TOPIC:

That's a good tip about pre-drilling them most of the way through with larger drill size - and reducing the overall length before drilling!

I was my own worse enemy. There were several reasons why I failed:

1) I couldn't find my drill press vice - so was holding MIG tip in pliers in a 6mm flanged nut

2) The 2.8mm drills that I ordered were really long so they were flexing as if to undo the twist.

3) I then tried pilot drilling with a 2.5mm tungsten carbide bit which started off really promising but then snapped off about 3/4 through.

I will feel better about having the Mikuni 280 jets in there. I will just have to wait a little longer for them

zed_thirteen wrote: I broke three drill bits trying to drill MIG tips so I've ordered some of the 280 jets that were recommended

If you're interested-
The reason you broke 3 bits trying to drill the copper mig tips is because the chisel face on the drill bit is too steep for cutting soft metals and the bit dug itself into the copper and stalled out and the drill over torqued the bit and broke it.

When drilling the softer metals ( copper, brass and even some of the lower series aluminum) or plastic, you need to flatten the chisel face ever so slightly. The easiest way to explain this is to watch this 2 minute video from of course Youtube. As a machinist, I was taught this trade related skill in classroom.

Flattening the chisel face of a twist drill for drilling soft materials

Kawboy wrote:

zed_thirteen wrote: I broke three drill bits trying to drill MIG tips so I've ordered some of the 280 jets that were recommended

If you're interested-
The reason you broke 3 bits trying to drill the copper mig tips is because the chisel face on the drill bit is too steep for cutting soft metals and the bit dug itself into the copper and stalled out and the drill over torqued the bit and broke it.

When drilling the softer metals ( copper, brass and even some of the lower series aluminum) or plastic, you need to flatten the chisel face ever so slightly. The easiest way to explain this is to watch this 2 minute video from of course Youtube. As a machinist, I was taught this trade related skill in classroom.

Flattening the chisel face of a twist drill for drilling soft materials

Thanks, that explains my problems. I have the Mikuni 280 jets in my bike for now but I have a spare set of carbs that I will try this on with MIG tips,

Cheers

Similar to kawboys alternative. Had the option of ordering a four-pack, so the choice was obvious - allowing me to Test one in my "shop-carb". Next time my carbs are off, I'll do the mod.

Kawboy wrote: This is a repeat of what I posted in "Kawboy's Restoration of a B2"

Popped the lid of of the carbs and started the mod. The thread size on the B42/55 is a 6mm x 1mm standard thread. The tap drill for that thread is a #9 which is .194" I measured the bore of the air jet in the body of the carb at .191" so there was no reason to have to run the #9 bit in the hole, just tap it out to 6mm X 1mm. Tapped it in 5 threads, cleaned the hole with a vacuum and pulled the choke plunger out to ensure no aluminum thread filings. Inserted the jet and Bob's your Uncle. So I've choked down the air to the choke circuit from .191" to .110" using the B42/55 X #280 jet.

I also like the nice clean look. If I decide down the road to change the orifice size, the B42/55's are available in #70 (which is .7 mm) to #290 (which is 2.9 mm)

Strangely these jets are not available in the UK in that size so ordered from the good old US of A. Looking forward to trying this mod

Read through all the posts on this and after the bike repeatedly failed to start after a few days I decided this modification was a priority.
Whipped the carbs out, took about 30 minutes - that's about 6 hours less than it takes to do the CBX ones:(

Took the tops off and found the take off


Measured the hole size in the casting and they were between 5.06mm and 5.11mm.
I know some have tapped the holes and used jets etc and they have the advantage of being removable but I didn't really want to tap a thread into the carbs because of the swarf from the tap, I know that it can be cleaned out but didn't have to take the chance of contamination.
Turned up some brass top hat bushes with a couple of thou' interference fit on each hole and a 2.5mm hole through the center. These also had a taper lead on the end, also made a small drift for installation purposes.


Tapped each bush into the hole, nice fit and they stop against the turned shoulder.


And the fitted bush.....


Put the carbs back together and back into the bike.
Turned the fuel on and took about 10 to 15 seconds cranking to fire up on first start.

The big difference is after a week or more of the bike being laid up I can turn fuel on, apply choke and the bike will fire up within a second, it really is night and day - fantastic fix.
Just got to get some better weather to get out on it now, cold and wet here - should be good for next month.
Russ