Re-jetting via drill

[ManagerMike]

By all accounts I've been able to assimilate here on CH11, jet size is a measurement of the diameter of the hole in the jet in mm's with the decimal point floated right two times, i.e. stock main jet for 78 is #137.5 which is equal to 1.375mm.
Each jet size change is 2.5 of a numbersize equalling 0.025mm, 0.025mm/25.4=0.000984" making each jet size just (about) one thousanth of an inch.

I've read the tip about using wire guage number drills to re-size jets and found it lacking in adequate resolution to deal with the metric sizing. Knowing that wire guage drills are also ground to metric sizes, I put my mitts on a 1.45mm twist drill bit which should allow me to resize my stock jets by three sizes (137.5 to 145) to allow for my pod filters and modified/customized exhaust without having to go to the #53 wire guage drill which would move me 5 jet sizes from stock (almost 0.005").

Any yea's or nay's on this one? Are my assumptions right, or did I miss something?

[natemoen]

Pretty sure mikuni is something more like flow rate of the hole and not hole diameter. I think dinojet is hole size but not sure

[TADracer]

The numbering system on true Mikuni jets is fuel flow in CC's per minute established under a certain testing pressure, not the diameter of the hole. So, a 137.5 main flows 137.5 CC/minute of fuel. Same for the pilot jets.

Other jet manufacturers might use a different method for sizing and that is why there is no really EXACT comparison between manufacturers and also why some aftermarket jets with the same number might perform different than a Mikuni jet.

[crazy steve]

I've always been told that drilling jets was to be done only as a last resort; something to try if correctly-sized jets weren't available (not the case here).

The explanation was that while drilling will enlarge the hole and change flow, the 'taper' into/out of the hole is also critical to flow as well as how 'smooth' the hole is and drilling will change that. It's particularly important on multi-carb setups as any flow inconsistancy between jets/carbs will make tuning very hard if not impossible.

This was a common fix 'back in the day', but went out of favor as manufacturing methods got better and more precise; few can duplicate the precision of the factory machining.

[yamahansolo]

I work in a quality lab and I drilled the main jets in my zrx carbs mainly because they were waaaayyy to small. I ussed a#55 precision bit I think... when I used pin gauges on them 2 were 1.31 and 2 were 1.32. Worked great on the zrx carbs. I posted a chart about month ago on the 3 main brands of jets.

[TomRodgers]

JMHO....I wouldnt drill ANY jets....for as cheap as new jets are....swap them out and done. not only that, but if you end up drilling them...and its too much....ya gotta buy new jets anyway. There are enuf members here that if you have/need jets, and want to swap...im sure there would be some takers. +1....if you go back to where you started, you have the originals to put back in!!!!

[T99Ford]

Drilling jets works just fine. Just be sure of your size before you start. I have done it many times with good success, and no ills to report.

[bikerphil]

I've drilled air jets before with good results, just cause they are harder to find than mains. A rough edge from drilling will cause problems though.

[petejw]

ive used this drill bit kit with good success,



ive also been able to use geuine mikuni main jets
to compare the sizes, 1.1 drill bit is equivelent to a 110 main jet
1.2 is equivalent to a 120 and so on, the only problem with the kit it goes
up 4 sizes at a time.

i see no problems with drilling the jets out, the only thing
i wouldnt do is to solder the hole up to redrill like u could
do with a car carby jet.

[ManagerMike]

Ok, I'll buy jets instead.
I plan to go with #145 mains and #45 pilots and raise the needles one clip notch to accomodate my pod filters and custom exhaust, which is 4 into 2 staight pipe with baffles, very free flowing baffles but baffles still.
I don't live at any significant altitude (SE Michigan), so I think this will ba a good place to start. Yea, nay? whaddya think...

[bikerphil]

I think your jetting sounds about right. Moving the needle clip I would disagree though. Every time I've strayed away from the center position, I've always ended up going back to the center. YMMV. BTW, I run the 45/145 combo with my Kerker 4-1 and drilled airbox (cut snorkel) w/K+N.

[coltrain5000]

Just some .02 cents here that I run my needles on the 4th clip. I went to the middle and my bike didn't like that. Just an FYI that the middle isn't for everybike.
I run a Modified air box with a jardine 4 into 2.