WR400 backfiring!!!!!!!!!!

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tonksy69
02/06/2005 at 19:30
my WR has started back firing for no apparent reason so im wondering where to start looking for the problem......(damn bikes,who'd have em)..
I love the taste of dirt in the morning,reminds me of victory..

www.ypc.co.uk/

www.andaluciantrailtours.com/

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Yamamoto
02/06/2005 at 20:41
Is it when you shut the throttle?
Yorkshire Where Men Are Men And Sheep Are Scared
WR_DAVE
02/06/2005 at 20:50
Hi Matey
Had the very same problem with my old WR,
Check Air filter and clean if dirty but this sounds like a mixture problem probably running too lean.
Check what slot the main needle is set at and take it down a slot to richen.
then you could always take the carb off and check the pump cover diaphram and then generally check all the jetts and also check the vacum slider plate at the back as these can crack and break up as mine did and then shag a couple of vavles and piston these also make idling and general perfomance all over the place.
also before you remove the carb check for any air leaks from the rubber collars that hold the carb in place .

Could always check Thumper talk web site for addition info.
Hope this helps?
Funners
02/06/2005 at 21:43
WR_DAVE wrote
Hi Matey
Had the very same problem with my old WR,
Check Air filter and clean if dirty but this sounds like a mixture problem probably running too lean.
Check what slot the main needle is set at and take it down a slot to richen.
then you could always take the carb off and check the pump cover diaphram and then generally check all the jetts and also check the vacum slider plate at the back as these can crack and break up as mine did and then shag a couple of vavles and piston these also make idling and general perfomance all over the place.
also before you remove the carb check for any air leaks from the rubber collars that hold the carb in place .

Could always check Thumper talk web site for addition info.
Hope this helps?


If its only just started doing it it wont be the needle....so leave it alone !
check for Air leaks, inlet and exhaust
check all the hoses that feed the air leak valve on the carb
check that the carb top isnt loose
then if that doesnt cure it, you need to be taking the carb out and checking the pilot jet/air jet to see if they are tight

the list goes on, but start with that lot and see what happens !
WR_DAVE
02/06/2005 at 23:56
This Problem was the main reason why i changed to the crf, i had a catalogue of problems which basically cost me about a grand in two months labour and parts, I had a 400 0n a 99 so if yours has the same carb do yourself a favour check the carb methodically with a manual to make sure its not an obvious easy repair which could save you £££ in the long.
Apart from that the bike was my first and got me into bikes and the supermoto scene so dont be put off by me but check that slider vacum plate for any damage if youn take the carb off!! this can save you aload of hassle.
good luck matey
tonksy69
03/06/2005 at 05:31
Yamamoto wrote
Is it when you shut the throttle?


more often than not yes....
I love the taste of dirt in the morning,reminds me of victory..

www.ypc.co.uk/

www.andaluciantrailtours.com/

www.walkinginandalucia.co.uk/index.html

Dunc
03/06/2005 at 09:43
tonksy69 wrote
more often than not yes....


Have you changed the exhaust recently for a better flowing one?
"One pound a week will supply water for an entire village in Tanzania" says Oxfam. So how come Essex Water charge me twentyfive pounds a month for my three bedroom semi?

Withershins
03/06/2005 at 11:23
WR_DAVE wrote
check the pump cover diaphram and then generally check all the jetts and also check the vacum slider plate at the back

What, and where, are these bits, as I don't recall seeing them on mine when I was spending months getting it to work as I want.
WR_DAVE
03/06/2005 at 11:36
Withershins wrote
What, and where, are these bits, as I don't recall seeing them on mine when I was spending months getting it to work as I want.

Pump cover is at the bottom of the carb 3 hex fixings , once the covers off theres a rubber diaphram that can split and collect dirt etc.
Vacumn slider plate is i think exhaust side of the carb can only see it if the carbs off the bike once off the bike activate the throttle linkagage and you can see the plate and slider moving but make sure the carbs out of petrol as this does squirt out so watch yr eyes
Worth checking the vacum plate as I've said earlier they're bout 90 odd quid from Allens,but if yrs looks damaged change it cos I had the top half rebuilt because of a damaged one.
Hope this helps but I'm no way an expert, carbs are still a grey area for me
will#224
03/06/2005 at 14:02
My yz pops and bangs like fook when i role off throttle. Everyone said it was too lean but i got it dyno'd and fueling is perfect.

My mates wr does the same.

Just enjoy the extra noise unless it really sounds wrong then check stuff out. Mine pops and bangs more since i got silencer repacked. Its calmed down more recently. Maybe the packing needed bedding in a little.

I'm told yz/wr's pop and bang more than most anyway.

Good luck!
__________________________
Yamamoto
03/06/2005 at 14:17
Mine pops like fuck on a closed throttle, fuelling is cock on but its the exhaust which is drawing air in somewhere as peeps on the BAR raid noticed when i lost my springs off the front.
Yorkshire Where Men Are Men And Sheep Are Scared
Withershins
03/06/2005 at 15:04
WR_DAVE wrote
Pump cover is at the bottom of the carb 3 hex fixings , once the covers off theres a rubber diaphram that can split and collect dirt etc.
Vacumn slider plate is i think exhaust side of the carb can only see it if the carbs off the bike once off the bike activate the throttle linkagage and you can see the plate and slider moving but make sure the carbs out of petrol as this does squirt out so watch yr eyes
Worth checking the vacum plate as I've said earlier they're bout 90 odd quid from Allens,but if yrs looks damaged change it cos I had the top half rebuilt because of a damaged one.
Hope this helps but I'm no way an expert, carbs are still a grey area for me

Aha! We are talking about the accelerator pump here, I am surprised that a defective one could cause that much damage . In summary, if you are interested, 4 strokes are rubbish at creating enough vacuum at low revs, hence the accelerator pump. As you wang the throttle open the small reservoir underneath that diaphragm forces a short jet at the engine to compensate the aforementioned vacuum deficiency. So... a defective accelerator pump will make the bike bog, due to fuel starvation under hard acceleration from low revs (incidentally I have mine set to be working at only half its stroke). If it is popping when the throttle is closed it sounds to me like it is running lean on the pilot circuit, and if it has only just started doing it (without any jetting changes) I would suggest turning the pilot screw out to richen it up a bit.

HTH
adam moore
12/08/2012 at 02:04

i had a similar problem on my 98 model wr 400, only i have blown two exhaust baffles out completely. the engine wont rev out , and the exhaust heats up instantly . It was running fine then on a ride lost power and blew the baffle out . I thought it was a fuel problem but have had carby apart , all seemed fine . It has a vortex cdi and i have changed to the yz timing , but it was running perfect . any ideas people ?

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