeah, it's not a bottomless supply and they sell it at the pump 20 minutes away from where I live..

Didn't get too much testing done last weekend. Throttle kept sticking open like the slide spring was binding or something, and couldn't get it to idle below 900rpm. I had my buddy steer it while I towed it back to the campsite with my truck, rolled it on the trailer and decided to spend the rest of my weekend over-indulging in adult beverages.. I'll take it all apart and maybe get some testing done this weekend..

Wise to have a backup plan. Buggy breaks ! Out come the adult beverages.

Yeah, I'd left it in gear so when I started it to test, off it went at full throttle, me diving in getting dragged like a cowboy with his hands caught in the reigns.. I think with all the taking the carbs on and off I might have stripped one of the clamps on the connector to the intake and I'm getting some air leak.. surges up and down and won't idle below about 800rpm..

hahahah I can laugh at this because this had happened to me a year or 2 ago started the yerf when not sitting in it and it took off on me and it just dragged me with it so now I don't start unless im in it.

I'm a dummy. I've had so much trouble getting this carb to work, I'd stopped looking at other parts on the throttle system. The adjuster near the gas pedal was all out of whack. Getting there. idle-25% throttle awesome, bogs and sputters mid throttle, runs like a champ 60%-WOT.

if everything in the carb is clean, try dropping the needle all the way in the hole and tune with the fuel screw. If you cant get rid of the mid range bog, then maybe go down one size on the main jet and bring the needle up to the middle notch .

You need to do a full throttle run, 900-1200ft. with throttle wide open. At end of run with throttle still to the floor cut the ignition. When engine stops close throttle, pull over, pull plug and read. Doing it this way tells you if main jet is too rich, lean or just right and takes the pilot jet out of the picture. Once you have the main jet just right then you can work on the pilot jet. When I done mine I had the clip on needle half way. Getting the main jet correct first makes tuning much easier and assures you the engine will not lean out at full throttle.

Yep, now that I have it running well enough (and safely enough) I can do a plug test. probably next weekend for that however. I got a bag of new plugs for testing and some good ones for when it's dialed in.

You don't want a new plug. Use an old plug it will show color better.

Oh, cool. For some reason I thought you needed new plugs. good to know.

Ha, looks like I did strip the clamps on the intake. Went to do some tuning today and my airfilter/intake stack kept coming off the carb. Tough to get it tuned with no air filter on.. Now to find where it fell off.. :banghead:

EDIT: $4.21 for four new No.24 clamps at NAPA..back to the dirt in a bit.

As it REALLY turns out, my new rear shocks were too soft and bottoming out causing the cargo rack to smack my stack and filter off the carb. Wrenched down the preload on the shocks as my as I could in the woods, and got some good runs in. After getting the floats right, I had to jet way up. currently #20 pilot and #180 main. Needle clip in the middle. Runs Awesome idle to 35%, Stumble/crackle/bog 35%-45%, and runs like a monster there to 100%. I just need to get that one pre-mid throttle area smoothed out, and I'll be good to go.

and as it turns out my needle is bent. That's why my carb was sticking open. The needle wouldn't go back into it's seat.. Holy crap. :cussing:

Buggy is running pretty good. I seem to have lost some top end, so I might bump up the main..