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weird miss sometimes - 2001 - LT-A500F

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So I've had my bro-in-law's 4 wheeler to get it running again. 
Ended up buying a new aftermarket carb, cleaning fuel tank, cleaning petcock and it works near perfectly .

put the petcock to PRI for about 5 seconds , turn it back to on, pull full enrichener , start it  up (starts right up), slowly release the enrichener and put it in gear and go up down the road. Slow, fast, idle etc all good

BUT then out of the blue it will start missing badly (by this time the motor is hot), to the point I can't move the ATV.

Then out of the blue, maybe 30 seconds later, it is back to normal and I can go up and down the road for testing just fine.

I'm trying to get the ATV in tiptop shape to return to him, but bugged by this miss, I'm sure this machine will come back to me if it keeps missing...

Thanks in advance


BTW I am mechanically inclined, but not an expert on this atv...

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Have you checked the plug to see if its rich or lean? Sounds like it's running out of fuel once its warm, I'm wondering if the plug shows it to be lean. Or maybe a plugged fuel cap vent causing vaccum lock on the tank?

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oh shit I think you nailed it.
"Or maybe a plugged fuel cap vent causing vaccum lock on the tank?"

I DID open the gas cap when it started missing then moved the petcock to PRI then back to ON.....

Thank you!! going to get a new cap...once I do one more test.


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took a ride and played with gas cap when it started acting up...don't think it is the cap or lack of gas

when it started happening I took the gas cap of and nothing changed

I changed the petcock to PRI and RES and nothing happened differently

then out of the blue it was fine for a bit, then happened again.

The video below shows that it doesn't appear to be the gas cap

Could it be the coil? 

Do you know the ignition system on this thing? What tells the coil to fire?
Starting with the stator/magneto wires must be going to the voltage regulator to produce the 14v+ to the battery, as well some wires must be going somewhere to provide 100-200v to something that has capacitors (like a boat's powerpack), then something to trigger that to send the volts to the coil that ups the volts to something like 20-40Kv that goes to plug....right?

Again I don't know this machine very well, so no clue what triggers the timing of the spark....I don't think the coil itself would know when to trigger, so a bit in the dark.


oooh found the ignition system for the quadmaster...it HAS a powerpack thingy...now where is it? 🙂

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I just recently went through a similar problem on my Suzuki Eiger.  
Turns out it was my flywheel.   My magnet configuration was off.  
The image you posted is called a cdi box.  The cdi is what gets and tells the spark/ignition  when to fire.  The pick up/trigger coil (located by the stator coil) gets its signal from the flywheel which has a small metal tab on it that passes by the pickup/trigger coil.   When is passes by the coil sends a signal to the cdi and that cdi delivers spark to the ignition coil.

In some instances the cdi fails when the diodes or the small capacitors inside go bad and the electrical current that's meant to flow in one one direction  no longer does that and creates a haywire flow of current.  
If its a capacitor that goes bad inside it usually kills the cdi completely (no spark) when a diode goes bad it allows current to go in either direction and that is where you get the misfire.  
unfortunately there is no way to test or repair the cdi box and its an expensive part to gamble on.   
Back to the flywheel, the magnets have a glue that holds them in place. That glue has failed on a run for about 3 or 4 years of the artic cat and Suzuki engines.  This allows the magnets to move around and cause havoc on the ignition system.  The way to inspect this is to pull the side cover off and inspect the flywheel magnets.   

Start with the flywheel and go from there.  
Then inspect the cdi for any moisture or cracks/damage.    It should be located in the rear under the plastics by the storage compartment.  

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Posted (edited)

Thank you so much for the detailed reply..

What you described is extremely similar to my 2cycle 225hp Evinrude.....so now I'm getting it...

on the johnson/evinrude too the magnets on the flywheel would either fly off (they were glued on) or slide towards each other, thus the voltage created by the stator would not be correct for the CDI/powerpack. When the magnets slid together the symptom would be lower than expected rpms and boat wouldn't get on plane....didn't miss per se, but no umpf

the flywheel is behind the pull start right?

Thanks again for the explanation...really helps



PS: I don't think the magnets are the issue as when the motor is warming up it runs great and fast....I suspect some thermal related issue....either CDI or coil.
Would the voltage regulator/rectifier come into play in the ignition? on my boat the regulator provides 14v to the battery, provides 12 v for the variou accessories and the tach puls for the tachometer....so I COULD remove the regulator and the motor would start...it would burn up the stator, but it would run.

Edited by cfauvel

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The regulator can cause these symptoms and easy enough to test or cheap enough to replace.   
The magnets have been known to move on the flywheels the only reason I'm agreeing with you about it NOT being your problem is the fact that you state it runs fine until hot.   In my case it would run like shit right out of the gate.   Your pick up coil could also be the problem when hot. Get your tester and check the spec book on it. Then go ahead and test the coil when its cold and then when the engine gets hot or starts to act up to see if the numbers are the same.      


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Posted (edited)

woohoo Update:

Test results

Ignition Coil
primary spec 0.1 - 0.8                                                 actual -> 0.000 ohm
secondary spec 10 - 15 kOhms                                   actual 17.x kohms  (not quite in spec)
cranking volts spec > 140v                                          actual >160+

Pickups and power source coils

pickup coil  spec > 0.5v                                                 actual = blue/green = 6.x+v
powersource coil spec  >0.7 v                                        actual - yellow/white = 0.5v - 0.6v  (not quite in spec)

pickup spec 170-250 ohms                                            actual 234 ohms blue/green
powersource spec .05 - .5 ohm                                       actual  0.0 - 0.1 ohms white/yellow


no load Voltage test (3 yellow wire at voltage regulator)
multimeter set to AC and engine running at 5000 rpm 

pin A to pin b, and pin A to pin C, and pin B to Pin c

Spec +60v                                                                Actual 69+ volts

Diode test...

.I screwed that up (just notice the multimeter is supposed to be set to Diode, going to re-test that for confirmation whilst cold and whilst hot)

The fact that the manual says to test the No Load Voltage (the three yellow wire connector at voltage regulator) whilst the motor is running and coupler disconnected I knew I could mess with that connector whilst it was running


waited 10+ minutes at idle and a bit off idle until I heard the motor missing...disconnected the 3 yellow wire coupler and it instantly got better ....plugged it back in...then a couple minutes it started missing again....and again unplugged the coupler and it got better instantly.

THE FIX - I am 99% sure it is the regulator though it is only 2 years old.

BTW I HATE how everything is buried...have to remove the gas tank to get to coil?! 

Edited by cfauvel
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looking at the parts layout...I don't see the pickup coil being separate from the stator, which is a bummer..... I'd like to replace the pickup coil stuff as it is a little off from spec...but since it runs so well slightly warm I think I will skip it.

The parts diagram lists two regulators model Y and model K1, not sure which is right for my application.

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it appears the model Y and K1 represent the years...apparently I'm working on a 2000 atv not 2001 like I thought

found an online vin decode page (not automatic)...



my vin comes out to 

J - japan
S - suzuki
A - atv
M - 400-499cc
4 - 4 stroke
2 - design sequence 2
A - design sequence A  "123456789ABCDEF...." - this is the 10th design sequence letter of A
0 - check digit
y - model year  2000(Y), 2001(1) (or K1)
2 - factory code
107100 - serial number

I accidentally bought the correct regulator

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Would be nice to be able to edit one's [previous posts....the values highlighted in yellow above were NOT achieved by either regulator...only the .4-.7 values were seen.
The new regulator stays cooler when the ATV is warmed up and doesn't sporadically miss.

I say this has been resolved.

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