CB450 Needle Jet

[whataulooknat]

Well upon tearing into my carburetors on my 74 CB450 I've come to the realization that the needles are not adjustable on the Keihin Carbs. What can I do about this? the needle bumps all the way against a seat so adding washers won't raise it. Anyone figured out how to richen the mid range on these old bikes without going mikuni. Also dirty carbs aren't the problem. Its lean because the engine is bored out. The bike has good color on the top end and good color at idle but steady mid throttle cruising is lean.

 

[teazer]

Shim the needle will raise it so that when the slide lifts, it's richer.

At low to mid range, try raising the FUEL level by 1-2mm

Another option is larger main jet and larger air jet so that it's richer at the lower revs and as revs rise, the air jet has more of an offsetting effect.

Larger needle jet is also possible, but it makes the whole rev/throttle range richer.

 

[teazer]

BTW, larger bores create more "suction" ie more gas velocity through the carb and that changes everything - it tends to make a bike run richer though, not leaner.

 

[whataulooknat]

I just dont see how a shim can be installed at all because the needle bumps all the way against a seat it can't physically be pushed up into the piston any further. The bike isn't extremely lean but I would just like it to be a tad richer at mid throttle. I've tried going a little bigger on the main jet but it just makes the engine bog at high rpm I think I'm as large as I can possibly go. The Main jet is a 165 so its rather large. but before the rebuild I burnt a hole in a piston due to being too lean so I always worry about not being rich enough for long periods of constant riding.

 

[AWDimprezal]

Have you tried going up a size or two in the pilot?

 

[whataulooknat]

Well the Pilot jet is a 38 I believe stock is a 35. How much difference would that make on the mid range? Doesn't opening up the fuel screw do basically the same thing or does that only affect idling?

 

[teazer]

Main jet on a CV applies all the way through the rev range, but at cruise the slides are not really lifting much, if at all. That's where you need to either find a way to lift teh needle by modifying something or raise fuel level slightly.

 

[CCRider]

I've had some success modifing a needle when I couldn't find a "richer" one. I chucked the needle in a drill motor and ran it against a coarse sharpening stone and carefully reduced it's diameter. That was on a single cylinder engine though, might be tough to get both needles exactly the same.

CC

 

[whataulooknat]

yeah might be a little hard to do but I do machining and have Micrometers. so I might order a couple stock needles to play with. Worst case scenario It doesn't work and a blow a little money. Sounds like it would be worth it to me. might try a little bigger pilot jet if that doesn't work. I read somewhere that the pilot jet size directly effects partial throttle. way more than adjusting the screw does. not sure how true it is but hey trial and error right?