Mikuni Carburetor Question

[Chakka]

I am running a 26mm Mikuni Carb on my resorted 74' Six Day and have some questions as I think some problems I am having might be related.
 
When I remove the gas tank from my bike and reconnect the fuel lines there appear to be air bubbles trapped in the line. The bike will usually take about 25 kicks to get it going after a tank swap. The Mikuni doesn't have a tickler so I can't flood the carburetor. Have any of you had a similar problem? It's almost like vapor lock. Once it starts it will only takes one/two kicks to fire it, even when the bike is cold.

 The fuel inlet barb on my Mikuni carburetor is located on the exhaust side of my Penton. Does Mikuni offer one located on the same side as the original Bing? I don't like the proximity to the pipe.

Last question, On the first long test ride a few weeks ago, my nephew rode about 10 miles to start out. The bike had a problem & died out, it would fire but would only run at idle if the throttle was wide open, otherwise it would quit running. After a few minutes he ended up "pop starting" the bike at which point it ran great, everything back to normal. He rode the same distance back to my house, not wanting to chance going deeper into the woods and the bike ran fine the whole way. Once back here it also started first kick and ran fine. Again that almost seems like vapor lock or a lack of fuel to me.

The engine has been freshly rebuilt and only has about 50 miles on it. What could be causing these abnormalities? I am no tuning expert so if these questions seem ridiculous chalk it up to a novice tuner at work.  

Thanks
Chris

[Dwight Rudder]

Check float level and make sure it is level with the bottom of the carb body.  Maybe lower it a tad .  Make sure your pilot jet isn't a size or so too big.  Sounds like it is loading up.  What is POP starting ?
IF it will only idle with the throttle full open turn the fuel cocks off and hold it there till it leans out and clears.
Dwight

Dwight Rudder
7 time ISDT / E medalist
8 time National Enduro Class Champion.

[Chakka]

Well I switched to an MX tank I have just refinished. New gas cap but older petcocks, I tested them and they flowed fine. The new cap might have made some difference as fuel went right through the lines and it fired up second kick. Not wanting to chance it I rode around the yard for approx 1/2 hr before it did the same thing as I described above. This week I will play around with the float setting, thanks for the tip Dwight. The pilot jet size is #60. It is a stock 26mm set up by Pro-Flo. The carb is the only thing I can think of, it has a new MZ-B ignition and everything else has been gone through.  

Chris

[Lew Mayer]

I'm not a Mikuni expert but doesn't a 60 pilot sound big for a 26mm?

Lew Mayer

[firstturn]

Chris,
  What I think Bob W is saying is to elminate the gas cap.  Take it of or loosen it.  It may not be giving any venalation.  Or just put another cap on that you know is good.

Ron Carbaugh

[Chakka]

Hi Ron,

I followed Bob's advice. The new cap eliminated the 25-30 kicks to get fuel into the carb. Once the new tank was on it fired right away...so Bob helped me to eliminate that problem. Still it wont run more than 30 minutes or so...its perplexing. What could cause it to run fine and then suddenly cut out? Tonight the only way to start it was to let the bike sit for a 1/2 hr.

Chris

[firstturn]

Chris,
  Not being there I just have to call on problems I have seen in the past.  Sounds like the carburetor situation may be resolved by your account.  Next I would be looking at ignition problem.  I don't know which system you have so I will say start looking for good grounds and tracing out that you have no resistence in your wiring.  Also if the bike is a a new restore you may have paint where the engine mounts to the frame causing resistence.  Give us more info and we will try to help.

