the latest carb technology from BoyGoFast

Discussion in '2-Stroke Engines' started by ThugBike, Jul 3, 2010.

  1. ThugBike

    ThugBike New Member

    I'm working with a recent BGF 49cc slant head kit, purchased around a month ago. So far, I've got it running, although quite rich, and I've got the needle set to the highest clip (leanest setting) as I am at around 5000 feet altitude. So the plug comes out black and wet, and black oil drips from the muffler, and there's also some black oil around the spark plug area on the head, and I almost always get smoke out of the exhaust. All signs of running rather rich, I believe. It will usually hold an idle although I had to turn the screw in quite a ways to keep it from stalling.

    Now, onto the carb stuff...I've attached some pictures to this post...the carb has the name "HUAZHONG" on it, and it has no intake tube, it just bolts right up to the engine. I don't know if this is the reason, but it is held on by some nuts (8mm I think) that are in a place so cramped that I can't even come close to fitting a combo wrench or socket around the nuts. (Check out the picture, they must use a paper thin socket at the factory, or something. And that slightly rounded off nut edge is courtesy of the factory as well.)

    So, question #1: How in the heck do I get these nuts off to remove and dissassemble the carb? Has anyone played with this model carb, and successfully removed it from the engine?

    There is some kind of "tube to nowhere" the comes out from where you might normally find a "float tickler", goes under the carb bowl (through a loop that appears to be designed to hold a tube), and then is pinched off at the end between the carb bowl and the bolt that the clutch cable goes through. (Please see the attached picture for my wordy description to make any sense.)

    Question #2: What is the function of this "tube to nowhere"?

    After running the engine for awhile, I close the petcock (attached to the carb) and the engine continues to run seemingly forever. So the petcock appears to be worthless. If I close the petcock at the tank, the engine will die within around 30 seconds. While the engine cools, I can hear some gurgling sounds in the carb, and after pulling the throttle cable, I was able to see fuel being slowly and randomly spit out of the jet. So usually I have gas dripping through the air filter out out onto the floor after shutting down. At there very least, there's always some gas visible when I pull the air filtre.

    Question #3: Is the petcock likely to blame for the excess gas spewing out, or could a faulty float needle set up also cause this?

    Since I'm running wet-plug-rich with the needle at the leanest setting, my gut tells me the float set up is not correct and I've got a small flood down there. But, since so far I've been unable to pull the carb, I'm not sure how to continue.

    Ooops, I ended up almost writing a book on this, thanks for listening if you made it this far. :sweatdrop:

    Attached Files:

  2. Scotchmo

    Scotchmo Member

    Since you do not have a manifold, you will have more gas spitting back through the carb than with one.

    To remove the carb - You need a very thin wall socket or try some needle nose pliers.

    You could pull the float bowel and check everything. When I got my engine, the main jet was falling out (and barely running and very rich). It might be worth checking. Check the float needle while your at it.
    Last edited: Jul 3, 2010
  3. GearNut

    GearNut Active Member

    You have to be patient and use an open end wrench to get those nuts off.
    When they are in a blind location like that, use a wrench that does not have lead in angles at the tip of the opening. A wrench where the flats of the wrench are actually flat all the way to the tip. I hope that makes sense.
    The hose is a float bowl vent. The hose is attached to the vent barb to reduce the chance of debris working it's way inside the carburetor and also to help prevent debris from clogging up the small hole in the barb.
    Last edited: Jul 3, 2010
  4. AussieSteve

    AussieSteve Active Member

    ThugBike, once you get it off, consider replacing those studs with allen-head screws, for next time.
    If you can't get the existing nuts undone, a well-centred 6mm drill bit would do the job.
  5. Al.Fisherman

    Al.Fisherman Active Member

    Those nuts can't be on there that tight. How I would get them off is to get an expensive 1/4" or 3/8" drive metric socket the proper size. Turn the outside circumference down with a bench grinder (make the wall thinner). Once done and removed Steve has the solution for instillation or reinstall the nuts and put the socket away for future use.
  6. Fulltimer

    Fulltimer Member

    The gurgling could be from sucking air somewhere. A bad seal. I had the same idea of needle noise pliers.

  7. ThugBike

    ThugBike New Member

    Thanks for the info.

    So it would seem that it was a bad idea for the hose to be pinched shut by that stud that the clutch cable goes through. Not much ventilating was going on... I wonder what effect that would have.
  8. ThugBike

    ThugBike New Member

    Thanks, this suggestion worked beautifully. In absence of a budget for the bench grinder, I pulled it off with a grinder drill bit and some artistic elbow grease. Talk about paper thin socket walls!

    So the stud size was M5 - 0.8, which was going to be difficult to find in the length I needed (around 50mm). Fortunately this size is pretty much identical to #10-32, so I got a small bag of those in 2 inch length and they fit just fine.
  9. ThugBike

    ThugBike New Member

    Once I had the carb all apart, I couldn't find anything wrong with the float or main jet setup. Maybe fiddling with it and putting it back together did the magical trick, because so far I no longer have gas dripping from the air filter.

    Inside the carb petcock there was a rubber "gasket" with 4 holes in it, that was a bit torn and chewed up, so that may be causing the petcock to act like a fuel line when it's supposed to be shut off. I'll be looking for a replacement "gasket", and BGF is the only obvious source to get one. Thank the Creator that the petcock at the gas tank works properly!

    I was hoping to find a size number stamped on the main jet, but there were no numbers at all. I'm still in the dark on the stock main jet size for a 49cc. The sticky on HT specs says either 0.70 or 0.79 for 66cc. Sike Bike Parts mentions a stock size of 0.70 but doesn't mention which engine, but surely they mean the 66cc. I searched a bit and can't find any numbers for the 49cc engine. Does anyone have this info? I assume the only way to figure out what I've got is to find a kit with a bunch of those tiny drill bits that are 0.02mm apart in size. The kits I've found on eBay are 0.05 mm apart... I guess that's a start.

    So, thanks everyone for the help. I can pull the carb with ease now, and there's no puddle of gas under the bike on the garage floor so far. Now it's time to get lean and mean with the proper size main jet.
  10. AussieSteve

    AussieSteve Active Member

    ThugBike, from what I've read, the 48cc and 66cc use the same carb with the same jetting.
    Best thing to do is get a known size and work from there. ie. Buy an 0.72-0.74 and see how it runs, then go up or down as necessary. If it's richer, you'll know the stock jet was 0.7mm and if it's leaner the jet was 0.79mm.
    Or as I did, buy a range of jets. I have almost every size from 0.72 to 0.79. This is the best option for tuning, especially if you're planning other mods. You can easily re-tune the carb as needed.
    Last edited: Jul 11, 2010
  11. ThugBike

    ThugBike New Member

    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:
  12. AussieSteve

    AussieSteve Active Member

    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.
  13. hayata

    hayata New Member

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

    ThugBike New Member

    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, 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. :dunce: 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! :veryangry: I guess I will be selling some Dellorto 5mm jets on eBay soon. :annoyed:

    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.
  15. AussieSteve

    AussieSteve Active Member

    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.
    Last edited: Aug 8, 2010
  16. RdKryton

    RdKryton Active Member

    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
    Last edited: Aug 12, 2010
  17. ThugBike

    ThugBike New Member

    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. :evilgrin:
  18. GearNut

    GearNut Active Member

    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.
  19. ThugBike

    ThugBike New Member

    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. :shout:

    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. :ack2:

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

    AussieSteve Active Member

    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.)