DAF-328 transition stumble

[blue]

Hi All,

Preface: I have Weber and SU expertise but not so much on the stock carbs.

I am helping a friend with a '72 510 1600 automatic with stock DAF-328 carb. I completely rebuilt it with new gaskets. The carb works great and the carbon fouled plugs are long gone. I just have one problem at transitioning from low rpm to high rpm under load. It is as if the diaphragm assembly that pulls the secondary throttle valve is not kicking in. By hand reving while parked,it is not until I am at WOT that I can see fuel coming out of the secondary's venturi....and I could only do this after I adjusted the accelerator pedal stop so that the linkage actually could open the primary throttle valve to WOT. I am guessing it is near 6000 rpm that I see fuel....I mean scary crazy high rpms lol.

When driving the car, I can baby the throttle from stop (650rpm) and nurse it to 60mph no problem without a stumble, however if I drive normally, it stumbles from ~22mph to 30mph before it gets enough momentum. It passes through a stumbling transition.

The only other way I found to get momentum and pass through the stumbling phase is to floor the pedal at 22mph when the stumbling occurs then release it at 30mph and drive normally...basically a spirited short burst.

I can go up a hill under load no problem as long as the pedal is floored.

My suspicion is that the diaphragm pulling the secondary is weak. I actually opened it and inspected on the rebuild. It looked ok but the membrane seemed stretched. I also put the small bearing in before the spring. I am hoping the bearing was not supposed to be on top.

This carb does not have any solenoids. It does have the larger BCDD like on the 280z EFI throttle body and there is a mystery needle valve just above the BCDD that I can not find info on how to set. I did notice that if I continue to turn the BCDD screw in CW, it will eventually allow air to bypass the throttle valve and the idle increases. I was hoping the BCDD would hold the RPM's high after breaking (like on the 280z) but it does not seem to do this on the 510. I left the BCDD and the mystery needle valve in their original places.

Well that is as good as I can describe... any help is appreciated for the transition problem and also practical advice on how to set the BCDD and the mystery needle valve.

I am curious to know if I should see fuel coming out of the secondary venturi earlier while hand reving and at what RPM.

Also the placement of the ball bearing in the diaphragm is of interest.


Thanks!
Philip

Follow up:

The only adjustment description I can find is in red below but I don't know what to "bend" . Can some one clarify this procedure? Thanks!

[James]

I don't have much to offer other than to ask - how is condition of timing, ignition set up etc.
I had a stock carb that the secondary butterfly was frozen. Is the accel pump doing its thing properly? Sounds like a lack of gas at that particular RPM....

[blue]

Thanks,

Everything works fine and moves freely. I just completely rebuilt the carb. There is no binding.
The timing is set at 7 degrees BTDC at 650rpm. Plugs, distributor and coil working fine. Plugs are nice mocha tan like a finely toasted marshmallow.


The diaphragm seemed stretched and "baggy" though not ripped. I am now guessing that it is not tight enough to pull the secondary throttle far enough.

[StrutlessWonder]

Did you review the DimeQuarterly downdraft carb tuning articles (2 parts) from DQ Volumes 1 issue 3 and Volume 2 issue 1? Second part is more about tuning DGV carbs.
They're in the archives: https://docs.google.com/folder/d/0ByCvx ... edit?pli=1

[blue]

Thanks Kurt,

Yes I reviewed that before rebuilding as well as a few other books with DAF328 sections. I'll take another look.

If someone has a DAF 328 and can check and tell me the RPM when the secondary venturi starts flowing gas when hand reved at the motor,that would help.

I now have a new diaphragm on order and will photograph the old and new when it arrives.

The only hiccup when rebuilding is that I could not remove the air corrector at the top of the main tube at the top of the tower in the primary. The whole tower turned. I just sprayed carb cleaner down it and hoped it would also flush the emulsion tube. Does the fuel flow symmetrically up into the tube from the bottom? I am wondering is the tower turning has cause an air leak or misaligned passages causing its reduced functioning? When I could not remove the air corrector and realized the whole tower was turning, I just turned backwards until it was firm like a screw that is seated well....so I don't think is is leaking air or fuel.

Philip