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Discussion Starter #1
Chasing a backfire on the sled. I’ve replaced: plugs (eya) plug caps, reeds (Boyesen), reed cage gaskets, in-tank screener and fuel line, tank to fuel pump fuel line, fuel pump to carb line, fuel filter.

I’ve cleaned the carbs 4xs, looks like one float was recently replaced, other original.
Adjusted fuel mixture screw 1.5 turns out.
Drained tank and filled over half tank with new fuel.
I’ve checked exhaust pipe and can for obstruction, no tears in carb boots or intake rubber.
I DID clean a nest out of the silencer box. Cleaned that as well as the tray the silencer box sits on top of.
Tested conduincy of stator and ground. Scrubbed and sprayed grounds with elec cleaner.
Compression test (110 both).
Prob done more but have done a lot. Sled starts on one pull and idles but it backfires (poofs) at carb exit (I can see smoke come from the manifold to exhaust pipe connection).

Stator ohms-out fine but still may be issue. Maybe it’s crank seals? Maybe it’s key way stuff?
The only thing I can think of is replacing the old float or tuning the problem out with mixture screw? Throw a coil pack at it?

Any ideas appreciated, thanks
 

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Discussion Starter #3
Further searching and I found the tm38 flat side carbs have air screws. I pulled the carbs and cleaned again, had some gunk come out of the air screw passages. Reassembled, set the fuel mixture screws to 2 turns out and the air screws at 1.5 turns out. Are these settings correct? I searched the forum and got 4 different settings for air screw. 1.5, 1.75, 1.25 and 2 turns out.

It’s still backfiring. It’s smokes too but it’s 60F today. Idles and doesn’t die but intermittent poofs. Idle slows and backfiring stops when I half choke. Disconnecting TSS makes no diff either way. The exhaust pipe moves a bit on throttle blip, exhaust escapes the gasket at the y-pipe/exhaust connection. I ordered a new gasket and new springs for fun.

Anyone? Thanks in advance
 

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Discussion Starter #4
I tested the ignition sensor (brown and green wires), came back 184.4 so I think that’s in +- range of 190ohms. Blaine video for the stator is a conduit test only and that ohms out fine including ground.

Found another settings for air and fuel screws. I went from 2 to 1.5 turns out on the fuel. Still backfires (half choke runs better but then dies).

Had to buy a long 1/8 straight slot to get to the air screw, going to set those from 1.5 turns out to 2. Saw a post saying 2.5 times out to compensate for tm carb issues? I’m addressing lean issue as that’s related to backfiring. Found a thread where more air equals more fuel.

I was able to bring a 98 zl500 back from the dead after 3 years sitting last month. None of the tricks are working for this

This thing on?
 

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I am on the ATV sides of Arctic Cats. A quick google search indicates a carb backfire is because it is lean. Did you let it warm up before revving it? They can run lean until warm. Maybe try upping the pilot jet, or rechecking it. Mine would idle fine but loose power in the higher rpm and speeds. I went from stock #38 to a #40 and it runs much better.
 

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Discussion Starter #6
I am on the ATV sides of Arctic Cats. A quick google search indicates a carb backfire is because it is lean. Did you let it warm up before revving it? They can run lean until warm. Maybe try upping the pilot jet, or rechecking it. Mine would idle fine but loose power in the higher rpm and speeds. I went from stock #38 to a #40 and it runs much better.
Thanks for your response! Yep, I mention the lean in post above yours.

I let it run for less than 5 minutes but longer than 2. I’m going to order a manual as I can’t find the definitive air/fuel settings. I’m also considering moving the needle up/down?

I’d really like to find stock settings for those things and then move to replacing jets and such. I assume I’d have to change carb settings with new jets and I don’t know what to set those to! :smile
 

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Discussion Starter #7
Manual doesn’t list screw specs. The e-clip on the needle is at middle position. I may drop it down one notch to see if that fixes the backfire. I’ll be running fuel at 1.5 out and air at 2 out.
 

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Discussion Starter #8
Better! Screws set as above. Left the clip at middle. First 20 seconds no backfire. Next 40 seconds it backfires a few times. Gave it a blip of throttle and let it run for 2+ minutes. No backfiring at iidle or blip. Can’t ride it to test but better than when it arrived.

So would you:
A) Go 1/2 more turn out on air In order to increase fuel flow?
B) drop needle one notch?
C) leave it. Give it back to your bro in law.
D)none of the above
 

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I personally would wait to do farther tuning until you can ride it again. The colder weather, assuming its a difference, will affect the carb tuning.
 
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