Topic: Weber carb set-up

[Bill/BentON Racing]

And  good coils, Dyna fine,G'luck, Bill

[Greggo]

And  good coils, Dyna fine,G'luck, Bill

3-ohm Accel coils!  Got them, and the Dyna from you I think Bill

[Bill/BentON Racing]

That'll work,  K, Bill

[Greggo]

Can anyone offer me a little education on the emulsion tubes?  What do the different letters and numbers mean, and what ones should I be trying to source for a mostly stock motor?

[BPellerine]

greggo,the diff numbers refer to the number of holes and the placement ,high or lower on the tube,I have f11 emusion tubes stock for now engine,rc webers came with f16 ,I do not have exp with trying other sizes as mine work ok.bill you should look up some stuff on the net or get a manual to get you started,helped me.

[Tintop]

Like all numbering with Webers, the emulsion tubes are not done sequentially. When they made one for a new application it got the next number.   The most useful ones tend to be the F9, F11, F16.  The position of the holes (high / low) & size, increase or decrease richness at different rpm ranges eg slow running / high accel or WOT. 

[scottly]

I also have F11s. You need the Weber secret decoder ring to know what the numbers mean.

[scottly]

I finally got the bike back up on the dyno today, and did a pull with no filters (did a base-line with the filters installed previously and found no major differences), then swapped to 32mm chokes, with no other changes. (I must work slower than Frank; it took me 35 minutes. ) I believe any gains were due to the leaner mixture, rather than increased flow, as the sag in the torque curve from 4000RPM to 6500RPM follows the A/F curve, more or less.
I'm going to swap the 3.5 aux for some 4.5s, just to see what happens. I don't remember what prompted my to install the 3.5s in '79, but I have a receipt that shows they cost $5.65 each then. 

[scottly]

I ran the comparison of the 3.5 vs 4.5 aux today. The Haynes manual states "By fitting a larger size auxiliary venturi, the main mixture will be delayed and vice versa." What I found was that not only did the larger size delay the main, it supplied more fuel from 4900 RPM up. Also interesting is that the fuel curve with the 32mm choke and 4.5 aux is very close to the 30mm and 3.5 from 5100 RPM up.

[scottly]

I could tell my accelerator pumps haven't been working properly (hard to start when cold, hesitation when opening the throttle while riding), so the first thing I did after pulling the carbs was a pump test. The #1 barrel merely produce a spritz of fuel, and the #4 was dry. I pulled the #4 pump jet, ran a tiny strand of copper wire through it's tiny orifice, and gave it another shot.

[BPellerine]

yes scottly,had the same problem with my pumps before,they don't mention the very tiny hole in the pump jet in the books,and my eyes don't see as well as they once did,but finally got it.billp

[754]

Not sure what mine have inside, but if you hold carbs at waist level, and open throttle, it will squirt 5 feet or more!

[scottly]

Strong springs, blank (no hole at all) spill valves, long stroke rods, and rapid opening of the throttle can shoot gas pretty far. One foot at ground level is still pretty impressive in my book; I've yet to see any carb that can match a Weber as far as the accelerator pump is concerned.

[BPellerine]

mine only squirt about a foot as well,little farther if you operate pump by hand.bill

[754]

 I think it's the key to the great response they give.
 EPA probably does not allow a stock carbon to expelled that much.
I suspect the 77/78 750. Carbs, squirt about the same total as one or two cylinders on the Webers...

[scottly]

Frank, the length I gave you for the MPD boots turned out to be on the long side, which would have left a sizable gap between the head spigots and the MPD manifold spigots. I cut a snug fitting arbor from some Delrin I had on hand, and ground down a utility knife blade to fit into a cut-off holder. The first try, which just shaved the end of the boot worked, but was a little ragged, and still too long. The next cut was much cleaner, cutting about .080 off the length. The target length should be about 1.040-1.050", depending on the casting irregularities of the MPD spigots. The new silcone boots still require some force to get onto the manifold spigots, but they sure are easy to get on the head spigots with a two-piece manifold.

