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Discussion Starter #1
We have a quadmaster 500 that will start and idle but when giving it gas it has no power and backfires
-has really good compression
-Carb rebuilt twice
-intake has been checked
-new stator
-new cdi
-new voltage regulator

Any ideas on what could be wrong with it ?
 

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Welcome To The Forum!
 

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Discussion Starter #4
Thanks for the welcome and thank you for your reply
No we haven't checked valve clearances yet we were also thinking it could be the rotor (magneto) we will check valves and see if not that would you try getting a new magneto? thanks for your help
 

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I have been working on one with the exact symptoms and have tried everything you have plus a coil and the valve clearance (except the cdi) without any results. The stator and regulator both tested bad and overcharged the system so I was hoping that would fix the problem but unfortunately it didn't. I'm down to either timing chain stretched, maybe bad cam, or keyway sheared. It runs great till you hit 1/2 throttle then it just starts backfiring and loosing power. Please update with any headway you make and I will do the same.
 

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Sounds like the timing is off or not advancing at all.
 

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Have you checked to see if your fuel petcock is bad..? When mine went bad on my 300 KQ it caused backfires, no power, hesitation, no idling, etc. May or may not be the issue, but it's worth checking it to rule it out as a possible cause. To check it, (With Atv Not Running). Pull the vacuum line off at the fuel petcock, (Smaller Line). If the line is wet, or fuel comes out that line or port where you removed that line, then your fuel petcock is bad and needs to be replaced. Again, may or may not be the issue. But never hurts to check it.


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When mine went the line was mostly dry but when i pulled it of and plugged the end it ran normally again.
 

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I have mine on a fuel bottle with the tank removed and the vacuum line plugged off so it's not the fuel valve. Intake gaskets all look good so thinking its a timing problem.
 

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LTA500F electrical problem

This is clearly an electrical problem, and feels like a rev-limiter. Backfire is at mid-throttle, and thru the exhaust. This quad had the common regulator/rectifier failure first (output voltage too high), and that, plus all related electrical components replaced.

The QuadMaster LTA500F was made for only 2 years, and still had a mechanical speedo/odo, without any speed feedback signal to CDI. The LTA500F Vinson replaced it in 2002, adding electronic dash, electric 4x4 engagement, different CDI, and other features.

I haunt ATVConnection, and found at least one other owner who has this problem. No solutions posted!
 

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I'm working on a 2007 Vinson 500 with the same issues. Cleaned and rebuilt the carburetor, still idles fine but backfires through the exhaust and air box just off of idle. disconnected the fuel valve vacuum signal hose and plugged it by hand while the fuel valve was on prime. still same result. verified that the alternator output was within specs, around 14.38 vdc. removed the air filter and hand choked it. engine revved out properly. this is definitely a main jet lean condition for this bike. I had some spare main jets laying around so I began to raise the main jet size. It now has a 136 main in it and runs out fine on the lift with just a bit of deceleration lean popping now. I've still got to do a brake job, rear axle bearings and seals and change the oil and filter before putting it back on the ground for a test ride, but it appears to be corrected. it strikes me as odd, however, that a completely stock engine is having to be jetted richer than oem specs for no apparent reason. I'd love to hear other's input and theories about this. Suzuki's in my shop are rare, usually I only see Hondas and the occasional Yamaha atvs with the bulk of my work being mowers and trimmers. The last Honda I had with this same issue ended up being an overcharging rectifier. This one had me baffled for a while.
 

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Thx for posting, FIRE.

This one has me baffled. I am a chainsaw and diesel tractor specialist, but I do a lot of ATVs and M/Cs. In your case, you got backfiring from exh and intake, correct? Any backfiring thru intake points to lean condition or valve timing issues, in my experience. Mine is all thru the muffler, pretty much like hitting the engine kill switch for a half second. Plug does get black, too.

I did also check the auto-decomp mech on the cam, but I do not see any problems with it. It is hard to set clearances properly (4-valve), but they are OK.

I bought this quad after an established shop failed to fix it. They even bought a new Mikuni carb for it ($$$$). I suppose I should check the main jetting, but adding fuel at the carb inlet does not help.

I have had all brands here, but have particular fondness for Suzies.
 

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Thx for posting, FIRE.

This one has me baffled. I am a chainsaw and diesel tractor specialist, but I do a lot of ATVs and M/Cs. In your case, you got backfiring from exh and intake, correct? Any backfiring thru intake points to lean condition or valve timing issues, in my experience. Mine is all thru the muffler, pretty much like hitting the engine kill switch for a half second. Plug does get black, too.

I did also check the auto-decomp mech on the cam, but I do not see any problems with it. It is hard to set clearances properly (4-valve), but they are OK.

I bought this quad after an established shop failed to fix it. They even bought a new Mikuni carb for it ($$$$). I suppose I should check the main jetting, but adding fuel at the carb inlet does not help.

