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

[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!!

[Stretch]

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.

[PLCGuy]

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.  ;)

[Stretch]

I bought an 83C that had a kerker, K&N and 150 mains and those plugs were blacker than the plug in my B&S with the air filter clogged with grass.

Do not go as high as 150's.

[Miecul]

I put a K&N filter and never changed the Jets. I never noticed any changes.  Billy, should I have? Would sea level make differences if so at what levels? Grab your coffee first and a fag first Billy.  :thumb :beer: :thumb

[PLCGuy]

You can't make more hp wihout more gas.
Adding a knN and 4n1 lets you pull more air and push more exhaust, now you need more gas to complete the triangle

[zarticus]

I put a K&N filter and never changed the Jets. I never noticed any changes.  Billy, should I have? Would sea level make differences if so at what levels? Grab your coffee first and a fag first Billy.  :thumb :beer: :thumb

I know Billy Likes coffee but not sure if he does fag's, With Billy you never know  :ryan:

[Billy C]

  We call them butts down here. No particular sex assigned to those butts though from high school French I recall they would call them le butts or la butts. No matter, both nasty habits.
  On a v4 with a collector box, swapping the filter to a free flow design gives the potential for the engine to pump more  air. Only a potential as the collector is the bottleneck. Add headers, ditch the collector and you just installed bigger fire hoses to your pump and now you are moving more air. More air same fuel gives a leaner mixture so post headers you gotta adjust jetting to get that mixture back up to 12.x:1 for optimal power. Theoretically, leaving the jets stock can yield more power with the leaner mixture. The old saying "lean is mean" applies. At the expense of more power you also get more heat  which is no bueno for those commercial built for a profit margin cylinder heads.  I remain convinced the reason many of the v4 models run larger mains in the rear carbs is for cylinder head cooling. But then you have the 83 which is 140 on all 4 corners? The basics are more air into the pump requires more fuel to maintain that power vs heat 12.x air fuel ratio. More air in requires more air out aka headers. Lower altitude or cooler temps means denser air ie more fuel needed. Our CV carbs are self adjusting  for temp altitude via the vacuum slides for the midrange, not so much idle and WOT. No really,  it is in the book. Confused yet? Me too. I am pretty darn good at making a rack into 120 parts in a bin, scaping all the grunge out and reassembling them back into a reasonable facsimile of what they resembled when they left the factory but I am not a master tuner. But, the guy that repairs the power plant down the road probably lacks a nuclear physics doctorate and manages not to wipe a medium sized town off the map if you count Chernobyl as an outlier event. Keihin carbs are wonderous mechanical devices that manage to mix air and corn juice at an appx 12.x:1 ratio from idle to 10k rpms reliably, unless you park them wet for too long and ya can't blame Soichiro Honda for that one.
  40/142 will do ya fine as frog hair. Gonna idle cool on an August EBGTX day and rev sweet to past redline if you find yourself chasing a litre class crotchie.

[Billy C]

  If you are old you remember that car exhaust reeked of raw gas. They ran them very rich to save the valves from the heat. They had notoriously bad valve seats which made for poor heat transfer from the valve face back to the cylinder head. They ran lead in the fuel which cushoined the valve on closing and aided heat transfer.
  50 yrs later valve seats are tougher, tolerances are tighter, aluminum alloy heads and they run leaner mixtures for power and emissions. Throw in $1000 worth of multi port fuel injection, computers and a half dozen sensors and they can maintain that lean mix right on the edge of the danger zone. Mechanical carbs, not so much. I would guess that 35 yrs from now when you try and restore that 2017 Cadillac CTxyz fuel management system it is going to be much more hassle and expense than shipping your rack to Canton or in the case of a '62 Fairlane ordering a new Edelbrock 4 bbl from Rock Auto.

[klopperg]

Thanks Billy for throwing some light on a very blurry subject.

Legislation has forced the manufactures to sharpen their pencils on emission controls.

30 years ago a carb was good enough and nobody looked twice at a rich running motor, but today is a different story altogether.

[Billy W., '82 Magna]

Thanks to all, but especially Billy C., for his excellent breakdown, and Clay for help with many things, but particularly tuning the rear shocks! I will report back on how it goes (already have the K&N, the Mac 4-1, the collector and front pipe gaskets. All I need now is the muffler to exhaust system gasket, which looks to be around 64mm (2.5"). If anyone knows the size of this, let me know, and I will search to see if I can find it too. THanks again!!!

[PLCGuy]

Did you fit the muffler to the header? Kerker don't require a muffler gasket,  trying to verify if the Mac does

[PLCGuy]

This leads me to believe it is a slip fit (no gaskets )
http://hondacb650.com/viewtopic.php?t=292

Based on this, look at the inside of the muffler mount for high spots and verify all welds