Jets for NT Carb found to be very inaccurate

Tim Bailey

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On two occasions I have found the jets I have purchased to be marked incorrectly. I bought 5 different sizes sometime ago and started jetting my NT carb for wide open throttle. Everything I tried produced no results or things got worse. I eventually bought a set of needle drills and used them to measure the jets. I was shocked to find that all five jets were the same size but had different sizes stamped in them. Just this weekend I took delivery on a Dellorto Type carb and it came with 15 different jets, sizes 61-75. I got my needle drills out and measured them and out of 15 there were only three different sizes. 71,72,and a 69. All the rest were the same size. I now solder the jets solid and drill them myself and this has made life a lot more predictable. I really like the Dollorto Type carb but that's a different thread.
 
Where did you get your needle drills? I got some a couple years back and none of them were marked correctly. Are you buying your jets from SBP? I have been using their 5mm jets for a long time, and have had good results.
 
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Just buy genuine Dellorto 5mm jets. I have a full range in single dight increments from #65 to #90. In South Eastern suburbs reigon of Victoria, Australia, using ethanol free 98 octane fuel, i have found that most of my engines run optimally on a Dellorto #76 main jet, though i have had one engine run optimally on a #70 and one engine ran optimally on a #86
 
I have a MSCDirect right by my house, cost was about $20.00 and they are in a nice case that is labeled. I use a mic to confirm the sizes. I got in trouble with an old set of wire drills which had about three drills all the same size. It was killing me, I was changing jets and getting no results or results that made no sense. the set looks like this, http://www.ebay.com/itm/20pc-HSS-Mi...346685?hash=item1ea5b86f7d:g:j6cAAOSwhcJWIDQa. Fabian do you know the size relationship between Dellorto jet numbers and the measurements in inches or MM? Thanks.
 
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#61 (.0390") #62 (.0380") #63 (.0370") #64
(.0360") #65 (.0350") #66 (.0330") #67 (.0320") #68 (.0310") #69 (.0292")
#70 (.0280") #71 (.0260") #72 (.0250") #73 (.0240") #74 (.0225") #75
(.0210") #76 (.0200") #77 (.0180") #78 (.0160") #79 (.0145") #80 (.0135")
 
I use a sewing needle to compare jet diameters.
Slide the appropriate sized needle into the orifice and compare how far it goes in.
Won't give an accurate measurement but will tell you if you are getting larger or smaller.

Jets are measure in flow rate, not hole size.
Some use the same size hole but different length orifice to achieve the proper flow rate.
Not very common, but this fact was part of our training in Automotive Mechanics many years ago.
I've never seen an application of it, but it is what we were taught. "Don't trust hole size for jet size".

Picture if this #0.9 hole was longer or shorter, it will flow less or more fuel, even with the same hole diameter.
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Steve
 
When I started tinkering with jets I read somewhere that some manufactures calibrate identifications with flow rates, while others use orifice dia.. The microbits I got were terribly missmarked, but with the mic. I found the .68mm in the batch. Now I see myself going to the sewing store with mic in hand sorting through needles. The lady's going to think I'm nuts, and that's exactly why I'm going to do it. I do need to replace a couple of buttons too.
 
Some good information, especially about length of bore and fuel flow. I hope it helps someone besides myself. I recently installed a "Dellorto type" on my 80cc and I'm close with the jetting. Unfortunately, I had to travel out of state and it will be a month before I can share any data.
 
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