Question for Avgas Users (100ll)

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5/4/2020 11:27 AM

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

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5/4/2020 11:30 AM

Check your reeds.

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Ed Johnson

5/4/2020 11:33 AM

lostboy819 wrote:

Check your reeds.

Ok I was thinking that might be a reason.....anything else I might be looking at?

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5/4/2020 11:35 AM
Edited Date/Time: 5/4/2020 11:37 AM

Avgas is trash for your bike.

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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5/4/2020 11:36 AM

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Non Gratum Anus Rodentum

5/4/2020 11:49 AM
Edited Date/Time: 5/6/2020 4:12 AM

An airplane’s most critical time of flight is leaving the ground and then upon landing. I’ve ran it in every 2 stroke I’ve owned for the last 15 years mixed with Motul 800 with absolutely zero issues. A good jetting spec is important on anything with a carburetor...check those reeds.

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5/4/2020 11:53 AM

UpTiTe wrote:

Avgas is trash for your bike.

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.

I ran it pure a week prior with zero pump gas mixed in and the bike ran absolutely incredible. 1 week later it does this so I doubt the avgas itself is the problem unless it absorbed a massive amount of moisture in 1 week which is why I'm asking.

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

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5/4/2020 11:55 AM

Avgas is legit. Id run it 4 gallons to 1 gallon of premium to keep it closer for jetting reasons.

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5/4/2020 12:03 PM
Edited Date/Time: 5/4/2020 12:16 PM

yz133rider wrote:

Avgas is legit. Id run it 4 gallons to 1 gallon of premium to keep it closer for jetting reasons.

It is mentioned in another thread that Avgas is Aviation spec and is designed as such, for altitudes etc etc...all of which is true.

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.

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Non Gratum Anus Rodentum

5/4/2020 12:07 PM

yz133rider wrote:

Avgas is legit. Id run it 4 gallons to 1 gallon of premium to keep it closer for jetting reasons.

Boomslang wrote:

It is mentioned in another thread that Avgas is Aviation spec and is designed as such, for altitudes etc etc...all of which is true.

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.

Bingo. Its higher octane, leaded, clean and consistent for sure.

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5/4/2020 12:26 PM

100LL isn't any more prone to absorbing water than pump, and is much less prone to doing so than the alcohol tainted swill we have in the PRC.

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.

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5/4/2020 12:30 PM

I’d be shocked if your issue was related to the gas, and if so you got a bad batch. There’s absolutely no reason a perfectly good tank of AVgas would start giving you problems after sitting for a week, something else is going on there and I doubt it’s fuel related. Myself and many others have ran it for years with no issues, you’re not going to light the dyno on fire with it but if you just want to keep detonation under control it’s a great option.

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5/4/2020 12:47 PM
Edited Date/Time: 5/4/2020 12:52 PM

Hey I've messed around quite a bit with 100LL and different premixes on my 07 RM250 and had a similar issue.

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.

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5/4/2020 12:50 PM
Edited Date/Time: 5/4/2020 12:53 PM

RMguy wrote:

Hey I've messed around quite a bit with 100LL and different premixes on my 07 RM250 and had a similar issue.

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.

That's partially why i suggested 4 gals of 100ll to 1 gal of premium also. Something about the specific gravity i believe it was. Makes premix seperate at low temps easier. 927 also seperates more than most premixes.

Did alot of reading on this in the past.

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5/4/2020 1:08 PM
Edited Date/Time: 5/4/2020 1:15 PM

UpTiTe wrote:

Avgas is trash for your bike.

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.

blah, blah, blah. Do you have extensive experience running avgas? Maybe you can direct him to a place, any place that he can buy non-ethanol 91. I'll save you the trouble. Nothing within 50 miles.
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.

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The older I get, the faster I was.

5/4/2020 1:12 PM

RMguy wrote:

Hey I've messed around quite a bit with 100LL and different premixes on my 07 RM250 and had a similar issue.

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.

I use Lucas Oils semi synthetic premix I'm fairly certain that between loading the bike up and the bouncing it took on the freeway for an hour and a half plus unloading would've shaken up the premix considerably so I dont think it separated, especially because it's not been (castor) oil I'm using

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5/4/2020 1:14 PM

UpTiTe wrote:

Avgas is trash for your bike.

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.

captmoto wrote:

blah, blah, blah. Do you have extensive experience running avgas? Maybe you can direct him to a place, any place that he can buy non-ethanol 91. I'll save you the trouble. Nothing within 50 miles.
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 fairly certain it's not the gas but had to ask.....other than the reeds, what else might I be looking for?

I'm going to make sure I check the reeds tonight as a starting point

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5/4/2020 1:45 PM

Avgas with amsoil 50:1 in the 50 and 32:1 in the 65 with no issues. Left it in the tank, and can for 2.5 months when the boy broke his arm. No issues.

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5/4/2020 1:56 PM
Edited Date/Time: 5/4/2020 2:28 PM

I have had the same thing happen as above when using 927 back in the day which made me switch to K2. The 927 completely separated and collected in a lump at the bottom of the tank and flowed into the carb plugging everything up. To trouble shoot I would pull the fuel line and open the petcock and see if fuel flows, if it’s all oil pull the carb and clean and while the tanks off shake it up and dump out... put fresh gas in mixed with a good synthetic oil at the ratio you have it jetted for and let it rip. silly as it seems I’ve had all kinds of dumb shit happen in the past 35 years of riding 2 strokes, improperly seated C clip on needle causing the needle to float and give erratic performance, main jet coming loose will give you an over rich condition and it’s happened to the best of us! How about a loose spark plug cap or loose screw cap on top of the plug... if it ran well last week without something catastrophic happening it has to be something simple that will make you shake your head for sure... Oh and as above check your reeds and air filter!

