Jets:

Some information and
How to tell when they've been used.

If you deal with carburetion, it's important that the jets that you use are of the highest accuracy. It's not much fun to struggle with a tuning job for a couple of days - only to find out that SOMEONE had drilled a jet to a larger size and left the original size marking on the jet - you only thought you had 4 #138's. Reality was that you had a bogus "#138 that was really a #142...." along with 3 #138's.

What's even worse is that, somewhere, there are probably 3 other bogus #138's in your jet supply  - but maybe not... but maybe......

But, now, you will have to find ALL of your #138's and measure all of them for 1.38mm sizing (works on Factory K Series, Keihin small, round, slotted, main jets). You should probably check all of your jets..... It's like a virus in your jet supply.

That's why many dealers don't take back or give credit for used jets. Other ones don't because they can sell you more jets, though I'd like to think they are concerned about accuracy. Seriously, most dealerships and the people who work there are trying to do the best job possible and would really feel pretty badly about giving you a badly sized jet.

Accuracy in jet sizes? As far as I'm concerned, the goal for accuracy has to be Keihin and Mikuni. From the best that we can measure, we feel that our 393 K Series jets are equal in sizing quality and repeatability to the best of the Japanese OEM suppliers.

Factory Mission: If it's got a screwdriver slot across the top, a size imprinted on the side and a hole down the center - it's correct.
Zero Defect Goal.

But go ahead and compare!

We avoid all non Mikuni / Keihin Japanese jets, Taiwanese jets, Chinese jets, American jets and jets manufactured in the UK.

It's not that they can't make quality jets, it's just that we haven't seen a consistent supply of properly sized jets supplied.

ID used jets? Installing and removing jets almost always leaves "witness marks".

Thanks for the "new" used jet from the guy that swore he didn't install it.
Gee whiz! A real sharp guy would have used a larger screwdriver that didn't quite hit the floor of the slot and was wider than the jet head to avoid disturbing the floor and walls of the slot. Oh, and also, the envelope really shouldn't smell like gasoline....