Dear Car Talk:
I have a '69 VW Bug. When it's just sitting still and idling, it runs great. But when I let out the clutch, it takes off fine, then bogs down to idle speed, then it takes off again and bogs down again. It does this continuously. I have replaced the points, the condenser, the rotor and distributor cap, and the plug wires. I've also rebuilt the carburetor. What do I try next?
How about a '79 Rabbit?
Let's assume you gapped the points correctly and set the timing correctly. In that case, the next thing I'd do is pull out the spark plugs. If they're all black, it means you're getting either way too much fuel or insufficient spark.
So if they're black, I'd replace the coil for $25. I'd also re-gap the spark plugs, because misgapped plugs also could cause weak spark.
If the plugs are clean, then I'd turn my focus to the carburetor's high-speed jet. And yes, I'm aware that the term "high-speed jet" is an oxymoron when talking about a '69 Bug, but that's what they call it. Even though you say you've rebuilt the carburetor, the high-speed jet may still be plugged up.
When you first step on the gas pedal, you deploy something called the "accelerator pump." That's sort of like a little water pistol that shoots a stream of gasoline into the throat of the carburetor. It lasts only a few seconds, but it allows you to accelerate while waiting for the carburetor's high-speed circuit to kick in and provide continuous fuel.
But if the high-speed jet is plugged, after a few seconds of acceleration you'll run out of the fuel provided by the accelerator pump, and the engine will fall back to idle speed. That's why you surge ahead, and then bog down, surge ahead, bog down, ad infinitum.
But rather than rebuild the carburetor again, I'm going to suggest that you just buy a new one. It's not because I lack faith in your carburetor-rebuilding skills, Jeffrey; I lack faith in everyone's carburetor-rebuilding skills.
New carburetors for this car are still available, and you can get one for less than 100 bucks. So why not? Put one in, and I think you'll be right back to doing those five-minute 0-to-60-mph runs.