Experimenting With Carburetors

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cannonball2

cannonball2

Well-Known Member
Oct 28, 2010
3,682
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Colonial Coast USA.
OK guess Im doing an Experimenting series kinda like the Dummies books thing.

So now its carbs. I have tried tuning the stock NT to some success and the RT ditto. I could never get the NT to run cleanly over the entire range. I believe the issue at least with mine is the needle profile. Jetting affected the running as expected as did raising or lowering the needle. Ran ok but never super. I got the RT to run pretty cleanly but it was always sort of mushy down low and was VERY cold natured taking a long while to warm up and run well. The lack of a venturi I believe contributes to this. Carb has to get pretty warm for the fuel to atomize well.
So now on to another carb. If you are expecting a hi-pro carb you will be disappointed. This is a carb with excellent running, easy starting, and superb idle. Right out of the box! It has a 12mm venturi so its smaller than most on MBs. This is what makes for the excellent throttle response I was looking for. Runs cleanly from idle(even extended) til it runs out of breathing around 28mph. That's only 2mph slower than the bike went with the RT. A larger main might regain that. It has and adjustable idle mixture which allows excellent low end acceleration due to the fact it can be adjusted a tad on the rich side. Idle is spooky low. Sounds like a heavy flywheel enduro bike. So what is it?

This: http://www.ebay.com/itm/Carburetor-...Parts_Accessories&hash=item58bf227ca3&vxp=mtr

I haven't seen a lot of info on these here, maybe I searched wrong, But if you are a rider like me that's interested in a good running stock CG, this is my best carb so far. Wont be much fun for you guys trying to squeeze 10K out of a CG.

Ignore the hastily thrown together look. It was just to try it out. Will add an enrichener cable and a proper cap for the vacuum take out nipple as well as an air filter. Didn't want to spend a lot of time if it was just going to wind up in the junk box.

Note: the opening for the manifold is 20mm. The sleeve from an RT fits it perfectly and the carb comes with a very nice rubber seal already installed.
 

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2door

2door

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Sep 15, 2008
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CB,
You mentioned "needle profile". Some years ago another member who doesn't post here anymore, experimented with altering the taper of the needle on the NT carb with varying results. His old threads are buried somewhere in the archives. If I can find them I'll post a link.
Seems to me I remember he had good luck with improving low and midrange performance but he also ruined a few needles with his experiments. I do recall him saying that removing material was critical and easy to over do it.

Tom
 
maniac57

maniac57

Old, Fat, and still faster than you
Oct 8, 2011
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Only issue I can see is that weird dual cable setup used on the PW for safe starting. If I remember right, you flip a lever to hold a set throttle opening for starting, and the bike won't rev until the lever is returned. Also won't start unless lever is engaged.
Does your carb still have this strange setup?
Edit. looked closer and see the second cable is gone. Apparently this defeats the weird stuff.
I have ALWAYS known a small carb improves throttle response. I used to drive all my buddies CRAZY swapping out the quadrajet four barrels on my street stock chevy v8's for a tiny two barrel with adapter plate and STILL outrunning them on the track. They could never understand that less overall power, but more IN THE RIGHT PLACE was faster than huge top end power.
I'd motor right on by coming out of the slick turns while they sat and spun the tires.
 
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cannonball2

cannonball2

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Oct 28, 2010
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2door I think in the past I have read some of the posts you are referring to. Seems he soldered and re-profiled or something. That's more dedication than I have.

M57, I just took a thumb shifter and cable and used it for a choke lever, its mounted on the left side of the handle bar. Easy to do and works well. You can also just use a short piece of cable and make a loop in it and leave it hanging at the guide tube on the carb.
I spent the afternoon riding and am so pleased with the running I will be ordering another for the shifter bike. $17 well invested!
 
bluegoatwoods

bluegoatwoods

Active Member
Jul 29, 2012
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You've just about got me convinced, cannonball.

Largely because I crave smooth running power at the low end. I never wind my engine out and I don't care how it runs in that range. I just putt-putt.

And if I'm understanding you right, then that's exactly where this carburetor is doing well. Have I got that right?

I've never measured a china girl intake. But memory makes it look like a bit less than 20mm. Is that correct? Does that mean that some sort of adapter is needed?

Wait a minute.....I see that your pics zoom quite nicely. Thanks for that, by the way.

And it looks to me as though the clamp is the carb's own intake clamp. Is that right?

