DVDA timing and tuning with 34-3

[sped372]

Jump to Observations if you want to skip to the nitty-gritty.

After reading much praise for the Philbin group I found a DVDA core and sent it to them for a rebuild. The Beetle has been running an ac.net SVDA for a few years now and although it worked pretty well I still always had a tiny flat spot. After buying the Ghia which came with the stock setup and is just all-around smooth I just had to get back to stock on the Beetle also. Enough backstory.

So yesterday I had the freshly rebuilt DVDA and a freshly rebuilt German Solex 34-3 freshly installed onto the 1600DP in the Beetle. I did a static time to about 7.5* BTDC and left the clamp loose for dialing it in with the light. After cranking long enough to pump fuel to the carb it sprang to life but sounded like a wheezing weed-whacker. I did a face-palm and correctly put the wires on the distributor cap (COUNTER-clockwise) and fired it up again. I had to back the mixture screw out about one turn and the bypass a turn or two but then it stayed running on its own. I let it warm up as much as idling would and then retuned the carb, find highest idle with the mixture screw and then adjust the bypass to get ~900 rpm. The screw on the throttle arm is just barely touching the fast-idle cam (which, by that point, was rotated fully open).

So, here's the deal. The car definitely seems to have the potential to run much smoother than before. Most of the time the transition was very very smooth on my first test-drive but I still dropped into a bog or two if I let the rpms fall too far on take-off (I think I need to revisit the bypass screw). Bear in mind that the car has old gas in it, like close to a year old. I did put fuel stabilizer in the tank, but still. I don't want to fully form any opinions till I have fresh gas in there.

Observations:
+Vacuum hoses off, and plugged.
+Engine is timed to 7.5*BTDC at idle.
+Timing advances another 10 or so degrees when engine is revved.
+Engine does not like to rev without the vacuum advance.
+Reattaching the advance hose does nothing to the timing at idle.
+Engine now advances to about 30* when revved.
+Seems slightly low for mechanical+vacuum advance.
+Timing falls back to 7.5*BTDC at idle.
+Hook up retard vacuum line.
+Timing drops way back to 5*ATDC at idle.
+Engine rpms drop dramatically and it runs rougher.
+Unscrewing the bypass screw on the carb brings the idle back up.
+Engine seems to idle OK, but not quite as nice as at 7.5*BTDC without the retard hose.
+Any disturbance of the throttle arm on the carb causes the timing to immediately jump from 5*ATDC back up to about 7.5*BTDC, then onward and upward with increased rpms - seems logical.

All of this seem correct? It seems right to me based on my understanding of what's going on. I plan to try and run this tank of fuel down, top it off, and run through the procedure again to fine tune everything. If that doesn't solve it I'll mess with the accelerator pump adjustment. At that point hopefully the flat spot will disappear completely.

One more thing, should I adjust the mixture screw with the vacuum hoses on (timing pulled back to the 5*ATDC setting) or off (up at 7.5*BTDC)? Or would the setting be basically the same either way?

[Amskeptic]

+Hook up retard vacuum line.
+Timing drops way back to 5*ATDC at idle.
+Engine rpms drop dramatically and it runs rougher.
+Unscrewing the bypass screw on the carb brings the idle back up.
+Engine seems to idle OK, but not quite as nice as at 7.5*BTDC without the retard hose.

should I adjust the mixture screw with the vacuum hoses on (timing pulled back to the 5*ATDC setting) or off (up at 7.5*BTDC)? Or would the setting be basically the same either way?

This is the point of having the vacuum retard. You must adjust the idle speed and mixture to the 5*ATDC timing. That is what the carburetor was calibrated to. You need the increased airflow required to idle a retarded engine at the 900-950 rpm range, then you need to provide adequate fuel to make a smooth 900-950 rpm idle by setting the mixture screw for best idle + 1/4 turn.
THEN you get the buttery smooth transition with no flat spot.
Colin

[sped372]

'swat I figured. Thanks for the confirmation. I'm still planning to get some fresh fuel in there and then reiterate the tuning procedure. Will post back with the results.

[VWBusrepairman]

THEN you get the buttery smooth transition with no flat spot.
Colin

Ah ha! I replaced my 34PICT-3 (Solex) carb with a new Pierburg ($155 wow) of the same size and only NOW have a flat spot. No flat spot with original carb, but it's throttle shaft was leaking fuel. Running the DVDA with compu-fire ignition. (1641 cc engine displacement; if that matters for "flat spot" removal)

I think I will start over with this procedure (above) to see if I can get the buttery goodness back upon acceleration!

additionally, must pump the accelerator a lot to get it to start, where as before, it fired up quickly with only one pump.

anyone else running the new Pierburgs? Heard they were great replacement carbs...

[Amskeptic]

anyone else running the new Pierburgs? Heard they were great replacement carbs...

Doesn't seem to be the case these days.
Colin
(but adjust properly, make sure choke flap is freely able to move and that it is closing when cold, and let us know if the new Pierburg is satisfactory)

[VWBusrepairman]

anyone else running the new Pierburgs? Heard they were great replacement carbs...

Doesn't seem to be the case these days.
Colin
(but adjust properly, make sure choke flap is freely able to move and that it is closing when cold, and let us know if the new Pierburg is satisfactory)

I've printed your instruction and will make adjustments and report back my findings. Then, the car is sadly for sale. The 'vert is gone and the '72 is next. this leaves me with the '78 bus and a '68 beetle. I need funds for a pole barn, so that I may collect more cars.

Of course, I'm running the DVDA distributor, so results may vary? I didn't have this flat spot with the German solex and changed nothing else, so we shall see. I even asked if this carb would be junk out of the box. Probably should have the Solex carbs re-bushed...

[VWBusrepairman]

and let us know if the new Pierburg is satisfactory

I disliked the carb; I called it junk out of the box. I returned it to the vendor who said he could only do store credit. He then took the carb, adjusted the acc. pump to the "+" side and drilled out the idle jet to 65, I believe. He said it would make a difference. At this time, the carb is wrapped in a plastic bag and hadn't been re-installed on any engines. $165 down the drain.

I put new gaskets in the Solex and it's been delivering roughly 28-30 MPG with buttery smooth acceleration, so I am hesitant to pull it off and put that Chinese carb on there.

Any time a part must be adjusted/ tinkered with out of the box, is that what things are coming to these days?

[ruckman101]

I've dealt with two hecho in Mexico 34 pict 3s out of the box, the first had a loose cover nut which created a huge vacuum leak and car no run until we figured that out and tightened it down. The second I immediately dismantled and found the top gasket was skewed and bunged up. But otherwise they were fine.


neal