Jetting Poll

[biknut]

Lets say we have a NT carb perfectly jetted, and then we change to a slightly bigger carb like say a Speed carb, or a DAX RT.

We're talking about a china girl 80. An NT, Speed carb, and DAX RT. Not any motor in general, or any carb.

What effect will this tend have on our mixture compared to the NT? Make it richer, leaner, or no change?

 

[rustycase]

it's a trick question
I can't answer that

a bigger carb will bring more top end (within reason) if original is too small for displacement, and worse bottom end power.

richer/leaner is the function of jetting adjustments respective to each carb.

Although the NT does not have the adjustment capability the cns does, it is apparently engineered superior to the cns and functions very well even though it does not have a low speed mixture adjustment on the exterior of the carb that is accessible to a user.

Perhaps ONE DAY I'll be able to adjust a cns properly, but there is something about the basic engineering of it that is not balanced properly...

The NT performs very well after simple jet adjustment.

I think the 'speed carb' is just a different flavor of cns????

Best
rc

 

[biknut]

Maybe I should explain. A jet is a little brass thingy with a hole in it, that fits inside the vassgassor. Vassgassor's and jets come in different sizes. Changing to different sizes effects the fuel mixture, which if done right, causes stronger pulses to come out of the blooey tube.

 

[headtrama]

Hard question to answer , a lot of things affect jetting. A change in the weather could have you going richer or leaner . A change in altitude Will also mess with your jetting . The NT carbs are kinda picky and a crap shot to get a good one , I have had good luck with the speed carb . I have since moved on to a better carb and couldn't be happier. And no I didn't get a cns carb .

Bigger isn't always better but i guess it depends who you ask. I don't think i would go bigger then a 16mm carb on these little motors .

 

[camlifter]

a bigger carb. aka larger dia. bore will bring in more air. if used on an engine that is too small for that size it will be leaner as it will suck in more air and lose volume there by drawing in less fuel. the cns carb is just to big for an ht engine. don't know what size jet is in it but it must be huge. the one that came on the skyhawk i just got is a pos. the air adjustment is blocked off with a plug and can't be used. it's so rich the idle screw is in untill the throttle is half open just to idle. the return spring binds up in the slide and jams it wot. i'm replacing it with an old nt carb.

 

[rustycase]

Maybe I should explain. A jet is a little brass thingy with a hole in it, that fits inside the vassgassor. Vassgassor's and jets come in different sizes. Changing to different sizes effects the fuel mixture, which if done right, causes stronger pulses to come out of the blooey tube.

Oh!
Well, in that case, bigger is better.
rc

 

[biknut]

So far everybody is overthinking this.

This is about a china girl 80, a NT, Speed carb, and DAX RT. That should make it a lot easier.

 

[biknut]

a bigger carb. aka larger dia. bore will bring in more air. if used on an engine that is too small for that size it will be leaner as it will suck in more air and lose volume there by drawing in less fuel. the cns carb is just to big for an ht engine. don't know what size jet is in it but it must be huge. the one that came on the skyhawk i just got is a pos. the air adjustment is blocked off with a plug and can't be used. it's so rich the idle screw is in untill the throttle is half open just to idle. the return spring binds up in the slide and jams it wot. i'm replacing it with an old nt carb.

I think I understand what you're implying. A bigger carb, flows more air, so the mixture should tend to lean out.

i guess the question then is, is a Speed carb, or a RT that much bigger to cause enough difference to make it leaner?

 

[camlifter]

every motor bike i've ever built, the carb that came with it was way to rich. the cns carb with the choke cable is the worse so far. i've never seen a china girl that needed to be richer. a bigger carb is going to have a larger jet to go with the larger dia. bore so your starting out richer anyway. once you lean out the jet to get it to run right it's still miss matched for the engine and most likly will have flat spots and boggs through out the power band. bigger isn't better, the cfm flow needs to be matched to the engine size. even the stock nt carbs are on the large size for these engines.

