Mikuni 24mm running too rich

thomas3120

thomas3120

Member
#1
#1
Hello,
I've done alot to my predator 224cc engine racing head, racing cam billet connecting rod flat top piston, mikuni flatslide 24mm carb....
Its running rich inside tail pipe is very very sooty. I ordered a bunch of pilot and main jets to try. Is it the main jet i need to downsize or both main jet and pilot jets?
Thanks,
Thomas
 
ole4

ole4

#3
#3
No way to tell based on the limited data you provided. The spark plug (only if it is the correct one) is what you should be looking at. The pilot jet is what controls the idle mixture and effects the mixture up to about 1/4 of the initial throttle opening. Then it is the combination of needle jet, and jet needle up to 3/4 of the throttle opening then from 3/4 up the main jet. Provide some data along with your question and it would help getting a better answer.
(
 
sparkwizard

sparkwizard

#4
#4
Simple back yard test: When do you see black smoke?
Idle, cruise or wide open throttle?
As mentioned above, more information is needed. If it was me, I would put a new plug in it to make sure all that soot wasn't from a fouled plug.
Then, after a good ride, look at the plug and tell us what you see.
 
thomas3120

thomas3120

Member
#5
#5
Here's some more info:
-Predator 224cc. engine
-performance billet connecting rod for clones.
-Fire265 camshaft
-ARC billet flywheel
-Gage ultra light roller rocker Arms 1.2
-chromoly pushrods
-billet valve cover
-Dual bearing billet side cover ARC #6057
-Tillotson racing flat top piston 212cc/70mm
-Stage 4 racing head for small blocks (milled) go power sports #pc-s4racinghead (has stronger valve springs)
-24mm Mikuni flatslide carburetor *Using 170Main jet and 20 pilot jet. (*oem jets: 130main & 15pilot).
-K&N style air filter with Mikuni 42mm adapter.
-Juggernaut super 30 series driver
-Using OEM predator spark plugs Used the autolite 3910X racing ones but after a while they won't run (Carbon/soot?). Also using the spacer washer for the milled racing head.
-Super Exhaust pipe from Go power sports but removed all baffles.
-using highest octane gas (93) and adding a bit of the octane booster stuff.
-Front sprocket 9T and rear sprocket 46T (#420)

Here's a picture of spark plug:
And more pictures of the bike:
https://oldminibikes.com/forum/index.php?media/users/thomas3120.49716/
SparkPlug.jpg
 
Last edited:
Minimichael

Minimichael

Well-Known Member
#11
#11
You can even use the ethanol stuff. Just to form a baseline you know? Then go to non-ethanol and maybe creep back up to 91 octane or even 93, but too much booster is likely to gum up the situation. Maybe even spoil the plug, too.
 
Minimichael

Minimichael

Well-Known Member
#13
#13
Maybe a combination of jetting too rich to get the octane jelly mixture to burn adequately. I'm just guess ing maybe you're down a rabbit hole with them both. You got a really nice sounding set up. There's gotta be a mikuni tuning manual available. There is for the round slide and lemme tell ya it's totally worth reading if you want to really dial things in.
 
ole4

ole4

#14
#14
Did you try the 130 and 15 pilot jet? What kind of riding were you doing? The plug insulator color is not terrible but it being so oily does not seem right.
 
thomas3120

thomas3120

Member
#17
#17
Ok, I complet;y emptied out the gas tank, sprayed just a bit of carb cleaner in the fuel inlet on carb. New filter, fuel line.
Trying the non ethanol gas (90 octane)).
I did notice the head bolts 1 or 2 were not tight (to torque). So retorqued all head bolts. Not sure if this could have been a problem with possible leak... but now just riding it around a bit.
Next time I ride it I'll ride it around 5-6,000 rpm's and turn off engine so it's not idling and check spark plug..?
Does this sound good? Btw, someone somewhere said when I cut engine off at higher rpm's to have throttle wide open. Should I do this or leave throttle alone?

Oh yeah, I also adjusted the idle 1500ish. then tightend the air mixture screw. then backed it out 2-1/2 turns, and found the point of highest rpm. Then readjusted the idle (screw).
Still using the 170 main jet and 20 pilot jet..
 
Last edited:
Minimichael

Minimichael

Well-Known Member
#18
#18
Ok. How's it running? Carb cleaner into the carb inlet is bad. It'll likely warp any rubber it contacts inside... Like the tip of a float needle. Just sayin

I would think a loose head might have had an impact on performance, er ... Runnin-richness.. How are you gonna know when it's all better? White glove test in the exhaust hole?

I really recommend a mikuni manual so you know you're tapping the carb's real potential.
 
Last edited:
You must log in or register to reply here.
Top