Need Mikuni help / idle speed too high

noseoil

noseoil

Active Member
#1
#1
Finally fired up the new motor & I have a small problem with the Mikuni 22mm carb. It seems to run a bit fast, as in really fast at idle. Here's what it does at idle...

Dirt.jpg
Dirt-1.jpg

I changed the TAV springs to white ones (3100 engagement) & it's idling fast enough to spin the rear tire at idle. Should have had a youtube of the start-up when I cracked the throttle, would have had a lot of "likes" as it was happily digging the holes & churning up a lot of dust & gravel in the front yard. Fortunately, I hadn't started the dinner (chicken) yet on the grill, so dinner should still be OK. I did set the front tire against a porch post (steel post set in concrete, porch is still standing) when it finally fired & took off like a raped ape.

My question is how the heck do I adjust the idle a bit lower? The slide is mostly down now, but there's still enough air moving through the gap at the bottom to idle way too fast. I've backed the screw on the outboard side mostly out to lower the slide, but it's still letting way too much mixture into the motor at idle. I don't want to ride this thing until I have this problem solved. The good news is that it seems to pull really well now.
 
TheDullCarbide

TheDullCarbide

Active Member
#2
#2
You built that engine, and now it wants to serve! This is what it was born to do!:scooter::thumbsup:

I'll be referencing my VM34 off of my 1980 snowmobile. It's the closest thing I have to gauge off of.

So if the idle speed screw is backed out and the throttle cable is not putting tension on the piston at idle and you don't have any vacuum leaks, the only thing left is your idle circuit. The amount of mixture that the idle circuit supplies should not be enough to get the engine to idle. The slide being cracked open by the the idle speed screw should make up the difference in order to get it to idle. Obviously in this case your idle circuit is giving the engine too much mixture. I would try jetting down the idle and turning in the idle air screw to compensate.
 
danford1

danford1

#3
#3
It sounds like your throttle cable is to tight and it is holding the slide up. Do you have an adjustment on your twist grip ? Try screwing the cable fitting in more after you loosen the lock nut. Do you have a threaded adjustment on top of your carb? Maybe it is under the small rubber boot? Loosen the lock nut and screw that in too. You want all the slack out of the cable without having slack if you know what I mean. The slide has to be all the way down. The only thing holding it up should be the idle speed screw. You'll have to screw that back in on the carb to set your idle speed where you want it.
Report back and tell us how it went.

Danford1
 
H

Havasu Dave

Moderator
#4
#4
A guess: The cable is too tight, not allowing the slide to come all the way down? I used the standard mini bike throttle cable with a soldered piece of brass tube on the end which fit the Mikuni slide. I was able to set my cable free length that way outside of the carburetor bore, with just enough spring tension to hold it.
 
delray

delray

Well-Known Member
#5
#5
does kind of sound like the slide tube might be up just a hair. look inside the carb and see where it is sitting and then unhook the cable to the slide tube and look again. there will probably a point where you will need to maybe change your pilot too. unless you got lucky and have the right one in there. wrong pilot will cause couple different problems,bike can surge at low speeds. also if the idle adjustment is less then a turn you need to go with a different pilot and same with over 2 turns. 1 1/2 (+ or - little) ideal setting. i really don't like the comet unit. get to much unwanted vibration when running a aluminum flywheel and the stall engagement tends to vary after unit gets real warm.
 
MB165

MB165

Active Member
#6
#6
yeah hopefully its just a simple adjustment, maybe check for vacuum leaks too, spray some carb cleaner, wd40 etc around the intake sealing areas while its idling, does it cause a change?
 
noseoil

noseoil

Active Member
#7
#7
Thanks guys, I'm doing some work this morning & won't have a chance to work on it until later today, but I appreciate the input. Will look at the cable, idle adjustment & slide to see what I can find.

