yzf popping problem

I have a friend i'm putting this question up here for he has a 04 yzf 450. The problem is that he has a popping that happens at wot and on deceleration bad, and fouls plugs every few rides. The plugs come out with all kinds of carbon build up. The main is a 145, pilot a 42, needle is stock position. Were at 5200ft elevation. It has an FMF Ti4 pipe and powerbomb header. No matter what we do with the jets the plug comes out black went down to 130 mj had real bad popping on wot, went up to a 165 fouled plugs out every ride. The valves were checked and their good. The only thing we havent messed around with at all is the needle. By changing the jets it never got rid of the carbon looking spark plugs when we were down the the 135 it was just a hint of brown but mostly black. I'm starting to wonder if the coil for the plug is bad or theres a bad ground on it not sure any info would be greatly appreciated.

thanks

PLug color on a 4stroke isn't as good an indicator of how its running as it is on a 2-stroke. Tuning a 4-stroke is more a seat of the pants experience.

With that pipe you might have to go bigger on the main and pilot. That poping on deceleration is a lean pilot circuit as is the poping at WOT I would imaging. I think going smaller on the jetting is the wrong direction.

Decel popping is:

> Mostly normal. A prpperly jetted YZ450 will do that to some extent no matter what.

> Caused by an air leak in the exhaust as least as often as any lean conditions. Check the mid pipe joint. It's a slip fit on yours. If it's leaking a lot, seal with a high temp exhaust sealer.

> Nothing to worry about.

As far as jetting goes, here's the setup being run by a TT member in Colorado:

Model: 2003 YZ450f

Edit Revision:7/23/05

Main Jet: 158

Pilot Jet:38 (edited)

Starter Jet: stock?

Leak: n/a

Fuel screw: Zyp Ty 1 1/2 turns out

Needle: #3 clip ( 1 up from stock, leaner)

Air box: Twin Air and Unis

Pipe/muffler: Stock header, FMF Q (older version)

Altitude: 5700 feet and up

Temp:current 40-80F

Humidity: 30%

Decel popping is:

> Mostly normal. A prpperly jetted YZ450 will do that to some extent no matter what.

> Caused by an air leak in the exhaust as least as often as any lean conditions. Check the mid pipe joint. It's a slip fit on yours. If it's leaking a lot, seal with a high temp exhaust sealer.

> Nothing to worry about.

As far as jetting goes, here's the setup being run by a TT member in Colorado:

Model: 2003 YZ450f

Edit Revision:7/23/05

Main Jet: 158

Pilot Jet:38 (edited)

Starter Jet: stock?

Leak: n/a

Fuel screw: Zyp Ty 1 1/2 turns out

Needle: #3 clip ( 1 up from stock, leaner)

Air box: Twin Air and Unis

Pipe/muffler: Stock header, FMF Q (older version)

Altitude: 5700 feet and up

Temp:current 40-80F

Humidity: 30%

Correct!

1) Mid Pipe Joint

2) Fuel Screw

3) Mid Pipe Joint Again

4) Jetting Expedition - (Greys Looks Good)

5) Mid Pipe Joint Again

Most important, don't forget to check the mid pipe, especially if you have an aftermarket pipe.

I solved my issue on step #5 after a jetting quest that drove me to the point of madness.

Anyway good luck.

Ride on!!!! :banghead::applause:

PS - Search this subject also there is ALOT of info that can help, but this is a good place to start.

we'll check out that but this was something that was going on before the exhaust was put on. that was only put on because the stock exhaust got demolished in a 25ft fall when the motor cut out on a triple jump and fell like a rock. It was fouling plugs before the exhaust just not as bad as it is now we put a 158 main jet and it was fouling plugs about every 3rd moto or so

I have a stock 2003 yz 450f with stock exhaust...165 jet...and it popss BBBADDDDDDDDDD on the decel...i checked the mid pipe and did everything i could have done. ANY ideas would help.

Before doing endless jetting sessions check your floatlevel. If that is off all your jetting will not give you the desired results. On my 05 450 the level was 2mm low from the factory!!!!

Alright.....if its low that will make it pop? and one more thing. pj isa 170 right now and i had it 1 1/2 turns out and it poped...i went out all the way to 4 turns out...it barely pops now but its FOUR turns out. when its 1 1/2 turns out with the idle completly off it doesnt pop even one time. but as soon as u make the bike idle it pops like no other on the decel. im at about 1300 ft elev. currenty man jet stock needle stock and pj is a 170. and you all know how it is.....u just want it to stop and u want to RIDE!

Alright.....if its low that will make it pop? and one more thing. pj isa 170 right now and i had it 1 1/2 turns out and it poped...i went out all the way to 4 turns out...it barely pops now but its FOUR turns out. when its 1 1/2 turns out with the idle completly off it doesnt pop even one time. but as soon as u make the bike idle it pops like no other on the decel. im at about 1300 ft elev. currenty man jet stock needle stock and pj is a 170. and you all know how it is.....u just want it to stop and u want to RIDE!
Yes, if the float level is low, the idle mix will be a little lean, and that will contribute to decel popping.

Your pilot jet IS NOT a 170. The stock pilot on that bike is a #42.

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