Measuring Jets/Atomizers

Has anyone ever measured main jets/atomizers with measurement kits?

Have you found any discrepancies found between jet/atomizers numbers and the actual size?

Just curious on this, seen a number of kits online and just wondering is it a worth while purchase.

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I’ve observed a variance in main jets, though relatively small. I believe the jets are calibrated based on flow, so I’m not sure the size difference really matters. I don’t have a pin kit, and I’ve never felt like it’s cost me a race win, but I know plenty of teams that use them. I’m curious to hear what others have to say and whether they see any true value.

I’m sitting on the fence right now, as if I use the same jets all the time i’m sure the varience would be found.

I can see the beenfit for large teams who maybe run a number of drivers.

My other side to this, is that I will be using a jetting app and weather station with RAD, so maybe having my jets calibrated to ensure I am running what the app is telling me to.

The plan is to perfect carb settings on a particular day (maybe even on a dyno) taking note of the RAD on the day, then allow me a base point to create a chart with carb settings against the RAD.

I’m not a tuning expert so my thinking would be dial in the app as perfect as I can, then run what ever it states, maybe with minor fine tuning. To allow me to focus on all the other varibles of race day.

Finding a baseline jet and adjusting based on RAD (or ADR or density altitude) is a pretty popular way to do it. Dellorto jets are numbered by diameter in 10/s of an mm (I think?). 150 is 1.5mm. However you’ll make RAD adjustments by the AREA of the orifice. Since it’s area, small discrepancies in diameter can have a large effect on flow/area.

Mikuni jets are numbered by flow… of course that’s not a huge amount of help to people running KZ.

Personally I think it’s worth baselining and checking your jets with pin gauges so you can be at least sure that the area is close to where it needs to be.

That does make sense James, I suppose it’s one more varible to rule out.

Just needed a reason to add to the collection of karting tools :joy:

Yes the dellorto are all off with discrepancies between 4 and 12 points. BUT there is no overlap.

Anecdotal evidence from buying them Mondo (so fairly fresh stock), shows that you never have any overlap (e.g. a 160 main that is actually bigger than a 165). This is because the 160 is say 170 real, and the 165 could be 173.
So for practical purposes, you can avoid measuring and will not get overly deceived

It’s semantics, but If you end up with a 160 that’s actually say, 164 that’s a big difference in area IMO.

1.6mm = 2.0106 sq mm
1.64mm = 2.1124 sq mm

By diameter that’s 2.5% difference, but 5% by area. A big jump for what’s supposed to be the same size jet.
You could use this to your advantage of course too.

The elephant in the room of course is that flow is not necessarily linear with area…. But going by orifice size seems to be good enough.

This is what I’ve done and had good results. You’ll of course need to dial in each and every day, and adding that data to the model will continue to strengthen it.

You may not that certain engines and setups will require different targets, whether different jetting to achieve the same output (EGT as one example), or different output ranges altogether.

I’ve the kits ordered.

It will be interesting to see how accurate things will be.

Atomizers were (mostly) exactly correct. From time to time you get a +1 but mostly a fluke and barely, barely over.

Main jets below, based on my current inventory


thats some big difference over there andy … i will take one correct measurement tool set for sure next payroll of mine …

Yeah not to belabor the point but it’s probably worth having a column for jet area as well.

Wow! Thats actually scary how off they were!

So even these variences alone would negate any jetting app.

@Andy_DiGiusto do you chart Relative Air Density?

From my expirience the Dellorto jets is that In the range we use them (122-134) they are 6-8 points larger then whats stamped on them.
Atomizers is close to whats stamped on them.

In the Rotax (and Rok GP) world, there’s a lot of us in North America who use the “Pro Flow” kits that Black Racing Engines assembles, after testing the flow of a large number of jets. In these kits, you might have 3 jets labeled by Dellorto as 120’s, but they might flow in a range of 118.5 to 120 to 121.5 (or something like that). They are nice for tuning with because there is more certainty of what step you might be taking by changing from one jet to another, and offer some “in between” flow rates that you wouldn’t necessarily know of without his flow testing.

Pinning jets might be better than just using a random numbered jet, but the only way to really tell what is what is by flow testing them (like Mikuni jets).


I’m curious how flow testing is done and what measurements are used. I guess measuring flow under vacuum would be best.

This thread is fascinating. Take nothing for granted, apparently.

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The real range of variance (5pts) is only one or two jet sizes, based on the range of variance vs actual of 8-13. Most people probably aren’t trying to jet on the edge so 1-2 sizes likely won’t make your engine go boom.

If some were 1-2pts off and others were 13pts off then we’d have a problem.

The pin kits arrived yesterday.

So I got busy measuring today.

All Atomoizers were perfect, apart from a DP265 that was actual a DP264 in measurment.

Jets… wow all over the place.

On average 8/9 sizes out per jet.

The most interesting aspect were the jets measuring the same. So my 160 and 162 measured as 168. Same for my 178 and 180 jets both measuring 187. That would leave head scratching at the track.

The rest followed a patten of always being 8 sizes larger than the jet.

Leaves some confussion now with the jetting apps. Do I use my actual sized values or the values of the printed jets. Would the software be in the know of jet varience and calculate as such?

I day of trial and error regarding dialing in my jetting chart system is in order now.

I guess I would record my baseline based on the measured size of the jet. Then any adjustment the app suggests should also be the measured size.

We might be splitting hairs here. But it’s interesting. The good thing that the actual difference between jets is not too bad. Ie they seem to be off by an amount that’s consistent.

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