Spark plug readings/mikuni tuning

Latent

Active Member
#1
How’s it going fellas,

Down to the last leg of the project, and that’s getting my chikuni 22mm tuned/jetted just right.I live in Colorado 5000-10,000 elevation when riding so keep that in mind as well.

My current build
predator 212cc
rtc-6 cylinder head with 27mm intake 25mm exhaust valves
stock automotive style retainers
26lb valve springs with .060 shims on intake and exhaust sides
stock rods
stock rockers
billet rod +.020"
hemi flat top piston
.010" head gasket
ngk spark plug
billet flywheel 32 degree timing
black mamba senior camshaft
Mikuni 22mm with 115 main jet 15 pilot jet (38 keihin pilot)
header

Currently I have a 38 Keihin pilot jet that came pre-installed in my carb which I believe is equivalent to a 15 mikuni pilot??? Also running a 115 main. I attached the spark plug reading after WOT runs to this thread.

The engine currently only start & stays running with enrichment on. Which leads me to believe at least the pilot jet is lean. I have a 17.5 & 20 pilot on the way. Also have a #95 main #115 main #130 main #135 main.

Currently at 115 main 38 keihin pilot (Equivalent to mikuni 15 pilot???)

Going to try 130 main 17.5 pilot from mikuni

I had this mini bike up to 59.6 mph with more to go with this current main jet 115 mikuni large round head... so i know it is close however suspect it to be lean (suspect 125-130 main to be optimum)

Just wanted some suggestions regarding my spark plug color & what the ideal color should be for optimum jetting?
890D25EF-7CF7-415A-838D-6457606FACDB.jpeg 53BD4311-ACFF-45E7-B744-57659913BF2C.jpeg
 
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Davis

Well-Known Member
#3
That’s a big jump in jets. My motor is similar to yours. I’m running a 12.5 pilot and a 115 main. Running at 5-7000 ft.
 

Davis

Well-Known Member
#5
Hemi block and piston standard arc rod. 28.5-25 17cc head built by @ole4. .010 head gasket dyno cams cl275 cam 1:3 champion rockers. Arc billet sidecase and flywheel. Headstuds 22mm chikuni carb custom pipe. Hilliard flame with bully conversion clutch. Running 6.25-1 ratio with 14 inch tall coker tires. 8210rpm is as far as I’ve taken it. 22CAD823-9A6F-4019-B073-5D7394D05D9E.jpeg
 

Attachments

Latent

Active Member
#6
Hemi block and piston standard arc rod. 28.5-25 17cc head built by @ole4. .010 head gasket dyno cams cl275 cam 1:3 champion rockers. Arc billet sidecase and flywheel. Headstuds 22mm chikuni carb custom pipe. Hilliard flame with bully conversion clutch. Running 6.25-1 ratio with 14 inch tall coker tires. 8210rpm is as far as I’ve taken it. View attachment 260323
Our builds are really similar. What do you have your idle set at RPM wise? Why did you go with a 12.5 pilot vs stock 15 ? Not sure why my engine only starts/runs with choke on. What is your needle jet set at? Thank you regardless :):scooter:
 

Latent

Active Member
#8
Update: My chikuni came with a keihin pilot jet, and the vm28/486 pilot jet I ordered did not fit. The vm28/486 pilot jet has a thread diameter of 3.9mm the pilot jets needed are keihin N424-74C or 3j which both have a 4.9mm thread diameter, they are basically the same. *

Currently at a 38 pilot bought the Keihin 40 & 42 pilot jets to test. Not sure how these convert to mikuni numbered jets, but we will give it a go. Also ordered a 120 125 main.

Main jet size - got a 115, 120, 125, 130, 135 main to play with.
Pilot jet size - got a 40, 42 keihin pilot jet to play with. Anyone have any past experience tuning a chikuni with a keihin type pilot jet??

Long story short jetting is a pain in the a$$, but we'll get there, lol.
 

Latent

Active Member
#10
Adjusted my needle clip position from 2nd from the top down to 4th needle clip position to richen the mid range 1/4-3/4 throttle region. Opened up the fuel screw to 3 turns out from previous setting of 2 turns out. Also installed a 130 main from a 115 main. Going to do some testing tomorrow & report back with results & another spark plug pic hoping for better results this time!
 

Latent

Active Member
#11
Update 2: Had a pretty successful day testing & tuning my mini bike. Found out the reason for the hard starting/not running without choke on was due to the mikuni manifold screws coming loose. I applied some loctite on them, and installed longer bolts to secure manifold to engine head which fixed the problem. Engine idles best at 1.5 turns out on the fuel screw, however the lowest I can get the engine to idle is about 2400-2600 range any lower than 2400-2600rpm with the enrichment off the engine stutters, and dies... Not sure why? When this was happening I put my hand over the air filter, and the rpm's would increase. Does this indicate that I need to up the pilot jet? Vacuum leak? I might get some JB weld, and weld the manifold to block...

I was able to tune the mini bike to be able to start on the 1st pull with the enrichment off each time. However can't get my engine to idle below 2400 with enrichment off.

I was able to tune the needle jet seems to work best in the middle position. However I didn't get to play around with any main jets. I just had the 130 main jet installed, and was able to hit a new RPM max of 7290 (13 inch tires, 59/13 ratio = 62 mph), but on deceleration I would hear exhaust pops, and when I was trying to come to a stop the engine would stutter, and try to shut off unless I kept flipping the throttle. Would this indicate that the main jet was too rich or a vacuum leak caused the pilot to become too lean after letting off the throttle @ WOT?

Would appreciate any insight, thanks all.
 
