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yz_ripper

08 YZ450 wont fire

5 posts in this topic

I just did the top end, new H/C piston and Pro Circuit head. The timing is dead on, piston at the top, intake and exhaust cams lined up with the TDC mark, all is perfect! I thought my jets had a piece of shit in them so i blew them out cleaned them, shit figured i'd just buy new ones as well. When you look from behind the carb and give it throttle you can see gas squirt into the intake, so why wont the bike fire? I want to think its timing but the plug doesnt get wet at all, need some help ASAP, im lost!!! Plug has plenty of spark, wont fire!!!!!

Thanks!

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Do you have your carb float installed correctly, or did the float get out of adjustment somehow during your maintenance? something is keeping fuel from your combustion chamber, since plug is dry.

Carburetor related, sounds like.

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Do you have your carb float installed correctly, or did the float get out of adjustment somehow during your maintenance? something is keeping fuel from your combustion chamber, since plug is dry.

Carburetor related, sounds like.

Man, i want to agree with you so bad but the carb is great. I used an air compressor to make sure all jets were fully clean, the thing is mint. You know what im talking about right? I have the subframe off so i can see the carb, etc. When i turn the throttle the cage lifts and the jet shoots a straight line of gas into the intake part of the head, how can it be the carb if thats the place where fuel is injected? Its obviously shooting gas into it. Would you think it has something to do with VP fuel? first time im running the stuff.

I've read some forums and people post stuff about valves and clearence, but i just got this head back from pro circuit everything is tuned. Sounds like a carb issue, not getting fuel.

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I've never done that "test" before (looking down carb throat and seeing the little mechanical pump work) but obviously fuel is getting to the carb, since you see it jetting into head.

It's probably not the VP fuel, but to eliminate that possibility just re-fill the tank with what you used before.

Be sure they set your valve clearances to specs in manual, and double check the intake/exhaust cam positions ahd that it's on the correct cycle (TDC on the compression stroke)

All that said, the plug should be wet if fuel is getting to the combustion chamber.

Since you had the head done at Pro Circuit , they might have some suggestions.

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... how can it be the carb if thats the place where fuel is injected?
That's one of 4 places fuel enters the air stream. (And it's not "injected").

There is the main nozzle where the slide needle runs, but that has no influence on starting. Fuel for starting come from two small ports just down stream from the slide, and passes through the pilot and starter jets. Both these jets and their passageways AND the pilot air jet in the mouth of the carb must be clean in order to pass fuel correctly.

Be sure you start out with at least 2-2.5 turns out on the pilot screw (adjust it once warm) and try giving 2-4 good cranks on the throttle to get a prime.

This assumes that you have good compression and spark, and you have verified that TDC as indicated by the flywheel is in fact TDC by the piston.

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