Mikuni VM22 on built Predator Hemi

#21
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
No one's panties are in a twist, and I get along great with Ole over the years. We have shared stuff back and forth, and he's one of the most knowledgeable guys on here, as well as Delray for that matter. Nope, no panty twists here.

Two turns out on the air screw is OK. I think I said that above. When you start going MORE than two turns, is when you need to be looking at a smaller pilot jet. Notice, I said MORE than two turns.

I thought my throttle cable was set properly too. I thought that three times, as I removed the slide and re-soldered it three times. Just saying. And it wasn't my first Chikuni.

So get the 12.5 and if everything is as you say, you're golden! If not, you can disconnect your throttle cable and try to start it and see if idle acts the same. Personally, I'd do that first, but that's just my most recent fiasco with silver solder talking.

By the way, the attached guide is the best Mikuni tuning and troubleshooting guide you will find. I ran across it a few years ago, when I was adjusting my first Mikuni on a flathead Briggs, and found it quite helpful in understanding the curcuits and the way in which they interact.

We're all friends here.

Edit: @delray as I recall, you wondered what the adjustable exhaust baffle would do to my carb tune? I forgot to mention in my build thread. Open, it is a bit more crisp. However, a main jet increase/decrease was as bad for either setting, as was a metering needle setting. So there is a slight difference, but not enough of a difference to be able to adjust the carb for it. Idle doesn't change. Big difference in noise though!

http://www.iwt.com.au/mikunicarb.htm
 
#22
Ole,

Yes, I have the 15 Pilot jet installed.

Delray,

Good point about the outside air temperature. The air screw is now slightly more than two turns out but the bike seems to run pretty good like this. It's just going to become richer as air temps go up in the spring. Mentioning the carb frosting up makes me recall my old 350 TPI engine back in the day. When the General only granted you a meager 245 hp, you tried every trick in the book to eek out a little more power. One of the tricks was to bypass the throttle body coolant circuit, which circulated engine coolant through the throttle body to warm the incoming air. I ran it like that for about a week when the air temps were in the 60's. The throttle body would be covered in frost after a 30 minute drive. Apparently if you ran it like this for a prolonged period it could cause the throttle valve to freeze in a partially open position. Not good.

I'd love to try it with an open pipe but the neighbors are already unhappy with the sound so the Briggs style muffler will have to remain. I will order a 132.5 main jet to have on hand for additional tuning. Good call since I haven't really started to tune for full throttle yet.

Dave,

Very good point about double checking the throttle cable. It will only take a few minutes. Going to be 33 degrees this weekend so I will have to wait to play some more. After verifying the throttle cable, I will install the 12.5 pilot jet and see what that gets me. If it's not good I can always reinstall the 15 pilot.

All,

Gotta head to work now, I'll be back here this evening. Thanks again for for all your help.

-Ray
 

delray

Well-Known Member
#23
Edit: @delray as I recall, you wondered what the adjustable exhaust baffle would do to my carb tune? I forgot to mention in my build thread. Open, it is a bit more crisp. However, a main jet increase/decrease was as bad for either setting, as was a metering needle setting. So there is a slight difference, but not enough of a difference to be able to adjust the carb for it. Idle doesn't change. Big difference in noise though
single baffle like you installed may not effect the carb so much and that might be a option for him to go with to keep his neighbors happy,but using a muffler I believe could affect the performance. good example of a real bad muffler are the factory 212 predators. right away when remove and installed header you need to increase the jetting.
either way ray your going to have to wait for warmer weather...…..
 
#24
single baffle like you installed may not effect the carb so much and that might be a option for him to go with to keep his neighbors happy,but using a muffler I believe could affect the performance. good example of a real bad muffler are the factory 212 predators. right away when remove and installed header you need to increase the jetting.
either way ray your going to have to wait for warmer weather...…..
Yeah, I agree. I'm only flipping over a many-holed flap. It "looks" like not much would happen noise-wise, but it has a surprising effect. I did try and tune with it on and off. (main jet and metering needle) I am positive that the larger mufflers vs. even a minimal header requires compete jet change on Chikuni. I can barely feel performance change, but it is there. I don't know that it quiets things down enough to make a difference between happy neighbor, and angry neighbor. I suspect not. :)

Ray, we are talking about this one.
 
#25
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
A bit late here. I wouldn't be to concerned about the throttle needing to be cracked open a bit at starting. The Lifan 125cc honda clone motor with a genuine Mikuni I have even says to crack the throttle open a bit at starting in the instructions that came with it. Every honda bike I have (3) with the mikuni or chikinu all need cracked open a bit at starting. The Honda GC 160 motor that I used a chikuni on does the same. Seems normal.
 
#26
A bit late here. I wouldn't be to concerned about the throttle needing to be cracked open a bit at starting. The Lifan 125cc honda clone motor with a genuine Mikuni I have even says to crack the throttle open a bit at starting in the instructions that came with it. Every honda bike I have (3) with the mikuni or chikinu all need cracked open a bit at starting. The Honda GC 160 motor that I used a chikuni on does the same. Seems normal.
I guess to add to my post..... It is a bit akward to pull the GC160 and hold the throttle open at the same time. Just one of the downfalls to deal with. Much easier with a kicksarter.:rolleyes:
 
#27
Dave,

I've seen that Robertson plate muffler on some vendor sites before. My first thought was that it can't possibly do much. Then it occurred to me that closing it would cause an increase in pressure just before the plate and an increase in exhaust velocity. This of course would increase the sound intensity (db). But it's the frequency of the sound that is changing as well. You're knocking out some of the lower frequencies with it closed while increasing the intensity of the higher frequencies. Some of the low frequency sound is nulified by the standing wave created due to the "restriction" that travels back towards the exhaust valve. Our ears would tend to perceive this as quieter, though not by much I suspect. You should post a video (if you already haven't) comparing opened vs closed. That would be interesting.

Anyway, enough of my geeky nonsense. That full-suspension Cat 250 is outrageous! I can't believe I missed it before. I've only looked at the photos shown on the page you linked. I MUST go back now and devour that thread. What a great build!

Oops, got off topic, I agree that any change in exhaust pressure will always require some carb tuning. Carbs just don't have the ability to adapt to changing conditions like the EFI we are all used to with our modern cars. And even then, EFI systems suffer from some of the same problems but they just have a wider range of adaptability before requiring further tuning.

Now on with my readings! :)

-Ray
 
#28
Littlebear,

Nah, I wasn't real concerned about the throttle crack starting method. Even my Tec H35 engine requires this. My technique is to sit on the bike, left hand across the bars gripping the twist grip and pulling the starter with my right hand. Perhaps not the safest technique, but it has improved dramatically with the Mikuni's wider throttle range. It was certainly more challenging with the stock carb.

-Ray
 
#29
One other thing to watch out for is the enrichment device can cause a high enough increase to engage the clutch or tc. You can set it halfway if that becomes a problem. It still works just not as high an idle.
 
#30
Ole,

Thanks for the heads-up. I will make sure to have the back wheel off the ground when I try it. At this point it's not helping but I bet once I lean out the idle jet a bit it will start to be more effective. It's gotta be safer than the throttle crack method. I wish the weather would cooperate, it's just too darn cold to play with this thing now. Guess I can't complain too much, got a brother who lives in Fargo, ND. I bet he's really enjoying his 2017 Mustang GT right about now. :D

-Ray
 

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