Mikuni vm34 "choke" no help starting

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850commando

850commando

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SO i kicked and kicked and finally found that with the choke off and a little twist of the grip the 850 would start. So whats wrong with my "choke" circuit ?

settings ;
needle 6dh3
main 260
air 2.0
slide 2.5
pilot 35
needle jet 159 -P0

air screw out 2.25 turns, once its running i can get a decent idle at~ 1100 rpm according to the tach
 
What's the history of the carburetor? Have you just fitted it?
For the choke circuit (enrichener) to work on a mikuni the throttle must be closed
 
Baz yes i know, about no twisting the throttle when engaging the enrichment circuit, if i just pull up the little knob i can kick and nothing happens. new carb bought by PO about 18 yrs ago. left in box just installed to get the basket case running.
 
Hopefully someone will chime in that has had this problem,
I know the enrichment plunger can deteriorate and not make a proper seal when it's in the off position but that's quite the opposite to what you have
Maybe try cleaning the enrichment circuit it could easily be blocked
 
Is this the first time starting from the machines basket days? What was the temperature when you had difficulty starting? Did the motorcycle sit with ethanol augmented fuel over the winter, if you didn't just put it together?

Plugs OK? Your main jet is too large unless you have open pipes, no silencers; not that the main jet influences a Mikuni cold start, just a suggestion. I generally jet the Mikunis for 850s with a 240 main, some customers have specified a 230, a 260 is what Mikuni put in at the factory.
 
If it's running rich, the choke won't help. Just make things worse. Sort your jetting etc. out, then it should work properly.
 
RoadScholar said:
Is this the first time starting from the machines basket days? What was the temperature when you had difficulty starting? Did the motorcycle sit with ethanol augmented fuel over the winter, if you didn't just put it together?

Plugs OK? Your main jet is too large unless you have open pipes, no silencers; not that the main jet influences a Mikuni cold start, just a suggestion. I generally jet the Mikunis for 850s with a 240 main, some customers have specified a 230, a 260 is what Mikuni put in at the factory.
Click to expand...

i have a 220 or is that to lean?

this is the first time starting the basket case so temp about 15c, and fresh fuel using an enema bag on a hook
 
Clean. Every. Passage. In. The. Carb.
Then, the enrichener will work.
 
I know that the manual says to not give the bike any throttle with the VM 34 Mikuni. On my VM34 equipped 850 If I don't give it any throttle it might start the first kick but it's just as apt to bitch slap me and not start. By giving it about 1/4 throttle it's a one or two kick bike. And yes, my 850 is set up just fine. Clean carb, good plugs, strong battery, and a pretty swift right leg. In 47 or so years of owning Nortons I can attest that no two Nortons are the same. What works on one is a total failure on another. Kinda like women.
 
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Every once in a while I feel compelled to ask about an acronym. It used to be they were rarely used. Everyone here in the states knew what USA meant, of course. But I have not been so long in this community of bike owners and the world of keystroke shorthand that I immediately recognize what to everyone else is as obvious as (insert acronym here). What's PO? as in "bought by PO". I'm guessing "O" stands for owner. Yes, I know I could have looked it up on the internet or, better still, searched the pages here. Just beein' old school and dumb is all.
 
Joe Schlaberdowski said:
Every once in a while I feel compelled to ask about an acronym. It used to be they were rarely used. Everyone here in the states knew what USA meant, of course. But I have not been so long in this community of bike owners and the world of keystroke shorthand that I immediately recognize what to everyone else is as obvious as (insert acronym here). What's PO? as in "bought by PO". I'm guessing "O" stands for owner. Yes, I know I could have looked it up on the internet or, better still, searched the pages here. Just beein' old school and dumb is all.
Click to expand...

PO Previous Owner.
 
Uh, Possible Offender. It gives us someone to blame for either our Norton's or our own shortcomings.
 
It could be that the choke plunger needs replacing. The rubber tip gets pounded in over the years.
Your pilot is about right.
The current 34 VM as offered by MAP or Sudco uses a leaner slide, a #3.
 
seattle##gs said:
It could be that the choke plunger needs replacing. The rubber tip gets pounded in over the years.
Your pilot is about right.
The current 34 VM as offered by MAP or Sudco uses a leaner slide, a #3.
Click to expand...
The failed plunger tips allows airflow all the time (to varying degrees), and causes a richer mixture. The symptoms reported are not consistant with that.
 
the carb has not been used until now, i changed the main jet from a 220 to the 260 based on suggestions here on accessnorton, but that should have no bearing on start ability, I can get a 240 and install it,

the float bowl was clean, no water or condensation, crude etc,

can the enrichment circuit get blocked? if i unscrew the mechanism and them blow it , and i blow the pickup tube too.
 
850commando said:
i have a 220 or is that to lean?

this is the first time starting the basket case so temp about 15c, and fresh fuel using an enema bag on a hook
Click to expand...

For an 850 a 220 is too lean, a 230 or a 240, then some plug checks. Your ambient is borderline, and at such temps a #40 pilot jet might be better, depends on how much air trim screw you have to turn in.

With a fresh engine any carburation will not work as it should. Your valves, if new or refaced and your seats, fresh cut or lapped will leak by until the two decide to get married properly. Once your new rings and valves/seats settle down you should be able to dial that Mikuni right in. The Mikuni enrichener is very sensitive to manifold vacuum, as others has already mentioned (no throttle while on choke---enrichener), it doesn't take much leakage at either of the intake valves to defeat this vacuum signal. The Mikuni's enrichener circuit looks lie it can dump gallons of fuel into the intake tract, but it doesn't; it has it's own vacuum breaker in the form of a passage that originates at the bell mouth and terminates just above the enrichener valve; your engine needs to take a pretty big gulp to get an enrichened mixture fro the "choke" circuit. A throttle valve that is set too high, on a new top end, (to provide a consistent idle) will also deprive the enrichener of the vacuum signal it needs to do its job. The "choke" will join the team when the other cast members have learned their lines...

A large part of having your new parts settle down will depend on judicious attention to the torque on the engine fasteners and the lash on the valves. Now that you have had it warm it's time to torque and set. I'd do it again after 50 miles then again at 500. I also recommend oil change service at each re-torque and re-set.
 
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