I've never worked on this type of carb before, so it's Greek to me for now. I'll report back to let you know what I find. It's so simple, I'm sure it must be something pretty basic I'm missing, but I'm going to try starting it on a stand this time. At least I don't have to worry about it walking around or making a hole in the house. Sure glad I had it against that post.

I've gotten the holes filled in. One of the lawn chairs had the seat covered in dirt & gravel about 15' away. If I ever figure it out, it should pull like a train. Hoping for somewhere between 15 & 20 hp. See my build thread for what's inside under clones/predator motors...
 
TheDullCarbide

TheDullCarbide

Active Member
#8
#8
I almost wonder if the slide was put in facing the wrong way. The first time I touched a Chikuni, it was a knock-off VM-18 on my buddy's Honda 200 ATV. I helped him get it running after he bought it. I did the carb clean and put the dang slide in backward:facepalm:. Everything fit except I didn't realize the locating pin in the side was holding the slide open at 3/4 WOT. Started the thing up (in neutral thank God) and it revved sky high.
 
noseoil

noseoil

Active Member
#9
#9
Hmmm, have to remember that one DC, but I got it running now. Basically two things working against each other ended up being the problem. The tip-off was some WD40 poured against the intake manifold which changed the idle speed.

1 - The bolts I had for the intake were just slightly long, so they felt tight but were actually bottomed. A hex head won't fit & turn.
2 - The gasket at the head (same connection) was bad & letting a lot of air through.

Went to ACE & got some metric, hardened cap screws with a larger drive hole.
Got a spare gasket, matched it to the intake (too small, a factory type) & sealed the leak.

Next is a bog at 1/2 throttle. Dark out now but I think it's just jetting or the clip on the needle which I can change. Pulls well to 4500 rpm, then it bogs, seems lean to me, so I'll have a look at that one tomorrow. Thanks again guys for the help, much appreciated!
 
TheDullCarbide

TheDullCarbide

Active Member
#10
#10
noseoil said:
Hmmm, have to remember that one DC, but I got it running now. Basically two things working against each other ended up being the problem. The tip-off was some WD40 poured against the intake manifold which changed the idle speed.

1 - The bolts I had for the intake were just slightly long, so they felt tight but were actually bottomed. A hex head won't fit & turn.
2 - The gasket at the head (same connection) was bad & letting a lot of air through.

Went to ACE & got some metric, hardened cap screws with a larger drive hole.
Got a spare gasket, matched it to the intake (too small, a factory type) & sealed the leak.

Next is a bog at 1/2 throttle. Dark out now but I think it's just jetting or the clip on the needle which I can change. Pulls well to 4500 rpm, then it bogs, seems lean to me, so I'll have a look at that one tomorrow. Thanks again guys for the help, much appreciated!
Click to expand...
Glad you got the idle problem fixed. That was going to be my next shot in the dark. Sounds like you could use a good needle raising and upjet. Then the bike will dig holes when you ask it too!:thumbsup::scooter:
 
noseoil

noseoil

Active Member
#11
#11
I'm at a 125 main jet now, with the needle in the middle slot & a 15 pilot. Going to richen the idle a bit & check that tomorrow first to see if it helps with the transition.

Then planning on moving the needle to the bottom & see how it does. The main jet may be a little too small still (130 next maybe?) but until I get rid of the stumble & bog at 1/2 throttle, I'll just work it this way. If the 125 is small, will go to the 130 & back to the middle position & try again.

I'll post results tomorrow when I find a better combination & report back on the tuning process until I get it dialed in.
 
TheDullCarbide

TheDullCarbide

Active Member
#12
#12
I couldn't tell you what jets to run on your motor, I just using the common sense I have with these carbs. If you raise the needle, that shouldn't effect your WOT A/F ratio. A plug check is never a bad thing to do either, it just takes longer.
 
noseoil

noseoil

Active Member
#13
#13
I posted this to my engine build thread, but hopefully someone will be able to help with my problem here...

OK, I've tried everything I can think of. Float level seems about right. Fuel level is about 9mm below the bowl-edge/carb body, so there should be plenty. Checked the needle valve & passage.

