Ignition or carb?

[Staffan]

Tested my bike after i had my new "custom build" exhaust system put on. Easy started and fine idle, good bottom but when i open the throttle more then half it just die. I don't know how to check spark at hi rpm. If i have to rejet the carbs which jet(s) to change? What does they do?

Staffan

[Night Vision]

I'm going through jetting experiments now beacuse of non-stock pipes.

first try adjusting your air/mixture (pilot) screws and check the sync.... if that doesn't help, then

try larger main jets, the ones on the sides of the carb behind the 10mm bolts... right now I'm at 127.5 front and back (the front is 122.5 stock, the rear is 127.5 stock)

and larger air pilot jets, the ones on top of the carb... right now I'm at 137.5 front and back

if your low end is good, then you may not need bigger fuel pilot jets, the ones underneath the carbs behind the screw caps.... if you do.... you'll have to make them yourself... right now I'm at 65-67.5 

[Staffan]

So the pilot jets handle fuel in the mid range, but what's the difference between the main air jet and main fuel jet. I thought that a bigger air jet would lean out the mix.

On a "standard" type of card like on an MX bike you just have the idle jet/screw, the main jet and the needle. Idle/low speed handling the first 1/4, the main jet the last 1/4 and the needle everthing else.

Some year ago i had the same type of problem on an HVA 250 MX bike. It turned out to be the trigger of the "high" ignition.

Staffan

[Night Vision]

the fuel pilot jets handle the idle, low end and the transition from the low speed jets (pilot fuel jets) to the high speed jets (main jets)

IF your top end suffers due to lack of gas... you need bigger mains..
to get the gas delivered through the bigger mains.... you need bigger air pilot jets on top of the carb to pull the gas..

 

[Staffan]

Thanks!

Now i think i get it.

Staffan

[fiat-doctor]

NV, you are close but not quite... the main air jets (called air correctors in Weber land) add air to the mix and larger air correctors lean out the mixture at higher RPM's.  Your explanation is otherwise right on.
Steve