Jetting question

[Mick Milakovic]

Hi All,
This is kind of off-topic, since it pertains to my Husky, but it's a Mikuni so others might benefit, also.  My 390 has always been a little hard to start at times.  When cold, I usually have to push start it, but after it warms up it kick starts right away.  Then when it starts to cool down it gets tempermental again.  I've been thinking it might neeed a larger pilot jet, but I don't know.  The bike runs outstanding through the entire range, and doesn't 4-stroke or spew unburned oil out the exhaust.  It runs really clean.  It's just a starting issue.  Any thoughts?

[paul]

Hey, Best way to check the main jet it to run it wfo somewhere you can do that and shut it of right quick. Go Fasst, pull the clutch in and kill it. Bring a plug wrench with you because next pull the plug and look at it. If its burns black to dk brown its to rich. Tan is what you want. As for the starting,pushing sucks...richen it up on low speed till the plug gets wet when kicking, you may have to go to a larger pilot. If the plug is wet  and you have spark you should not have to push SIZE anywhere. Damn, you must be getting in shape pushing that beast. I'd make sure everything in that carb is clean especially the choke ect. Miks dont usually act up. Did that bike come with a Bing? Good luck, I have a square slide 38 Kehien or 40 Mikuni round slide you can try. Easy, TTP

[TGTech]

Mick,

I'd say that the larger pilot jet is the issue based on what you said about it not starting cold but OK when it's warm. If you screw with the air screw, you may be able to find out how a jet change may affect the idle circuit.

When I'm trying to check the idle circuit, I get the bike warm and then  only turn the throttle a 1/4 or less quickly. If the bike doesn't respond immediately, properly, then there is definitely an issue with the idle jet.

In the Japanese carburetors, the air screw should be between  1 and 2 turns out, so if it is out of that range, the jet probably needs to be changed.

Dane

[Ernie Phillips]

from "Starting Secrets of Round-slide Mikuni"  -Rick Siemen

"Air flowing into the starting carburetor makes fuel come out of the bowl to join the airflow, and this is caused by the high vacuum, or suction, because you are cranking the engine. The piston is moving up and down gasping for air, but it can't get very much, so vacuum is high.
What makes high vacuum behind the throttle slide, is the fact that you are cranking the engine with the THROTTLE SLIDE CLOSED. Leave the throttle completely closed until the engine starts. If you can't resist opening the throttle with a spastic reaction every time your kicking foot moves down, try putting your right thumb in your mouth.
If you insist on opening the throttle, the vacuum behind it disappears and the starting carb will not work. Most of the hard starting of Mikuni-equipped bikes is caused by this right-hand syndrome."

Mick, To ensure fully closed throttle, back out the idle screw (the one that holds the slide up) and add some slack to the throttle cable.

http://www.superhunky.com/feb01rsVintageMikuniTech.html


Ernie P.
Chattanooga, TN

[VICTOR MONZ]

I placed a 30 mm brand new Mikuni on a fresh Sachs D motor, it was hard to start or required bump start when cold.  Sudco suggested going down one size on the pilot, did so and it starts first kick.  Problem solved.

Victor Monz

[john durrill]

Mick,
 I would listen to Dane. The pilot jet should do it. If your Mikuni uses a starter jet for the enrichner circuit ( choke ) you could go larger on that to help with cold starts. The air screw will let you know which way to go with the pilot ( once the bike is warm ) like Dane said. Changes to those 2 circuits should not effect the way it runs except at idle and just above idle.
 My 1/2 cent:)
 John D.

[Merlin]

You may need the next larger pilot or a slightly smaller slide cutaway.

Quote: Thomas Jefferson, We are all born ignorant, some work to remain that way.
 Quote:Merlin, "it is impossible to teach those that wish not to be taught".

[brian kirby]

I have a Mikuni on my Can-Am 175 and its similar to Mick's situation. If I jet it where it runs perfect and clean (35 pilot), its hard to start. If I go even one size bigger on the pilot (40 pilot) it starts on half a kick, but blubbers and loads up on the bottom. I prefer to leave it lean so it runs correctly when warm and just deal with the hard cold starting. Laying the bike over on its side before I start it will allow me to start even at mid-30s temps, but it will NOT start unless I do that to prime some fuel. I also have a Mikuni on a '79 Maico 440 and it is similar, but worse since it takes a lot of power to get the big Maico spinning fast enough to light off. Once warm both bikes start easily.

