Mikuni VM22 on built Predator Hemi

fhpe77

Active Member
#1
I finally went and did it and it was a great decision. The increase in power is amazing and it noticeably improved throughout the range. My Predator Hemi has the usual billet rod, billet flywheel, governor delete, SS valves, 26 lb springs, milled & ported head, 10.5:1 static compression ratio, Black Mamba Jr cam, and pipe. I built it last year and ran a jetted stock carb (40.5 main & 18 pilot). It did well and had plenty of power. It also seemed to run out of steam at ~6600 rpm, just wouldn't pull much after that. This was sufficient to get my bike and 230 lb self up to 50 mph pretty quickly but I suspected there was more performance to be had. I recall looking at the stock carb while porting the head and thinking that the carb was going to hold the engine back from it's full potential. This certainly was the case.

I purchased the NR Racing "Chikuni" kit with short adapter. Jetting is 130 main and 15 pilot. Now it keeps pulling and I'm not sure where it tops out. It is so scary that I don't think I will ever find out what it's capable of. Going from part throttle at 30 mph to wide open pulls the front wheel off the ground. This occurs with me seated forward and leaning over the handlebars. It's terrifying.

One of the problems that plagued the stock carb has been corrected. It had less than 1/8 turn from idle to full throttle with my direct linkage. While I got used to that, it did make for a twitchy ride. The Mikuni throttle setup has 3/8 turn from idle to full throttle. It seems a lot calmer until you go WOT.

I had a year to think this through. Initially I wanted to go with a VM22-133 "real" Mikuni. After lots and lots of reading I concluded that I like the "Chikuni" more because of its popularity/availability, lower weight, smaller physical size, and readily available jets. I'm glad I made this decision. Upon opening the box, I was greeted with a beautiful part. The castings are of excellent quality. Fit and finish is spot-on. You read a lot about folks having trouble getting these setup. That was not my experience. I opened the bowl, verified the float level, and installed the 130 main jet. It came with the 15 pilot already installed. Based upon various readings I concluded that the 15 pilot was a good place to start with a modified engine. It seems a little rich at idle and I may install the 12.5 pilot when it warms up outside. Adjustments involved placing the bike up on wood blocks to get the rear wheel off the ground and alternating between the throttle screw and the air bleed screw. It was a brief iterative process that took less than five minutes. With the idle where I wanted it, I took it for a spin. Throttle response is great - no lag, no hesitations throughout the range. And of course, the power is outrageous. Not a little improvement, a huge improvement. Oddly enough, the engine has a sharper exhaust note and the cam chop is more pronounced. Sounds absolutely beastly now.

I regret not doing this sooner. If you were on the fence about the Mikuni, just do it. You will not be disappointed. When you have a little motorcycle, running a lawnmower carburetor does not make a whole lot of sense.

-Ray

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delray

Well-Known Member
#2
yes, it's day and night when comparing a lawn mower carb to a mikuni carb...

i'm surprised that engine doesn't take more jetting.
 
#3
Same jetting I have on an HS40. My metering needle is centered. Note that right now NR is closing out Chikuni's for $20 each. I've had two of these carbs now, and plan to use one again on an even smaller engine.
 

fhpe77

Active Member
#4
Put a new spark plug in it yesterday so I could get a baseline A/F reading. It seems to be developing nice a tan color towards the center and a little sooty on the outside edge. My nose tells me its a bit rich at idle and the spark plug reading seems to confirm this. You may certainly be right about it needing more jet, I've just begun playing with the bike again and this is uncharted territory.

-Ray
 

delray

Well-Known Member
#5
need to get some good pulls up 6000+ rpm's and then shut down the motor. don't let it idle at all. shut it down..... and then you will see the color your looking for......

see a lot back forth talk about the mamba JR I believe isky rates them at 6000+ and some guys claim they pull good up to 6500+
 

fhpe77

Active Member
#6
Delray,

Perhaps you are right about the cam. I use a GPS app on my phone to measure the speed then plug that value into a spreadsheet to get the RPM's. The fastest I went before chickening out (with the old carb) was 50.1 mph which means the engine is turning ~6,600 rpm. It seemed like it really wasn't pulling that hard just up to that point. Maybe that's all it has regardless of the carburetor.

