Mikuni specs

motoddrob

motoddrob

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Location
S/W Australia
Just picked up the new jets for the mikunis. At this stage I'm trying
127.5 mains
20 pilots
3 new matching mixture screws.
Just dropped the needle back to middle position.
Needles are 90ZH5-50.
What needles do you mikuni users have in your carbs?
 
Exactly those Rob...

122.5 main, 17.5 pilot if memory serves me right. Good chance the needle is in middle notch as well.

Went up and down slightly with the mains, these gave best seat-of-pants dyno results. Used a 3km long uphill stretch of unrestricted autobahn with a service area conveniently placed atop the hill for plug chops.

78mm pistons, 4C cams, fat headers with custom collector and SFC750 silencers, empty airbox with smooth runners to carbs. On a whole, pretty pleased. ;)

piet
 
Hi Red, Having another closer look at that needle number you are right, it's a D.
So carbs are finished and ready to go back on. Next track day I'll go armed with a set of 130 mains in case the 127.5s are still looking lean on the plugs.
 
Rob, as an indication of variation in jetting requirements between motors, I set my FCR39s for initial start and running based on Lee's racer (same carbs, 750 hi comp pistons, ported head, long inlets, 6C cam, 2/1). Main diff to mine is 880cc, 5C cam, 2-1-2 exhaust, different port shape and OS inlet valves. First run on Rex Ws dyno and he said the main was dangerously lean on the A/F ratio - from memory I started with 152 mains and we ended up fitting 162s.

Obviously Miks and Keihins are different, but it highlights the jetting variation for different motor configurations: eg, 1000cc triple running std bores, same on hi-comp, 1115 hi-comp, Jota mufflers vs 3/1 etc. It's why dyno runs are so useful.
 
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On my race bikes I lift the middle carbs needle by one notch so it is slightly richer then the two outer carbs
I also use B9ES plugs
 
redax5 said:
On my race bikes I lift the middle carbs needle by one notch so it is slightly richer then the two outer carbs
I also use B9ES plugs
Click to expand...
When Matty is on the gas (most of the time?!) would running a size up on the middle main also be beneficial, Red? Or does the needle still pay a part at full throttle on the Miks?
 
Data for the Mikunis on my Jota if it's of any use. (Normal jota spec: 1000cc, high comp Mondial pistons, 4C cams, big bore exhaust, rearsets, orange paint, etc).

I bought them on eBay from some dude in USA as a set of used carbs off a Suzuki. The photos on eBay showed a nice clean set of carbs and they were described as being in excellent condition. What I got was a filthy set of damaged carbs with dirty water in the float bowls and some parts missing. One of the float bowls had split open from the water inside it freezing and one throttle slide was stuck half-way open because the needle was bent (how the fuck does that happen?). The accelerator pump piston was stuck in its bore as well. The good news was they hadn't seen much use so they weren't worn out and none of the idle screw tips were busted off, but they took a lot of work to get them into useable condition. With a bit of help from Red and Mikuni Oz, I managed to assemble 3 decent carbs out of the pile of junk.

I did the re-jetting on a dyno, so it's pretty close to optimal for my engine. The bike runs nicely and is more economical on fuel than it was with the Dellortos.

Main jet 130
Needle jet P-4
Jet needle 5790 2H 5-50
Slide ?
Pilot Jet 17.5
Float height 20
Idle mixture 3/4 turn

I have not recorded any data on the starter jet, float valve jet/needle or accelerator pump.

Looks like I may have mis-read the numbers on the jet needle too. I've recorded it as 90 2 but maybe it's 9D Z. I'll have a closer look next time I have them off the bike.
 
Hello all,
Rather than start a new thread I tried to find one that suits, and this was close.

I fitted my triple with a set of mikunis provided by Red a few years back, and all was well out of the box. Red did warn me on collection that they were set with the accelerator pumps to actuate from the stop, which was as per his race bikes. I never experienced any issues with that setup, of the jetting as provided.

After the COVID layup, the bike developed a bit of a misfire and more exaggerated low end hesitation off of idle. Sometimes it will stall if slowly rolling on the throttle. I was wondering if this was symptomatic of a bad/fouled pilot jet and circuit, or if it may be down to it being over rich and bogging down due to the pump setting?

I have just had the carbs off for a clean and they were not unduly dirty. Pulled out and cleaned through pilot jets and blasted cleaner through all the galleries.

I will have it back together later once the air filter oil has soaked in properly

While Ive got easy access to the adjustment screw, what is the commonly accepted wisdom for the proper pump actuation setting or should I just leave as it is and see if cleaning has fixed my issues?
 
The Mikuni manual that came with the ones I bought for my Gpz1039R said to set the accelator pump actuator rod to come in from 1/4 throttle opening to 3/4 throttle opening. However, that's likely not your issue. I'd remove the pilot air screws, primary jets and go though the primary circuits with carb cleaner AND compressed air to ensure that there are no restricted or blocked passageways. I've just had to do the same with my Gpz which was running like crap with poor idling and pick up.
 
Had a similar issue with my track bike carbs, couldn’t get it to run right and had a track outing approaching. Short term fix was steal the carbs off the Jota, change the jets over to suit the track bike and all was good. Still left me wit the issue in the track bikes original carbs😢. Solution was to contact Mikuni’s Oz and after a pleasant chat with the resident expert I purchased new pilot and main jets. When the track bike returned home I rebuilt its carbs after rigourously cleaning all the passageways in the carbs, installed the carbs back on the track bike and ran like it should. Perfect😊👍
Tom
 
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