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12/12/2007
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I bought 2 gallons straight from the pump at the airport the day before I went riding and my KX250 had never ran better.....100% avgas used for premix no 91 pump gas
Exactly 1 week later same track, same gas that was still in my bikes gas tank, I'd get less than a half a lap in a go to grab a handful or chicken-wing it and the bike sounds like it was running under water on mid to top with a burbuling sound and the bike isnt jerking when it does this, it doesnt sound like a bog or a mechanical failure either. Itll clean up for a split sec and act up again.
No excessive smoke and it only happens while being ridden under a load or when I'm about to land from a jump and crack it open. In neutral I can wick it wide open and nothing changes. Air filter is clean, I changes the spark plug at the track with the same BR8EIX plug, drained the float bowl and pulled the main jet. All was clean and the problem persisted. Plug tip was a slightly darker version of tan, it was try and not oily.
I checked the carb vent hoses they seem to be clean and the gas cap vent seems to flow semi ok
Bike has an ever so slight minuscule deeper sounding tone at idle but it's barely noticeable and it idles for a bit but will drop off and quit after about 20 seconds.
Any help here, can 100ll avgas attract moisture in a week to cause this or am I missing something here?
Bike is a 2003 KX250
Exactly 1 week later same track, same gas that was still in my bikes gas tank, I'd get less than a half a lap in a go to grab a handful or chicken-wing it and the bike sounds like it was running under water on mid to top with a burbuling sound and the bike isnt jerking when it does this, it doesnt sound like a bog or a mechanical failure either. Itll clean up for a split sec and act up again.
No excessive smoke and it only happens while being ridden under a load or when I'm about to land from a jump and crack it open. In neutral I can wick it wide open and nothing changes. Air filter is clean, I changes the spark plug at the track with the same BR8EIX plug, drained the float bowl and pulled the main jet. All was clean and the problem persisted. Plug tip was a slightly darker version of tan, it was try and not oily.
I checked the carb vent hoses they seem to be clean and the gas cap vent seems to flow semi ok
Bike has an ever so slight minuscule deeper sounding tone at idle but it's barely noticeable and it idles for a bit but will drop off and quit after about 20 seconds.
Any help here, can 100ll avgas attract moisture in a week to cause this or am I missing something here?
Bike is a 2003 KX250
It’s designed to burn at high altitudes at 35-4500 RPMs. Gas is not gas, they all have different blends designed to burn different ways.
If you ran 91 non Ethanol gas, it would run better than the trash you have in it. Drain your tank and put in good pure gas.
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Something important was mentioned in an avgas thread and that is that the most crucial part of a plane's engine performance is at takeoff isnt it? Therefore the gas is also perfectly fine to run at altitudes on terrestrial earth so I dont thing it's an altitude problem with the gas either.
I'm going to check my vforce reeds but I may try pump gas first and see what happens. One things for sure, that kx250 pings bad on 91 pump gas and didnt ping a lick on the avgas
Avgas is also very clean and consistent (as approved by the FAA), the same cannot be said about pump gas.
A stock 2T should have no issues when using Avgas provided that it is jetted correctly.
It's actually a very stable fuel, capable of sitting in an airplane's tank for long periods of time prior to use. When kept in a sealed metal container, it stays potent much longer than pump fuel does.
I've been running it for years in my 2 strokes, and on occasion have run it in highly built thumpers too. Since it's not oxygenated, it runs a touch richer than the lightly oxygenated pump fuel.
What are you running for premix? I bet the premix seperated from the gas and is sitting at the bottom of the tank and gunked up the petcock or clogged your pilot jet. 927 will seperate from 100LL pretty easily.
When I first got the bike I wanted it to run on 100LL for the higher octane, consistency and no ethanol. Not long after getting it, went to Southwick for a practice day. Shook the jug, filled the tank with 100LL and 927 premix. Made the 2 hour drive and when I arrived the bike ran exactly like you describe. The float bowl had collected premix and the pilot jet was clogged. Cleaned everything out, drained the tank and refilled. Ran fine after.
After a few years the best combo I found (without running VP C12 which is AWESOME) was a 50/50mix of 100LL and 91 MO Gas (no ethanol) mixed at 32:1 with K2 premix. Bike is jetted for it, premix doesn't seperate from the gas and I wouldn't change a thing.
Good luck.
Did alot of reading on this in the past.
I never had any problem running it any 2 stroke I've owned. From a YZ465 to my weed whacker. If you rode my CR250 or YZ465 and weren't told it had avgas you would be happy with the way they run.
My string trimmer sat for 6 months, started on the third pull.
OP, agree you need to check your reeds, it's got to be something other than the gas.
I'm going to make sure I check the reeds tonight as a starting point
Pit Row
Checking reeds tonight
Spark plug cap WAS loose on the plug tip which I had thought would've alleviated the problem but it didnt ironically.
When I pulled the fuel line off the tank the petcock lever caught something and opened up and fuel flowed perfectly fine till I re shut it off again (doh!)
Main jet was snug.
Oddly enough .....one thing I did do was when I put my new fastway f4 footpegs on right before this last weekend's problematic ride, I shut the fuel off and had to lay the bike all the way over on its side because on the KX250 in order to take the footpeg pin OUT you literally have to remove the clutch cover! I'm thinking mayyyyyybe just maybe that float bowl clip might have moved from that? I doubt it but if my reeds are good I'll check that too.
OH ... and yeah check the reeds!
My spark plug cap WAS loose and probably had been for a while and my old plug on the lead tip was missing a piece so I'm thinking there was possibly some arcing going on between the plug cap and the spark plug tip.
Could this have ruined my coil, if so what are the symptoms of a failing coil?
Avgas is more oxygenated so it actually run leaner than pump gas and that is why it feel better
Its designed to run at slower rpms at altitude so it has a slower burn rate, so if you running in higher RPMs, you’re spitting unspent fuel out of your exhausts which causes heat.
Avgas is measured differently then regular gas, in reality it’s about a 96 octane compared to regular gas but the oxygenation makes it feel so much better. Your bike will run better than regular pump, but it will run much better and cooler with a clean 92-98 octane, and it’s cheaper.
Another thing I’ll add, octane doesn’t mean horse power.
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