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#12
You didn't cover up the air bleed hole at the bottom of the carb to manifold flange? also your idle speed is too high, once the slide is above a certain point you start drawing fuel from more than the pilot, it may be that you have the slide too high, the proper way to adjust the idle air screw and idle air speed is to get the best idle like you are doing then lower the slide adjustment screw a little bit, then go back and re adjust the idle air bleed screw for best idle then in a bit to slightly richen it then repeat till you get the idle where you want it. The isky black mamba is a pretty serious cam and it is not going to idle like a stock cam or mild cam.
 
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Latent

Active Member
#13
You didn't cover up the air bleed hole at the bottom of the carb to manifold flange? also your idle speed is too high, once the slide is above a certain point you start drawing fuel from more than the pilot, it may be that you have the slide too high, the proper way to adjust the idle air screw and idle air speed is to get the best idle like you are doing then lower the slide adjustment screw a little bit, then go back and re adjust the idle air bleed screw for best idle then in a bit to slightly richen it then repeat till you get the idle where you want it. The isky black mamba is a pretty serious cam and it is not going to idle like a stock cam or mild cam.
No I didn't cover up the air bleed hole I had the air filter off, and had my palm covering 50% of the carb when I did this rpm's increased. Less air made the rpm's increase meaning there might be an air leak somewhere(lean condition).

I suspect its due to not having an intake gasket also did not have an exhaust gasket. I am going to get this from the auto parts store, and didnt know there were no gaskets on *face palm*. Going to seal intake manifold to cylinder with some jb weld to get a complete seal. I do not run a fuel pump so no pulse lines to leak from.

My gear ratio is pretty low 59/13 about a 4.5 ratio.. going run a 5.6 ratio, but after I can get this engine sealed up right, and tuned in on these slicks.
 

Latent

Active Member
#14
Update 3: Got the exhaust gasket on, and intake gasket on. Also used red loctite on the intake mounting bolts so they shouldn’t be coming loose again it’s super tight right now. if they come loose again the manifold to engine head will get welded. New pilot jets/main jets coming in this week, and will test/tune again this weekend. If I don’t get the results I desire (8000rpm & crisp responses) will get genuine mikuni as the fakes are a hit or miss.

These corrections should get rid of the air leak, and hopefully I can finally get my carb dialed in this weekend. If I am unable to get it tuned in going to get a genuine mikuni & try my luck with that.

Most likely will get a real mikuni as this chikuni has gave me nothing but headaches. Probably worth the extra bucks. Will update again. F88456B1-EDB2-44C2-B1F3-BDADF88C7761.jpeg FBAE52C1-8B12-4E50-BFDD-279DEDEA4AB1.jpeg 41E19B63-E9BD-4F94-997E-E995E3E12334.jpeg 52BB9970-0708-4581-B7C2-279C022D4989.jpeg
 
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#15
I'm in the same boat as you. I decided yesterday and ordered a real Mikuni cause I want something that can idle halfway decent without choke and get good response.
 

Latent

Active Member
#16
I'm in the same boat as you. I decided yesterday and ordered a real Mikuni cause I want something that can idle halfway decent without choke and get good response.
Right... these clone copy’s are either a hit or a miss.. what site did you order the genuine mikuni off of?

Update: Just ordered a real mikuni vm22-133 hopefully itll be here by friday for weekend testing. Got 115/120 main 12.5/15 pilot jets to try out. Hopefully this real mikuni will make a difference, and solve my issues with high idle/tunability.
 
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#18
Right... these clone copy’s are either a hit or a miss.. what site did you order the genuine mikuni off of?

Update: Just ordered a real mikuni vm22-133 hopefully itll be here by friday for weekend testing. Got 115/120 main 12.5/15 pilot jets to try out. Hopefully this real mikuni will make a difference, and solve my issues with high idle/tunability.

Just received my real Mccoy mikuni and I can say I'm impressed just by comparing it. Choke lever is more solid, round bore is smoother and much larger, fuel bowl drain plug is a bolt with larger surface area for sealing, much heavier, and other small things I noticed. Plus it will be much easier to order jets and not worry about getting the wrong type. Let me know how yours turns out.
 

Latent

Active Member
#19
Just received my real Mccoy mikuni and I can say I'm impressed just by comparing it. Choke lever is more solid, round bore is smoother and much larger, fuel bowl drain plug is a bolt with larger surface area for sealing, much heavier, and other small things I noticed. Plus it will be much easier to order jets and not worry about getting the wrong type. Let me know how yours turns out.
Mine will be here by end of day friday 4/17 so just in time for more test/tuning this weekend. Look for an update on Saturday afternoon. Hopefully the real mikuni, and sealing of intake/exhaust make a difference. Good luck! Let me know how it turns out when you install, and run it.
 
#20
Update: My chikuni came with a keihin pilot jet, and the vm28/486 pilot jet I ordered did not fit. The vm28/486 pilot jet has a thread diameter of 3.9mm the pilot jets needed are keihin N424-74C or 3j which both have a 4.9mm thread diameter, they are basically the same. *

Currently at a 38 pilot bought the Keihin 40 & 42 pilot jets to test. Not sure how these convert to mikuni numbered jets, but we will give it a go. Also ordered a 120 125 main.

Main jet size - got a 115, 120, 125, 130, 135 main to play with.
Pilot jet size - got a 40, 42 keihin pilot jet to play with. Anyone have any past experience tuning a chikuni with a keihin type pilot jet??

Long story short jetting is a pain in the a$$, but we'll get there, lol.
HI I know this is quite late, but im struggling to find jets to fit my chikuni. Where did you order yours from thanks.
 
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