1978 R100/7 Flat top Carburetor Rebuild

kmkahuna · Apr 5, 2011

Okay, gang, here we go with the complete disassembly and rebuild of two Flat top, Bing CV Carburetors off of VIN# 6147992. Carb numbers are 64/32/223 and 64/32/224 respectively.
http://s1142.photobucket.com/albums/n606/kmkahuna/BING%20Flat%20Top%20Carburetor%20Rebuild/

Before....

And after the initial disassembly...

The details are all on the website above... One unique aspect of the Flat Tops though, is that you have to warm up the nylon diaphragm holder...the Bing tech said just use a light bulb, and it worked like a charm!

30064 · Apr 5, 2011

Kurt,

Did you mark the butterfly position on the throttle shaft as they were originally assembled? The edges of the butterflies are specially chamfered so they close tightly upon the veturi walls. If not try to mark them with a light scribe or crows-foot in their original position. Keep us posted on how things turn out.

-Mike V.

kmkahuna · Apr 5, 2011

30064 said:
Kurt,

Did you mark the butterfly position on the throttle shaft as they were originally assembled? The edges of the butterflies are specially chamfered so they close tightly upon the veturi walls. If not try to mark them with a light scribe or crows-foot in their original position. Keep us posted on how things turn out.

-Mike V.

Yep, Mike, I marked them....now, we wait...
http://s1142.photobucket.com/albums/n606/kmkahuna/BING%20Flat%20Top%20Carburetor%20Rebuild/

kmkahuna · Apr 9, 2011

Kits are here.

Got the kit from BING today, and finished carb #223 tonight:

Boiled the nylon diaphragm collet and it popped on no problem:

Don't forget the silicone grease and tape on the threads when the O-rings go on... Notice the cool list that Bing gives you identifying each o-ring..

Main Jet, aux starter circuit and the throttle shaft in...

Still need to set up the floats correctly tomorrow, but got the left side done besides that..

Lots more pictures on my photobucket website... Nite all.

kmkahuna · Apr 9, 2011

Float adjustment and measurement

Searched the forum for the proper, safe way to adjust the tang on the floats, read through Snowbum's extensive, extensive.....Extensive, explanations, and cannot find something like "The best way to bend the tang is to....blah blah blah...."... Anyone out there that can tell me, after I put the carbs on today and check the flow when they are in the correct position, the proper way to bend the float tang so it's parallel to the carb body?

20774 · Apr 9, 2011

kmkahuna said:
"The best way to bend the tang is to....blah blah blah...."

I don't know that I've seen anything, but maybe..."carefully"! I would consider using say two needle nose pliers and grab the flat portion that's between the pivot pin and the float itself, at either end of the tab. Then use the pliers to make the adjustments on the tab by rotating your wrists in or out to bend the tab. I wouldn't push or pull on the float at all.

I noticed you went back with the white floats. Bing sells, for more $$, gray floats which are direct replacement for the white ones. The gray floats are alcohol proof.

dennisdarrow · Apr 9, 2011

For me, I like to get 22mm of fuel in the float bowl when it is held level and I measure to the bottom of the little ring in the center of the bowl.......Some folks go with 27mm and some as little as 18........

I use a small screwdriver to pry the small tang open or close........yes, a very small needle nose will work to close it up................

This parallel business is to confusing for this dyslexic old guy to figure out upside down as I bend over, so I find a measurement works best for me.

YOU are getting there......FANTASTIC.......God bless.......Dennis

30064 · Apr 9, 2011

Kurt,

I take a piece of clean fuel line and attach one end to the carb, the other in my mouth. While holding the carb upright, gently blow into the tube while carefully lifting the float. When the top of the float gets parellel with the carb base, air from your mouth should stop flowing from the fuel line. If not, adjust the tab accordingly.

Make sure the float moves easily and freely on the hinge pin, take up any excessive adjacent movement in the float at the hinge pin by CAREFULLY squeezing the float hinge with a pair of needle nose pliars decreasing the diameter of the hinge.

Might sound silly but this has worked perfectly for me and I don't have to make messy measurements or have fuel all over the shop with multiple carb bowl removals and reinstallations. This tip was given to me by an experienced old Airhead. Good previous tips on adjusting the tabs in the previous threads.

kmkahuna · Apr 9, 2011

Vroom!

Just filled up with fresh gas, and pulled the left bowl to check the float, and it's dead on level... Took the bike around the block and whoooom! Wow, what a difference squeaky clean carbs make! Now, I've got to balance them and we're in business. Thanks so much for the advice guys, I know just what to do if I do need to adjust that tab now.

My next ominous problem is a mechanical bucking in second gear....when under torque, the bike will buck right at 2800 rpm, and one or two more times as the rpms go up. It's only in second gear too... Note, I did NOT rebuild the tranny on this bike. I did fill and swish with kerosene and the magnetic plug only picked up small shavings, nothing big. Anyway, will drain the gear oil at 100 miles after this rebuild and see if anything else it in there.

20774 · Apr 9, 2011

The "float parallel to the body" setting might be a starting point, but Dennis referred to the proper way. You want a certain height of fuel in the bowl. Fill the bowl, turn off the petcock, and then drop the bowl. Snowbum says to measure the height of the gas right in the center of the bowl. For the 32 mm carburetors, use 24 mm fuel height.
For the 40 mm carburetors, use 28 mm fuel height.

radar41 · Apr 10, 2011

Well Done!!

