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Old 01-30-2005, 07:51 AM   #1
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Hi, Last year my sled would sometimes backfire out the exhaust when stating it after it was warmed up, as soon as I pull the rope it would make a big bang in the exhaust and then run but it almost all the time would have a fouled plug after that. After rebuilding the motor this year the problem was gone till yesturday at radar run, I started it and let it run to warm up, took it for a little ride on the lake to clear it out and went to get it inspected for the radar runs. Well when I went to start it it banged so loud people jumped so I got pissed and went and put new plugs in it but almost every time I started it the same thing but it ran ok and never lost a plug. To me it almost sounds like it is firing a plug in the intake stroke when the exhaust port is still open and banging out the exhaust, I don't know if this is possible or not but it is very anoying anyway. Has anyone out there ever had there sled do this? I would love to know what the problem is. I put on a new coil and spark plug ends and tried a different used CDI box but it is the same. Could it be the pickup on top of the recoil housing causing it? Or maybe the sparkplug wires on the wrong cylinders? Will it even run with the wires backwards? I am out of ideas on this one. Tom
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Old 01-30-2005, 08:22 AM   #2
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A friend and I were discussing this last night... The problem is that raw fuel fumes/vapour is being ignited in the pipe when starting up.

When you stop riding, the idle rpms will be a little high until the pipe cools down. After riding, let the engine idle for a few seconds until the rpms drop... Then turn off the motor. It may not prevent the problem, but should minimize it.

When riding, the pipe is hot... Which keeps the piston hot... Which induces a leaner burn condition than when the pipe is cool. This temporary situation is what causes the idle rpms to be higher immediately after riding.

If you shut down the motor immediately after riding while the rpms are high, what happens is the motor is still drawing in fuel but without a spark it is not burning. The raw fuel vapour ends up in the pipe. You start the sled, the fuel vapour in the pipe gets ignited, and there's your big bang.

It is not a backfire, and is not ignition devices failing. Just let the rpms drop before you shut her down hot, and the problem will be minimized or gone.
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Old 01-30-2005, 10:22 AM   #3
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AG is right just and just let the rpms drop before shutting it down. The worst sleds I've found for this were the 500 and 600 twins. But last week my ZRT let a couple bangs out of it but it backfired when I shut it down, not on startup.
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Old 01-30-2005, 06:57 PM   #4
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Hi, I do let it idle a little before shutting it off but maybe not enough. Now that you say this it kinda fits, when I would take it out for a ride and shut it off it would bang on startup, when I was in line to run and had to start and stop it alot it would bang the first couple times but as it sat there awhile it stopped doing it, also when I started it after sitting so long in line I would let it run awhile to keep things warmed up so I was not beating on a cold motor and it didn't bang after it idled a while. But last year every time it banged it had a bad plug after. Yesturday it banged alot at start but ran fine after. To change subject a little, what kind of stuff can I do to make it faster without costing a fortune? I plan on getting heads cut some and I have boysen reeds in it. Would polishing the ports do much without changing the port timing?
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Old 01-30-2005, 07:17 PM   #5
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 Quote:
Originally posted by 94Wildcat@Jan 30 2005, 07:52 PM
Hi, I do let it idle a little before shutting it off but maybe not enough. Now that you say this it kinda fits, when I would take it out for a ride and shut it off it would bang on startup, when I was in line to run and had to start and stop it alot it would bang the first couple times but as it sat there awhile it stopped doing it, also when I started it after sitting so long in line I would let it run awhile to keep things warmed up so I was not beating on a cold motor and it didn't bang after it idled a while. But last year every time it banged it had a bad plug after. Yesturday it banged alot at start but ran fine after. To change subject a little, what kind of stuff can I do to make it faster without costing a fortune? I plan on getting heads cut some and I have boysen reeds in it. Would polishing the ports do much without changing the port timing?
If you're getting too much of this 'banging', is there a chance you may have a choke plunger that is not completely seated 100%? That could add fuel to the fire too!

Yeah, if a person was to hit the kill switch during a really fast idle, a lot of fuel still goes through the motor until it spins down to a stop. Carbs don't stop when the spark does.

If you're in the lineup to run someone, a hotter pipe is better than a cold one anyway, because optimum jetting would be based on a hot pipe at normal engine operating temperatures.

A cold pipe means it will be a little richer than optimum until the pipe heats up.

Ideally, you want that optimum jetting right from the start if you're going to do a drag race... Pipe insulation/sheilding or ceramics should help keep the pipes warmer and closer to optimum jetting than cold.

Just cleaning up the transfer ports makes a difference... But results will probably vary greatly on many motors. As for port timing, it will be unchanged unless you modify the openings.
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Old 01-30-2005, 08:10 PM   #6
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Its a EFI, so no chokes, I might be wrong but turning the key off should also shut off the injectors since it kills the power to the ECU so in shouldn't be injecting fuel as motor stops I would think, unless its leaking out of a injector till pressure drops.
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Old 01-30-2005, 10:53 PM   #7
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I have efi on my sled and if I shut it down when the rpm is up, it will backfire when I start it, so doesn't matter if efi or carb
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Old 01-30-2005, 11:34 PM   #8
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I guess I will not worry about it then, sorry if I sounded bad in my last post, I just wanted to make it clear that I am dealing with EFI and not carbs, I am glad for the help. Tom
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Old 01-31-2005, 02:23 PM   #9
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Before you spend any money on machining heads, which I wouldn't do on an old EFI, work on clutching, gearing and traction. Make sure you can get all available horsepower to the ground before throwing money at making more horsepower. Why spend many hundreds of dollars to get 5 more horsepower at the crank when you will only get 2 of that to the track? Motor mods, heads, porting, pipes, etc., will require fuel delivery mods, which on EFI usualy means computer changes.
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