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Don’t piss off the old people - the older they get the less “life in prison” is a deterrent for them!

2020.5 KTM 450 SXF FE
2006 KX250

5/4/2020 2:57 PM

bvm111 wrote:

I have had the same thing happen as above when using 927 back in the day which made me switch to K2. The 927 completely separated and collected in a lump at the bottom of the tank and flowed into the carb plugging everything up. To trouble shoot I would pull the fuel line and open the petcock and see if fuel flows, if it’s all oil pull the carb and clean and while the tanks off shake it up and dump out... put fresh gas in mixed with a good synthetic oil at the ratio you have it jetted for and let it rip. silly as it seems I’ve had all kinds of dumb shit happen in the past 35 years of riding 2 strokes, improperly seated C clip on needle causing the needle to float and give erratic performance, main jet coming loose will give you an over rich condition and it’s happened to the best of us! How about a loose spark plug cap or loose screw cap on top of the plug... if it ran well last week without something catastrophic happening it has to be something simple that will make you shake your head for sure... Oh and as above check your reeds and air filter!

Air filter is clean
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.

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5/4/2020 2:58 PM
Edited Date/Time: 5/4/2020 2:59 PM

One thing I'm incredibly worried about is if it's an all of a sudden electrical issue bec those aren't easy to diagnose and they can be expensive

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5/4/2020 3:13 PM

I think you can safely say your problem has zero to do with the avgas.

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5/4/2020 3:25 PM

What's great about a avgas is the consistant Octane ratio. It must be tested daily prior to being sold.

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5/4/2020 3:29 PM

bvm111 wrote:

I have had the same thing happen as above when using 927 back in the day which made me switch to K2. The 927 completely separated and collected in a lump at the bottom of the tank and flowed into the carb plugging everything up. To trouble shoot I would pull the fuel line and open the petcock and see if fuel flows, if it’s all oil pull the carb and clean and while the tanks off shake it up and dump out... put fresh gas in mixed with a good synthetic oil at the ratio you have it jetted for and let it rip. silly as it seems I’ve had all kinds of dumb shit happen in the past 35 years of riding 2 strokes, improperly seated C clip on needle causing the needle to float and give erratic performance, main jet coming loose will give you an over rich condition and it’s happened to the best of us! How about a loose spark plug cap or loose screw cap on top of the plug... if it ran well last week without something catastrophic happening it has to be something simple that will make you shake your head for sure... Oh and as above check your reeds and air filter!

Lightning78 wrote:

Air filter is clean
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.

Good deal...Process of elimination working simple to difficult, did you blow all the circuits in the carb with cleaner and air? For sure could be stuck float valve and also check for spark with the plug out and threads against a head bolt ... just to be sure, also that the plug cap is fully seated! I have had the cap pop up just enough on my KX250 and it was cutting in and out with vibration took me a couple minutes on the side of the track and for grins jammed on the cap and it clicked into place... shook my head on that one!!!!

OH ... and yeah check the reeds! wink

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Don’t piss off the old people - the older they get the less “life in prison” is a deterrent for them!

2020.5 KTM 450 SXF FE
2006 KX250

5/4/2020 4:13 PM

Its not the avgas. But dont waste your money on it. Me and everyone i know runs 93 pump gas with ethanol. Zero issues. If you have a modded bike you may need higher octane. The ethanol issue really only applies gas that is left to sit. Also 93 last way longer than 87. I no longer buy 87 for lawn tools, chainsaws etc. ethanol is a problem with boats but not in a dirtbike anymore.

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5/4/2020 7:26 PM

Reeds look fine, cylinder and top end look fine but compression is pretty low likely my rings are worn out. I can keep my thumb pretty easily over the spark plug hole and crank it over and hear either pushed back into the intake sort of.

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?

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5/4/2020 8:06 PM

Drain the carb after riding. I run T2 and do it just to avoid having jets clog.

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5/4/2020 8:25 PM

You could simply drain the av gas and try 91 again and see if it runs normal again.

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5/5/2020 8:24 AM

UpTiTe wrote:

Avgas is trash for your bike.

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.

Lightning78 wrote:

I ran it pure a week prior with zero pump gas mixed in and the bike ran absolutely incredible. 1 week later it does this so I doubt the avgas itself is the problem unless it absorbed a massive amount of moisture in 1 week which is why I'm asking.

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

Sorry, I didn’t mean to say it’s bad for your bike, it isn’t. It won’t hurt your bike but imo it’s trash.

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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5/5/2020 8:32 AM
Edited Date/Time: 5/5/2020 8:36 AM

UpTiTe wrote:

Sorry, I didn’t mean to say it’s bad for your bike, it isn’t. It won’t hurt your bike but imo it’s trash.

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.

there is not a shred of evidence that backs up anything you’ve claimed. You literally just write anecdotes and opinions to pass them off as fact.

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If it can't be fixed with a hammer, it's an electrical problem.