I have had all brands here, but have particular fondness for Suzies.
what led me to the main circuit was hand choking the inlet at the air filter. i removed the filter and used my hand as a restriction. the "choke" on these carburetors is an enrichener, not an actual butterfly, so simulating an intake restriction helped me to verify that i had a fuel delivery issue off of idle. if it hadn't been idling so well i would have suspected the idle circuit, but it was idling too well for that to be an issue. no smoke and no stumbling without throttle input. the problem started off of idle. with my hand over the intake throat it would rev out correctly so the only logical conclusion would be the high speed circuit since a vacuum leak would have also affected idle. correct me if i'm wrong, but hand choking would not have an effect on a valve lash or valve timing issue. it definitely wouldn't help an ignition issue, especially without it blowing black smoke and flooding. has me confused for sure, but the larger main jet has corrected the problem for now. after i get it back on the ground and give it a good high speed run i'll know for sure if it's corrected
 

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correct me if i'm wrong, but hand choking would not have an effect on a valve lash or valve timing issue. it definitely wouldn't help an ignition issue...
Yes, correct. I use the hand-choke method, too, when trying to fix carb issues.

I also wondered about my auto-decomp mech. Have you messed around with those before? Not sure if that can malfunction and open a valve at midrange.
 

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Yes, correct. I use the hand-choke method, too, when trying to fix carb issues.

I also wondered about my auto-decomp mech. Have you messed around with those before? Not sure if that can malfunction and open a valve at midrange.
I've never had one fail to the point of causing running issues, just starting. I usually see this on briggs engines with overhead valves. most of the time it's still not the decompression function but valve lash out of spec. I've only had one come in with a bad cam.
 

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I've never had one fail to the point of causing running issues, just starting. I usually see this on briggs engines with overhead valves. most of the time it's still not the decompression function but valve lash out of spec. I've only had one come in with a bad cam.
Yes, that is very common. Folks do not realize that small OHV engines on their mowers must have the valve lash adjusted. If electric start, too much clearance can cause the pinion gear to break, due to excessive compression. That is for Briggs, Honda, LCT, and clones with plastic pinion gears.

Our ATVs with auto-decomp mechs are made better, but I notice that this LT-A500F has a decomp lever on the cam with very little spring tension.
 

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Yes, that is very common. Folks do not realize that small OHV engines on their mowers must have the valve lash adjusted. If electric start, too much clearance can cause the pinion gear to break, due to excessive compression. That is for Briggs, Honda, LCT, and clones with plastic pinion gears.

Our ATVs with auto-decomp mechs are made better, but I notice that this LT-A500F has a decomp lever on the cam with very little spring tension.
I know this is old but I am having the same problem with my quadrunner ltf500f and I tried the hand choke and it ran perfectly but took my hand off and it backfires out the intake and exhaust has anyone fixed this problem or know what I should try??
 

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Hey guys, I came across this thread about a year after I had your same problems and finally figured it out. I was fixing my dads 2001 quadmaster 500 that was backfiring and running very poorly. Black smoke, etc... I changed coil, wire, spark plug, petcock, cleaned original carburetor. Still the same. Ordered a new carburetor and installed, same thing. Installed original coil, same thing. Checked compression, Valve clearance, everything. All were fine. Turns out that the ignition switch was bad and not allowing the proper voltage to the electrical parts, therefore causing weak spark. Put a new oem switch in and it runs like brand new.. was fairly cheap also. I think I paid 55.00. Hope this helps...
 

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All I habe to say is oh. My. God..... okay, not really, lol. However I signed up just to chime in with my amazement at this thing. Maybe I'm on the wrong machine. But just about the same, as I javent memorized the model numbers. I have a 98 quadrunner500, which I bought with the description that it "burned oil".... well, new battery(jumper box) and it fired off the choke and idled perfectly clean with a new plug (they had a 6 in there, iirc, whatever is 2 numbers off from whats called), but same symptoms. Idle only. Tries to stall and backfire with throttle. I thought "okay, classic main jet or similar circuit malfunction... even pilot, since it needed choke to run. Cleaned the carb tonight, and aside from the usual crud in the bottom of the bowl, everything clean and clear. Richness up the needle one click, then later leaned it because of the bogging and exhaust backfire from unburned fuel... reading this thread, I of course did the hand choke after cleaning out and studying the stupid auto feed petcock, and it ran better with my hand. Instead of replacing, I pulled apart the ignition switch, and belive me its like a brand new switch going back together (use notebook paper to clean electrical contacts to make them new, unless the are rotted or arced away) then, I went to the battery terminals and cleaned them up, and lowe and behold, it began to run 10x better. I cant belive its as sensitive as a new car. Of course it didn't run great because
tththe
aaiair
bbox and snorkel isn't installed, and yes, that makes a HUGE difference in the main jetting.... and this is a good point to segway to some things I would like to point out, but first, ill out line my next steps. I noticed the relay mounted to the cargo box side (not the front) has been bypassed, so im going to fix that, and then go over all the ground and ignition distribution connections... but thanks to the previous post, I now know that these systems are overly sensitive to poor connection, just typically built so well that its not an issue. Lol.
And now to the points: when you have an issue with poor spark, be it weak, or incorrect time, or you have an issue with coressier ring
 
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