 

[Goat Herder]

Here practically half way up the Himalayas ''altitude wise'' at about 6000 foot above sea level I have seen a 19mm carb work perfectly on a AKA pos H.T. motor. The majic is in the tuner's hands....

The H.T motors bareley breath to began with becuase of the way the jugs are designed. Realy Not Much can be done about that.

 

[biknut]

This poll starts with the assumption of an NT carb that is already perfectly jetted. So the question pertains to what effect there will be when we change a bigger carb.

I guess this is confusing because I didn't say what size jet is in the bigger carb. I was thinking compared to the same size jet as the NT.

So far the poll indicates a bigger carb will make it richer.

 

[camlifter]

Here practically half way up the Himalayas ''altitude wise'' at about 6000 foot above sea level I have seen a 19mm carb work perfectly on a AKA pos H.T. motor. The majic is in the tuner's hands....

The H.T motors bareley breath to began with becuase of the way the jugs are designed. Realy Not Much can be done about that.

yeah, at 6000 ft you probably could use a bigger carb, less air up there. but remember, down here a 19mm carb will run a 10hp engine. the little ht has maybe 1.5 hp. a 12 mm carb would probably work better.

 

[Goat Herder]

yeah, at 6000 ft you probably could use a bigger carb, less air up there. but remember, down here a 19mm carb will run a 10hp engine. the little ht has maybe 1.5 hp. a 12 mm carb would probably work better.

I beg to differ on this I beleive it is quite the opposite of what you are saying at a high altitude? I could be incorrect? I beleive a 19mm would work even ''way better'' at sea level? I do agree that it helped me immensely for my high altitude tho.

I have to lean my carb out for the thinner air. But the same car can be ran richer at sea level..

I need all the increased atmoshere I can get! Yet because my air is thinner the venturi effect with a bigger carb would be more unforgiving. Such as a guy going to a carb so big as the motor cannot operate it becuase it does not have enough air volocity to pull the venturi. . The density of the air in of itself has a role to play in the proper operation of the venturi. Having the air thinner in this equation I say/think might prove to make this much more unforgiving?

I have traveled down to the valley here to a lower altitude like 4000. I did not notice any thing that significant. It always did seam a little more peppy.

I guess don't know how to understand that logic from where I am currantly at? If I put a blower on a car it does not mean I have to go to nearly half the size of carb too.

From here: http://www.strokerengine.com/BDSBlowers.html
The size of carb(s) or CFM required for a given application can be calculated by the following formula A: {(CID x RPM) / 3456} x {Boost: 14.7) + 1} = CFM required. The amount of CFM required will determine carburetor size and quantity. If you try to use a carb with less CFM than required, performance and economy may be greatly reduced.
If a bike at sea level gets more power why stop there?

I know of folks running 21mm carby's on 49cc Morini's at sea level and they are doing perfectly fine. The little H.T. is bigger in cc's than that.. I know the jug is still horribly tuned. While I am not going down to sea level just to try it lol. I am sure someone here has run one 19mm?

I used this link to show what I think a increased atmosphere does I don't use any blowers lol.

 

[biknut]

I thank you guys. I always enjoy good discussion about carb tuning, and hearing about other peoples experiences.

I'm not exactly a light weight when it comes to the subject, but sometimes I get a little full of myself LOL.

Even though I know the principle is the same, I don't have much experience tuning these tiny little carbs, or 2 stroke motors. I've found that on this site at least, it's a lot easier to ask than try to reinvent the wheel.

 

[machiasmort]

Elevation is a consideration, but how about compression... Leave it to me to throw the thread a curve! I got my motor blown up and nixed my upper and lower gaskets! Torque is crazy! 2,000 miles on it!

 

[biknut]

What about this. I'm running an NT that's gassing well with a 60 jet. The venturi measures 13.74 mm.

I want to install my new Speed carb. It measures 14.78 mm. Should I start out with the same size 60, or should I go bigger at first to play it safe?

 

[Goat Herder]

Elevation is a consideration, but how about compression... Leave it to me to throw the thread a curve! I got my motor blown up and nixed my upper and lower gaskets! Torque is crazy! 2,000 miles on it!

