yzf brutally backfiring


16 replies to this topic
  • Cadoewen

Posted March 13, 2009 - 09:35 PM

#1

so i rebuilt my yz450f new piston / rings JE 13.5:1 and added a gytr flywheel in anticipation of racing this year. my first race is sunday so i pulled the carb cleaned it all up richened the pilot to a 48 from a 45 because its going to be cool this weekend. it started up on the first kick and immediately ran horrible.
it was idling rough and it wont idle without the choke on. i twisted the throttle a little and it BELCHED flames after i let off the throttle almost as if it was extremely lean on the pilot/fuel screw. i adjusted the fuel screw and the poping wouldnt stop.


i know for a fact the bike is timed right because thats the first thing i checked. i inspected the carb and everything is spotless. valve adjustment is within spec. the only other thing i can think of is if it is shitty gas. the i put in it was pump gas. 93 octane. i just bought it today and there was no stale gas in my bike because i drained it.

Do i need race gas? is my problem detonation? i cannot think of anything else please help

  • CyclePro238

Posted March 14, 2009 - 04:52 AM

#2

anything over 13:1 needs race gas. Did you notice any funny noise coming from the motor? Like a pinging sound.. You might be ablt to get buy running Aviation gas, I belive its 100+ BUT I would run no less than 110 in my motors.... But thats just me...

  • 642MX

Posted March 14, 2009 - 05:24 AM

#3

anything over 13:1 needs race gas.


Not exactly true. Lots of new bikes come with a 13.5:1 compression ratio and they recommend pump gas. You only need enough octane to keep the bike from pinging, too much octane will slow it down.

  • 642MX

Posted March 14, 2009 - 05:28 AM

#4

so i rebuilt my yz450f new piston / rings JE 13.5:1 and added a gytr flywheel in anticipation of racing this year. my first race is sunday so i pulled the carb cleaned it all up richened the pilot to a 48 from a 45 because its going to be cool this weekend. it started up on the first kick and immediately ran horrible.
it was idling rough and it wont idle without the choke on. i twisted the throttle a little and it BELCHED flames after i let off the throttle almost as if it was extremely lean on the pilot/fuel screw. i adjusted the fuel screw and the poping wouldnt stop.


i know for a fact the bike is timed right because thats the first thing i checked. i inspected the carb and everything is spotless. valve adjustment is within spec. the only other thing i can think of is if it is shitty gas. the i put in it was pump gas. 93 octane. i just bought it today and there was no stale gas in my bike because i drained it.

Do i need race gas? is my problem detonation? i cannot think of anything else please help


Running with the choke on typically means the pilot circuit has a restriction. Is it possible that a small piece of dirt got into the carb while you had it apart? Also check the obvious things, like the carb boot clamps, hot start, and the float level.

How did you adjust the fuel screw?... When you turned it all the way in, did the bike idle down and try to die?

  • TIG88

Posted March 14, 2009 - 06:09 AM

#5

+1 on thehot start!

Also what elevation are you at?

  • Cadoewen

Posted March 14, 2009 - 07:02 AM

#6

im at 1000 ft i just check the carb clean as a whistle. the bike is popping so bad out the exhaust its hard to hear anything coming from the engine. the bike wants to die if left idling on its own. i havent had it started for more than 5-10 seconds out of fear. its not violent its just like decel popping on steroids keep the ideas coming im checking the hot start again

the bike is popping badly even with the choke.

  • TIG88

Posted March 14, 2009 - 07:10 AM

#7

You def have carb issues, or you botched the install somehow.

Id suggest taking the carb off the bike, and go over with a fine tooth comb. If this shows nothing, rip the engine apart again.

Check your timing again. It couldnt hurt.

  • grayracer513

Posted March 14, 2009 - 09:46 AM

#8

I'll guess a fouled hot start cable, given that it runs better with the choke on.

