Jet sizes

Bob Mac 18704

New Member
Jan 24, 2010
127
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penna.
First of all I would like to say this about these jets. Their not what they appear to be. I was having the standard problem of bogging at 3/4 to full throttle. From reading I figure I need to rejet. I go on line and find a dealer who could supply me with some jets. I'm told the the standard jet is a #70. Without knowing which way to go (larger or smaller) I order 3 jets from an unnamed supplier. I order a #68-#72-#74. Let me first clear this up, the higher the 2 digit number the smaller the hole in the jet. The #68 jet is supposed to measure .031 but it measures.028. The #70 jet which was the stock jet in the carb was supposed to measure .028 but actually measures .026. The # 74 jet was supposed to measure .022 and it measured .029. As you can see these have no consistancy what-so-ever. I used my micro bit set that I calibrated with my starrett micrometer and the bits are spot on. what i'm getting at here is if you think your putting a larger or smaller jet in your carb. Chances are you mite not be. The numbers are stamped on the jet itself and 100% of the jets i bought are wrong. I took the jet out of my carb and replaced it with another one that came in with what I ordered and it works great. I don't know how big it is because I didn't mic it before I put it in. but I can pretty well guess it's not a #72 I'll have to check it next time I have the carb off. As for the answer for the bogging problem. I would say get yourself a micro bit set (about $15.00) and get a few #74 jets. those jets will be the smallest you would ever need. Then start working your way up the drill index until you have the problem solved. you only need one jet to keep drilling bigger and bigger. If you get it too big use another jet and go back one size. You can always solder the holes shut on the jets you drill too big. Any questions. others mileage may very. This is just my observation.scratg
 

escout

New Member
May 11, 2011
14
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Wyoming
Here's a link to my thread...I just ordered some jets for my cns gen B carb. and posted a link to Dellorto Direct. I also have a link to SBP that sells jets for NT carbs. Good luck.
 

Bob Mac 18704

New Member
Jan 24, 2010
127
0
0
penna.
Im going to say SBP but it has nothing to do with the quality of the parts he sells. They are a victim of no QC just like the rest of us. This stuff is made by people that make $3.00 dollars a day and don't give a damn about what they ship out the door. From what I discovered, I wouldn't doubt most of the problems with these carbs is the jetting. Their's nothing consistant about jetting. You never know what your getting in that carb. Oh I just read your post. I didn't realize you had a cns carb.
 
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mapbike

Active Member
Mar 14, 2010
5,400
20
38
Central Texas
I have had to solder and redrill the jets on every engine I have, it seems that they all come with jets that are to big, they may be ok with the 16:1 30wt motor oil they probably use in china but not here.
 

bigbutterbean

Active Member
Jan 31, 2011
2,417
2
38
Lebanon, PA
I personally have never had throttle issues. While I would say that for the most part I have never had exceptional throttle response (except for on my brand new engine, the throttle response is beyond exceptional), I have never encountered bogging or any other throttle or carburetor related issues. My personal suggestion when it comes to jet sizes is if you aren't happy with youir throttle response and want your carb jetted differently, then figure out how to modify your existing jet yourself. Good luck.
 

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
6,641
468
83
Los Angeles, CA.
In general, I have to re-jet one size smaller on every 'performance' HT engine that I build!
I'm lovin' the RT carbs these days, but they too need one size smaller. (one size smaller is a #71 drill bit or 68mm jet.)
 

mapbike

Active Member
Mar 14, 2010
5,400
20
38
Central Texas
In general, I have to re-jet one size smaller on every 'performance' HT engine that I build!
I'm lovin' the RT carbs these days, but they too need one size smaller. (one size smaller is a #71 drill bit or 68mm jet.)
What fuel/oil ratio and what type of oil do you run and tune for?

I run Opti 2 and #71 is still to big on mine, normally #73 is about right on my engines, the plug is still nice and brown and it still 4 strokes a bit at part throttle while not under a load, some dont even like this but I dont want to get them too lean so I stop at #73 and they work good there for me in this very dry warm/HOT Texas Climate.
 

mapbike

Active Member
Mar 14, 2010
5,400
20
38
Central Texas
is a 1/16th bit too big? its the smallest ive got..
Yes way too big........here is a link to where I got my wire gauge bits from, you will like ly need to solder and redrill with the #72 or #73 if you have the same results as I have, I live in a pretty warm to hot and sometimes fairly humid climate here in central Texas so the altitude and the weather will be something to consider, I would get a set of these bits first and then you can start the tuning proccess with a combination of jet size and clip position on the metering needle.

