Burn Down, Pics

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wls

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Mar 8, 2005
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What do you think happened. yamaha 440 all stock, other then boyseen reeds, brown paper bag colored spark plugs, won heat race, leading final by big margin. just put rings in, this was the first race, broke in w/dino oil. The bottom end had new seals, halves had yamabond on, 9 months ago, topend new gaskets thru out. premium gas 40-1 klotz. Wanted to mention, during breakin, in shop, I sprayed some either around seals and all joints, did'nt make a difference.
 

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looks like lean fuel mixture. can you give more info like what 440 you have....
 
Here is a pic of the plugs, left plug is the burn down cylinder. I'm going to take the other cyd head off to have a look at it. I took a compression check of the good cyd, when I got back to the shop, it was the same as before.
 

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RJH said:
Where on the track did it go - be exact?

What did you do to the timing?
Click to expand...

We had run 2 hot laps, then a 3 lap heat, and was on the final lap (5th) of the final, coming out of turn 2, about 1/2 way down the back straight, started losing power.

I did nothing to the timing, just put it back to where it was set last.
 
might have got some air in the fuel line coming out of the corner. How much fuel was in the tank?
 
Good on the timing.

If you locked up going into a turn - it could be too small a pilot jet.

I forget - does your carb have a a high speed air jet? This is located at the bottom of the intake and runs into the emulsion tube area? There may be just a hole there.

Just thinking - is there any chance you have a coolant leak at the top?

Boyseen reeds are great.

What concerns me is - it ran well in the heat race.
Did the air change a lot?

Tell me more.
 
RJH said:
Good on the timing.

If you locked up going into a turn - it could be too small a pilot jet.

I forget - does your carb have a a high speed air jet? This is located at the bottom of the intake and runs into the emulsion tube area? There may be just a hole there.

Just thinking - is there any chance you have a coolant leak at the top?

Boyseen reeds are great.

What concerns me is - it ran well in the heat race.
Did the air change a lot?

Tell me more.
Click to expand...
I had thought the jetting was good, light brown, it has a air jet, but i think that is for the lower rpm's. It does have a power jet also. it got wamer as the day went on, maybe even 10 deg, hotter at the final, then the heat race. the carb is a keihin slide, like a mikuni, also this is a fan cooled motor. thanks
 
The secret to this carb is it runs a little lean at 3/4 throttle you need to pull the needle up all the way and set your air screw at 1 turn out... hope this helps out!!! Race on!!!871 exciter :2strokes:
 
Ya – I’m not talking about the low speed air jet or screw – I’m talking about the secret high speed air jet – this little hole works in conjunction with the mj only. It is in the bottom of intake track. Leave it there – if it’s a fan. Maybe tell me what number it is ….e.g. 0.7?

Fan cooled – ohhhhhh – most of them need fuel – you can’t get optimum jetting on some of those models. It needs to be rich – you willll go faster.

You may have been on the edge – the track would have been slower – air was better – but putting extra load on the engine will increase the heat – something Yamaha products don’t like.

I also think your carb has a fixed emulsion tube designed to work with a water cooled performance engine with a different pipe. The mid range is configured to a performance engine that burns fuel efficiently….not a fan.

Not sure of the rpm – but I would check to see if the rpm is high enough to make the PJ even work?

It’s almost impossible to help on line…..but you might start with getting the needle richer and up on the main. This combination may make performance worse than the stock carb…

Go to 50:1 - this will make it richer as well – slightly.
 
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