Grubee Skyhawk GT2b carb question

Kickstart70 said:

Heya all,

I've just completing my first assembly of a bike motor (it's been complex...mainly in deciphering the instructions!) and it's basically all together except for one thing...

On the carb, facing toward the rear and coming out of the bottom of the carb, is a clear plastic line about 4 inches long. I can't figure out what it's for. The air intake is obvious, the fuel goes in a nipple at the top of the carb.

Someone give me a hint? Thanks!

Click to expand...

I was thinking that's the float bowl drain.

I am having the same issue with the carb on mine. I just can't seem to get it running right. I was running rough at start, then a few days later it was running perfect. Now it is back to running crappy at WOT and bogging down.

I was just out playing with it and it seems like it might be starving for gas at WOT. I pulled the choke a little and it seems to straighten it out. But then letting off the throttle, it craps out. Any ideas?

Here is a pic of my install.

Kickstart : Your cable is not tight enough, your pucks could be stuck to the plate, or your plate height needs adjusted.

Throttle adjustment is just cable tension - what else do you expect?

The two screws adjust the pilot jet's air/fuel mix and idle. You will only see any major change at idle.


MrPowers : The major problem with the CNSv2/GenB (red air filter) carb is the seal at the intake. There is an inherent air leak around the outside of the seal due to the junk white plastic shim/spacer. Seal that first, then tune your idle, then set your throttle needle.

MRPowers said:

I am having the same issue with the carb on mine. I just can't seem to get it running right. I was running rough at start, then a few days later it was running perfect. Now it is back to running crappy at WOT and bogging down.

I was just out playing with it and it seems like it might be starving for gas at WOT. I pulled the choke a little and it seems to straighten it out. But then letting off the throttle, it craps out. Any ideas?

Here is a pic of my install.

Click to expand...

If you haven't gotten your carb problems worked out, take a look at the thread I just posted about what I did to get mine working. While tuning I had several sets of problems, mid way through tuning I was where you are at now, bogging at WOT and starving for fuel, choke helps get her going a bit. I detail what I had to do with my float, jets, and needle to get rid of the bog. From what you describe I think you can skip past the float and check the jet size and make sure it is unobstructed to fix your bog at WOT and then adjust the needle if you still bog mid throttle. Read the entire post for more details...

http://motorbicycling.com/f4/cns-carb-working-great-32736.html
Happy motoring!

DuctTapedGoat said:

Kickstart : Your cable is not tight enough, your pucks could be stuck to the plate, or your plate height needs adjusted.

Throttle adjustment is just cable tension - what else do you expect?

The two screws adjust the pilot jet's air/fuel mix and idle. You will only see any major change at idle.


MrPowers : The major problem with the CNSv2/GenB (red air filter) carb is the seal at the intake. There is an inherent air leak around the outside of the seal due to the junk white plastic shim/spacer. Seal that first, then tune your idle, then set your throttle needle.

Click to expand...

The plastic spacer is not the seal, it simply holds the carb in place. The black rubber thing inside the carb at the end of the neck is the seal. When you put the carb on the intake slide it back as parallel with the tube as possible, then when it gets to the end kinda move it around a little while pressing to make sure the intake seats on that black seal that last little bit. I always press the carb firmly onto the intake while I tighten the screw to make sure it gets a tight seal.

Kickstart70 said:

Heya all,

I've just completing my first assembly of a bike motor (it's been complex...mainly in deciphering the instructions!) and it's basically all together except for one thing...

On the carb, facing toward the rear and coming out of the bottom of the carb, is a clear plastic line about 4 inches long. I can't figure out what it's for. The air intake is obvious, the fuel goes in a nipple at the top of the carb.

Someone give me a hint? Thanks!

Click to expand...

The clear plastic line that comes from the bottom of the carb is the vent/overflow tube. It connects on the bottom but connects to nothing at the top, just goes upward under the plastic air filter retainer.

Kickstart70 said:

Followup...I was so freaking nervous about my first attempt to get it started, but it fired right up on the first go!

I definitely need a larger sprocket on the back to handle hills better, and I had to tweak a few things after a couple minutes (note: the bolts on the idler/tightener are absolute garbage and need to be replaced). The choke is really, really tight. I pretty much can't get it pulled right in to lock it on with one hand. Related to that, I still haven't managed to pull the choke and have the engine idle while standing still. It stalls immediately.

To me it still doesn't feel that it runs smooth enough, but I blame the fact that it's pushing my 240 lbs and a heavy mountain bike frame.

Click to expand...

As for your choke (or any other cable) being hard to pull. Make sure your bends in the cable are large and sweeping rather that short bends. Also, make sure you put some light weight oil down your cable housing to lube the cable, make a huge difference. Also, these cables that come with the kit are not lined, if you can't get a smooth pull by adjusting the bends and lubing the cable try getting a new cable that has the insert sheeth inside it, they pull much easier.

From how you describe your motor running it sounds just like what I had at first run. My float needed adjustment, after I did that I was able to get a nice idle. Do this little test, take a ride and turn off the fuel valve while riding. If after a short period the bike starts to run much better (and even start to idle better) then dies (cause it ran out of gas) you probably need to adjust your float. Take a look at the thread I just posted about all I did to get my CNS carb running great, I linked it a few posts up in this thread.
Good luck with your bike, if you need help feel free to post and I will try to help you out.
BTW I weigh over 200Lbs, not quite 240 but pretty damn close. My bike will go over 35MPH on flat land and can hit 25MPH going up a steep hill pulling my big @$$......

I have the same carb. It came with the kit which is my first attempt at this. I am using the SBP jack shaft kit which is nice. However I can't get this silly carb tuned. I've read through all the posts that I could find on it and am tempted to purchase the NT carb. I hate to spend another $25 on this though. The main problem is that the engine bogs down if I use the throttle at all. I can get it to idle fairly well though. I took the carb apart and made sure that nothing was plugged. The little "E" clip was on the second to the top position. I tried the top position with no change so then I went to the third. This runs the best so far. The engine doesn't completely die when I use the throttle now, but it does still bog down. I have the last bit of RTV sealant drying around the area where the carb mounts to the intake. I'm pretty sure that the carb is ALL the way on. Should I try to put the "E" clip in the fourth position? What would cause the engine to bog down when the throttle is used, too much gas or not enough?

Here is the kit I have. You can see all the details of what I have done so far if you want to by reading my posts on the jeep forum that I usually hang out at.

Bike Motor (moped) Build

Well It is running much better now. It still idles too fast with the clutch pulled in, but other than that it is pretty good and the throttle is responsive. I figure I would just run it this way while I use the first tank of gas and just shut it off when I come to a stop. I ended up having to turn the idle screw all the way in, with the "E" clip in the third notch from the top. Once the break in gas is gone I will try to tune it a bit more.

The only problem I seem to have with CNS carb. now is the plug carbon fouling after several days. Considering my riding habits...normally low to mid-range throttle...I'm wondering if the plug isn't reaching its self-cleaning temp. I'm running NGK BP6HS at .30 and I'm wondering if a colder plug is in order....any suggestions ?