Carburetor jet size

F

fischer550

Member
I have a new motor I just built. It has a ported cylinder, high comp head, and tuned pipe. I live in Colorado at elevation 5,000 ft above sea level. I also put this carb on it. http://www.ebay.com/itm/Racing-Carb...467?pt=LH_DefaultDomain_0&hash=item3f34802313 So, the carb came with 6 jets, 60, 65, 70, 75, 78, and 80. I started with 70, and it ran OK, but did bog just a bit and just seemed like it wanted something more. Me being in such a high altitude, I thought smaller jet. I tried to 65, and the bike would basically not run, it would not respond to throttle. So do you think that my engine actually needs bigger jets b/c of the port work? Is that common?
 
Jet size is probably more dependent on your exhaust than your other mods,
I have found .65 works well at close to sea level.
But I would not trust that the jets you have are labeled correctly.
Just been my experience.
try it on the .75 check the plug.
If it's lean it may bog, if it's rich it'll 4 stroke.
A little rich is better than too lean, especially if your doing any hill climbing.
 
Engine fires maybe every second time, runs rougher, with less power.
you get a blat blat sound instead of the smooth 'zing' that a 2 stroke should make.
It should probably 4 stroke a little as you back off the gas.
 
Yes, the blat blat blat sound is exactly what it is doing, especially at lower end. I was doing it with stock NT carb, and this one. when I ran it with the 70, it did it at low throttle. With the 75 (I tried it too) it was blatting at all levels, and the 65 just didn't work. weird
 
I'd make sure the .65 is actually .65.
I'm not much of a fan of the Delorto Rep carb, might be ok on a standard motor, not had much luck with them my self.
much prefer the NT or Speed Carb, all my motors have run better with any carb other than the Del rep.
Some like them, probably cause they can't be bothered to tune their NT properly.
 
The Del reps come with 5mm jets which will work in the NT, But also sometimes come with 6 mm jets which will not work.
Are your Jets 5mm or 6mm?
The NT will have a jet somewhere between .65 and .68mm
I have found with certain set ups the standard jet to be a little rich and the .65mm better.
 
I hwve NT cabs on two bikes tuned well and I like them, I have the Dellorto Clone carb on two other bikes and they both run well, im at 1240ft ASL so that may make a difference, my engines have big hogged out ports and free flowing exhaust and I run 100:1 Opti2 fuel/oil mix.

The 70 jet in the RT/ Dellorto carbs works great with my engines, only 4 stroke a slight amount at low throttle low load on engine but at cruise speed and acceleration there is zero 4 stroking, now all said, the fuel mix ratio will also have an effect on jetting needs, I can run a smaller jet in my bikes because I run a very rich fuel mixture of 100:1 and Idont buy different jet sizes when tuning any of my carbs, I solder my jets and redrill them with wire gauge bits, its a very simple process, a small torch or torch type cigarette lighter will work to heat the jet and I buy my bit set at www.widgetsupply.com the bits and pin drill chuck for bit is about $10 or less best I remember, I remove the jet, then I test the jet hole by seeing which bit will just fit through the jet hole and then I will place a very small amount of small wire solder over jet hole after jet has been cleaned with a good spray of carb and choke spray cleaner or brake cleaner, place a very small piece of solder over hole on side that has screwdriver slot and heat jet from bottom just until solder melts and flows to fill the hole, then let it sit and cool for a couple minutes and then drill carefully with next size smaller bit than the one that would just fit into original jet hole, for my setup the NT carbs have worked great with a jet soldered and redrilled with a #72 or #73 wire gauge bit, in my experience both NT and RT carbs work great, the Dellorto Clone carbs do seem to give a little better low to mid range power and throttle response for myself and other have said they have noticed the same effect, a well tuned NT carb set up right on a well ported engine with a good flowing exhaust and correct gearing has yeilded a little better top end speed for several people not a huge increase but a couple mph in my experience on one of my engines, bike would do 45-46mph with well tuned NT and only 42mph with well tuned RT Dellorto clone, the engine had better throttle response and a little beter torque with the RT/Dellorto Clone though, so really there isn't a huge performance difference and the NT carbs are easier to tinker with and about half the cost, I like the NT Speed carbs the best because I can use 1/4" fuel line instead on the 3/16" line that needs to be used on th standard NT carbs.

http://www.widgetsupply.com/mm5/merchant.mvc?Screen=PROD&Product_Code=BLU01&Store_Code=WS


http://www.widgetsupply.com/mm5/merchant.mvc?Screen=PROD&Product_Code=WB05&Store_Code=WS

http://www.widgetsupply.com/mm5/merchant.mvc?Screen=PROD&Product_Code=SGS3-841DH&Store_Code=WS


Map
.wee.
 