-Ray
 
#7
You probably need more cam if you want higher revs. Not necessarily more lift, especially with a HEMI, but more duration. I'd suggest the NRR Mongoose cam.
 
#8
When you say it’s rich at idle where is the idle air bleed set? If pilot is correct you should get the best idle quality between 3/4 turn out to 2.5 turns out. If you have to go more than 2.5 you need leaner pilot, if you are at less than 3/4 then richer pilot. This is assuming you set the idle slide height correctly otherwise all bets are off.
 

delray

Well-Known Member
#9
Delray,

Perhaps you are right about the cam. I use a GPS app on my phone to measure the speed then plug that value into a spreadsheet to get the RPM's. The fastest I went before chickening out (with the old carb) was 50.1 mph which means the engine is turning ~6,600 rpm. It seemed like it really wasn't pulling that hard just up to that point. Maybe that's all it has regardless of the carburetor.

-Ray
still sounds like a good cam for you project build. I like to try that cam in a built hs-50 . I would have to send tim isky a core first. I was very please with the first cam(different profile) he ground for my ohh motor. that little guy was turning 8000 rpm's at 125 ft.
I think once you get that carb dial in you will be good to go. question for you when you start that engine cold. can you click the choke on and first pull does it start up and stay running? I have found out once I have the correct idle/pilot/ jetting with that type of carb. it pop's all day on the first pull. I found when the low ciruit is very close or spot on the choke works well. it's just something I have experience on my last to built motors. but then again they are tecumsehs and they don't count...lol
 

fhpe77

Active Member
#10
Ole,

I have the air bleed screw at two turns out. My perception that it is rich at idle is due to the smell of the exhaust. It's not awful and there's no visible smoke. Kinda like running a Holley double pumper on a cammed street car. Those things always had a rich idle circuit out of the box. You can tweak them a little by drilling the primary metering block air bleeds to lean it out more. It's supposed to be 50 degrees and dry out on Sunday. I'll go through the adjustment process again and see what that gets me. Presently the slide is not set very high so I think I'm close.

-Ray
 

fhpe77

Active Member
#11
Delray,

What? Tecumseh's do count! :) I love my H35. It is the reason why I swapped out for a Predator. I figured why beat up and wear out a low hours classic engine that is no longer in production? I got it running great with a new carb and now it sits waiting for some future project, maybe even put it back on this bike after the Predator finally scares me enough.

8000 rpm in 125 feet? Bet you had to hold on for dear life. I just plugged in 8000 rpm into the spreadsheet. That would get me over 60 mph. I had considered going with the Black Mamba initially but chose the Jr for better all-around ridability. Knowing what I know now, I don't want to go 60 mph on this little bike.

One article that I read stated that once you get the VM22 close, you don't need the choke/starting enrichment to get it started when cold. Presently I just crack the throttle and it starts in one pull. Next time I go to start it I will try the choke and not crack the throttle.

Yes, please have Tim grind that cam for your HS50. I would love to hear that run.

-Ray
 

fhpe77

Active Member
#12
I spent some time yesterday playing with the carburetor adjustments. To get it started, I tried with the cold start enrichment on. It would pop with each pull but not start unless I had the throttle cracked slightly. I guess this indicates a rich condition?

After some warm-up time, I slowly turned the air valve screw in and observed an idle speed increase with each 1/4 turn. I kept turning it in and the idle speed didn't begin to drop until the air screw was almost turned all the way in. I backed the air screw out to about 1/4 turn and then went to adjust the throttle stop screw and found it to be almost all of the way out, that is, the slide was almost all the way closed. I can lower idle speed by turning out the air screw to about 2-1/4 to 2-1/2 turns out. I took it for a spin around the block and it seems to have nice throttle response.

Would this indicate that I need a smaller pilot jet?

-Ray
 
#13
It sounds like your throttle cable is too short, from the point it enters the top of the mixing chamber, to the indentation where the ferule goes in to the round slide.

This condition does not allow your slide to go down far enough. That is why you have the mechanical stop needle so far out.

The result is that your carburetor is seeing a "high idle" setting, and you are compensating it by first leaning the air bleed (that is why "idle" RPM increases which turning it in,) and then choking the air off, as the bleed becomes closed off.

It sounds like your pilot and main jets are correct, and that your metering needle is adjusted correctly as well. I had the "exact" same thing going, but knew where to look, since I make my own cables.
 
