NT jetting, with too many numbers


Active Member
Oct 30, 2018
So I finally got some jets so I can start dialing in my carb on my newish HT build.
Since my memory is utter garbage, and I thought my newbie experience might be useful, I'm documenting and posting it here for both my reference, and yours.

It's a stock generic supposedly 40mm stroke engine, but I believe it's a 38mm stroke, GT-5 type engine based on the ZAE-80 rod it came with. Purchased from eBay seller in 2018. Stock exhaust.
It has an NT carb. It seems to be running rich on the top end at WOT, so I wanted to try jetting it down a touch.
I wasn't 100% sure of how the jet sizes worked, so I just worked through it below. Here is a drill bit size chart.
And here are the sizes pertinent to my change.
Micro Drill Index sizes
Index      mm
#69        0.74
#70        0.71
#71        0.66
#72        0.64
#73        0.61
#74        0.57
#75        0.53
I bought the set of 5 jets, sizes 60, 65, 70, 75, 80. They are the round head jets for Dellorto SHA clones, but they seem to fit. I'm not certain the thread pitch is exactly correct.* They go in a little tight, but they do go all the way in and seat without any noticeable damage.

First, I checked my stock jet. It has a hex head and is not marked with a number, but a drill #71 fit (a little too tight), while #70 was much too big.
The size 65 jet from my set seems to be the same. The 0.66 mm bit will go in both but it drags substantially.
The crossover between index numbers and mm sizes between #70 and #71 makes it easy to confuse what the numbers mean in this range of sizes!
I chose the next smaller jet in the set to replace my stock jet, so that is the 60. A #74 bit fits this one loosely.

What I gather from this is that the jet sizes are in hundredths of millimeters. (I wasn't sure - I thought they might be flow based.) Also, my cheap eBay micro drill index seems to be pretty close to the correct sizes.

I haven't put it back on the bike yet. If the weather is OK and I have time I'll try it out tomorrow and share my results.
To summarize, I went from a stock 0.65 mm jet to a smaller 0.60 mm jet.
Also, I'm still running a 16:1 fuel from the break in period, so things will change, but at least I'll get to see what the effect of this change is.

Edit #1 Some jet measurements for reference
Overall length 8mm
Hex across flats 6mm (stock)
round head 7mm (Dellorto, SHA clone jets from eBay)
thread 4.9 mm
*thread M5 x 0.8 mm

Edit #2 First round Dec 13
I'd been sick and the weather was crappy, so it took a while before I got back out to try the new jets.
Sucks having a chronic incurable condition that lays me so low so often:(.
I tried them in the following order:
#60, #70, #80, then back to stock #65

Unfortunately, I also had to change my plug and plug wire at the same time and I seem to have developed a high RPM misfire.
It occurred under load and not under load, and with all the jets. I expect it is related to spark, but I don't know the source yet.
I was feeling too lousy to continue, so I put it away for now. I'll swap the CDI next time I get a chance to work on it.
Plug setup is NGK BR6HS with non-resistor crimp-on boot on copper core wire and original stock CDI.
Plug is gapped to about 0.026"
I also need to pull the magneto cover and check the gap on my magnet because I remember pulling it and not gapping it carefully when I put it back in.
Even with the hi rpm problem, I feel like the stock jet ran the best, but I was unable to see which one would let the engine rev out the best on the top end.
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