Sure cure for all the NT carb troubles and jet changing

xseler

Well-Known Member
Apr 14, 2013
2,883
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OKC, OK
Just ordered one...........This will make 5 different carburetor brands that I have in my 'tuning arsenal'!!

The previous carb I bought was supposed to be tunable without changing jets ---- nope. I'm glad I can see the 2 needle valves. This is how it should be!
 

xseler

Well-Known Member
Apr 14, 2013
2,883
130
63
OKC, OK
I am sure you won't be disappointed........Curt
It's all an adventure. If there weren't any bumps in the road, smooth wouldn't be appreciated.

Hard to get the 'gumption' to work on the bike lately. Temps have been bouncing from 70's one day to freezing precip the next. A good project like this may help get things moving!
 

cannonball2

Well-Known Member
Oct 28, 2010
3,662
152
63
Colonial Coast USA.
The manifold is the main part of this conversion.
The 3/4 copper water pipe is a good start as it matches the carbs bore. My gut feeling is the transition from the 21mm bore to the stock manifold accelerates the flow improving throttle response. Fairing this in is a must.
Be interested to see what you come up with.

The pulse port in inside the carbs throat so no case/manifold drilling for a pulse line is required. The nipple on the diaphragm cover is a vent probably went to the saws air filter box. Just leave it open.
Use a small cotter pin to attach the throttle cable to the carbs butterfly. Pop it over the cable stick it thru the hole bend the tangs back and clip em off. Instant swivel. The stock throttle cable works however it has more travel (1/8"). Position the cable bracket so the slack is below the cotter. This has a bit of slop but doesn't over pull the butterfly. Or shorten the cable.
You will spend a bit of time dialing it in. The carb has the ability to choke it or starve it with fuel. It will scream on top end when too lean. Stay on the rich side of this point, or bye bye piston! Work the lo/hi together to get a decent mid range.
This carb has NO throttle stop adjustment. It has a small window in the butterfly for idle speed. Works perfectly with a good idle on the 66cc.
Finally when properly set will take a bit of time to warm up and likes a partial choke.

Hope this works as well for everyone else as it has for me. Feel free to ask any questions
 
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xseler

Well-Known Member
Apr 14, 2013
2,883
130
63
OKC, OK
The manifold is the main part of this conversion.
The 3/4 copper water pipe is a good start as it matches the carbs bore. My gut feeling is the transition from the 21mm bore to the stock manifold accelerates the flow improving throttle response. Fairing this in is a must.
Be interested to see what you come up with.

The pulse port in inside the carbs throat so no case/manifold drilling for a pulse line is required. The nipple on the diaphragm cover is a vent probably went to the saws air filter box. Just leave it open.
Use a small cotter pin to attach the throttle cable to the carbs butterfly. Pop it over the cable stick it thru the hole bend the tangs back and clip em off. Instant swivel. The stock throttle cable works however it has more travel (1/8"). Position the cable bracket so the slack is below the cotter. This has a bit of slop but doesn't over pull the butterfly. Or shorten the cable.
You will spend a bit of time dialing it in. The carb has the ability to choke it or starve it with fuel. It will scream on top end when too lean. Stay on the rich side of this point, or bye bye piston! Work the lo/hi together to get a decent mid range.
This carb has NO throttle stop adjustment. It has a small window in the butterfly for idle speed. Works perfectly with a good idle on the 66cc.
Finally when properly set will take a bit of time to warm up and likes a partial choke.

Hope this works as well for everyone else as it has for me. Feel free to ask any questions

Thanks for the pointers! Things like this are the real fun in this hobby! Yeah, riding is good, but making odd things work is what toggles my switches.

Had a thought that I might try some heat shrink tubing over whatever I come up with as the manifold --- should help with potential air leaks. I guess I'll have a better idea with parts in hand....
 
Last edited:

xseler

Well-Known Member
Apr 14, 2013
2,883
130
63
OKC, OK
Carb came in today. Got several ideas on how to fab up the manifold mount. Finally excited to tinker in the garage! May have some time Sunday. Thanks again for the idea!
 

cannonball2

Well-Known Member
Oct 28, 2010
3,662
152
63
Colonial Coast USA.
Sure no problem!
Still waiting on my third one.