#14
Addendum: You can quickly verify how deep the throttle slide is going into the mixing chamber by disconnecting your throttle cable at the throttle. Next, remove the mechanical idle adjustment screw, pull it and the spring out, and visually check the position of the adjustment slot that is built in to the slide with a flashlight. It should be near the top of that "detent."

Normally, throttle slop of free play at the throttle grip would tell you if you are bottoming out the throttle, but it won't tell you if your throttle slide is hanging up.

If you were only describing the air bleed symptoms, I'd say that you are too rich on the pilot jet. Rule of thumb is that if the air bleed needs to be more than two turns out for best idle, you need to go to a smaller pilot jet.

I apologize for not being more simplistic. Checking this is easier than writing about it. If you require a size 12 jet for your application, OK, it's definitely on the small side. I just recommend checking the slide first, to make sure you are not trying to adjust a carb that is above 1/8 idle, which is opening up the main jet slightly.
 
#15
As dave said above, but make sure you have the enrichment off. Plunger should be down, If not you are letting extra air and fuel in. When using the enrichment device it does raise the idle rpm considerably.
 
#16
As dave said above, but make sure you have the enrichment off. Plunger should be down, If not you are letting extra air and fuel in. When using the enrichment device it does raise the idle rpm considerably.
I didnt say that! LOL! I said that it sounds like the SLIDE is not all the way down, hanging up, or because the throttle cable is too short. Also said how to verify. If that is good, then yes, go down a size on pilot jet. Tune to two turns out on air bleed. Mechanical stop on slide should never be nearly falling out to adjust for idle. So easy to do in person. Much harder over the internet, troubleshooting by telepathy with no photos, and no hands-on. Just trying to help.
 
#17
My grammar was off what I meant was what you said was correct but if he had his enrichment on instead of off even with the slide completely closed it would still idle fast as that opens a big hole behind the slide letting in fuel and air, so if he was trying to adjust with the enrichment on the adjustments would all off. Sorry for the confusion. I sometimes have trouble converting what I am thinking to typed words.
 

fhpe77

Active Member
#18
Easy there fellas. I really do appreciate all of the help and attention that you kind folks are giving me. You guys are the best! No sense in getting our collective kinckers in a twist.

That said, some of this is my fault. I did leave out some details that may be adding to the confusion. One important one is that I soldered a piece of 1/8" tubing to my throttle cable after careful measurement to get it just right. Just right is within the range of the cable adjustment on the twist grip so that the slide can be all the way down (closed with the slide screw backed out) without there being any slack in the throttle cable. I paid close visual attention to the slide while setting up the throttle cable. Rest assured that the cable is not holding the throttle slightly open at all, while still having some adjustment in both directions, in the twist grip. These things expand and contract with ambient temperature. You always have to leave in a small degree of adjustment.

I agree that it is difficult to troubleshoot via an internet forum. It's so much easier to put your eyes and hands directly on the patient. I also think I understand how the enrichment control works. Off = lever up/plunger down. On = lever down/plunger up. Anyway, I suspect that I made all adjustments with the enrichment off.

If my understanding is correct, the more you turn out the air screw, the more you lean out the idle. The fact that I need >2 turns is an indicator that the pilot jet is too big. So in order to get the air screw closer to say 1.5 turns out, I will need to install a smaller pilot jet, which will effectively allow it to be properly dialed in with less turns on the the air screw. At least that's how I'm thinking it works. So do I understand this correctly or am I way off base?

-Ray
 

delray

Well-Known Member
#20
(fhpe77,I purchased the NR Racing "Chikuni" kit with short adapter. Jetting is 130 main and 15 pilot)

yes ole4 15pilot is what he stated early. i'm with ole4, 2 turns out is not bad. but if you think you can get it closer to 1.50-1.75 out ?
I think maybe one of the other big problems is now . trying to get it dial in when the weather is cold out side. you might be waiting for away to get it spot on. if it's cold enough, it doesn't take much for the carb to frost up little inside and can really screw things up when trying to dial it in or I should say it can miss lead you into other things and if your in jersey you might be waiting for away. my opinion 70 degrees is a safe temperature. did you every get a chance to go with a 132.5 main jet?
are running a muffler on that? you might want to try a good 3 stage pipe without a muffler and then dial the carb in.....
 
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