Mikuni and Dynojet mainjet size comparison

[KevinC]

I had measured some Mikuni main jet orfice sizes once, and got different sizes than the number (i.e.: 140 was not 1.40 mm). Srinath assured me the number was the hole size, but this site claims that it is the flow rate in cc/minute (i.e.: 140 would be 140 cc/minute, presumably at some fixed vacuum).

I never got around to re-measuring them.

This site has a comparison of Dynojet and Mikuni mains, but only down to 140 Mikuni:

http://vmaxoutlaw.com/tech/dyno-mikuni.htm

A 140 Mikuni supposedly has the same orfice size as a 149.3 Dynojet.

[The Buddha]

If that's the case... 134 mains - the biggest supplied with the GS500 kit will be ~125-126... same size as canadian stock... I have actually seen that site before... something about the vmax is different. I have these number drills... the largest that fits in the 134 DJ seems to measure out to 1.34mm in dia. I'll measure it tonight. Or you do if you have numbered drills. But that site is wrong or not applicable to other bikes for one reason or another.
Cool.
Srinat.

[KevinC]

The site does say that the DJ numbers are the hole size, so 134 would be 1.34 mm. It is the Mikunis that the number isn't the hole size. Supposedly.

I don't have any Dynojet jets anymore. I've got to get my gauge pins back to measure the Mikunis again.

[The Buddha]

Yea I have also measured Mikuni's (a long time ago as well...) a 1/16th drill bit will fit the 152.5 quite handily... wont fit the 150's unless the drill is a bit worn. I can shove drills into them tonight... we'll know.
Cool.
Srinath.

[KevinC]

I suspect we need to measure a few sizes, since it may be possible that one size happens to corespond to the millimeter dimension.

I used to have a set of precision plug gauges, but I lent it out.

[KevinC]

I happen to have an assortment of music wire sizes on hand, so I've been able to bracket some Mikuni large round jet orfice sizes. I measured the wire sizes with a micrometer. Calipers aren't accurate enough.

A wire that measures 0.0555", 1.41 mm, goes in a 137.5 jet, but not a 135.

A wire that measures 0.0627", 1.59 mm, goes in a 155 jet with a fair bit of play. My 150s are in my 36 mm carbs, and I don't have 152.5s.

According to that chart, a Mikuni 150 jet should be 1.60 mm. I'll have to pull my 150s out, and see if my 1.59 mm wire will go in. I think it will, given the play a 155 jet has on the wire.

I would say the Mikuni numbers are *not* the orfice ID.

[The Buddha]

Ok I have been shoving drill bits in it.
.046 inch = 1.16mm is loose in a 122.5... dont have a smaller mikuni.
.052 = 1.32 is very very tight in a 130
.055 = 1.39 mm is very very tight in a 137.5
.063 = 1.60 is very tight in a 160
The 16 bit that it in the 152.5 was a used drill so no idea what it measure to exactly.
I broke my .059 which fits snug in a 150... but that was from a earlier test.
I didn't use my vernier to test anything for diameter, and I also as putting in the back side of the drill in... but the drills are new and have never been used so no chuck marks to worry about. I believe they are close enough cos drills will make a hole atleast their diameter. So basically if you want a 1mm hole you cannot drill with a 1mm drill bit. So a 1.3 mm drill bit was used to drill out the 130 mikuni. So its a bit bigger than 130. I dont know... I think its close enough to be called by the drill diameter. The 160 was 160 in my test. I believe drilling will make a hole atleast the diameter of the drill, and measuring with a vernier or micrometer will measure the maximum possible thickness it can be. With these assumptions I think mikuni just drilled these with a 1.3 mm drill and called it a 130. drills wobbling ate may account for the extra hole dia but guess what... they didn't expect anyone to measure them. You were supposed to say 127.5 is lean... lemme try 130. For that the will work as advertised cos the 127.5 is more like 130 and the 130 is more 132.5 in ID.
Just my theory...Maybe DJ has better drills that wobble less cos they make smaller quantity. They can make more precision. I'd say close enough. But the Vmax site is plain too far out to be within drill wobbling error. 1-2% is ok not 7-10% IMHO.
Cool.
Srinath.

[KevinC]

Srinath,

Are you sure the drill shanks are the specified diameter? I'd feel a lot better if we had some measured OD's, since your results seem to be very different from what I'm getting. They obvioulsy do forming work to produce the drill flutes, so maybe the blank OD (shank OD) isn't the finished drill diameter.

Anyone else out there have a set of jets and a micrometer?

[The Buddha]

I was going to measure the shanks sometime... but currently too busy getting rained on... right before and after my 16 hours at work.
Cool.
Srinath.