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  • wellsy1

Posted March 14, 2009 - 10:45 AM

#9

My bike has done that before to. When I first got it, it would backfire really bad! Flames and all. Took it back to the shop where I bought it to check the value adjustment for the first time they told me that the bike came from the factory with the intake cam one tooth off. maybe check thats not happened to you. You never know. They did show me the tooth that was off, It ran so bad I could'nt ride it hard enough to jump a tooth on the chain. Still dont know why my bike was like that. Hope you get to the bottom of it.

  • CyclePro238

Posted March 14, 2009 - 12:35 PM

#10

Not exactly true. Lots of new bikes come with a 13.5:1 compression ratio and they recommend pump gas. You only need enough octane to keep the bike from pinging, too much octane will slow it down.



Well I guess I learned somthin new today.

  • MX926

Posted March 14, 2009 - 12:58 PM

#11

Check your Fuel Screw/O-ring. I recently rebuilt my 08 450 and swapped the pilot for spring weather just as you did. Upon starting it it I had same issues you are having. Backfire, and only would idle with choke. Turns out my fuel screw o-ring had a slight tear in it, and was not in the proper position. It was smashed up inside the carb, plugging the fuel srew opening.

-I also checked my timing, it was good.
-And checked and lubed hot-start


-After cleaning the small pieces of o-ring out of the carb and installing a new o-ring everything runs great.
-Be sure to look up inside the fuel screw opening and check for any debris, might need a magnifying glass.

Good luck.

  • grayracer513

Posted March 14, 2009 - 02:33 PM

#12

...too much octane will slow it down.

While you're right about the rest of it, that part is also "not exactly true". In the truest sense, too much octane is simply a waste of money, and has no effect on anything.

http://www.thumperta...893#post2827893

http://www.faqs.org/...line-faq/part3/

  • 642MX

Posted March 14, 2009 - 03:37 PM

#13

While you're right about the rest of it, that part is also "not exactly true". In the truest sense, too much octane is simply a waste of money, and has no effect on anything.

http://www.thumperta...893#post2827893

http://www.faqs.org/...line-faq/part3/


The "slow it down" part came from personal experience when I was running a 13.5:1 stock bore. I tried some 118 leaded race fuel and the thing was really slow to rev out. It seemed like it just couldn't burn the fuel... Thats why I said that.

FWIW, I'm back to a 12.5:1 with the 444 kit. Running 89 or 91 depending on the gas station I stop at. Bike runs great! .....Now, I just need some decent weather so I can ride. :)

  • Cadoewen

Posted March 14, 2009 - 08:46 PM

#14

just got back from my garage, i disconnected the TPS and it ran "good"

10x better than with ti plugged in. it runs good except when i let off the throttle or pull in the clutch. it doesnt stall but its hard to describe. its like a "reverse" bog it comes down off the RPms wierd. it will idle ok and the backfiring is gone since i disconnected the TPS.

when you say fouled hot start cable what do you mean? my hot start cable is a little gummy but its not stuck open. im racing tomorrow regardless the bike runs fine but definately needs some work. took me about 4 hours of scratching my head re-checking everything from my cam timing, to checking to see if the woodruff key was still there on the flywheel. its been a long day.

  • grayracer513

Posted March 14, 2009 - 09:36 PM

#15

The "slow it down" part came from personal experience when I was running a 13.5:1 stock bore. I tried some 118 leaded race fuel and the thing was really slow to rev out. It seemed like it just couldn't burn the fuel...

That may well have happened, and it may have been the fuel that caused it, but the octane number was not the attribute of the fuel's chemistry to blame for it.

  • grayracer513

Posted March 14, 2009 - 09:40 PM

#16

when you say fouled hot start cable what do you mean? my hot start cable is a little gummy but its not stuck open.

By that I mean hooked or snagged on something in such a way as to prevent the plunger from seating. If you say that's not happening, then OK.

If it runs well without the TPS connected, the TPS may be defective or way out of adjustment. You can run it without the thing and not have much trouble, really. The CDI just assumes you are always at full throttle.

  • rufusz

Posted March 17, 2009 - 02:28 AM

#17

The CDI just assumes you are always at full throttle.


Than just go always full throttle :)

Did you check if the carburetor clamps are fitted correctly? If it sucks in false air there, than you've got a problem.





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