Here's what's really strange, one article I found this afternoon said to REMOVE the main jet & run the idle mixture & pilot jet only to set the idle. I did this. The article said to run the motor without the main jet & it should run up to 3/4 throttle if it's right, but then load up at WOT. If it sputters at 1/2 throttle, the jet needle is wrong. I still can't get past 1/4 throttle without it shutting down like it's out of gas.

Do I need a fuel pump?
 
TheDullCarbide

TheDullCarbide

Active Member
#14
#14
noseoil said:
I posted this to my engine build thread, but hopefully someone will be able to help with my problem here...

OK, I've tried everything I can think of. Float level seems about right. Fuel level is about 9mm below the bowl-edge/carb body, so there should be plenty. Checked the needle valve & passage.

Here's what's really strange, one article I found this afternoon said to REMOVE the main jet & run the idle mixture & pilot jet only to set the idle. I did this. The article said to run the motor without the main jet & it should run up to 3/4 throttle if it's right, but then load up at WOT. If it sputters at 1/2 throttle, the jet needle is wrong. I still can't get past 1/4 throttle without it shutting down like it's out of gas.

Do I need a fuel pump?
Click to expand...
First of all, that article has a pretty good idea there. As for the fuel pump, I wouldn't be surprised if you need one. The fuel tank isn't too far above the carb bowl and the fuel line loops just a little above the bottom level of the tank. Maybe try to shorten it and if not try a fuel pump?
 
noseoil

noseoil

Active Member
#15
#15
Still stumped. I changed the float setting slightly. It was set at about 23mm, so I changed it & finally came to rest at about 24.78mm (between 24 & 26mm is what it should be). When I put the bowl drain tube against the bowl & open the screw, fuel level is at about 2/3 down on the two small casting bumps on the top area of the bowl (7-8mm maybe). There should be enough fuel in the bowl to at least begin running on the slide beyond 1/4 throttle, but it's still not happening.

Two things which I had forgotten to mention, not sure what it means.

1 - The bike will die at idle when it's leaning on the kick stand over about 20-30 seconds. Not sure why but I suspect fuel level with the angle changing. It's fine when the bike is sitting level & stays idling.

2 - The bike won't start with the the choke plunger in the down (starting) position. To start it, I have to shut off the ignition switch, pull the rope twice with the plunger down, open the choke to the running position (up), turn on the ignition, open the throttle 1/8 turn & then it will start.

Not sure what this means either, but that's how it works with this carb.
 
danford1

danford1

#17
#17
It sure sounds like your carb is jacked up. Do you have another carb or a friend with one? If the other carb starts and runs good then
get another carb. I spent Way to many hours playing with a carb I was determined to fix. I never did, it beat me. I got a cheap China carb from eBay and it ran great...
Here is a cheap one for less than $15 with free shipping from Dayton, New Jersey.
Danford1
22mm Carburetor Carb for Motor Honda Engine XR-50 CRF-50 XR-70 CRF-70 48mm Mount | eBay
Danford1
 
noseoil

noseoil

Active Member
#18
#18
Found one up in Phoenix for the right price. Will try picking it up later this week. Too many things happening today now.

Lever "down" to start, "up" to run, but still no luck. Sorry I mis-spoke about the lever vs. plunger thing Ole.

Aaron at GPS was kind enough to walk me through a different scenario on the phone. Did the three things he asked:
1 - Reset gap at flywheel to .060"
2 - Use NGK plug instead of Autolite "X" plug
3 - Change main jet size

Did all, still no luck, maybe a bad carb? Aaron said that even with the jet wrong, it should still get up on the throttle, at least until the bowl is drained of fuel which would take a little while.
 
noseoil

noseoil

Active Member
#19
#19
Ordered a new carb. Starting over, something wrong with this one. Don't want to go back to a "stock" carb, flows 1/2 as much as the Mikuni (thanks Ole for the info on cfm)...
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