SHENG WEY carb ID and tuning info

[1968Cub]

Just got a 1968 Honda 50 Cub running, with a China motor 90cc(PO installed) and a SHENG WEY carb.
It will start great on full Choke and idle but dies as soon as the choke is opened at all. I can ride it that way but it will die as soon as off choke and have to play with the choke a little to keep it running. Has good power but real hill will kill it.
To get it running after it sat for 2 years with the PO I:
- Drained and replaced all the fuel.
- Completely disassembled carb and cleaned every part and orifice with carb cleaner and air.
- Reset the 2 adjustment screws as they were.
- Lower screw to the outside was 1/2 turn open.
- Upper screw near middle of card was 4 1/2 turns open.

So I have build several cars from the ground-up including engines and trannys but realize I need guidance here.

Is there:
- I place/way to ID the carb? (pics attached).
- A place to find instructions on setting up the carb?
- Were do I measure to determine the carb size? I've seen discussions talking sizes. Mine is 30mm at main round opening, 20mm at the wide part of oval inner opening(top -to-bottom), and 15mm the narrow way on the oval opening.

Also what do I need to do to covert the headlight to 12v? There is no bulb in the glass now and there are 2 wires not attached behind it. I'll b checking for voltage at those wires while it's running and the switch is on.
Thanks.

[cheesy]

If the engine only runs on choke, you've got an air leak somewhere. Check the mounting flange of the carb and both flanges of the intake pipe flatness with a straight edge. Might as well check the intake side of the cylinder as well. Also check the intake pipe for any cracks. If all good there, I'd punt and spring for a new carb.

[dirtbkr188]

The important place to measure the carb is where it bolts up to the intake manifold, right at the flange, that's how the Chinese carbs are measured. From the smallness of the air intake end of it, I'd guess it's not going to be any more than a 16mm (if that)measurement. ShengWay also has a 20mm carb which is the same style as the one you have, I use them on a lot of pitbikes.

You might have to have the carb cleaned in an ultrasonic tank to clear all the minute passageways inside the carb itself.
The lower left screw toward the rear of the carb is the air screw, the other screw in the center is the idle adjust screw.

[1968Cub]

Originally Posted by dirtbkr188

The lower left screw toward the rear of the carb is the air screw, the other screw in the center is the idle adjust screw.

Thanks for the quick responses guys.
I'm going to pull the carb off again and measure it the correct way and check and maybe mill(file) the spacers. You may have noticed, the PO has 2 spacers between the carb and manifold to get it to clear the engine.
The lower left screw is actually on the front of the carb the way it's positioned on the cub.

[1968Cub]

Thanks for the fast advice. Just measured every thing, carb, spacers, manifold, head port(see pic). The lower left screw is actually at the front the way the carb mounts on the Cub. I ran as small a wire as needed to clean every passage but good tip on the ultrasonic.
Measurements are:
- Carb 20mm
- Spacer to manifold (2 of them to clear head) both 20mm.
- Manifold 26mm
- Spacer to head 20mm
- Head port 20mm
I thought I read some where that size changes/steps in the intake path are not good? Like the 20mm up to 26mm back down to 20mm like I have. Your thoughts?
If that is true, I suppose I could taper the manifold to head spacer to make it more like a venturi, using my head porting kit from when we built my son's '67 Mustang.

[1968Cub]

Update.
Checked for indications of air leaks like broken spacers or manifold, etc. all looked OK.
Filed the manifold to head spacer to ensure it was flat.
Also filed both ends of the manifold.
No O-ring on the manifold to head spacer so I made a gasket for each side of the spacer.
Cleaned the O-rings on both carb to manifold spacers and filed the non O-ring sides.

Still runs the same. Starts great on choke, barely off choke it starts to run on regular idle(very fast if I have it set a little higher) then dies once the choke is open much more than 1/8.

I'm fine with getting a new carb if needed, as long as it's an air leak and not the carb. Any other places to check for air leaks?

What carb would be best to replace this and bolt right in and where to buy?

[dirtbkr188]

A common test for air leaks around the carb/manifold area and the manifold/head area is to start the bike and while it's running, spray contact cleaner or brake cleaner in both of those areas. If there is an air leak, the RPMs will go up or down, and then return to idle. A light coating of wheel bearing grease on the o-rings and gaskets will help do away with air leaks and make a better seal.



A slightly taller manifold will do away with one of those two heatstops the PO has in place in front of the carb.

[franque]

It could also be that the pilot jet is plugged/way too small. Have you taken off the float bowl?

[dirtbkr188]

You can find a replacement carb right HERE


extra main jets HERE


extra pilot jets HERE

[dirtbkr188]

Here is the baseline setting that I use to tune the ShengWay 20mm carb...

The 20mm ShengWay carb comes with the #89 main and a #35 pilot jet, both of which you can buy the extra jets from TrailBikes or any TB sales rep. As a baseline setting, I use the #90 or #92 main jet, the #38 or #40 pilot jet, set the jet needle e-clip in the 3rd or 4th notch from the top, and the air screw at 1-1/2 to 1-3/4 turns OUT from seated. Make the necessary air screw adjustments IN to richen it a 1/4-turn at a time and see what the bike does. With some of the carbs, I've run it to within a 1/2-turn of being all the way IN to get rid of 98% of the off-idle hesitation. You might have to raise or lower the clip on the needle, depending upon your altitude, I'm at sea level.

[1968Cub]

Originally Posted by franque

It could also be that the pilot jet is plugged/way too small. Have you taken off the float bowl?

I had the whole thing apart and cleaned every orifice with carb cleaner, different size wires and compressed air. Then sprayed cleaner in the jets and saw it come out of all the tiny holes.

May take it all apart again for a 2nd cleaning and try the carb cleaner or propane test for air leaks.

[1968Cub]

Originally Posted by dirtbkr188

A slightly taller manifold will do away with one of those two heatstops the PO has in place in front of the carb.

Where would I find a taller manifold, Tbolt?

[dirtbkr188]

Yes, you should be able to find one HERE.

[1968Cub]

2nd cleaning of carb did the trick. Been making adjustments, needle height(now in nest to last slot towards pointy end) and air screw. Have it running pretty good, might need just a little more to the rich side. Got 70KPH out of it with both my wife and I on it on level ground and 80KPH with just me.

What are the best sources out there for info and parts on a 1968 Honda C102? I've been in touch with Beatrice Cycle(DrATV), but would also like to find a few more resources.

Any vintage Cub C102 folks out there. I've been fixing things the PO did wrong or left off like bolts! Still need to sort out things like why/how I can't get the top 1/2 of the chain guard on(not room to slip it between the rear sprocket and trailing arm(missing a spacer?).