Carb problem or ???

[cdm]

'73 Super with one of the Chinese knockoff carbs installed. The symptom of the problem is that it will run for awhile and then suddenly start coughing and sputtering like running out of gas. But there is plenty of gas. Then it dies. On restart after sitting it will run great for a while and then repeat the death rattle and die. Sometimes won't start again until sitting for some time.

I have checked valve adjustment twice. They are to spec. I have tested for vacuum leaks exhaustively (no pun) and found nothing. No exhaust leaks. Timing is 7 btdc at idle and 28 btdc at 3000 rpm. Compression is 100 psi with 2 psi variation from highest to lowest. It hasn't changed. Carb heat tubes are connected and open. Manifold is getting heat.

I have cleaned the carb twice. No change. Am I missing something or is the assumption the carb is faulty in some way I can't see legitimate? Out of ideas here. Thanks.

[Cusser]

Check the automatic choke of the carb for proper operation.
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1970 VW (owned since 1972) and 1971 VW Convertible (owned since 1976), second owner of each. The '71 now has the 1835 engine, swapped from the '70. Second owner of each. 1988 Mazda B2200 truck, 1998 Frontier, 2014 Yukon, 2004 Frontier King Cab. All manual transmission except for the Yukon. http://www.thesamba.com/vw/forum/album_page.php?pic_id=335294 http://www.thesamba.com/vw/forum/album_page.php?pic_id=335297

[ashman40]

What Cusser said, check that the choke is working and understand if the problem only starts when the choke turns OFF (choke butterfly fully open and the fast idle cam rotated so the throttle arm screw resting on the lowest level of the cam).
If the problem only appears once the choke has turned OFF, it could be a vacuum leak. Test as follows:

Warm up the engine so it is not running on the choke.
Remove the air cleaner housing.
If the engine cannot idle on its own, rotate the fast idle cam under the throttle arm screw just enough to keep the engine idling.
Place your flat hand w/ fingers closed over the top of the carb to block off 50% of the opening. You are basically acting like the choke butterfly.
Slowly move your hand over to the opening to close off more and more of the carb air flow. At some point you will start to restrict the amount of air flowing down the carb.
For a properly running carb, this is the same as closing off the throttle plate or in the case of 34Pic carbs you are screwing in the bypass screw. You are cutting off the air flowing down the carb, you expect the idle rpms to drop. If instead the idle rpms smooth out and increase... you likely have a vacuum leak.

When you have a vacuum leak your engine is running of an air-fuel mixture that is too lean and the carb cannot adjust because that extra air is not flowing down the carb throat where it can meter the proper amount of fuel to maintain the air-fuel mixture. By choking off the air flowing down the carb you actually richen the mixture and bring the air-fuel back into balance. This is why the rpms go up instead of down as you choke off the air flow.

If the above test indicates you have a vacuum leak, you need to start spraying all the common suspect leak areas with carb cleaner to find the leak. Put the red straw on the spray can of carb cleaner and with the engine running, focus your sprays anywhere that air could leak into the intake flow. When you spray carb cleaner and hit a leak the engine rpms should change. You should hear a change in the engine pitch.
The common leak spot for old Pict carbs is the throttle shaft bores (but some cheap new carbs will also leak here). You need to point the carb cleaner straw at the small gap between the throttle shaft arm and the flange on the body of the carb where the shaft exits. If hitting this spot while the engine is running causes a change in the rpms, you have found a leak. Also spray all vacuum hose ends; all gaskets; all boots. Anytime the rpms change when you spray it means the carb cleaner is making it into the engine air-fuel mixture. Ideally, except of the opening at the top of the carb there should be no ways for air or fuel to lean into the path between the carb and the cylinders.
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AshMan40
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'67 Beetle #1 {project car that never made it to the road }
'75 Beetle 1200LS (RHD Japan model) {junked due to frame rot}
'67 Beetle #2 {2019 project car - Wish me luck!}

[Starbucket]

Remove the top of the carb. and look for machining crud which works it's way over to the jets and after sitting it falls away until sucked back in, a common problem with China carbs.

[cdm]

[cdm]

[cdm]

[Jimbug57]

You might consider getting one of these - https://www.harborfreight.com/fuel-pump-and-vacuum-tester-62637.html. They can be useful in diagnosing issues like you're having. and an inexpensive and useful tool to add to your collection - it can perform multiple diagnostic functions.
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Repeat after me "I am smarter than metal!"

[Starbucket]

If you have points it could be a bad capacitor since your carb. seems ok. You can test yours or just replace it as they are cheap.

[ashman40]