New carb question -- Secondary butterflies

[Honolulu]

Since we're having the most informative discussion I've EVER seen on the subject of carbs, (THANK YOU 61porsche), I have to ask whether there is a way to know when or even if the throttle secondary butterfly valves are opening. I get lots of nice noises from under the hood when I mash it, but how do you know if they are working together?

Secondaries are of course vacuum operated and you can't generate sufficient vacuum (any way I know of) in a shop setting while you can bend over and see things going on. IIRC the vacuum passages are internal so there is no way to pull a vacuum using a Mityvac or similar. By the time intake manifold builds to the point that the secondaries are supposed to open, you gotta have the pedal pretty much to the metal.

Unlesss......? Open to any suggestions.

 

[61porsche]

Carb tips continued... secondary actions

You posed several questions:

1. How to tell if they're opening- place a small paper clip on the rod, Closest to the box and go for a drive. If the paper clip moved, then compare carb to carb.

2. How do you know how far they opened; As DQ pointed out, the rod length determines this. Not just close, but exact on the factory measurement. A cheap digital micrometer is your friend.

The accelerator pump shot and the secondaries work together. First get the pumps squirting and not dribbling. That's what you feel when you mash the pedal for that split second or two.

Trick- look out back at the exhaust briefly- keep increasing the squirt ( adjust the arm) until you see the puff of black smoke. Then back off a little. For those with a modified engine or running 3.5, there is a jet for the accelerator pump. Increase the size of the jet with a reamer or drill slowly till you get your neck to snap so to speak.

Any lag when accelerating is generally going to be the squirter.
( acccelerator pump discharge.)

An owner should get into the habit at tune up time to grab the secondary rod or adjustment lever and make sure the secondaries are working and not stuck. I find that by periods of just sitting or non use they tend to not want to open and just need a coaxing once in a while.

Tip 2 on secondaries- there are two tiny holes on the primary , below the butterfly. make sure they're clean. One is for the choke and the other - secondary vacumn. If you've had a car that's backfired a lot due to non use, timing out, lean, etc. Guess where that backfire goes beside out the top or exhaust?


Keep asking and I'll do my best to respond.

 

[hans3]

How can one make the accel pump jet spray versus just dribble? Should they spray when testing them with engine off? Or is the ability to spray based upon engine temp and throttle opening rate?

 

[Stevehose]

I have been through this accel pump issue - when all is well they should spray simply by lifting the linkage, whether the engine is running or not. If they aren't you can try cleaning the nozzles (carefully pry them out of their housings) or adjusting the pump linkage by bending the lever (both covered in the service manual). If that doesn't work you'll need new pumps (or seal skirts if you can find them but go for the whole pump - the seals in the rebuild kits are low quality/fit), and they make ALL KINDS of frustrating starting and acceleration issues go away. If you can't find them under BMW parts then search for similar year Mercedes with Zeniths - same part.

How can one make the accel pump jet spray versus just dribble? Should they spray when testing them with engine off? Or is the ability to spray based upon engine temp and throttle opening rate?

 

[61porsche]

Carb accelerator pumps

The pump can be operated manually by lifting the arm.

It is lenear in it's volume. The more your throttle opens, more pump and fuel.

First check the operation by operating the lever several times. Try to determine if the lever has pressure both on the down stroke and return as the valves or bores can get stuck which can mean you need to polish the bore. if the arm is not touching the linkage at rest, there needs to be an adjustment.

If no fuel comes from there at all, the pump has a replaceable gasket but I always try to make the original work better as it's a thicker material. The check valve that allows fuel up the bore and seals it to squirt can need cleaning. The jet which meters how much fuel comes into the bore can be clogged. The small brass tube in the carb throat can get clogged.

I haven't found one yet that I haven't been able to function reasonably well.