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AV GAS (100 leaded) & jetting changes

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A friend of mine's got a 98 xr 400. He lives right down the street from a small airport and he's going to start running it in his XR. He's says the bike idles fine and runs great 3\4 to full throttle but... has some kind of sputter/bog in the Mid range.

 

He's got the Stock Exhaust with Opened Ground Out Header, Uni Air filter with Open Air Box, Running 100 Octane Leaded & 700 ft above sea level.

 

162 main

62 idle

3rd clip position

J6DF needle jet

Fuel screw? No idea...

 

Does 100 octane leaded gas require jetting changes?

 

Anyone have any jetting recommendations? Any help is much appreciated!

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100 LL should not need a different jet.  It has essentially the same BTU content and density of regular gas.  The high octane rating is a total waste of money unless you have raised the compression ratio significantly with special pistons or heads.  What the 100 will do is increase fouling of spark plugs and drive the requirement for more frequent oil changes.  The Rotax aircraft engine folks want oil changes at 100 hours if running non-ethanol gas and only 50 hours if running the 100LL.

For longer life of fuel components, less corrosion worries, and a reasonably high octane rating, I'd recommend ethanol-free gas.

tom

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You know an awful lot about your friend's bike.

I know everything bout my friends...I ride ktm's he can't log in to his account. But thanks
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Used to run Avgas in Vintage MX class, had to lean a little usually.  

Dont know if it made any gains to be honest, but fairly sure the XR500's started easier.

Edited by Muzz67
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I agree with geezer, waste of money and no advantage.

A stock compression XR400 running 87 octane should be able to tow the KTM all the way back to the repair shop.

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Does it sputter/bog in the Mid range when he runs pump gas in it? If it does then it isn't the avgas causing the issue. 

 

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I agree with Bad Moon. What's is run like on pump gas. Octane is a detonation inhibitor. 100LL is probably too high for the XRs low comp engine/low intake pressures to be able to vaporize sufficiently at certain rpms of the engine.

I think his jetting is too rich. His exhaust can still being stock (unless it's Gordon modded or the like) is a bottleneck. I'd have him drop his main to a 158 or 160.

Edited by Trailryder42
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Does it sputter/bog in the Mid range when he runs pump gas in it? If it does then it isn't the avgas causing the issue. 
 

It does sputter on regular pump gas... he's gonna switch back to regular. Any jetting suggestions?

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I agree with geezer, waste of money and no advantage.
A stock compression XR400 running 87 octane should be able to tow the KTM all the way back to the repair shop.

Lmfao ! That was funny.
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62 tells me he may have a blockage in the fuel screw passage.  Should be a 58 or 60.  Does turning the adjustment screw have no affect?  Does the bike start cold without choke?

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1 hour ago, Baja Rambler said:

62 tells me he may have a blockage in the fuel screw passage.  Should be a 58 or 60.  Does turning the adjustment screw have no affect?  Does the bike start cold without choke?

Agree. But didn't say anything because he has no complaints about that throttle range.

J6DF needle? Never heard of it. It's not stock so where did it come from? Who, why is it installed? If he keeps that needle, may need to play around with clip position to try tuning its mid range taper to suit better.

 

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Avgas cleans the varnish out of the fuel system like no other. If your buddy opens up the carb, he'll see everything below the "waterline" in the bowl will be like new clean. All that crap went somewhere. Overhaul the carb.

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I was told by anotherracer that AVGAS burns too slowly and will actually reduce your acceleration power. This could be mistaken for a "bog". At most, mix AVGAS with pump gas 50/50% and try again.

As a plane owner, I have burned lots of AVGAS. It smells wonderful and it is pretty. But it was designed for max horsepower high altitude constant RPM engines.

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2 hours ago, Firestone said:

 I have burned lots of AVGAS. It smells wonderful and it is pretty. But it was designed for max horsepower high altitude constant RPM engines.

Oh ya it is that pretty blue and has that lovely smell fresh or burning, one thing about flying some folks don't know is that plane carburetors your always leaning or enriching them out to the proper mixture as your flying right from the seat for every altitude and temp change, you get a good ear and you eyeball the cylinder head temp and tack to dial them right in.

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