jetting- great read

[g0zer]

The jet size problem


some selected jetting wisdom:

The jet number is linear dependent from the fuel flow which means a #120 main jet has 20% more fuel flow compared to a #100 jet. Do not mistake fuel flow with jet bore diameter! This dependency is not linear!

some of my jets had a serious mismatch between label and fuel flow of upto #30 numbers

For this reason I’d advise anyone who is going to change jetting to make his own jet measurements. The values will be different but the intention is to compare "unknown" with "known" jets and to judge if the jet label is in a valuable range.

I used a rinsing bottle for battery acid (diameter about 70 mm, about 180 mm height) with an 80 mm hose with 5 mm inner diameter (to screw in the jets). Pour in an exact amount of water using a small kitchen measuring jug or letter scales and measure the time in which the whole amount has flowed out.

im going to create the same table for all my mikuni jets, my tunes are going to be sooo perfect

[Gangus]

Some good info gozer!
The problem is worse swapping between manufacturers.
Pattern jets are like Russian roulette.
Do report your findings......

[g0zer]

this might take a while :p

[g0zer]

my jets arent patterns, genuine mikuni

so far the results are encouraging enough i think i can't be bothered testing them all

its not linear, but the pairs are close enough to be within the accuracy of how long im willing to sit there measuring and the next size up is always higher than the previous.

im glad i tested them tho, i guess if anything its a good advertisement for paying extra for oem

[smoker]

Great stuff, so from what you found do you think the genuine Mikuni jets are going to be consistent enough in production to simply accept correcting with needle/clip the lean condition once you cross above 220 mains?

[g0zer]

not sure what you mean with correcting lean with needle/clip once you are above 220 mains

i wanted to know if my pairs were flowing exactly the same amount of fuel, which would affect the carb sync

[smoker]

Yeah, sorry, bit of a tangent. Just the results always seem to look like the production process puts out closer to or above expected flow rates on the smaller jets (apart from the variation between individuals) and larger seem to fall below more consistently. Of course a couple of bucket loads of jets could disprove this.

[Jamathi]

[D.J]

I have the Sudco Mikuni tuning manual (general part) on PDF if anyone needs it, I also bought the book for the spare parts info if someone gets stuck also. I think I get the point Bert, most road riding uses the needle jet and needle for metering rather than the main jet (unless your a rev head ).

[Veefore]

Great stuff Gozer.
Tuning carbs seems to be a bit of a dying art.

[EEKNOWS]

I hope you didn't test with the jet in the tube thread first? Mikuni jets are calibrated to flow one way the way they do in the carb.