Ron Carbaugh

[3putt]

Carry a new plug and wrench, and when it quits the next time, after it's hot,immediately pull the plug, and check to see if you still have spark.  If not, or poor, then you know it's a problem with ignition, and proceed to run it down.   Do you have a second ignition system that you could use for replacement.  Good Hunting.   wnm

[Dwight Rudder]

quote:Originally posted by Chakka

Well I switched to an MX tank I have just refinished. New gas cap but older petcocks, I tested them and they flowed fine. The new cap might have made some difference as fuel went right through the lines and it fired up second kick. Not wanting to chance it I rode around the yard for approx 1/2 hr before it did the same thing as I described above. This week I will play around with the float setting, thanks for the tip Dwight. The pilot jet size is #60. It is a stock 26mm set up by Pro-Flo. The carb is the only thing I can think of, it has a new MZ-B ignition and everything else has been gone through.  

Chris

Pro Flo doesn't sell pre setup carbs.  You have to put in the correct jetting.  Most run 35-40 pilot jets not # 60.
Dwight

Dwight Rudder
7 time ISDT / E medalist
8 time National Enduro Class Champion.

[Chakka]

Thanks for the ideas guys, Dwight I will order a new pilot jet this week. What would the correct set-up jets, and all be for a 125 6b running at sea level to 1000 feet? I know it has been posted before but I couldn't find it when I searched. This bike is used for light trail riding, I have no plans to race it.

I do have the original Bing which is in really good condition. I suppose I could also use that to isolate a carb problem vs. an ignition problem.  When I reassembled this bike I was carefull to sand all the motor, coil & ground mtg points. Still It could be an electrical problem. I hate tracing electrical ghost's, I just traded in an Audi A4 that had a bad ground and would lock and arm it's alarm system at random...Here we go again!
Thanks Guys and Happy Easter!

Chris

[Dwight Rudder]

Do a search on Mikuni Jetting on this forum.  You will find 3 pages of stuff. You can find what you need in there.  Or at least enough to get you started.
Dwight

Dwight Rudder
7 time ISDT / E medalist
8 time National Enduro Class Champion.

[Gavin Housh]

I too have the Mikuni on my bike and I also have the fuel line closer than I would like to the exhaust pipe. I zip tied the fuel line in several places to the ignition leed so it stays away from the pipe . It still passes within 1/2-5/8in of the pipe as it crosses over the top left hand side of the carb. Could the fuel line be getting so hot that it is causing a problem? Next time the problem happens put your hand on the fuel line and see how hot it is. If it's getting too hot to touch then you need to move it and or insulate it with that aluminum heat wrap tape. I run a #25-30 pilot.

[socalmx]

I took a different approach to hooking up the fuel line to my Mikuni carb. I used fuel line from the petcocks that go into a section of 1/4 hard tubing that was formed to curve down, over and down to mate (with a short piece of fuel line) to the carb spigot. I thought that this would be the best way to protect the fuel line from the exhaust pipe. I have ridden the bike like this in one Grand Prix (lots of full throttle sections) and it seems to supply sufficient fuel flow (was concerned about vapor locking).

[Chakka]

Socal, can you post some pictures of your fuel line placement? I fabricated a short steel strip with a 1/4" hole at the end, I mount it to the frame using the hole for the engine strut and tie wrap the fuel line to that. It keeps the hose approx an 1.5 inches away from the pipe. Also I searched board for jetting and set-up for the 26mm carb. The only comprehensive setup I could find was on the 30mm Mikuni. Could Anyone currently running a 26mm Mikuni forward how they have set the carb up? pilot jet, needle ect.  

Thanks
Chris

[socalmx]

Before I went with the metal line, I tried using a plastic clamp mounted to the strut hole like you mentioned. In my case, it put too much of a pinch as the hose bent to go behind the carb toward the spigot. My ability to attach pictures is extremely hampered because I use my company computer and they frown on installing personal software for photo type activities. My hard line goes at a 45 degree angle from the single (post t connector) fuel line to just behind and below the carb cap the bends to the left going straight behind the carb then 90 degrees down to the spigot where it is mated up with a very short piece of fuel line. Use this as a start and use a piece of hanger wire to figure out what would work best for you then bend the tubing to match. Hope this helps.