73 carb issues

[Mr Gumby]

During the latest shakedown run, I discovered a bit of an issue. Idles great and hums along nicely while cruising but any more than 1/3 throttle and it just falls on its face. I bought the carbs used but "gone through". I had an old Datsun with SU carbs that have a similar design that did that before I had proper filters. I've been using some extra K&Ns I hade laying around from my VMax and have UNIs on order. Does it sound like a carb internal issue or or other wizardry at work? I'd imagine that it's stock jetting inside them. Should I be upping he jet sizes with the open filters and Emgo shorties? I've read it both ways.

 

[TwoManyXS1Bs]

Download the 'carb guide' and do the diaphragm leakdown test written in there...

 

[gggGary]

Verify the installed jetting is correct for the carbs, many rebuild kits have the wrong brass in them, aftermarket brass is a always a crap shoot as far flowing the marked value.

 

[5twins]

Note that the needle position for your carb set is slot #4, not the middle #3 slot that's normally used. If your needles are set in the #3 slot, that's a step leaner and could account for the flat midrange. Yes, many times you do lean the needle setting but that's only when running larger than stock mains. Doing it with stock jets will make things too lean.