This set comes with a pin vise which makes it much easier because these bits are very small and will break very easily if a person doesn't take care in using them.

Widget Supply 20pc HSS Twist Drill Bits, No 61-80 and Pin Vise

This page just has multiple listings of the wire gauge bits and a couple set without the pin vise.

Widget Supply: Wire Gauge Drills


Hope this helps, any more questions just put them here and I'll bet someone can get-em answered for ya.

Peace
 

mapbike

Active Member
Mar 14, 2010
5,400
20
38
Central Texas
I'm confused here. So those .68mm and .66mm jets I bought from SBP are actually
larger than the stock jet on the NT carb?
No they aren't larger....

the stock jet is likely a .70mm so you should put the .68mm in first and see how that works and be sure to do a plug reading after the ride.

The numbered bits are not jet sizes...... the jet sizes are in metric measure.

I dont know everything there is to know about jet measure but the conversion tool I use says that a .70mm jet is 0.028" or #70 wire gauge bit size & the .68mm is 0.027" which is closer to the #71 wire gauge bit size which in wire gauge bits the higher the number is the smaller the bit & the .66mm is 0.026" and is closer to the #72 wire gauge bit size.

Just install the .68mm jet and possibly tinker with the needle clip and check the plug after a good little run down the road a ways and back and se what you have.

best wishes and hope my very limited knowledge on the subject helps in some way..........I'm sure someone who really knows their stuff on the jet sizing topic will ring in here and set us both straight on this subject.

Peace
 

treyoh5

New Member
Apr 29, 2011
36
1
0
Miami, Fla.
Gonna check my local hobby shop for those wire bits. Any suggestions on soldering shut my current jet, was thinking I'd use my 30watt soldering iron and melt some solder down the jet.
 

camlifter

Active Member
May 4, 2009
1,034
2
36
acme labs marion ohio
i always end up with a #73 bit for best performance. it will all depend on where you live. the higher the elevation the smaller the jet will need to be. i live at 1000' elevation, the stock jet is always to rich. #73 drill size brings it into a good a/f ratio using a 32-1 mix at this elevation. always check your plug. you want a nice dark tan color.
 

turbojoe

Member
Mar 30, 2011
57
0
6
Little Rock,AR
Thanks for the the clarification. I already put the .68 in my carb and it's still fouling the plug. I'm gonna put the .66 in today and see how that works. I live in Little Rock, AR and
the elevation is about 300ft. It gets fairly humid this time of the year, so, I don't know if that has much effect, too. If that doesn't work then I suppose I could try the drilling my own jet thing.
 

mapbike

Active Member
Mar 14, 2010
5,400
20
38
Central Texas
Thanks for the the clarification. I already put the .68 in my carb and it's still fouling the plug. I'm gonna put the .66 in today and see how that works. I live in Little Rock, AR and
the elevation is about 300ft. It gets fairly humid this time of the year, so, I don't know if that has much effect, too. If that doesn't work then I suppose I could try the drilling my own jet thing.
just a small propane torch is perfect for heating the jet and thenadding just a very small amount of solder to fill the hole, just make sure that the jet has been cleaned good with either carb or brake cleaner to remove any oil that might hinder the solder from flowing in the hole and sticking to the brass properly.

Peace
 

mapbike

Active Member
Mar 14, 2010
5,400
20
38
Central Texas
i always end up with a #73 bit for best performance. it will all depend on where you live. the higher the elevation the smaller the jet will need to be. i live at 1000' elevation, the stock jet is always to rich. #73 drill size brings it into a good a/f ratio using a 32-1 mix at this elevation. always check your plug. you want a nice dark tan color.
#73 works good for me also, I have one drilled with #72 right now since it still in break in mode, this engine is gonna get one of Duanes RT carbs I have for it real soon and then the jetting will start all over until I get it just right, I will get my exhaust the way I want it first and then tune the carb.

I live in central Texas and honestly dont know what my elevation is here.

Peace
 

rustycase

Gutter Rider
May 26, 2011
2,746
1
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Left coast
"I live in central Texas and honestly dont know what my elevation is here."

You can google your town's name and your elevation will be in there somewhere...

or

There is usually a brass/bronze survey monument pretty close to most post offices that has the elevation stamped in it along with the long & lat.
rc

I just googled my own 'zipcode, elevation' and it came back with 855ft.
 
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