[john durrill]

Brian,
 Does your Mikuni use jet in the starter , enrichner circuit?
If it does going up on that jet should help or doing what i have threatened to do on my Lectron chuckle chuckle!! Putting a primer kit in it like a Bing has.
 john D.

[brian kirby]

John,

Nope, no jet in the enrichener circuit. Its not a big deal with the Canned Ham 175, all I have to do is lay the bike over to spill some fuel into the intake and it fires right off, but you can kick all day without doing that and it wont start. At Combs the temp was at or just below freezing in the morning and it started after two kicks, but I had to repeat the lay over process 2 times before the engine got warm enough to stay running. The bike, once warm, is jetted perfectly so I will gladly do lay over thing to get it started.

The Maico is another story. I only weigh 140lbs so I have to work at getting that big piston spinning fast enough to start. I have been kicking around the idea of adding an outboard style primer bulb on the fuel line. The Mikuni enrichener circuit, if you forced fuel into the float bowl with a small primer bulb, would flow that fuel through the circuit into the intake. You would essentially be forcing the fuel under pressure instead of engine vacuum doing the work, meaning less kicks to prime and easier starting. Any big two stroke need quite a bit of fuel to start cold, which is the one huge advantage to the Bing with its tickler.

Brian

[firstturn]

Merlin,
  I agree with you on the following:

You may need the next larger pilot or a slightly smaller slide cutaway.

But I think you need to explain you comment so that people that new to setting up a carburetor will understand what a smaller slide cutaway or cutting a slide does to to the performance of a carburetor.

Ron Carbaugh

[john durrill]

Brian,
 If its worth the time to you then , with what you have already tried you could do what Merlin suggested get a 1.5 slide and try it. Play with the pilot jet. If its not right trim .5 mm off the cutaway. The start point on the cut would be closest to the slides center and taper to the added .5mm at the rear of the slide. If you have a #2 now then take 1 mm ,that would be a 2.5, and play with the pilot jet. We did this on reed kits we installed years ago to get the jetting right. I had a bunch of 1.5 slides and a set from 1.5 to #3 . I would trim the 1.5 to whatever that engine needed useing the set to find the right slide cutaway. It helped me by not having to stock a bunch of different slides.
 Mikuni's should start the Can Am with the right set up. The big Maico too but piston port engines that big maybe its not unusual.
Mick may just need the jet change. He wont know till he gives that a try first.
 It some trouble to dial a carb in just right for a motor but if you ever get one " right " it will spoil you rotten.
Sorry Ron i was typing as you were posting
 John D.

[Mick Milakovic]

Dane, thanks for the advice on the larger jet.  I'm a little confused by your term "idle jet".  Is that the same as a pilot?

John and Brian, I pulled the pilot jet today and it's a 35 so I'm going to order a 30 and 40 tomorrow.  It does have an enrichener circuit.  How do I get to that jet?  I'll Keep you posted.

Ernie, I'll try to keep my handoff the throttle!

Thanks to all, Mick.

[john durrill]

Mick,
Sudco shows the starter jets at the bottom of the online catalog page with  parts for the vm round slide. The note beside the list says for TM 32-38 but i have seen them in round slides over the years. It depends on when the carb you have was made and for which manufacturer . Yours may not have a jet that can be changed. Its usually where the brass pick up tube hangs down from the carb body and located in the float bowl. It has to be between the fuel in the float bowl and the pickup tube. It can be just a hole drilled in the casting. Thats what Brian has from his description
 What size and model is the carb ( that may let us see what it came with stock ) and what does it have for a slide cutaway ?
 A call to Sudco's tec line might help. Its possible they could tell you how to change the enrichners fuel delivery to work better with a big single 2 stroke. Its possible opening a fixed jet like Brians with the right drill size would help solve that cold start problem. I would use caution with that tract though. There would almost have to be a point  of diminishing returns on how big the hole is and  to big could cause  problems. Best to get some advice from people that should know whats possible with the carb  starting system you have first if you can.
 John D.
 

[VICTOR MONZ]

Sounds like this Mikuni advice is all over the place !  That is why I replaced the new Mikuni with  a 32 mm Bing down sized a little on jets for region specifics and it out performs and starts much better than the Mikuni.....the Mikuni made a neat sound last night as it hit the trash can....bing, bing, bing !

Back to Bings for my Pentons !

Victor