Hi, joined up long ago, and finally had some trouble with my quad to come ask for help. If you make it through my epic story of symptoms, i thank you in advance for looking. [img]i/expressions/face-icon-small-smile.gif[/img]

ATV in question, 1998 500cc Suzuki 4x4 quadrunner, model lt-f500f. It's a manual shift transmission, with about 3000km, have had since new.

I've been having a lot of trouble with poor idling and starting this spring and summer. I've also noticed the exhaust smoke seems to be blacker than i recall, enough that you can see it in the air, but not, like say a diesel engine under load, plus i get a modest smell of unburnt fuel when getting on the throttle while testing it in my shop. That would indicate it's running a little rich to me. ???

It didn't sit too long in storage last winter, and i use fuel stabilizer. I put a new air filter in it this spring when i discovered it wasn't running right, and oiled it sparingly, less than the old one was.

I've taken the carb out and dismantled it completely 4 times now, and either blew compressed air or pushed a wire through as many passages as i could, to confirm none were gummed up. I've been adjusting the float slightly, leaning it out, and then it idles a little better, but backfires when you hold the throttle open steadily. I'm just in the process of incrementally raising the float level to try and find the best spot.

I checked the sparkplug, seems carboned up more than a normal one would be, it's new as well, installed same time as new air filter, and only a couple of hours of use since. It has a nice blue spark, but i noticed it will very rarely jump from the center electrode to the side electrode, mainly out to the threaded body of the plug. I thought maybe a weak coil, and checked it out with my ohmmeter, and it was a little out of spec. According to my clymer service book, should have 10-15k ohms secondary resistance, and 0.1 to 1.0 ohms for the primary resistance, (or maybe it was the other way around) mine was 16200 ohms, and 1.2 ohms. So, i ordered a new ignition coil, but it had the exact same readings. Either i got a bad one, there's actually nothing wrong with it, or maybe my ohmmeter is in error. With no returns on electrical parts, i'm not keen on ordering yet another one to try again.

So, what other options are there?

Would the CDI unit go bad just enough to cause symptoms like this? I would think it should work or it won't, not maybe/maybe not. I've given the wiring harness a visual inspection, and don't see any fraying, open wires. I guess i'll remove it next and unwrap it all for a closer inspection. Battery was nicely charged, 12.5+/- volts, but it is original.

I checked the valve clearances, and they seem just fine. Ignition timing is non adjustable according to service book.

I didn't see anything visually wrong with the carb jets, passages, float, intake boot, hoses, etc. I took the fuel selector out of the tank, and while there was a little crud at the very bottom, it didn't seem to be restricting fuel flow. I cleaned it well and reinstalled the first time i removed the carb to dismantle, and checked it again the 2nd time too.

I should say, it would occasionally let out a very low sounding "poof" of a backfire when shutting down since it was new. It didn't bother me, my snowmachine does that, and IIRC, my old Honda ATC's did that too.

It was running just fine last year, and then this spring it's been running like crap. I've only used it a few hours in the last 5 months since it runs so poorly.

Any ideas? Thanks for reading this far.
Ken

 

My first guess would be the rings, did anything happen to get anything sucked through the carb, like dirt or mud? The black smoke seems funny though, would think it would be blue.

You ever rebuild your top end or know of a repair manual for these bikes that can be downloaded from the net?

 

Thanks for response,

I'm kind of leaning away from a ring problem, it's definately not burning any oil. The black smoke is more like partially combusted gas, which is why i was thinking of a weak ignition component, what with the carboned spark plug.

I've been working on it all day so far, raising the float level a little at a time. I've got it to run without backfiring, but still won't idle, and is getting harder to start after each adjustment. I have to hold the throttle open about halfway while cranking, and then keep opening it up to keep it running at idle. If i turn up the idle screw, then it idles way too fast.

I checked the cdi pickup coil (inside the engine case by the stator) resistance, per shop manual instructions, and it was right in the middle of the range.

I'm going to have another go at it later, but i think i'm just going to drop it off at a small motor shop next week and see if they can figure it out.

As for a manual to download, i couldn't tell you. I picked up the one i bought on ebay a couple of years ago for $12.
Here it is - it covers a complete teardown and rebuild, plus maintenance. Maybe not as good as a factory manual, but worth the money. If only i could figure out what i am missing.
http://www.amazon.com/gp/product/089...16520&v=glance

 

Just thought i'd update for the sake of anyone in the future with similar problems.

I finally tracked it down to a worn out fuel valve assembly. It was getting fuel into the vacuum side of the valve, and since it was getting dumped into the engine on the front, engine side of the carb, that is where the extra rich mixture was coming from.

So, if you happen to pull the hose from the rearward fitting of the fuel pet **** valve, and find gasoline running out of it, it's defective. Now that i've replaced it the bike runs very smooth and idles perfect again.

For some reason the Clymer service manual doesn't include much if any info on the operation of this engine vacuum activated, fuel valve. Pity, maybe i should write them a letter for the next edition. [img]i/expressions/face-icon-small-smile.gif[/img]

Ken

 

Timberjack's description fits my 1998 F500F very well. I am not well versed on the nomenclature and can't find the fuel valve assembly in the book are you talking about Part #8 (Diaphragm) in the carb layout. I fount a Diaphragm kit listed on line for about $128 or so. I will pull the Carb out and take another look. I put a Carb kit in it about 3 weeks ago did not help much if any. The carb kit did not include that diaphragm. Thanks for any information.

JR