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alexszl

Back firing when cold

13 posts in this topic

Good Day.

I have a 2006 WR450.

I have recently replaced my 3 inlet valves and had the seats cut, new piston, rings and cam chain.

The inlet shim clearance are 0.09mm, 0.11 and 0.10.

I checked the timing marks are spot on - the exhaust cam gear mark on 9 o'clock and inlet at 3 o'clock, level with the top of the head surface - the cam lobes point in 10 o'clock and 2 o'clock. The flywheel marks line up with the 'I' of the 'HI' .

When I start the bike from cold it backfires. Once it's warmed up it goes well. This it didn't do before the inlet valve job.

Any clue where I can look what could be the problem ?

next I plan to clean the carb and replace the spark plug - although I don't think it's that.

Oh and count the cam chain links - what are they for the WR 13 links, 12 for the YZ ?

Thanks in advance.

Alex

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Hi mattfidaho.

No I haven't changed the jetting. I spoke to the guy who did the valve job and he sugested fiddling with the Fuel/Air mix screw. I looed on the bike and in the manual - but can't locate the fuel/air mix screw.

The only thing I changed to the carb was a new throttle cable. I think it might be too tight - i.e. no freeplay. Now thinking that the cable is pulling slightly and not closing the carb completely. Although it does idle normally.

??

Alex

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If nothing else has changed (pipe etc.) and you had no popping before the work you have an airleak at the header gasket or midpipe junction or your pilot jet is clogged/partially clogged.

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Hey Alex

The location of the fuel screw can be found by looking at the exploded view below.

934161_570758052957670_1623795173_n.jpg

Turning the fuel screw to 2 turns out or an additional 1/4 of a turn should be fine.

One request- I am looking to upgrade to 2006 WR450 rims because they are the nice Excel ones that never seem to crack, but I am unsure if the front rim has the tabs on the rim for the speedometer. So- Can you have a look at your front wheel and tell me if there is three locking tabs for the speedometer please. If you are unsure where to find them they will be located on the right side and be on the bearing housing. If you can do this for me it would be greatly appreciated.

Cheers

Jarrah

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Thanks for the reply's. I will check tomorrow re air leaks on carb and exhaust mid junction. As to head gasket - I fitted a new one and torqued it to spec - hope it's not that. And will fiddle the fuel/air screw - now that I know where it's located. Thanks for the detailed instructions.

As to the front wheel speed sensor - mine has 2 locking tabs. See image.

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Thanks for the reply's. I will check tomorrow re air leaks on carb and exhaust mid junction. As to head gasket - I fitted a new one and torqued it to spec - hope it's not that. And will fiddle the fuel/air screw - now that I know where it's located. Thanks for the detailed instructions.

As to the front wheel speed sensor - mine has 2 locking tabs. See image.

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JDLowrance - when you mentioned header gasket you mean exhaust gasket at the head? (Not the head gasket between cylinder barrel and head). Thinking of it now - that was a bit bent and wobbly when I installed it.

Guess I'll get a new one then.

Thanks

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Thanks for the reply's. I will check tomorrow re air leaks on carb and exhaust mid junction. As to head gasket - I fitted a new one and torqued it to spec - hope it's not that. And will fiddle the fuel/air screw - now that I know where it's located. Thanks for the detailed instructions.

As to the front wheel speed sensor - mine has 2 locking tabs. See image.

Ahr yes I meant two lol.

I cannot see any image but that would be great to see how it looks. Does your tabs look like the ones in the pic below???

321621_570964942936981_1696683437_n.jpg

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JDLowrance - when you mentioned header gasket you mean exhaust gasket at the head? (Not the head gasket between cylinder barrel and head). Thinking of it now - that was a bit bent and wobbly when I installed it.

Guess I'll get a new one then.

Thanks

Yep...bent and wobbly would indicate the need for replacement.

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Barra8 - Yes. Mine looks like that!

And .... got the backfiring sorted!.

- I took the exhaust off and saw there was blow-by on the exhaust manifold gasket with the cylinder head. Got a new one and Viola! I'm happy.

Cheers, thanks for all the advise.

Alex

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Barra8 - Yes. Mine looks like that!

And .... got the backfiring sorted!.

- I took the exhaust off and saw there was blow-by on the exhaust manifold gasket with the cylinder head. Got a new one and Viola! I'm happy.

Cheers, thanks for all the advise.

Alex

Thanks Alex :thumbsup:

Your help on the wheel subject is greatly appreciated.

Keep it wheel!

Jarrah :)

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Barra8 - Yes. Mine looks like that!

And .... got the backfiring sorted!.

- I took the exhaust off and saw there was blow-by on the exhaust manifold gasket with the cylinder head. Got a new one and Viola! I'm happy.

Cheers, thanks for all the advise.

Alex

No worries, happy to help.

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