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The engine is rebuilt completely stock 1973, 1600 dual port. I replaced the 30/31 PICT carb that was on it with a NEW 34PICT-3 Pierburg two years ago with the engine work.

I recently replaced the exhaust, adjusted the valves, replaced plugs, points (stock distributer)... basically a complete tune up. However, now I have an awful "flat spot" when driving. At idle everything is fine. Good idle at 900rpm etc. Slap the throttle down and it follows up real smooth. Drive it on the road and she stumbles between N & 1st, then to 2nd. 3rd & 4th are fine.

Using engine brake to slow coming down hill, I hear a pop-pop out the exhaust. I thought this indicated a lean condition, so I re-adjusted both screws on the carb. Some of this went away, but....

To prevent the stumble, or at least smooth it out some, I have to give it a squirt of throttle and back off to where I want the throttle to be on take-off.

I have adjusted and re-adjusted the carb. I don't know what this is problem. I have checked for vacuum leaks, checked the advance of the distributer and redone the timing...both static and with a strobe. No change.

Any ideas? Or are these after-market replacement carbs awful?

Mine drives pretty much the same, I just figured it was normal for an old VW. It seems like it just needs more throttle between 1st and 2nd on mine to keep from stalling.

I have a 73 beetle with the same stock engine set up, but it doesn't do the stumble.

If I could just figure out what is causing this...

What exhaust system did you install? If it's a header or aftermarket "sporty" muffler it won't yield as much preheat flow as a stock muffler, and NONE if it came with the riser pipes undrilled as many do.

I bought the one you can get from Mid America... It fits similar to the '74. I had the after market one that is similar to the original '73 (mufflers on the outside L/R) from Thing Shop. But, it is an absolute pain in the rectal tissue to adjust your valves with the way the Thing Shop has that thing rigged. There is not enough clearance to get your tools in there. You can barely snap off the valve covers. Getting your flat-head driver in there AND your hands AND still work the feeler gauge, is next to impossible...

I drilled out the riser tubes with a 1/4 inch drill bit... do I need to go larger? How would heat risers affect the engine in the warm winters of Tennessee?

The Mid-America Thing exhaust, like 99% of all aftermarket exhausts, has nothing in its design to promote any flow through the heat riser pipe. Look at a stock muffler and you'll find that one end is routed to a low-pressure point inside the muffler; on aftermarket systems with each end connected at an exhaust port all you really have to move any heat is convection.
AFAIK The Thing Shop's system isn't any better in this regard so this may not be the only factor at work here, but it is a heck of a coincidence, don'tcha think?
I'd drill the holes out as big as possible short of hitting daylight...the intake manifold still won't be getting as warm as it does with a stock muffler.

Check the aim of the accelerator pump. Most of the time it's best if aimed as far out as possible without hitting the venturi wall. Double-check that the points haven't closed up on you, too (too much dwell => retarded timing can give similar symptoms).

Two things to check. First what distributor do you have? It just says stock. Stock as in DVDA(two vacuum hoses)? Is the diaphragm good in the distributor? Suck on both hoses(not at the same time) and see if the diaphragm is still good and not blown out. If you have no vacuum advance(the rear side of the distributor), you may have a flat spot on acceleration.

Second thing to check, where is the accel. pump nozzle in the carb venturi? If you initial squirt from the pump is not in the correct place in the carb venturi, it will stumble. Ask me how I know.....BTDT