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I never bother trying to tune one of these until there is about 300 miles of run-in on the engine - as the rings begin to seat, it will change a lot
 
crassius said:
I never bother trying to tune one of these until there is about 300 miles of run-in on the engine - as the rings begin to seat, it will change a lot
Click to expand...

Nothing wrong with that approach at all..... I dont wait since I have a good idea from the get go what my engin s and set do good on, I give mine about 10 miles breakin period and then ride them the way I plan to ride rhem and on the fuel/oil mix I plan to run the life of the engine, only one failure back in 2010 with a bgf engine that the lower bearing failed in and I wasn't running Opti2 oil yet at that time, since switching to Opti2 I have never had another engine failure.

I wont say these results will be the same for all and especially if a cheap or inferior oil is being used, if a cheapo oil is being run I would also recommend a slowed breakin process or even better yet run Opti2 and then in my experience the premature wear and failure problem has been solved, if the engine components are in good shape and you replace the china made upper bearing with a high quality Japanese bearing.

Map
.wee.
 
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I have a set of drill bits from amazon that have worked well, I had to drill out the main on the first bike I built that had the cns2. With the nt I can get by with the stock jet size even on my 48cc engines as long as I run a free flowing exhaust, a good low restricting exhaust will help solve a lot of tuning problems.
 
You're welcome Greg... that is the best place and prices I have found on bits and all else for my rotary tool, I get my deburring bits and all my drum sanding stuff crom them also, good stuff at great prices, its the stuff I use to do all my port work on my jugs and. Anything else that needs to be ground down or sanded that needs a small tool for the job.

I have never been able to get by with stock jet in the NT carbs down here where the air is hot and thin alot, I can with the RT/ Dellorto Clones for some reason but a #70 bit is always the size of the jet in my NT carbs and I solder and redrill to either #72 or #73 before I get that good plug chop and the 4 stroking is limited to low speed low load riding.

I finally got that expansion pipe running satisfactory on the Kulana, it will cruise at 42 mph now and its not crazy loud doing it, my other pipe got me an additional 3 mph but it was way to loud to ride into town, the pipe I speak of is one I picked up off ebay for $10+$11 shipping best I remember, about $21-22 total, I feel bad because someone here on the forum pointed me to it and it slips my mind now who it was, I bought one off amazon.com too, but I think the one off ebay might be a little better... im no sure but Im gonna try the other one on another bike and see how it does.

But even with the little bit of extra restriction my carb tune is still working fine, I thought I might have to jet down one notch but didn't.

Map
.wee.
 
This thread contains some very good tuning informatiom that should be required reading for anyone interested in properly tuning their carburetor.

Like mapbike, I'm a Opti-2 devotee and also solder and drill my jets. His advice is solid. Check the orifice size against a known drill size then decrease it by steps, one step at a time until you hit pay dirt.

crassius is also right on with his 300 mile theory. I do port and piston mods first on a new engine but I don't expect them to pay off until there's some miles on the odometer. These little two strokes are like a fine wine. They just get better with age.

Greg, gearnut, Theon, thank you all for your usual valuable input.


Tom
 
Re: Carburetor je

Greg58 said:
Map it wasn't me this time, I know I've used your advise a lot.
Click to expand...

Im th same way, I've learned loads of valuable info from the members here, Tom, Dan, you Greg, Bairdco, Paul, Barelyawake, Nashville Kat and many more jumped in and answered many of my questions when I firat found this forum, many on here were here for me with good thoughts and prayers when I almost lost my youngest son....... and on and on...... in my opinion we have the best group of people on this forum that exist on an forum..... and I have always appreciated all of the help and advice even if I do or dont always listen or agree... never overly offended and feel like Im with family when Im on here.

Thats what its all about people helping people and enjoying this great hobby.

Map
.wee.

I think it was allen_stanley.... but Im not sure..... ??????

Map
 
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