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Yz400f Backfires
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Posted by: FOUR 41---------------------
I just picked up a 1999 YZ400f. When I kick it with the choke on it fires up right away. It goes into the normal high rev that a choked bike goes into. After about thirty seconds it idles way down, almost like you pushed the choke in. When you blip the throttle it dies. If I can get the bike to rev it revs perfect and sounds fine, but as soon as I let off the bike backfires severely.
The bike has not been run very much in the last three years. I cleaned the old fuel out of the tank and the bowl. I cleaned the air filter and reoiled too as well as changing the plug. The one thing I did notice is that between the head pipe and the silencer the clamp is gone and it was replaced with a hose clamp. I am sure there is an air leak there.
It seems to me the bike is running rich. Does anybody have any ideas as to what the problem could be?
Posted by: IrishEKU---------------------
Carb, clean out the jets, replace the clamps and it should run fine.
The bike is set to run as is, have there been any add ons?
Posted by: FOUR 41---------------------
The only thing I've noticed so far is a Twin Air filter. The pipe is stock.
Posted by: Hick---------------------
When you say "blip the throttle" is this with the bike in gear? It is pretty easy to stall a YZF in neutral by quickling opening the throttle.
Also, a few pops w/ the throttle off during deceleration is normal. A large amount of louder pops could be an indication of an air leak in the exhaust somewhere. I would think a hose clamp could do the job in place of the stock exhaust clamp provided the gasket is in good condition.
Is the jetting stock? Is the accelerator pump working properly (even easier to stall a YZF w/ a non-functional pump)?
I guess if it were me I'd check that exhaust gasket and replace if necessary, take the carb back off, make note of all the jetting, and check to see if the accelerator pump is working (just lift the slide, gas will stream out of carb throat. Watch your eye).
Then, starting from stock jetting, I'd ride the bike again and see how it runs.
Hope this helps.
Posted by: Hick---------------------
I thought I should add that the popping on decel, absent an exhaust leak, is generally an indication of a lean pilot/pilot air circuit(s).
Sometimes if you try to eliminate all the popping by richening the idle mix you can kill your throttle response. So IMO a small amount of decel popping is okay provided it isn't constant and consistently loud.
Also, just flat-out ignore whatever your bike does when cold, especially in the winter. Give it time to warm up fully before trying to determine how it is running.
Hope this helps (and why can't I edit the typo in my last post?)
Posted by: FOUR 41---------------------
Hick,
I've have a yz250f besides the 400f. The 250f doesn't pop at all. The gasket between the two pipes doesn't look real good. So I'll have to try replacing that. Why does an air leak in the pipe cause the bike to backfire? Aren't the exhaust valves closed when it backfires?
Thanks for all of the help.
Posted by: Hick---------------------
Quote:
Originally posted by FOUR 41
Why does an air leak in the pipe cause the bike to backfire?
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I'm not exactly sure, but I always thought it was sucking air in that was then combusting with unburnt fuel in the exhaust. I do know that an exhaust leak will almost always result in that popping. So will an overly lean pilot/pilot air circuit.
You should also check the accelerator pump, sometimes the pump mechanism on the YZ 400 carb can collect dirt and bind, stopping the pump from operating.
Hope this helps.
Posted by: WRSTEVE---------------------
[QUOTE]Originally posted by Hick
[B]
I'm not exactly sure, but I always thought it was sucking air in that was then combusting with unburnt fuel in the exhaust.
That is exactly what it is doing, trust me on this. I have cracked my headpipe twice now and on both times, everytime i decel. a huge POP blows out the back of my muffler.
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