the latest carb technology from BoyGoFast

[ThugBike]

Hey Steve, have you seen a "value pack" of jets for sale anywhere? All I've found so far are individual jets at Sike Bike Parts for $4.59 a pop. Since I don't even know my starting point, it would be nice to get a good deal on many sizes.

I forgot to mention, my 49cc carb bore measured 13mm, FWIW. It sounds like 14mm is the typical size for a 66cc. I'm not sure if this difference is how they get away with using the same size jets. Or, maybe you just never know what you'll get when you order from BGF. :laugh3:

 

[AussieSteve]

Nope, no 'value packs' that I know of. Everyone charges about $5 each.

You have the newer carby, I think. Interesting that it's 13mm. Previously, both 48cc and 66cc engines had the same carb with, as you say, a 14mm bore.

 

[hayata]

that carb is ol school i have the same carb on my 49cc mini bike from 4 yrs ago..

 

[ThugBike]

that carb is ol school i have the same carb on my 49cc mini bike from 4 yrs ago..

So the "latest carb technology from BoyGoFast" is at least 4 years old. Good to know. The thread name was somewhat sarcastic, and sort of came from the fact that every picture of a NT carb I had seen had an intake tube and mine didn't. It seems that few people around here have been toying with the 48cc like myself.

So, back to the topic of main jets. I got some 5mm Dellorto main jets from dellortodirect.com, and was foolish not to measure my jet before hand. My 48cc BGF's HUAZHONG labeled carb with 13mm bore and NT labeled air filter housing evidently has a 4mm main jet. The 4mm jets are even harder to find out there so it seems the solder-and-drill method will be put into use very soon.

I have another kit from BGF purchased earlier this year, which is a 66cc that I haven't installed yet because I need a mountain bike for a container. Just for fun, I pulled apart the carb in that kit (which looks like all the pictures in the NT carb tuning thread and has an intake tube). The BGF 66cc carb does appear to have a 5mm jet, but the Dellorto 5mm jet would only screw in 1 and 1/4 turns before stopping. So there's a difference in pitch or something! I guess I will be selling some Dellorto 5mm jets on eBay soon.

So, Steve, when you got the range of jets, were they Dellorto 5mm? And did they fit properly? Or if anyone else with aftermarket jet experience could chime in, I'd appreciate it.

I think the biggest lesson I've learned is there's so much gosh darned variance in the sizing of these carbs and components, it's hardly worth the trouble of trying to find and buy proper jets. Maybe it's a BGF thing, maybe it's a HT thing, who knows.

 

[AussieSteve]

I've bought 5mm Dellorto jets from OzMiniMoto, Rock Solid Engines and ZBox.
Sick bike Parts also sell them.

My carbs, (2 of them, intended for 66cc engines), are both standard NT, labelled 'NTTC' with a 14mm bore, and the 5mm jets fit perfectly.

Haven't heard of 4mm jets before.

 

[RdKryton]

I was just wondering if you have might have a fuel boiling issue as well? Having the carb that close to the cylinder has to be cooking the heck out of the fuel. This causes all sorts of problems too when tying to sort out the correct jet size. Whizzer had this issue and they developed a spacer to move the carb away from the cylinder and guess what, it worked. It is one of the few problems they actually corrected although the engineering was done very poorly. The idea though was to move the carb away from the intense heat and this made selecting the correct jet size much easier.

Good Luck
Jim

 

[ThugBike]

Jim, I may have boiling fuel issues indeed. With every tiny drill-out (enlargement) of the solder hole, I get pretty inconsistent results. Reading a new plug, it appears that I'm running leaner than with the smaller hole I started with. Then, on my last drill-out, boom, I'm running pig rich now. I'll have to look into the spacer in hopes of bringing some sanity into the equation. Thanks for the info on that.

BTW, about 4mm jets, I found one (and only one) supplier. It is definitely a 4mm jet in this carb, but the jet I bought is much shorter in length and probably wouldn't sit low enough in the bowl to take a drink. So solder-and-drill is my only option. And Mr. BoyGoFast is aware that he's losing out on replacement 4mm jet sales.

 

[GearNut]

ThugBike,
The needle jet (brass tube between the carburetor body and main jet that the slide needle goes down into) is long enough for you to use a physically shorter main jet if that is what you are talking about.

 

[ThugBike]

Yeah, that's what I'm talking about, although that aftermarket jet is many millimeters shorter, so much shorter that it would be skimming the very top of the fuel surface, and that's with the float all the way up (closing the needle valve). The other bummer is the jet sizes are all 0.05 apart, so tunability is very coarse. So I came to the conclusion that I was better off with the solder-and-drill method.

Then I found that method very frustrating, where at one point I'd be running lean and hot, then just barely file out the hole (with the exact same size micro bit) a tiny tiny bit bigger, then boom, I'm suddenly running pig rich. Grrr.

This touchiness makes me think Jim's right about a fuel boiling issue.

So I'm planning on getting an intake tube as well a more standard carb that I can actually get a good variety of jet sizes for.

The BGF 48cc engine has held up so far (over 150 miles), other than I finally stripped the spark plug threads and had to order a new head. Too much plug chopping and maybe too much elbow grease when putting the plug back in.

Thanks again for the ideas, and I'll be back later with updates.

 

[AussieSteve]

The hot/boiling fuel issue has been on my mind for a long time, especially since fitting the RSE hi-comp billet head. Even with a stock inlet on a 66cc engine, I've noticed that the carb is much hotter than it should be, and more than any of my previous 2-stroke bikes.
In the case of 48cc engines without an inlet tube, this MUST be a real problem.
A rubberised inlet would help, like on Jap dirt bikes.
I haven't ridden mine for a while, but next time it's running I'll measure the carb temp out of interest and compare it to the boiling point of fuel, whatever that is. (A Google search is in order.)