With Wings As Eagles: Craig P. Steffen's Blog

Finally, a sufficient condition: correct(-ish) throttle plate

2012 July 08 12:10

As I talked about in the last entry, my struggles with my 34PICT-4 carburetor continued this spring, even after I fixed the fuel pressure. People had bugged me about it; I'd ignored it at first, but then I figured it couldn't hurt. I bought the gauge, tested the pressure, modified the configuration so that the pressure was reasonably correct, it still had the same problems. Humph.

I again spent a bunch of time digging around on thesamba.com. I'd encountered a few posts of people talking about putting a rivet in the hole in their throttle plate, or even soldering it shut. That seemed like a bad idea to me, but I at least tried to find the posts.

I found this post on holes in the throttle plates that gave me the idea that different sized holes would match to different other parts. Another evening of banging around and I finally found this post by carburetor expert keifernet which explicitly lists the diameter of the holes in the throttle plate. In case this URL ceases to work someday, I'm quoting the post here:

The DVDA is approx 5/32" and SVDA is 1/8".

Hard to believe that difference does anything but trust me... it makes
a big difference in the way the carb runs/tunes when your using a DVDA
flange # German carb meant to be timed at 5 ATDC with an SVDA timed at
7.5 or an 009 timed at 10 BTDC.

I find the pop rivet easier than taking the plate out and soldering
the hole shut, grinding it flush and re drilling it but that's a
personal choice. It can also be "undone" in a matter of minutes if
someone wants to go back to a DVDA and does not alter the carb in any
other way.
By the way, SVDA means a "Single Vacuum Dual Advance" distributor; one that has centrifugal timing advance and vacuum advance (but not a vacuum retard). DVDA means a "Dual Vacuum Dual Advance"; a distributor with centrifugal advance (I think), please vacuum advance and vacumm retard.

Here's how this all works. Here's a picture of the business end of my 34PICT-4 carburator.

This is the end that bolts to the intake manifold flange; the brass-colored circle in the middle is the throttle plate. When you push down on the accelerator pedal, it opens and allows lots of air and fuel through to make the engine run fast. When you let the pedal up, it closes, as shown here.

The 34PICT series carburetors is unusual in how its idle circuit works. The carburetor idles with the throttle plate completely closed. There is a "bypass drilling" that goes from above the throttle plate, through the body of the carburetor, and then exits back into the carburetor throat just below the throttle plate, indicated by the green arrow. Inside the bypass is where fuel is added to the air for idling. Some air also goes through the throttle plate through the hole, indicated by the red arrow.

Although the idle mixture is controlled by adjustments on the carburetor, the base mixture (and the mixture ranges that are possible) depend on the balance betwen the amount of air flowing through the hole in the throttle plate and the amount if air/fuel coming through the bypass. The correct mixture at idle is different depending on what the idle ignition timing is, and that in turn is different between the SVDA and the DVDA distributors.

So having found the above posts, I measured the hole in the throttle plate in my 34PICT-4--and it was slightly larger than 5/32". This is not surprising; the 34PICT-4 was originally sold with a DVDA distributor, but I'm using a SVDA distributor (it's simpler and you can time it statically). So my throttle plate/distributor combination was wrong which is why I was having so much trouble. It explained why my 34PICT-3 carb worked Ok (it had the smaller hole throttle plate). It also explained two different 34PICT-4 carburetor bodies had the same problem (I used the same throttle plate both times).

I didn't mess around with rivets or solder--I just took the throttle plate out of my other carburetors with the smaller hole, put it in the 34PICT-4...and IT WORKED! Holy crap! A single change, of what seems to be a very small thing, and it ran like a totally different engine. The idle still wanders a tiny bit, but not badly, and I can tune the carb properly with the adjusting screws. Whoo-hoo!

necessary but not sufficient: fuel pressure

2012 July 08 10:54

I struggled since last fall and this spring getting my vintage Beetle to run right. It would drive Ok, but the idle was never smooth. I had a 34PICT-3 carburetor that was just about right, but I had the absolute dickens of a time getting the California-spec 34PICT-4 to work at all. It had always had an unstable idle. It seemed to me like it was running too lean; maybe a leaking intake manifold, or perhaps leaking around the throttle shaft.

To try to combat this problem, I had new bushings put into a couple of carburetor bodies. Although the throttle shafts were much tighter, it didn't change the behavior significantly.

People on thesamba.com kept bugging me about the fuel pressure. Obviously if fuel pressure is low, then the car won't run right because it's not getting enough fuel. However, too much fuel pressure is also a problem. If you have a lot too much fuel pressure, the float in the carburetor bowl has to rise higher to shut off the fuel flow, and so the fuel level in the bowl rides higher than it should. THe level in the bowl is a very important part of how the carburetor runs, so too much fuel pressure can be a significant problem.

It turns out that Harbor Freight has a fuel pressure testing kit. I bought bought one and set it up to measure my fuel pressure. I put a T in the line from the pump to the carburetor, and had the gauge set up the read the pressure:

I started the engine and looked at the pressure. It's supposed to be in the range of 1.5 to 3.5 psi:


So the first thing I did was remove the incorrect-spec fuel pump that I had on the engine. I fought with a carburetor problem last year, which turned out to be the carb float not fitting. During that process I'd changed the fuel pump a couple of times, and ended up with one that I'd gotten from a not-very-good VW parts place that wasnt' quite right. It worked, but the hose fittings were the wrong size, and judging by this pressure test, weren't the right internal specs either. Once I put a proper-spec pump on the engine, the fuel pressure was much lower, but still slightly above 4 psi.

The way to adjust the fuel pressure is to shim the pump up. Here's the pedestal that the fuel pump rests on:

The brown thing is the plastic pedestal that the pump sits on. It insulates the pump from the engine block so that the pump body doesn't get hot and cause vapor lock. The steel rod in the middle of the pedestal moves up and down as the engine turns, actuating the pump. You add shims on top of the pedestal, which raises the pump body so that the steel rod can't actuate it as far.

You can stack up gaskets on the flange, but they're pretty thin and it can take a lot of them. I instead made my own out of a sheet of plastic; I cut them out with razor blade. It only took about 5 minutes per shim.

Here's all four shims on the pedestal ready to install the pump:

Changing pumps and shimming didn't make the engine run right; there was no noticable change (although the shims did bring the fuel pressure down into the correct range). I don't if the high fuel pressure would have prevented the engine from running right once I got the carburetor fixed, but now I know that it's within the proper spec.