Kurt,

Excellent photo's!!

Isn't it amazing how well the Airheads run when the Carbs are correct??

kmkahuna · Apr 10, 2011

Carb tuning...

Radar41 said:
Kurt,

Excellent photo's!!

Isn't it amazing how well the Airheads run when the Carbs are correct??

Thanks, Radar41; yes, it pulls very strong on the freeway now. I still need to synchronize them:Here's my 4$homemade tool: from Hugh Kenney's airhead post:
http://www.airheads.org/content/view/183/98/

Note: I had a lot more kerosene in there...all the way up to that half way mark on the yardstick..
One tube, a yardstick, and kerosene. Only problem is: the kerosene gets sucked up and into the carbs. And yes, I mean sucked up 3 feet! I figure it's because of the viscosity of the fluid, but I hesitate to use anything thicker: I don't want to gum up the hard work I just did, or have smoke billowing out into the neighborhood.. Any suggestions? I figure I can either borrow someone's synchronizer, or figure out a better fluid...maybe that guicci looking one that fourcats has on this thread??
http://forums.bmwmoa.org/showthread.php?t=51998

dennisdarrow · Apr 10, 2011

Great work Kurt........Totally rare for a guy to come on here and actually do the work you have done so well........

For me, I use ATF. That's automatic transmission fluid........The viscosity seems to work and not move much and I like the red color that makes it easier to read for these old eyes..............My tubing is perhaps 10 feet per leg so that is a very long way to suck; but truly this has never been a problem as it only moves an inch or so...........Anyway good work Bud.........God bless......Dennis

radar41 · Apr 11, 2011

$4 Carb sync tool

DennisDarrow said:
For me, I use ATF. That's automatic transmission fluid........The viscosity seems to work and not move much and I like the red color that makes it easier to read for these old eyes

Dennis

Kurt,

I too use ATF (red color), but mixed it about 10% with 90 W gear lube , slows the movement a bit.

kmkahuna · Apr 11, 2011

Two stroke motor oil!

Radar41 said:
Kurt,

I too use ATF (red color), but mixed it about 10% with 90 W gear lube , slows the movement a bit.

This is Hugh Kenny's source for the cheap manometer...
http://www.powerchutes.com/manometer.asp
He used two stroke motor oil, in the "remote" chance that it's ingested.... Let's rephrase that to "Likely" in my case...
Will try the 2 stroke oil this week..

kmkahuna · Apr 14, 2011

What causes small backfires?

I've re-adjusted the valves after 100 miles of initial break in after the rebuild. The left exhaust valve was at .013mm vice .02, but other than that, all other valves were still fine. After a warm up run, and synchronizing the carbs at idle (and leaving the idle mixture screws at the recommended setting: 1/2 turn), I still get some popping, during decelleration. I checked and made sure the floats were "level" with the bottom of the carb when fuel comes out of the new needles, but I did no further adjustment to them after that. (The floats are new).
So: what has changed since the rebuild?

New O-rings, main needle, main jet, float needle, clean innards, and floats that are more buoyant, yes?
I'll take a look at Snowbum's small novel concerning the subject, but if anyone has any ideas, let me know!

P.S. I also tried to upload an audio file of the backfires, but youtube doesn't seem to play mp4's that come from iphones...

20774 · Apr 14, 2011

Do you mean 0.2mm instead? It's 0.2mm or 0.008 inches.

Have you adjusted the floats the other recommended way? That is, the proper amount of fuel height in the bowls. The float-parallel-to-body is mostly just a starting position.

Are you talking "backfire"...through the carbs, or "frontfire"...out the exhaust. Popping in the exhaust probably means some unburned fuel is getting into it. Backfiring would probably mean a timing issue.

Recheck the valves if not 0.15mm/0.2mm.

kmkahuna · Apr 14, 2011

20774 said:
Do you mean 0.2mm instead? It's 0.2mm or 0.008 inches.

Have you adjusted the floats the other recommended way? That is, the proper amount of fuel height in the bowls. The float-parallel-to-body is mostly just a starting position.

Are you talking "backfire"...through the carbs, or "frontfire"...out the exhaust. Popping in the exhaust probably means some unburned fuel is getting into it. Backfiring would probably mean a timing issue.

Recheck the valves if not 0.15mm/0.2mm.

Yep, I meant .2mm, got the decimal wrong. It's definitely popping out of the exhaust, so how is the fuel getting "not" burned? Too rich, I take it. Guess I need to get out that Bing manual again and try and get my head wrapped around the CV theory!
Mike V's recent post of the dynamics involved might help this thread...

20774 · Apr 14, 2011

You could be running rich with the floats set too high. Suggest you measure the height in the float bowl per the numbers posted earlier.

Typically, setting up a carb is not really a difficult thing to do. If using the proper jets and settings plus going through a proper carb synch, things should be fine. Recheck the idle mixture screw.

Beyond that, it's timing and tight connectors between carb and head.

kmkahuna · Apr 14, 2011

20774 said:
You could be running rich with the floats set too high. Suggest you measure the height in the float bowl per the numbers posted earlier.

Typically, setting up a carb is not really a difficult thing to do. If using the proper jets and settings plus going through a proper carb synch, things should be fine. Recheck the idle mixture screw.

Beyond that, it's timing and tight connectors between carb and head.

Thanks Kurt, I'll do just that. I've pretty much eliminated everything but the float height.

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