[BPellerine]

vast improvement overall though ,mine is one piece and works well.bill

[scottly]

I did a few runs today after changing the 125 mains with 120 mains. The first was with the same 32mm choke and 4.5 aux as the last run, and the second with 3.5 aux. It is still rich in the middle RPM range, but much better. I need to get some 115 mains, and then I will have to address the tendency towards a lean condition at higher RPMs with the air jet.(I'm currently using 185, but I do have a set of 175 on hand.) I will re-test with the 30mm chokes before changing anything else first.

[BPellerine]

have you tried the bike on the road yet ,for any change in the response with the bigger chokes and aux?bill

[scottly]

Hey Bill. No, I haven't tried the bike on the road yet, it's been parked on the dyno for a couple of weeks now. From what I've seen, the larger 4.5 aux draws more fuel from the main than the 3.5 regardless of choke size, and this starts from about 5100 RPM and continues as RPMs increase. One of the reasons I have been comparing the 4.5 to 3.5 is that most of the Webers fitted to our bikes had 4.5s; you and I may be the only ones running 3.5s.

I probably. Tried at least 10-20 different combos.. In a day.

The Wierd part ( and I tell people this when they tell me their motor runs fine,  or they think their jetting is fine)..is that the motor felt fine to me before and after..even though I picked up about 3/10 in the 1/8 mile..(Thats about 1/2 second in the 1/4).
 Our track is about 4000 feet..and once jetted up there, it still felt ok in town..about 1200 feet.
 I have at times heard Webers are altitude sensitive, but 4000 feet difference does not seem to affect it to the point you should be changing anything...my thoughts.

Frank, I thought my carbs were jetted fine, but the dyno tells the truth. I wonder if anyone has ever really dyno-tuned Webers before on a CB 750??

[Swoop]

Hi guys,
I picked up a set for the Seely.
Starting to go through them....a bit of a strange set up. They came off an 812 Yosh race motor.
45F9
130/122
200
F15
35
He was running 122 mains one the outer cylinders and 130's on the inner cylinders?
He was also running #1 aux. on the 2 insides and #2 on the outside.
Running 30 chokes across the board.
So it came with 4 each 30 and 32 chokes and one isnt stamped, but measures 30.
4 each of #1 and #2 auxiliaries.

Any thoughts on that?

It came with an MPD that is in two pieces. He said that he did that because they were easier to fit.....maybe he just broke them and went with it...who knows?
Any thoughts on running a 2 piece manifold....as long as I secure them well at the entry end?

One more question....what are most of you running...filters (is there enough room?) open air horns or air horns with mesh or filter socks on them?

Okay one more question. I read on here about a ball to be careful with....I have found none but there are a couple of 1/2 inch metal pieces, rounded at the ends, that may be doing the same thing? Is that common?

Thanks for any help that you can give me.

Steve



Thanks

[754]

I never tore mine completely down, never needed to...yet.

The split manifold is far easier to install. Often the far end is supported with a rubber mount. On Mine with this the spit in the manifold would move around, affecting to idle speed. I never worried about it too much, but this may fix it. File a half round in both sides of the split to accept a #10 screw. Then use the screw with abthick washer either side of split, and tighten down to stop movement.
 Later versions were made in 2 pieces, with a provision to bolt them together once installed..

[Swoop]

Thanks Frank

[scottly]

Okay one more question. I read on here about a ball to be careful with....I have found none but there are a couple of 1/2 inch metal pieces, rounded at the ends, that may be doing the same thing? Is that common?

There are small steel balls under those brass pieces; the brass parts act as weights to prevent the accelerator pump from squirting with small or slow throttle movements. There is also a small plastic ball inside the spill valve, but it is locked in place.
Some aux have a 1,2,3,or 4, but that isn't the size. Look at Greggo's thread; I posted pics of the location of the size.
I run foam Uni filters over air horns. The air horns are required to hold the whole throat assembly in place.

[754]

Tintop has started me good info on some airfilters....and pierce manifolds carries the bugeye Lynx type.... They fit my stock frame with modern manifold... Had to notch the sidecovers a bit.