Tuning issue - need second opinion

My wife's '05 YZ 450 has just developed a problem with idling (it won't when warm), low end acelleration (stutters) and backfiring (both accel. and decl.). It was running fine during most of a ride, and started the above mentioned symptoms towards the tail end of the ride. The bike was dropped a few times during the ride, however the symptoms did not start until over an hour after the last time it was dropped. The bike was operated at very low revs for most of the day (slow technical trail).

When I got the bike home, I changed the oil (in case the bike had taken in water)....was very slightly whitish....but not extremely so. Also changed the spark plug just in case it was bad...it wasn't. When I restarted the bike (cold) it ran fine untill it started idiling down...then tries to die. If I don't stay on the throttle (1/8 to 1/4), it will bog down and die (whether in gear or in neutral).

All my forum searching leads me to either the carb or valve/timing issues. I imagine over time I will end up trying to troubleshoot both, however I am hoping to save some trial and tribulation from having to tear apart both the carb and the top end. I am leaning toward valve clearance being the culprit, just because I find it hard to believe that the carb could get "off" in the middle of a ride with no real event to blame.

Any ideas would be greatly appreciated. (I purchased the bike used, from the looks of it,was well used as a MX race bike, however I have no idea of it's maintenence history)

Thanks for the advice. I will check all that out. I have confidence that I can work on the carb. The valves seem a little inimidating for me right now (not to mention all the $ to spend on specialty tools). I guess if I can't sort it out with the carb I will take it to a shop and have them mess with the valves.

Update.....

Just a quick update.... Thanks to all who have replied. Everything becomes a bit less intimidating with moral support! :nod:

Cleaned the carb....was a little gunky. Took the bike to a friend's (who knows thumpers). Checked the valve clearances. One of the exhaust valves was out of tolerence (too tight). Deciding whether or not to tackle shimming myself or have someone experinced do it....It looks straightforward in the manual....but maybe it's over simplified. Hmmmmmm
:whoa:

The Surprise Ending

So, I took the bike to a shop to have the valve shim pad installed. The shop concurred with my clearance measurement. The mechanic also adjusted the needle up a clip on the carb. When I came back to get the bike, the mechanic said that the bike had race fuel in it, which was throwing the carb adjusting off. He explained to me that race fuel, having a high octane rating, provided more oxygen to the combustion mixture, thus throwing off the carburator adjustment.

So, I knew the bike did not have "race fuel in it". It had pump gasoline (91 octane) with Klotz lead free octane booster in it. I mixed it in to raise the octane level to about a 96 (the manual specifies 95 octane, which is not available at the pump....at least not here in Montana). Though I am not a chemical engineer, I do know (or think I know) that octane rating is really just a measure of how well the gasoline resists detonation from high pressure (which is why high compression engines need higher octane fuel so they don't knock, which is caused by predetonation). I'm pretty sure that octane has nothing to do with oxygen in fuel.

At any rate, the mechanic said all's fixed, bike runs good, just start putting regular pump gas in it, no "race fuel" unless you want to totally re-tune the engine. I, knowing all about octane and such matters, smiled, and took the bike home.

When I got home, I unloaded the bike, fired it up, and rode it around the block a few times. When it warmed up, it acted exactly the same way as it did before all the work was done on it.....backfiring, dies when idling, and misses on low end accelleration :bang:

So, as a last ditch effort, before I woke up way grouchy tomorrow and called the shop and chewed out the mech for ripping me off, and posting one of those typical "can't figure this out, please help, I wanna ride" threads, I drained the "race
fuel" out of the tank, and put regular pump fuel in it, and rode it around. I ran great. Like it did before it had the "problem". Idled fine, acellerated smoothly, didn't backfire. I guess it was the octane booster, not necessarily the dirty carb or the tight valve.

The thing that still boggles me, is why. It can't be that otane booster gives more oxygen to the fuel so it messes up the carb settings, can it? :coocoo:

Rich,
Thanks for the explanation. I was fairly well baffled, now it makes sense (even to me!). After reading the your old thread I think I will stick to pump gas for our non-competitive putting around that we do.

I really did think that I needed octane boost for my fuel to meet what was stated in my owners manual. I imagine that all the OTC fuel additive companies make a good profit off that kind of ignorance.