S&S Super B Mid Range Flat Spot Shovelhead

[HogsRear]

I recently picked up a used Super B and wanted to try it out on my bike. It's a kick only stock displacement 74" shovel chopper with points ignition. Timing is good, and seemingly no intake leaks (checked with an unlit propane torch). I had a functioning Super E on the bike but I kinda like how the B's look and how simple they are in and out. Plenty of people hate this carb, but I've seen just as many people stick up for it so I wanted to see if I could make it work. I took it apart, used chem dipped it to get rid of the gunked up old gas, and blew out the passageways with carb cleaner and compressed air. It cleaned up really nice, and I replaced the throttle shaft, butterfly valve, and spring with a S&S kit.

I'm having the same issue a lot of folks have – relatively hard starts, followed by falling on its face in the mid range then picking back up again closer to 3/4 throttle and up. After adjusting the float height to spec, I have tried probably 5 different jet combinations so far using the S&S baseline of .0295 intermediate and .068 main and working around it. I've read that this flat spot is caused by the transition of the fuel through the intermediate to the main when they are both being activated at the same time causing an overly rich condition which then ideally leans back out as the main jet fully engages. Is this accurate? To try and nail down the issue I tried leaning out both intermediate and main, but the problem got worse if anything. Then I tried the opposite beefing up both jets to the higher side of the recommended range, but the stumble was still very noticeable when riding. I've also read about drilling the air bleed in the side of the carb to delay the onset of the main jet, but I'm not sure I can do that on my own. There has to be a way to tune around it, just curious if anyone running a B on a similar motor can chime in. Thanks!

[JBinNC]

Making the main air bleed larger is the way to get rid of the flat spot, if it's flat rich blubbering in the transition. It's really a pipe problem, and you just have to tune it as best you can for the pipes. Changing from a straight pipe to a pipe set with mufflers will help usually. If you want to see how good it can be, use a set of drag pipes 30 - 32 inches long. (Loud.)

Drilling out the main air bleed is relatively easy since it is readily accessible. The standard size is .042, and you usually need to go up to maybe .060 for the usual drag pipe/Super B combo. You need a selection of small drill bits, the number set from 1 to 60 will include the sizes you need. And you need a drill holder so you can drill the hole by hand, no power drill here.

Jim

[JBinNC]

You did not mention throttle shaft bushings in your list of replaced parts. Most Super Bs have throttle shaft holes that are worn out. You can rarely get by with just replacing the shaft. That will lead to hard starting and an off-idle stumble.

Jim

[Dragstews]

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[HogsRear]

You did not mention throttle shaft bushings in your list of replaced parts. Most Super Bs have throttle shaft holes that are worn out. You can rarely get by with just replacing the shaft. That will lead to hard starting and an off-idle stumble.

Thanks Jim. I was surprised when I replaced the shaft there was virtually no play at all, in fact it went in a bit tight and then was able to spin smoothly once fully assembled. I would have a hard time believing the throttle shaft holes are part of the issue here.

I currently have drag pipes on the bike, not super long but not super short. No exhaust leaks at the heads. I'll look into drilling the air bleed. By drill holder do you mean a drill press? I haven't been able to find any other detailed instructions on how this is done online.

[HogsRear]

Thanks Drags. I haven't seen this Power Tube before, have you used one with relative success? They are making it sound like the greatest invention since sliced bread.

[JBinNC]

By drill holder I mean a little round pin vise that you can clamp the drill bit in and turn with your fingers. That is what you want to use when drilling out jets, and the air bleed too.

Jim

[Dragstews]

[QUOTE=HogsRear;846380]

Have you used one with relative success?

Maga .....

[HogsRear]

[QUOTE=Dragstews;846396]

Have you used one with relative success?

Maga .....

what?

[Dragstews]

Mega >>>> a world of em ...

Try one .... I'm sure you'll be more than pleased with the results .... !!

They left out one thing .. (Improves Fuel Mileage)
Expect about 20 or 30 more miles out of a tank of gas ...

[Dragstews]

Nothing shows on the outside of the main jet nozzle ... It's all internal ...



Bird's eye view .. ^^

[markwade74]

Not having any issues but I believe im gonna try one of these

[Davestune]

if you want to dump the flat spot put some lolly pops in the pipe, then if you still have flat spot after tuning carb play with air bleed

[Dragstews]

Air Bleed on a "Super-B" .....

** Mid-Range Tuning **

The Air Bleed is an air passage connected to the Main Jet emulsion tube that allow air to mix with fuel before it is pulled into the carb throat ..
You may have found now that by increasing the low speed jet size, the motor responds better off idle but now has a stutter or hesitation (flat spot) as the rpm rises to a level that recognizes the carb's main jet ..

All Super-B's are manufactured with a fixed size air bleed passage. By modifying the carb to accept jets will allow you to adjust the main jet air bleed size for more control over the main jet circuit signal timing and fuel volume ..

Takes drilling, tapping and spot facing to accomplish the install of the air bleed jet ..
The original air bleed size is .042"; suggested range is .054-.072" .. Use S&S main jets for this application ...



Bore with a 1/4" drill until you reach the cross drill in carb body. (DO NOT GO PAST CROSS DRILL !!)
Spot face the area with a 1/2" end mill for mating surface of the jet.
Use 5/16-24 starter tap in hole and then remove to use a 5/16" bottom tap to finish the job ...

[HogsRear]

Alright I ordered a set of smaller drill bits and will try opening up the main air bleed to .060 – also planning on trying the lollipop in the pipes trick. I'll check back in once I get it together. Thanks!