Starting/Bog Issue

Hardwrkr13

Hardwrkr13

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DIrtbike (18 KTM 300 Carb) usually fires right up instantly when cold. Today I turned fuel petcock on and it cranked for a minute or so until the battery was dead then I gave it two hard kicks on the kickstarter and it fired up (it coughed a for a second a couple times when at the end of the battery cranking so it seemed about to start). Ran perfect for the following 3 hrs of singletrack work. On the way home near the end of a long gravel trail/road straight held around 3/4 throttle for let's say 10ish seconds it bogged down like it was out of fuel so I pulled in clutch and feathered the throttle and the revs came back up a few seconds later and it worked fine the remaining 5ish minutes home (went slower and kept revs lower).
Fuel (non-ethanol, marine premium) is less than 6 months old. Iridium sparkplug has around 25hrs on it.
Since it worked perfect for the 3hrs between issues I don't believe it's a carb issue.
I'll pull the sparkplug anyways and take a look and also check the vent hose on the fuel tank (fuel tank was around 1/2 level on way home). Battery is also new this year so I’ll double check those connections as well as the ground as I’ve heard the bike won’t run without a battery connected (battery was replaced with identical oem one).
 
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Both hard starting and occasional bogging can come from a variety of things on a 2t.

If I was going at it, my first guess is fuelling. The spark plug will tell a story. If it’s white, you may have a clogged jet or air leak in the intake boot or crank seal. Assuming it was running well - jet sizes and fuel/air should already be dialled in.

- let it sit for an hour and pull the fuel line at the carb and check for good flow
- send the fuel thru the petcock into a clear cup to check for water
- pull the carb and check for gunk in the bowl and jets. Blow out the passageways.

A dirty pilot jet makes for hard starting, gunk in the main jet or emulsion tube can cause bogging. Gunk if up carbs can be intermittent, in the beginning.

Other less likely possibilities:
Clogged air filter or exhaust
Stator, CDI, coil
Low compression (rings).
Electrical connections and sensors
 
I like Mike's list.

I'd look for an air leak. Something like a split boot may lean you out when starting with the button but when you kick it, things move and the crack could close. I'm surprised it worked so well when riding but it could have been closed more often than it was open. Back to steady state and it got pulled open and not jostled to close it again.
 
Lightcycle said:
Does it have an inline fuel filter? Maybe clogged/dirty.
Click to expand...

Mad Mike said:
Both hard starting and occasional bogging can come from a variety of things on a 2t.

If I was going at it, my first guess is fuelling. The spark plug will tell a story. If it’s white, you may have a clogged jet or air leak in the intake boot or crank seal. Assuming it was running well - jet sizes and fuel/air should already be dialled in.

- let it sit for an hour and pull the fuel line at the carb and check for good flow
- send the fuel thru the petcock into a clear cup to check for water
- pull the carb and check for gunk in the bowl and jets. Blow out the passageways.

A dirty pilot jet makes for hard starting, gunk in the main jet or emulsion tube can cause bogging. Gunk if up carbs can be intermittent, in the beginning.

Other less likely possibilities:
Clogged air filter or exhaust
Stator, CDI, coil
Low compression (rings).
Electrical connections and sensors
Click to expand...
Fuel filter is a couple inch screen on the other end of the fuel petcock that sticks inside the tank. It’s clean, had it out a year ago.
Air filter is clean, only ridden it once this season and wasn’t following anyone. Compression feels good on kicking it, plus it had normal “tried to kill me” power again today.
I’m also thinking it’s a fuel flow thing and maybe the petcock isn’t opening fully so I will check flow as suggested and see if the sparkie tells me anything.
Boots/sealing all look as new without issues but will give them a more thorough look. Thanks for the tips as there’s a few in there I hadn’t thought of.
 
I would put in a small inline filter into the gas line. They trap a lot more than the petcock screen will and you get a sight glass into fuel flow.

If the petcock is sticky or not completely opening, that’s normally gunk or a failing o-ring — both arch enemies of a carb.
 
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