I see that your intake also has a small 'branch' running down. That wouldn't seem to have anything to do with this carb. But I'm still curious: what's that all about?
 
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cannonball2

cannonball2

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Oct 28, 2010
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Seems as such so far SB. Carbs are so cheap these days and there are so many choices its not the issue it used to be trying different ones, like when they might approach $100 a pop.

BGW, it attaches just like the stock NT carb. It has a 20mm opening compared to the 19mm CG intake. I just used a 19mm RT carb sleeve. It fit perfectly into the 20mm opening on the new carb adapting it to the manifold. The nipple on the manifold is part of another ongoing experiment unrelated to the carb.
I have my engine pretty well sorted out all around but as I stated earlier was less than happy with the carb situation. This little carb has run the best right out of the box. It is as said not a performance carb(though it performs well-lol) but if your riding is done on the south side of 30 its a good choice. I felt pretty good this clone was pirated from a Yamaha design, seems they did a good job.

In the interest of accuracy this engine has had the timing retarded which also improved lowend/midrange. Even with this however both the NT and the RT carbs were still found somewhat lacking. This one not so.

It will need an RT carb cable and the RT sleeve to install. Dax has both. You can mod the stock cable if you know how to make cables. And it will need an air filter with a 1" clamp on opening. These are available in the aftermarket.
For $17 shipped go for it!
 
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Venice Motor Bikes

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
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Here's the thing with installing ANY carb on ANY engine.... You're probably going to have to re-jet it for the best air/fuel mixture.

With the stock NT carb, it's almost guaranteed that you'll have to re-jet it for optimum performance... Just adjusting the c-clip wont get it close enough.

It's most likely that this new carb you installed simply had a jetting that better matched the engines needs.... (something a $2 jet could've fixed).
 
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Venice Motor Bikes

Venice Motor Bikes

Custom Builder / Dealer/Los Angeles
Mar 20, 2008
7,272
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Los Angeles, CA.
Also... For the guys claiming that a smaller carb is sometimes faster; IT'S VERY TRUE!!

Many of the 2-stroke racers are running 24 & 26mm carbs, & I just stepped down to a 22mm Mikuni & my bike's never been faster!! (^)



image-1539688167.jpg
 
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cannonball2

cannonball2

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Oct 28, 2010
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Colonial Coast USA.
Venice Motor Bikes said:
Here's the thing with installing ANY carb on ANY engine.... You're probably going to have to re-jet it for the best air/fuel mixture.

With the stock NT carb, it's almost guaranteed that you'll have to re-jet it for optimum performance... Just adjusting the c-clip wont get it close enough.

It's most likely that this new carb you installed simply had a jetting that better matched the engines needs.... (something a $2 jet could've fixed).
Click to expand...
I have gone through both the NT and the RT pretty carefully to the point of drilling my own jets. The NT is about as good as I can make it. Its really between three jets and none is totally satisfactory. A .6 is too lean, .63 is ok, ..65 way rich. The .63 runs the best but still has ranges of richness(4stroking). Has to be the needle. I have tried another needle 5 slots vs. 4 with little success. Enough of that.
The RT runs pretty well except on the very bottom end off idle. If I jet it lean enough for good bottom the top suffers, rich for the top the bottom suffers. It ran the best while the check valve was installed on the manifold. I suppose I could solder and redrill the low/midrange metering hole. but the carb has other characteristics I don't care for so the search began for another carb.
I expected better throttle response with this carb being smaller, but was really pleased that it carburates cleanly over the entire range. This was one of the few carbs I have seen for sale that listed the specs on it. It indicated a .67 main jet. Of course its not marked as most factory jets. And I haven't verified that. Might be even better with a slightly richer jet for top end. As it is its fine.

What I really like is it starts easily, warms quickly, and idles and runs well. That's what I was looking for.
 
F

frank66

New Member
Jan 15, 2015
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canada
cannonball2 said:
I have gone through both the NT and the RT pretty carefully to the point of drilling my own jets. The NT is about as good as I can make it. Its really between three jets and none is totally satisfactory. A .6 is too lean, .63 is ok, ..65 way rich. The .63 runs the best but still has ranges of richness(4stroking). Has to be the needle. I have tried another needle 5 slots vs. 4 with little success. Enough of that.
The RT runs pretty well except on the very bottom end off idle. If I jet it lean enough for good bottom the top suffers, rich for the top the bottom suffers. It ran the best while the check valve was installed on the manifold. I suppose I could solder and redrill the low/midrange metering hole. but the carb has other characteristics I don't care for so the search began for another carb.
I expected better throttle response with this carb being smaller, but was really pleased that it carburates cleanly over the entire range. This was one of the few carbs I have seen for sale that listed the specs on it. It indicated a .67 main jet. Of course its not marked as most factory jets. And I haven't verified that. Might be even better with a slightly richer jet for top end. As it is its fine.

