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Carb jets to optimize power with new Mac 4-1 on '83 v65 Magna

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Billy W., '82 Magna:
Hi Billy! I recently purchased a new (to me ;-) 1983 v65 Magna to go with my 1982 v45 Magna, and with Clay's guidance ( :thumb ) I'm installing a Mac four into one in hopes of gaining some power. The mufflers that came with it turned out to not be stock, and though they fit onto the bike when mounted, the part that connects to the collector was too small to use the stock gasket. The exhaust was (not surprisingly I suppose) leaking at all of the connections, and since I had just returned a set of progressive shocks (after Clay showed me how to adjust the stock shocks properly) I just used the return credit to get the Mac exhaust. Clay said that I would need to make adjustments to the carb if I wanted to take advantage of the Mac, and he said that you were the man to ask what I needed to do. Thanks, in advance, for your help Billy!!

A headder alone isn't really enough to warrant upping the jets. If you go to a K&N air filter then you'll need to make the change. If you do change the jets, go up one size first because 2 sizes may be too much and you may end up fouling plugs.

I don't remember if anyone has ever dyno'd their bike before and after the headder/filter change. There are a lot of people that say power increases according to the ass dyno but no hard data.

Billy C:
  YMMV but in my experience that bike should go to #42 slow jets and #142 main jets.
  I remember Kevin (rocksalt) madly swapping out jets and running the dyno multiple times at Gils BBQ. That was a loooong time ago. I think you will find every bike has a slightly different taste for jetting. But 42/142 should work well.
  I was running 42/142 on my stock 83 big Magna and was very happy with it. Since then I added Kerker cans but have not ridden it with them. I'll tune it to what it likes with the baffle plates on the end of the cans.

I always thought it was 40 and 142

Billy C:
 :wal:  Sorry, I did mean 40/142.  Looks like I picked a bad day to quit huffing super glue.  ;)


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