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2004 backfires under cold start conditions

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Hey guys,

First of all - what is a backfire?

Second - I went to start the bike cold without priming it first, could this have led to backfires?

Thanks

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Theres no primer on these bikes but if your referring to blipping the throttle or engaging the choke then no there is no way you caused any problem.

BUT if you left your choke engaged (That is, in the position that it is pulled away from the bike) when you try to ride it it could very well back fire and generally run poorly.

AND if you blipped the throttle too many times the excess gas I believe could cause a backfire but I dunno if this would happen on a cold start as the exhaust has to be hot enough to ignite the un burnt gas(Right guys?)

I don't know exactly how to explain a backfire as I'm not sure the exact line between back fire and deccel pop but to the best of my knowledge a back fire is when you hear POW! come out of your exhaust, usually accompanied by a small flame out of the exhaust.

You should also add more info like how the bike runs when you ride it? Is it just back firing when you try to start it but stops once it starts?

Jet sizes?

Elevation?

Add some more info and I'm sure one of the Mechanic Gods on this site will help you out.

Also this is probably better suited for the jetting forum.

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A backfire can be the cause of one of 2 things , USUALLY , a backfire thru the carb is a rich condition and its usually too much fuel by too large of jets or improper air/fuel screw adjustment (Air 2 stroke/Fuel 4 stroke),and it slightly floods the motor , it stumbles and the fuel gets pushed back into the carb and into the airbox then the motor fires again , poof ! , and a backfire thru the exhaust is usually a lean condition , usually indicated when letting off the throttle , usually indicating either too lean of jets , or a air leak somewhere in the exhaust system

Not a scientific explanation , but that is basically what causes a backfire

In reference to your question....see above

If the temp is dropping where you live , then you most likely are getting a lean backfire thru the muffler , the pilot jet needs to be richer and/or the fuel screw needs to be turned OUT

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I haven't checked the valves yet, planning on converting to ss this winter so I was just going to let it ride out until then. - could this cause any harm?

The backfire was through the exhaust so you are probably right 450XJim - it was very lean without giving it any fuel before trying to start it.

Thanks for the help fellas.

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I haven't checked the valves yet, planning on converting to ss this winter so I was just going to let it ride out until then. - could this cause any harm?

The backfire was through the exhaust so you are probably right 450XJim - it was very lean without giving it any fuel before trying to start it.

Thanks for the help fellas.

no harm, but you simply won't get the bike started if your valves zero out (no clearance). You might be able to bump start it, but that won't do you much help if your by yourself..

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Check your valves they are probably out of wack. I just ordered new kibblewhite intake valves for my 03 crf450 because they were real tight. They OEM valves will wear away and pull into the head and the smallest shim out there won't help you.

Last time I did my valves I had to use the smallest shims in my hotcam shim kit. I only did one race and I was having problems with backfiring and not starting when cold.

Sounds like your valves.

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I believe worn valves will eventually damage the seats, so you'd need to get those cut in addition to the stainless valves.

well lets explore that.. When valves wear and "zero out", there is no clearance from the bucket to the top of the valve. When this occurs, the valve face is not even making contact with the head seats. So how would the valve damage the head seats at that point? cut ss valves? waste of time and money when they only cost $25 or so each anyway..

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