backfires like crazy when you let off the throttle

i was just wonderin if anyone new why my bike is back fiering when i let off the throttle and i also noticed a lack in power if anyone has any sugjestions please post them up thnx

Incorrect jetting. Do a search on this site and you will find a million threads about it answering your questions.

Something that I don't see enough mention of here is float level. I have never seen any two FCR carb with the same float setting from the factory since I've been working on them (since '99).

The factory spec for the FCR carb used in the Yamahas is 8mm (allways has been). My brand new '06 YZ450 was set at 7mm (caused it to bee rich at low throttle). My girlfriends brand new WR250 was set at 8.5mm. I have seen the float levels range from 7-11mm in carbs that never had the float level set.

If your float level is off you'll have nothing but headaches trying to get the pilot/low speed jetting set.

My personal preference is 9mm. Set-it, jet-it. No problems ever.

I don't understand how the float level being 1mm off could cause the jetting to be off? The air pressure difference caused by the venturi in the carb draws the fuel up through the jets. If your float level is too high the extra will run out the overflow tube.

If jetting is adressed then you need to check your exhaust system for air leaks, especially where head pipe attaches. My 07 450 f was exactly as you describe when I first got it. I did some jetting and a quick shot2 acc. pump cover and it's helped but still does it a bit, drives me crazy.

I don't understand how the float level being 1mm off could cause the jetting to be off? The air pressure difference caused by the venturi in the carb draws the fuel up through the jets. If your float level is too high the extra will run out the overflow tube.

I know it may not sound logical, but the float level being off by 1mm does affect jetting. If you dont believe me try playing with the float level yourself. Road race guys have known this for a long time.

I worked on a bike a few years trying to sort out the jetting and after I checked the float level and reset it (a DRZ400 w/FCR carb) my mixture problems went away.

Now, I ALLWAYS check float level before trying to diagnose a carb problem.

It's actually perfectly logical. Obviously, if the float level is too high, it will overflow. But if it's too low, it creates a need for a higher level of vacuum at the discharge nozzle to draw fuel into the air stream by having to raise the fuel farther from the lowered level.

Vacuum is most commonly measured, remember, in "inches of mercury" or "inches of water", for lighter levels of vacuum. This simply means that a vacuum of one inch of water is a condition in which the air pressure at the vacuum source is enough lower than atmospheric to raise a column of water one inch in a vertical tube connected to that vacuum. If you need to raise the water 1.05", you need more vacuum.

In a carburetor, that means that the engine will have to pull more air through the carb (run faster) to get the same amount of fuel if the float level is lowered.

If it's backfiring on decelaration under pressure like going down a hill, no throttle, or heading into a corner fast then letting off then we are not talking about main jet/needle but rather the pilot circut. I think an air leak between the head pipe and cylinder would be what I check first, then consider increasing the size of your pilot jet.

For some reason i hear a lot of new yz450s popping like that at the track. Do they come lean from the factory? Not looking to start trouble, just need the info.

For some reason i hear a lot of new yz450s popping like that at the track. Do they come lean from the factory? Not looking to start trouble, just need the info.

The stock idle mixture screw is a bugger to get to. I think a lot of owners just don't wan't to fiddle with it..............that's why the popping.

my wr did that till i removed the ais

For some reason i hear a lot of new yz450s popping like that at the track. Do they come lean from the factory? Not looking to start trouble, just need the info.
Decel popping in and of itself is no big deal, and the truth of the matter is that a correctly jetted YZ450 should pop at least a little. If they don't ever, they're too fat.

But, yes, the '06 in particular, and YZ's in general, tend to be a little lean as delivered.

It's actually perfectly logical. Obviously, if the float level is too high, it will overflow. But if it's too low, it creates a need for a higher level of vacuum at the discharge nozzle to draw fuel into the air stream by having to raise the fuel farther from the lowered level.

Vacuum is most commonly measured, remember, in "inches of mercury" or "inches of water", for lighter levels of vacuum. This simply means that a vacuum of one inch of water is a condition in which the air pressure at the vacuum source is enough lower than atmospheric to raise a column of water one inch in a vertical tube connected to that vacuum. If you need to raise the water 1.05", you need more vacuum.

In a carburetor, that means that the engine will have to pull more air through the carb (run faster) to get the same amount of fuel if the float level is lowered.

Low fuel level= leaner/ higher= richer

mine was popping a bit untill the other day it started popping a lot upon further inspection my header pipe had developed a crack almost all the way around it!!! You had to look very close but sure enough it was cracked. So I would say that minimal popping is ok but if it popps a lot check for air leaks

air leaks, is there a sertain way to check for them, and aslo could this poping bullshit be becouse my valves need to be reshimed. i aslo apretiate all the help with this guys thnx

The stock idle mixture screw is a bugger to get to. I think a lot of owners just don't wan't to fiddle with it..............that's why the popping.

I hear that. I think they should all come with the finger adjustable after market style. That's one of the first changes I made. Gray, do you see any benefit to running an air cut off valve, or is it just not that big a deal? I know some popping on decel is normal. I have a buddy with an 07 yz 450 and it seems like it pops a lot even when he just revs it in nuetral. I told him it sounds lean but he is one of those don't fix it till it breaks kind of guys.

Gray, do you see any benefit to running an air cut off valve, or is it just not that big a deal?
If you run an air injection system, it's a must. You just think you've heard decel backfires until you've heard an engine with AIS and a malfunctioning anti-backfire device (which is what the air cut valve is). I'm talking muffler damage. Without AIS, you don't need the ACV at all.

On decel backfiring itself:

http://www.thumpertalk.com/forum/showthread.php?p=3350613#post3350613

Thanks for the info, you're the dude.

Before you tear your carb apart change the gas and put in a new plug!!!!!!!!!!!

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