Getting more air in a motor raises commpression too. RE bigger carb.
When I messed with the Morini motors this is an old quote to what I found.

My S6S with a larger 21mm phbg Dellorto may be running higher compression than stock with just the carburetor upgrade. Reason I say this is because I have peaked inside my S6S 9.4 hp. and my S6T 5.8 hp.

Their heads jugs pistons etc look identical. The only difference are their ignition, smaller carburetor and smaller reed valves. What I want to say is just by making these breath a little better raises the compression because a bigger breath of air is more air to compress. Like turbo charging a car.


The S6T 5.8 I have came with a very restrictive reed valve ''single wall 3 peddle medal '' and a very restrictive small bore carburetor. Verses a 4 peddle on two walls carbon fiber.


My S6C is getting a 21mm Del for sure!! The S6C must have a reworked head



S6C INTAKE Reed valve w/4 flaps in crankcase COMPRESSION RATIO 15,5 +/- 0,5 : 1
S6S INTAKE Reed valve w/4 flaps in crankcase COMPRESSION RATIO 14,5 +/- 0,5 : 1
S6T DISPLACEMENT 49,8 cc INTAKE Reed valve w/3 flaps in crankcase
COMPRESSION RATIO 12,5 +/- 0,5 : 1

What about this. I'm running an NT that's gassing well with a 60 jet. The venturi measures 13.74 mm.

I want to install my new Speed carb. It measures 14.78 mm. Should I start out with the same size 60, or should I go bigger at first to play it safe?

Go to a 61 eh? Dunno can't tell ya. In experimenting with tuning with my Morini tho..when I went from a 19mm to a 21mm I needed a bigger main jet my safty zone at wot was noticably too lean. This was acheived by simply transferring all the jetting parts right over from the 19mm PHBG. Both models of carbs were PHBG models all the jets etc crossed right over. The 21mm had a much less restrictive rear opening it breathed better and I felt better throttle responce immediately. .

I made real good freinds with the dellorto PHBG carbs. The needles, idlejet, main jet, atomizers can be totally changed around. It can be a pita learning this carby but man I think they are pretty versatile now. I am certain by now I can definitly match one up to where all stages of the throttle would be perfect! Its all in the jetting!

One my buddies here is running a 19mm carb on his H.T. forever here now and it is a great good runner. He thinks he could get a 21mm on his at sea level. Talked to him last night his and my consensus we both think this carby thing to go a little larger is by far way way more forgiving at sea level.

He ran a slightly longer intake than stock and the diameter of it matched the carb perfectly. Last with a tuned pipe..

I am absolutly sure a 18, 19mm will kick butt at sea level on a H.T..

 

[machiasmort]

If it's made the same as your stock carb? I remember my HT had 4 possitions for the needle. Which one are you on now? And could you open it up to let more gas in? I would try that before drilling! Your spark plug will tell the tale very quickly! Start with a new one!

 

[biknut]

Go to a 61 eh? Dunno can't tell ya. In experimenting with tuning with my Morini tho..when I went from a 19mm to a 21mm I needed a bigger main jet my safty zone at wot was noticably too lean. This was acheived by simply transferring all the jetting parts right over from the 19mm PHBG. Both models of carbs were PHBG models all the jets etc crossed right over. The 21mm had a much less restrictive rear opening it breathed better and I felt better throttle responce immediately. .

In that case I think I'll go back to a 64, or 66, and probably work my way down from there.

 

[biknut]

If it's made the same as your stock carb? I remember my HT had 4 possitions for the needle. Which one are you on now? And could you open it up to let more gas in? I would try that before drilling! Your spark plug will tell the tale very quickly! Start with a new one!

Thanks for taking time to offer your advise. Usually my NTs end up with the needle on the next to leanest, or leanest position.

The needles job is mainly gassing the low, and midrange. That's when you're riding with less than full throttle. It has little or no effect on full throttle operation, no matter what position the needle clip is in.