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Discussion Starter · #1 ·
Just got this machine used. Been sitting quite a while.
I have:
New Plug, air intake boot, clean air filter, new fluids, and completely disassembled and cleaned the carb (twice because doubted myself).
Now she starts correctly with the choke, idles great, has good throttle response in almost all situations.
Now to the confusing part. At 2/3 - 3/4 throttle in gears 3-5 in high range it wants to stall out and starts coughing and backfiring. Briefly apply choke and it wakes right up, remove choke and back to same condition like it's starving for fuel. This scenario does not happen in any other gear. I am wondering if the venturi lifting diaphragm is too stiff from sitting and will not open the throttle properly. I am pretty good at turning wrench but have never worked on a CV carb. Is the diaphragm supposed to have a natural wrinkle in it when removed? Maybe I need to raise the needle.
Any help would be greatly appreciated. I just want to ride and know it will bring me home.
 

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Discussion Starter · #2 ·
Think I might have it figured out. A little time on You Tube never hurts. Carb on the bench I manually raise the venturi, plug off the choke and vacume chambers, release the venturi and it drops right away. I thing the diaphragm is either leaking or not sealing properly. I know it doesn't feel very pliable.
 

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It's a mikuni or keihen
 

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Think I might have it figured out. A little time on You Tube never hurts. Carb on the bench I manually raise the venturi, plug off the choke and vacuum chambers, release the venturi and it drops right away. I thing the diaphragm is either leaking or not sealing properly. I know it doesn't feel very pliable.
That is normal operation. It will close quickly under spring pressure. It takes manifold vacuum from a running engine to hold it open at the correct distance for current engine load and RPM.

You could however have a problem in the diaphragm. Hole in diaphragm, cap to diaphragm seal problem, or diaphragm to carb body seal problem

In nearly all cases of failure to run properly after a cleaning attempt is a result of incomplete cleaning or improper assembly or perhaps missing a part during assembly like the needle jet that may still be in the cleaning bucket. Also possible that the float fuel level adjustment got knocked out of specs during cleaning or assembly. Also is the PMS adjustment back to were it should be. All jets, ports and passageways in the carb body must be perfectly clean and installed back to factory condition.

The PMS screw operation on a Mikuni carburetor is backwards from a Keihin carburetor. Mikuni controls air, more open = more leaner, Keihin controls Fuel More open = more richer

At 2/3 - 3/4 throttle in gears 3-5 in high range it wants to stall out and starts coughing and backfiring. Briefly apply choke and it wakes right up, remove choke and back to same condition like it's starving for fuel
This does point to a potential problem with the needle height or fuel delivery at mid to upper part throttle ranges. Also could be a problem with getting fuel through the main jet since all fuel metered by the needle must pass through the main jet first.

On Keihin carbs its easy to mix up start and main jets and install them backwards. that would cause this problem you are seeing if it was a keihin carb. Not sure if your Mikuni has start and main jets that can be interchanged or not???? perhaps???????

If that year and model has reverse and or netural rev limits built into the CDI a diode installed backwards in the fuse box cam mimic carburetor problems. ??????
 

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Discussion Starter · #6 ·
All very good points.
I have worked on a LOT of small and large engines including their carburetors. All my friends say "Take it to Billium, He can fix it" and I always do. I just have not experienced the CV type carburetors in the past. The jets on the Mikuni are not interchangeable. All jets and passages are clean and visually inspected. The diaphragm doesn't appear to have any holes or cracks, that I can see, but it doesn't seem as pliable as it should. I think it should be a pliable as silicone rubber.
IMHO it just feels like it has become too stiff and will not open all the way under low RPM. These carburetors are quite simple and there just doesn't seem to be any other explanation.
If it were a rev limiter issue wouldn't that show up as a constant in all gears?
I put the PMS screw at the exact setting as before and set the float correctly.
It seems there is only one variable situation; high gear + low RPM = vacume too low to lift the old/stiff diaphragm.
Please let me know if you disagree as these things are not cheap $100.00.
 

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Discussion Starter · #7 ·
Jumping back on my old post.
When I got the machine it had a 132.5 main jet. Installed a 142.5 and saw good improvement but still lacking power and response on the top end.
Put a couple hundred miles on it last weekend and being loaded it really didn't have the power I think a 400 should have. I would use the enrichment circuit to gain power in the higher gears or climbing steep hills which makes me think I am still running too lean.
Just installed a 145.0 main jet last night, more improvement. I will raise the needle on notch tonight and see what happens. The exhaust now smells like it is getting a good fuel mixture.
Any other thoughts/suggestions are gladly accepted.
 

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The diaphragm should be pliable like rubber should. If it's stiff or brittle then replace it. What does the plug look like? Does sound like a lean condition to me.
 

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Discussion Starter · #9 ·
I raised the needle two notches and now have what I feel is 400cc power.
I can actually spin all four from a dead stop 1st gear high range. And the exhaust smells like it should.
The diaphragm is a little stiff IMO and will get replaces in the near future.
For now I am happy I can ride with out "messing" with it all the time. Cat happy
I am wondering if one more jet size would make it that much better but the next size up is 10 points. I think I will stay right where I am at.
 
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