With a bit of fitment the carb will fit on a standard 32mm flange manifold(the engine side). Am thinking of joining two manifolds. I bought a port matched big bore manifold. It fairs/flows nicely to the 22mm carb. Thinking of stubbing it down and bonding it to the BT80s stock manifold with a section of copper pipe. It would be a double ended 32mm manifold. Hope the carb gets here tomorrow. Anxious to see how the BT80 does with it.
 

xseler

Well-Known Member
Apr 14, 2013
2,883
130
63
OKC, OK
Think I'm gonna try a 3/4" washer and a bit of an old used seat post (where it transitions from smaller to larger). The larger inside mates to the washer, which will mate to the carb. With a little filing, the small side will slide over an intake opening. Two welds and voila, hopefully it will work!

I did notice that the holes of an intake almost match those of the carb. With the manifolds I have, I don't know if I would be able to make a smooth transition between the two bore sizes ---- there's not much room to make a large taper. With the seat tube over the manifold, I can 'sharpen' the intake tube to maximize the flow. There's also 2 step downs instead of just one. We'll see......
 

cannonball2

Well-Known Member
Oct 28, 2010
3,662
152
63
Colonial Coast USA.
Seat tube sounds like a great idea. I used a large counter sink to sharpen as you say the edges on my other manifolds which maximized the flow thru the transition.

Got my carb and got started. I trimmed my two manifolds down a bit to make it shorter overall. Strangely the BTs manifold bolt spread is 35mm. Looks like and easy mod except on the carb end a tiny bit of the pulse hole is left exposed due to the narrow shape of the carb mounting flange. Pondering how I want to handle that. It's such a small portion I may just give it a small dot of JB being careful not to plug the hole.

Thinking the BT is really gonna like this carb.
 

cannonball2

Well-Known Member
Oct 28, 2010
3,662
152
63
Colonial Coast USA.
So last night while working with the pulse port issue I dismantled the pump side of the carb and found to my surprise the port is not drilled thru into the pulse side of the diaphragm! It is drilled all the way to the chamber but not thru to it. I had earlier stuck a wire thru the port to determine it was drilled and concluded the port was active. WRONG!

The nipple on the pump top cover IS a pulse connection. Both running carbs have run perfectly with no pulse on the pump. I have done numerous WOT runs to a plug chop with no issues. I can only assume that with the high head pressure from the fuel tank the carbs are gravity feeding.

SO I will be adding a pulse inlet, a small brass tube, into this latest manifold and running a pulse line to the carb. As far as the two running carbs. I see no reason to change them as they presently run.

Just wanted to point out my error on the pulse situation and advise adding a pulse inlet.
 

curtisfox

Well-Known Member
Dec 29, 2008
5,231
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minesota
Yes they work just fine on gravity feed, LOL but would do better to use the pump. And with the pump you can mount a tank lower. Post a picture if you get time, after hookup. That is good to know ........Curt
 

xseler

Well-Known Member
Apr 14, 2013
2,883
130
63
OKC, OK
Haven't had a chance to physically work on this project..... Tomorrow looks like a go to actually fabricate something! The extra time has given me the opportunity to ponder through some ideas, and I believe what I've got in mind should work perfectly (as I click my heels together 3 times). Fingers and toes crossed!
 

xseler

Well-Known Member
Apr 14, 2013
2,883
130
63
OKC, OK



I'm thinking that this will probably be the way forward for me. Still have some filing/sanding/shaping to do, but this is gonna work. I'll fill some of the gaps of the 1/2" EMT set screw connector with a touch of JB Weld and then have the 3/4" washer tack-welded to the connector. The connector will be grinded smooth to the face of the washer and tapered into the washer. The entire assembly, including the carb and filter will be about the same length as the NT setup.

If you should see any major issues with this, please let me know! For me, this is the 'fun' part of our hobby!
 

xseler

Well-Known Member
Apr 14, 2013
2,883
130
63
OKC, OK
Hello. What would make my engine bog down when I turn the throttle ,and then rev real high when I let off the throttle? I didnt know how to post questions on here, so I jumped in on your conversation. Sorry.

You've got an air leak somewhere..........check that the carb is secured on the manifold. This is the most common culprit for this symptom. Good luck in finding your issue!