delayed throttle response

[gossamerjohn]

Hi everyone.

I'm noticing that when I pop the throttle on my R60/6 (Bing 26mm carbs) that there is a half-second to full second delay before the engine responds with more revs. On my old bike with Keihin CV carbs this was something that some people encountered but I never had a problem with it. I hit the throttle and it was all right there instantaneously.

This bike had sat for about 10 years or so before I got it. Under a cover in a garage. So there was a LOT of sticky in it. I've gone through and thoroughly cleaned and reassembled the carbs. The plugs are a pretty nice shade of caf?® ol?®, maybe a BIT whiter than brown but not too far off. Could I be a little to lean still? I'm new to BMWs and Bing carbs and wonder if this is typical or if there is a tuning/adjustment tip for this. It doesn't feel mechanical as the throttle turns freely. I'm on the 2nd groove in the needles, everything is stock in terms of jets, etc..

If this is just par for the course for the BMW I can deal with it, but if not I'd like to clean it up.

After this if I can just figure out the infuriating odometer digits (all of them) (completely dissassembled and cleaned once already) I'll be in good shape.

Thanks

 

[NEALART]

The idle mix screws on the 26mm carbs are supposed to be turned out .5 turns from seated according to the Bing catalog.

Also, your throttle cables should only have 1mm of slack in them before they start to evenly pull on both carbs.

Also, be sure your choke cables are turning off the choke completely when you are at normal riding temp.

 

[20774]

Under what circumstances does this happen? From a stop or while riding along at 40 or 50 or 60 mph? If you're at speed, I'd think the problem is more in the area of needles/jets.

What are the carb numbers? 1/26/123-124? My Bing manual says that the needle clip position should be 3rd from the top. You might be running fairly lean in the 2nd position.

What are the rest of the settings? My Bing manual says:

idle mixture - 1 turn (the manual says 0.5 turns for the /5 series, carbs 111-112)
main jet - 140
idle jet - 35

Are your floats set right? If the gas level in the float bowl is too low, it will also create a lean condition.

 

[NEALART]

Sorry, you are correct - 1 turn out from seated for the idle mix screw.

 

[gossamerjohn]

Under what circumstances does this happen? From a stop or while riding along at 40 or 50 or 60 mph? If you're at speed, I'd think the problem is more in the area of needles/jets.

What are the carb numbers? 1/26/123-124? My Bing manual says that the needle clip position should be 3rd from the top. You might be running fairly lean in the 2nd position.

What are the rest of the settings? My Bing manual says:

idle mixture - 1 turn (the manual says 0.5 turns for the /5 series, carbs 111-112)
main jet - 140
idle jet - 35

Are your floats set right? If the gas level in the float bowl is too low, it will also create a lean condition.

Those are the numbers on my carbs.


It mostly occurs at a dead stop. I don't know how to check the float heights on these carbs. I suppose that's in the bing manual I don't have? On my Keihins I used a clear tube and threaded insert and measured from the float tops.

I'll try the third notch. I just left this as it was when I got it for now. But I'm open to tweaking.

 

[20774]

I don't know how to check the float heights on these carbs.

There's fuel height numbers that's been posted for the constant velocity Bings. After letting the float bowl fill up, you turn off the gas and quickly remove the float bowl. Then a ruler is used to measure the depth of fuel in the center of the bowl. On the CVs, it's supposed to be something like 22-24mm.

On the slide carbs, I don't know...maybe it's about same. The classic way (this is described in Clymers and Haynes manuals) of setting the floats is to make sure that the floats shut off the fuel flow when the body of the white foam float was parallel with the lower flange of the carb body. The foam floats have a line running along them which is part of the molding process. With the float bowl off, you use your finger and lift up on the float so that the needle is closed. Turn on the petcock. Slowly lower the float so that fuel flows. Then raise the float until flow stops. What is the relationship of the float line with the carb body? Adjust the small tang as necessary. Done carefully, this can all be done on the carb.

Barring anything else, I'd start there.

You may also need to consider a carb synchronization. As mentioned there needs to be a certain amount of slack in the cables. The amount of slack needs to be adjusted so that just off idle, the slides pick up at exactly the same time. You may be able to judge this for yourself by removing the air intake tubes and looking into the rear of the carb throttle. I believe someone described using "pencils" for this test. Get something you can stick in the rear of the carb...something that will move with the slightest movement of the carb. Then slowly open the throttle. The two "pencils" should show movement at the same time. If not, then the slack is not the same.

The best way really is to do this with the engine running. Have someone operate the throttle. Put your head behind the bike with your ears centered between the muffler. Have your assistant slowly open the throttle. Listen for the increase in RPM on either side. That will tell you which side is picking up first. Loosen the side that picks up first. If in the end you have more than 1mm or slack after the adjustments, you probably need to readjust to less slack and start again. If there's gobs of slack, that could be the reason for a delayed response.

 

[tkbaker4]

" I believe someone described using "pencils" for this test. Get something you can stick in the rear of the carb...something that will move with the slightest movement of the carb. Then slowly open the throttle. The two "pencils" should show movement at the same time. If not, then the slack is not the same."

I use 4" sections of zipties on my Triumph to do this. Works great with the "Hump" under the edge of the slide. I can then see both move and adjust the cables so they move in synch.

DO NOT do this with the engine running. They may get sucked into the cylinder and knock a pushrod out of alignment. DAMHIKT

 

[James.A]

Many of the mechanical Bings have an accelerator pump inside the main jet stack. I have just gone thru the book on type 53 Bings and there is a spring loaded pump piston with a reed valve set up inside the brass column where the main jet is fitted to the bottom. While the cable sync is important, when pulling the slide up off of idle, this pump provides fuel to be atomized when the slide opens and the vacuum changes. The idle settings are no longer in play when you crack the throttle. It's a purely mechanical action. Also, on /5 carbs, the correct initial needle setting is position 3, 3rd from the top. Hope that helps. Do yourself a favor and get the Bing book. Here is a link to a post where I illustrated setting the float level a few months back. All the info in this post is correct and helpful. http://forums.bmwmoa.org/showthread.php?t=26530&highlight=Float+level

 

[gossamerjohn]

Many of the mechanical Bings have an accelerator pump inside the main jet stack. [/url]

Aaaaahhhh. I remember that when I took apart the carbs for cleaning. I also remember that one of the springs inside one of the jet stacks (left one as I recall) was slightly buggered up and I put it back in just to start form ground zero. Thanks for reminding me about that one.

I wonder if this could account for less than instantaneous reaction when the throttle is burped? I've got no experience with the mechanical pump system so I'll have to do some research.

Tomorrow I'm going to check the float heights and I suppose Monday I'll be ordering a new spring and a book from Bing. My neighbors to the west.