Topic: Main jet size CB750K2 stock airbox MAC 4-1 Header

[willbird]

Getting my K2 around and running. Originally my dad put an action 4's header on it in the 70's, I remember him rejetting then. He passed away in 86 and the bike became mine.

 I being younger and less wise then put KN pods on it and re jetted, obviously it "worked" because I went on to put 30k miles on it. A few years later the action 4's header was rusted through so I did a frame up on the bike and put a MAC 4-: header on it. I remember raising the needles one notch then due to a lean out at 60mph cruise speed.

So the MAC 4:1 header remains, and I have a stock air box coming, what main jet size should I need ? MAC header has their simple canister muffler. 1.5" primary tubes on the Header. I really miss the Action 4's header, from memory it had smaller primary tubes and got better mileage.

[Jerry Rxman Griffin aka MuthaF'er]

Good chance stock size mains will work

[willbird]

Good chance stock size mains will work

Thank you

[HondaMan]

The K2 came with 2 different carb sets, and the jetting changed sightly between those.

If yours has "657a" series carbs, it used #110 mainjets with the needles in notch #3 or #4 (#3 with the TEC spark advancers aboard and #4 with the more common AD125 Hitachi spark advancer).

If yours has the "675b" series carbs, the mainjets were dropped to #105 with the needles in the #4 slot, regardless of advancer type.

The "657a" carbs also had staggered brass floats inside, with one (the furthest from the sidestand) set at 26mm and the other set at 25mm.
The "657b" carbs had plastic floats and both were set to 26mm.

[willbird]

The K2 came with 2 different carb sets, and the jetting changed sightly between those.

If yours has "657a" series carbs, it used #110 mainjets with the needles in notch #3 or #4 (#3 with the TEC spark advancers aboard and #4 with the more common AD125 Hitachi spark advancer).

If yours has the "675b" series carbs, the mainjets were dropped to #105 with the needles in the #4 slot, regardless of advancer type.

The "657a" carbs also had staggered brass floats inside, with one (the furthest from the sidestand) set at 26mm and the other set at 25mm.
The "657b" cabrs had plastic floats and both were set to 26mm.

OK that is very good info. Memory is telling me plastic floats but I will double check that when I have a look at them.

Bill

[willbird]

Ok Plastic Floats and presently 130 jets.

Checked emulsion tube holes and a .033" gage pin fits.

Looks like a #61 drill would achieve the suggested .039"

The smaller holes that get left alone are .022"

[HondaMan]

Those holes are likely a bit "crusty", so to speak, if looked at under high magnification. The years of MTBE gasolines partly dissolved their edges, which then collect minerals and sediment to make them slightly smaller in the end. The 0.022" holes should be 0.025" in 'real life'. They only act above 1/2 throttle, though, so you probably won't notice very often.