where is grayracer.. backfire question

[snowboardstylee]

Hey, i am more than sure gray racer could tackle this question . he should be on jepardy i swear he knows all.

Im wondring if this could be an exhaust leak. or something else. (jetting or valves or carb)

this started to happen last time i rode at gorman and it started to sprinkle. i noticed on downshifts and hard decelerations the bike wants to pop and backfire. is this an exhaust leak or due to humidity in the air. it was cold tonight when it was doing it also.

[joshb]

Your jetting is lean. Cold air leans out the jetting. Try turning your fuel screw out a little and see if it helps.

[falshh]

sound like you are running slightly lean on the pilot.Back the fuel screw out about 1/4 to 1/2 turn.In colder weather I always back it out to take popping away when I let off of the throttle, making it slightly richer, and on a hotter day Ill turn it back in, making it leaner.

You should do this on a regular bases to adjust for weather and elevation differences, by a zip ty fuel screw off of thumpertalk store for $20, that way you can change it with your hand on the track.

other thing that causes popping is if your exhaust where your header connects to the midpipe is loose or gasket is bad.

[Drunken Monkey]

Your jetting is lean. Cold air leans out the jetting. Try turning your fuel screw out a little and see if it helps.

Wrong! cold air does not lean out the mixture, quite the opposite!

high humidity and temperature cause less oxygen in air, so less fuel required.

[ovrrdrive]

Wrong! cold air does not lean out the mixture, quite the opposite!

high humidity and temperature cause less oxygen in air, so less fuel required.

Cold air leans out the mixture. He's right. Colder air is more dense, more dense air has more oxygen in it.

To the original poster, right before it rains the bike gets rich, but after it starts raining it gets lean. Go figure.

[SureBlue]

Wrong! cold air does not lean out the mixture, quite the opposite!

high humidity and temperature cause less oxygen in air, so less fuel required.

Wrong Sir, the Gentlemen here are right, cold air contains more oxygen and leans out the mixture, so you need more fuel. You need a larger pilot, very possibly also a larger main. Backing out the fuel screw may not increase fuel enough. I would suggest to run a bit richer instead of too lean, even though modern thumpers are not too sensitive on lean condition. Running the bike constantly lean raises the burning temperatures and may possibly lead to premature valve and seat problems within time.

[SureBlue]

Hello ovrrdrive and Happy New Year!

[biznet1]

Wrong! cold air does not lean out the mixture, quite the opposite!

high humidity and temperature cause less oxygen in air, so less fuel required.

Not exactly. On the surface it makes since but it is wrong to assume that colder weather requires a leaner setting to keep heat in the engine and vice versa. Cold air is denser than hot air. The denser, colder air packs more oxygen into the engine, so going from hot weather to cold needs a commensurate increase of fuel to balance ratio of fuel-burning oxygen and the fuel itself. The opposite is true in hotter weather. Going from cold to hot weather requires a leaner mixture setting.

[SureBlue]

And you too biznet, Happy New Year. Are you guys having breakfast already? We are preparing for the celebration, it is 4 PM already here and we will head out for a party in 2 hours.

Sorry for going off topic. I hope our answers are good enough even though help was needed from grayracer only, I think grayracer has passed out already LOL.

[biznet1]

Happy new year to you too! It's only 8:00 AM new years eve, but hey! Like Jimmy Buffett says, "It's 5 oclock somewhere!" Gimme a beer! I think Gray is probably studying up on his tech manuals. I'll bet he's designed his own Yamaha powered, 55HP, new years ball.

[ovrrdrive]

Hello ovrrdrive and Happy New Year!

Happy New year to you too SureBlue!

With any luck, sometime this year (06 of course) you'll get a care package from me. ?

[snowboardstylee]

You guys are great. I do need to rejet my bike since putting my aftermarket exhaust on it. The tech guys at FMF said to go to a 165 main and a 42 pilot. but keep the stock needle and put it on the 3rd clip. i will be pulling my carb soon as i get time .

i just wanted to make sure that it was a jetting issue and not a exhaust leak. cause the exhaust leak is way easier to fix than the jetting.

thanks for all the great responses. its raining here in so cal today . so im sure the cold moist air last night was helping it run lean.

[grayracer513]

Where Grayracer was was running around in the Superstitions for two days with several friends and about 10,000 other people of various levels of intelligence, skill, common sense, and sobriety. A good time was had by all.

To the question: Yes, it could be an exhaust leak. It could also be lean idle mixture. The path of least effort runs like this: back the fuel screw out a quarter and if it doesn't start hard and the pop goes away, quit. If it still pops, stop messing with the carb and check the midpipe clamp, if you have one, and the header joint at the head. Once you're convinced that the exhaust is OK, go back to the idle mixture. That will usually keep you from chasing your tail by trying to fix a leaky exhaust with the carburetor, or vice-versa.

And a Very Happy New Year to all of you guys.

[snowboardstylee]

Well gray racer. like you i have the same exhaust. no mid clmap. i put a generous amount of orange RTV high temp silicone on it when i put it on the bike. still have the stock headder. so i doubt that its loose. but i will try backing off the fuel screw 1/4 turn and seeing if its still popping. I know i need to rejet my bike it should be running lean. still running stock set up after putting my exhaust on a few weeks ago.

I hear that there is this stupid little plastic deal you have to put under the main jet when you rejet, its a part that comes in the jet kit is that true, or can i just thow in the recommended main jet. . .

[falshh]

Ive never had to rejet when I put on on a pipe on a motocross bike .The stock pipe is a full open pipe, and typicaly the aftermarket is open enough to require rejetting.maybe if you are at higher elevations it is more sensitive?

[grayracer513]

I hear that there is this stupid little plastic deal you have to put under the main jet when you rejet, its a part that comes in the jet kit is that true, or can i just thow in the recommended main jet. . .

There's a plastic fuel baffle already in the carb on both my FCR's that will stay in place unless you remove the main jet, pilot jet and starter jet all at once and then pull on it. That may be what you're talking about if you have an earlier bike. Or you may have gotten that advice from someone who once left it out of his own carb by mistake.

It's generally true of YZF's that the need to rejet is more due to the fact that the original jetting was wrong in the first place than because the new pipe requires it. Not always, just usually.

And the other thing is that you shouldn't get too worked up about trying to get rid of every trace of decel popping. A properly jetted 450 with a well sealed exhaust will still pop a little.

[snowboardstylee]

Grayracer, you are in yourself a walking talking book of MX Knowledge.

that is the exact part i was talking about. from reading whatdynojet and FMF have in their tech support on their jet kits, they make it seem as if that part doesnt come stock, i know that it comes n the jet kit. however i was not sure if it came stock. so for now i think i should just buy a main jet $4.99

and see what that does. a lot cheaper than the 56 dollars for the whole jet kit. i wont be using the slide springs or needles .

i will beinvesting in a fuel mix screw tht stays on the bike.like the pro-tec or the zip tie..thanks for all your help people. ???