What I really like is it starts easily, warms quickly, and idles and runs well. That's what I was looking for.
Click to expand...
exactly what i have seen with all my nt carbs on various engines including my tanaka 3hp.
i had a the nt running perfect once but the replacement needles are expensive holy.
on that note im looking for a straight answer. the stock carb could be great. its so close....

------
cannonball how does that 12mm carb mount again? thanks
 
S

Slogger

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Sep 8, 2014
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Racers size carbs to get all possible air into the engine at top RPM. The fact is 99% of your riding or driving is done at half that speed. You take the displacement times half the redline rpm, then you will know how many CFM (cubic feet per minute) your racer needs at speed. (in 4 strokes)

The smaller venturi maintains high incoming air velocity at moderate RPM so gas is pulled from the jets better, atomized better and crammed into the cylinder better.
The only time the big carb wins is at redline.
Everywhere else it causes low velocity, poor atomization, bogging and stumbling around.
Racers are for the race track.
 
cannonball2

cannonball2

Well-Known Member
Oct 28, 2010
3,682
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Colonial Coast USA.
Well said Slogger! I have seen so many people install carbs that are too big. That was especially apparent with folks "upgrading" their carbs on the gas R/C airplanes. They would install a carb with too big a bore then wonder why it didn't rev cleanly and would cough and die in an emergency throttle up.

F66, the carb mounts just like an NT or the RT, a clamp on. Being as its a 20mm opening at the intake it will require sleeving in some way to make up the extra 1mm difference. A sleeve for the RT carb works perfectly. Also any thing you could make would do as long as its air tight, like maybe wrapping up some shim stock and gluing it to the manifold with gasket sealer. I glued the RT sleeve to the manifold with Yamabond. The carb itself has a very nice rubber seal installed to seal against the manifold.
 
cannonball2

cannonball2

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Oct 28, 2010
3,682
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Colonial Coast USA.
The Walbro is a good choice if it has been properly selected. Seems like most of the ones I have seen are for high pro applications, with little to none for stockers. I had hands on Walbros , Tilliys, Zamas, etc. daily in the small engine business. They are what I would call a simple/complex carb. They generally elude the skills of the public, don't tolerate old/contaminated fuel as well as some carbs, and require s GOOD understanding of setting the needles or you may wind up with a melted piston on the exhaust side in short order. I would consider these advanced carbs to experiment with.
All that said the ability to change the high and low ranges to suit the engine/mods/conditions is second to none for a small engine. The pump is nice for low tank locations, though the throttle interface can be fussy. Also you can drill and tap the manifold with a small nipple for pulse, or just get the model carb drilled to take the pulse at the carbs flange. I would have no problem paying $100 for a good specified carb with the accompanying manifold, filter and cable and a GOOD fuel filter. Buy a few diaphragm kits and one is set.
 
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frank66

New Member
Jan 15, 2015
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canada
cannonball the airscrew on your carb is great right.

ffv8 the walbro BUTTERFLY carbs are really frustrating with cable spring mounts/ hangers and fasteners. notorious infact.
wait till walbro wises up and offers a factory cable friendly assembly.


the walbro BARREL carbs are ok and do not have the cable problem because the cable mount is excelent.

----- again i love the walbro BUTTERFLY. but trying to add a good cable mount is as EASY and TROUBLE FREE as it is putting an engine on a bike :)
 
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F

FFV8

New Member
Oct 29, 2013
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Spring Valley NV
frank66 said:
cannonball the airscrew on your carb is great right.

ffv8 the walbro BUTTERFLY carbs are really frustrating with cable spring mounts/ hangers and fasteners. notorious infact.
wait till walbro wises up and offers a factory cable friendly assembly.


the walbro BARREL carbs are ok and do not have the cable problem because the cable mount is excelent.

----- again i love the walbro BUTTERFLY. but trying to add a good cable mount is as EASY and TROUBLE FREE as it is putting an engine on a bike :)
Click to expand...
You might want to look at the link I posted.
 
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