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Carb / jetting issues

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jason
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bohac
Helping a friend with his kids’ 90cc Honda ATV. It recently developed a problem when going WOT.
If you punch the throttle from idle it stalls and dies, but you can ease into it and get to WOT. It also doesn’t seem to eager to take a load (like an adult riding it up a ramp into a truck).
He had been tinkering with it, and i finally got frustrated and put jetting back to stock, and i pretty sure i have the idle and air screws at stock turns out. Still having the same problem, but it’s idling much better now. I also gave the carb bath in simple green HD and in the ultra sonic cleaner. We’re at about 6,000 ft, and I’m not sure how much that affects these little ones.


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Have you pulled the plug to see what it looks like? From what you’ve mentioned, it sounds like the main is a little rich (whacking the throttle, dumping too much gas). If the needle on the slide is adjustable, you might try lowering it (raise the clip a couple of notches). If not, maybe a smaller main. But, I’d check the plug first to see if it will confirm it’s too rich.

Also, check the air filter (if you haven’t already). I have a friend who’s son had an XR200 that wasn’t running right. When I checked the air filter, it had turned to something akin to jelly. Never had been cleaned. Removed it and the bike came to life. Many times play bikes like this never see any maintenance until they die.
 
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Was tinkering this afternoon an after talking to a friend tried a smaller main jet.
With the smaller main jet, it won’t start or idle with out full choke (its started and idled perfect with out the choke stock). And the needle isn’t adjustable.
I got to looking at the parts breakdown and noticed the air box lid is missing a restricter (#7 in the pic), so i may be getting too much air. I’m going to source a bigger main
3f16fc3e2ba434ff621eef2cdb78cf2e.jpg
jet and see what happens.


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Kinda strange as the main shouldn't effect idle.

effectiverange.gif
 
Kinda strange as the main shouldn't effect idle.

effectiverange.gif

That was my thought as well. Main jet/needle should not be affecting idle. My KLR 650 and KTM 530 EXC both had similar symptoms when I had forgotten to drain the carbs before letting them sit for more than a few weeks. The alcohol in the gas causes them to gunk up faster than they used to. The pilot/idle jet would get clogged, restricting gas flow. They would idle on choke, but always die if I turned off the choke, even after they were warm. Trying to go to more throttle would kill them every time. I tried cleaning the idle jet, but eventually found that it is FAR easier to just buy spares and keep them on hand. It takes a few minutes to install a new one and have the bike run great versus jacking with trying to get one cleaned, reinstalling it only to find out it is not clean enough, taking it back out and trying again, etc,....

Even when you try to go wide open on the throttle, the idle circuit is still contributing to the overall mix. It is possible that it is restricting the fuel flow enough that when you try to go wide open, you lean out and it dies, especially since your air box is open and allows for more air. I would put the main/needle back to stock and install a new idle jet. Then make sure you have the air/fuel screw properly adjusted (if you can adjust it). Some carbs have a small metal cap over the air/fuel screw to keep you from messing with it. On some carbs, the screw controls air flow and on others it controls fuel flow. Either way, there is usually a basic setting, like two turns out from the screw being bottomed out. This basically lets you fine tune the mixture as altitude changes; whereas, changing jets and needle settings are more of a larger change. Then you need to tweak the idle speed knob just to smooth out the idle (basically that raises/lowers the resting position of the slide but does not control the actual air/fuel mix ratio). I'd also put that air restriction plug back into the air box. That might be an option installed by dealers for ATVs purchased in locations that are above sea level.
 
Did you check the timing and valve adjustment? That little tiny engine has tiny valves and like wise tiny valve clearances. Didn't look it up but I would not be surprised if its as little a 0.002' - 0.003". Oh BTW, that carb is very basic, just a can with jets. Before I spent hours and hours on it I'd do all the other basic test and adjustments first.
 
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