running problems

2g00d4u

2g00d4u

New Member
Apr 22, 2010
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Smithville, ON, Canada
So, I finally got spark on my moto4 200, (sure everyones sick of hearing about the pos) It is ridable, but it heats up quickly, covers a plug in carbon after an hour of use, backfires a lot when gearing down or revving in neutral, and when you first start it after it has been sitting, it revvs high on idle, so you turn down the idle speed to a reasonable RPM, then ride for a while, then it won't idle, you have to turn it back up. the problem repeats. Also, if I play with the mixture screw even a little, it won't start. It has to be within a few millimeters of one spot to work.

What could cause all these little issues and what can I do to fix it? I'm thinking a carb rebuild would work or a new carb. been cleaned a few times all the way through but little difference. It has a FMF Shorty, that has no back pressure, so that could be part of the problem.

so, the original question, What could cause all these little issues and what can I do to fix it? and if I need to buy a new carb, should it be a different size?

thanks all
 
the backfiring issue deffinitely sounds like a carbs issue to me... as well as the "carbon plug" back firing while gearing down in my exp. is the slow/pilot jet ..need to up the size of it 1 or 2 sizes...but with all your other issues i would start with a complete rebuild kit...if youve cleaned it sevral time already.

but if your poppin/backfiring off throttle you more than likely need to go up on the pilot ....


just my opinon
 
Last edited:
If you have the YFM 200 Moto4 with the built-in front rack between the front fenders, and the carb # is Mikuni VM22SH ID # 52H-00, then the stock main jet is a 115, the needle jet position is # 3, and the pilot screw turns out 1.5 turns.pilot jet size 20. If your carb # is Mikuni VM22SH ID # 1NU00, then the stock main jet is 117.5, the needle jet setting is the same, and the pilot screw is 2 turns out.

But for the record, valve clearances are: Intake - 0.05-0.09mm Exhaust - 0.11-0.15mm

jet sizes are on the actual jet itself very small etched numbers .....
pull out your pilot jet look at the little number on it and order the next 2 sizes up

u can get them from many places....

Jets R Us
Mikuni - BikeBandit.com

or ebay ... where ever just google your bike model number or name and put carb jets after it

heres a diagram for you to refer to for all the carb part numbers and names

OEM Parts Yamaha MOTO-4 (YFM200DXU) 1988 CARBURETOR - Cheap Cycle Parts
 
also once you get it runnin right you need to "read your plug"

Normal: Grey to Light Golden-Brown Color
This condition is ideal, the spark plug and engine air/fuel mixture are operating properly.

Dry: Black Soot Buildup
Air/fuel mixture is too rich, the carburetor settings are incorrect, or the flame arrestor is dirty or has mounting problems.
Spark plug heat range is too cold for the operating conditions.
Ignition system problems causing a weak or intermittent spark.

Wet Fouling: Shiny, Wet, Black Appearance
Excessive use of the choke (gas fouled)
Prolonged low rpm operation (gas or oil fouled)
Fuel to oil ratio is too rich (oil fouled)

Excess Deposits: Bumpy, Chalky Buildup
Poor fuel quality
Oil leakage into combustion chamber
Improper oil used for premix/injected

Overheated: White, Blistered, Melted Electrode
Lean air/fuel mixture
Spark plug heat range is too hot for operating condition of the engine
Plug is not properly gapped and/or torqued onto head
Overly advanced timing

A detonation problem would show signs, such as silver specs, black specs, or melting or breakage at the firing tip.

Signs of fouling or excessive heat must be traced quickly to prevent further deterioration of performance and to prevent possible engine damage.
 
it "should" ... but i cant promise it will as i dont know your quad ....

fyi. a rebuild kit will come with ALL NEW jets ... i would simply rebuild it with what comes in the kit .. unless you have mods done to it ... aftermarket pipe,filter,engine mods...etc etc ... if you do then u will deffinitly have to "rejet" with bigger jets ... if your quad is stock u shouldnt need to rejet at all ....
 
just a pipe alone shouldnt cause that many issues.... seeing as your are allowing the engine to exhale better, BUT if your filter/airbox and lid is stock then your engine cant inhale better .... long story short upgrading the pipe and not the filter too doesnt really do much but add noise.... it will affect it some but not enough to cause all the problem your having. rebiuld your carb ... u should deffinitly see a diff. put a new spark plug in while your at it and check the valve clearance....if you do all this it should deffinitly take care of it
 
but .... technicly(yes i cant spell) u should rejet when u add a pipe. if you want to take full advantage of the pipe, you really should upgrade your air filter.

then you will have areason too rejet

fmf's site should have the info on that, if you cant find it let me know ill look it up for ya
 
have the valves ever been adjusted on it?? if not or if its been awhile then u really need to take care of that as well
 
adjusting valves when there is a cam timing chain involved is such a pain. I would only do it as a last resort. Right now, I have the air filter removed with the box open, and the Shorty pipe. does that change anything?
 
2g00d4u said:
adjusting valves when there is a cam timing chain involved is such a pain. I would only do it as a last resort. Right now, I have the air filter removed with the box open, and the Shorty pipe. does that change anything?
Click to expand...

yes ... rejetting will be nessary it that case


valves will cause chaos as well if there not set right
 
Mikuni VM22SH ID # 52H-00
Mikuni VM22SH ID # 1NU00

look at your carb and see which model u have ... that will help alot
 
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