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Discussion Starter #1
I have a 92 Yamaha Phazer with a 483 fan cooled engine.

It runs like a bag of garbage (coughs and splutters over 4000). I take the air box off--it runs great. I put the air box back on, it goes back to garbage. I replaced the main jets, no improvements.

 

Steve
 

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Take off the carb bowl.  There should be 2 plastic plugs and a center chamber should be open.  Hard for me to describe.  3 orfices in the rough shape of a triangle with the center orfice open.
 

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Actually there are 3 holes surrounding the center jet tube where the main jet would slide up in. Of the 3 tubes or holes, one has the pilot jet in it with a rubber plug over it, the center one not the big center one with the jet tube is how the fuel gets to the choke and has a rubber plug, the next hole as far as I know is not used and is plugged internally and has no rubber plug.

When did this problem start?

It sounds like it is too rich.
 

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Discussion Starter #4
Ok, Here's the long story.  It's a customer's machine  (I'm a self-employed mechanic) so I'm not sure of the history.  Here's what happens:  With the machine up on a stand or on the trail when it gets up around the 4000 rpm mark it misfires.  Blows lots of smoke.  Pull the plugs out and they are wet and black.  I've cleaned the carbs and set everything to factory specs.  When I did all of this I ran it without the airbox and to my delight it ran super!  Problem solved! Right?  Put the air box on and tried it again and it bogs right down at the 4000 mark.  So...I thought maybe dropping one jet size would help.  No improvement...if anything its marginally worse.  Compression is about 90 psi.
Puzzled and frustrated.  

Steve
 

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What elevation are you at?

What size jets?

The compression is low, should be at least 110-120 depending on elevation.
 

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Discussion Starter #6
We're in Northern Ontario, so for all intents and purposes I would say about sea level.  the jets were 143.8 (stock) and now they're 147.5 (next size down).
 

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Well for one thing a stock Phazer has 145 jets so the next size down would be 142.5 Did you take the air box apart could have a mouse nest in there ,
 

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Actully a Phazer II has 143.8 mains jets.I would check the plastic plugs in the carb first.One covers the low speed jet and the other is in the center center hole (not the main jet but the little one) compression is also VERY low time for a rebuild I would say.When it is on a stand there is no load on the engine so it will rev right out.As for the airbox you oculd also take it apart and check inside for anything.

BJ White
 
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