the latest carb technology from BoyGoFast

[ThugBike]

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.

 

[Scotchmo]

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.

 

[GearNut]

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.

 

[AussieSteve]

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.

 

[Al.Fisherman]

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.

 

[Fulltimer]

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

Terry

 

[ThugBike]

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.

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.

 

[ThugBike]

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).

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.

 

[ThugBike]

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.

 

[AussieSteve]

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.