I cant figure out why it's sputtering, backfiring and hesitating

[WojoBeetle]

Hello Forum Members, I need your help. I have a stock 1973 Super Beetle Conv, stick shift with a 1600 engine. Recently, all of a sudden, the car started bogging down upon acceleration, sputtering, backfiring and running rough. It starts fine and idles well, but as soon as I start pressing on the throttle, it either dies or stumbles. I can start driving if I go very gradually, but as soon as I hit accelerator a bit harder, it bogs down. If I feather the throttle, I can get it to advance to high rpms (4K+) and it runs great at steady rpm, but as soon as I let go of throttle, it falls down to idle, but stumbles upon pressing the throttle again. Here's what I did so far:
- Installed a total of 3 separate carbs (all ran fine in the past)
- Three different distributors (also previously tested)
- Checked manifolds and their rubber sleeves, repositioned and tightened
- Replaced a total of 3 coils (two used in the past and testing fine and one new)
- Set up timing with a strobe gun, but also experimented with various setups
- Checked valve clearance and was per specs
- Tested cylinder compression on a cold engine/opened throttle. I got 100psi with +/- 3psi variance at most, between the cylinders. Before you say it's too low, I got a cheap cylinder compression kit off Amazon. For comparison, I tested my Honda CB750, which runs fantastic and the reading was only 75psi, so I blame the kit and have to assume compression is not the issue.
Absolutely nothing I did above made any difference. The engine behaves the same way with every set up and it's essentially undrivable. The engine was rebuilt 5 years ago with all new parts and I put only 10K miles since then. Until the above started happening, it had run very strong, always started easily and consumed no oil whatsoever. At this point, I am out of ideas and don't even know what else could be causing this. Any ideas or suggestions?

[67rustavenger]

Set the crankshaft at TDC and check your firing order.
1-4-3-2.
If you get 3-4 mixed up. It will run as you have described.

Timing fools the best of us. I get the 3-4 mixed up every once in a year.
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[mukluk]

Pull the spark plugs and check each one to verify they're all actually sparking.
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1960 Ragtop w/Semaphores "Inga"

[ryohey]

check the points thoroughly. Make sure there are no nubs/peaks in the middle of them. take them out and open them up. Its definitely an ignition issue.
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69 bug restoration thread http://www.thesamba.com/vw/forum/viewtopic.php?t=387230&highlight=

[VW_Jimbo]

Vacuum leak? Base of carb. Take a 13mm wrench and snug them up.

Accelerator pump working?
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Jimbo

There is never enough time to do it right the first time, but all the time necessary the second time!

[virusdoc]

You say this vehicle is stock, and you seem to have access to multiple known-good parts for diagnosis. So I assume you know what you're doing or have had help doing all these parts swap outs. I'm going to assume that there hasn't been something careless done during this process and that the things you've ruled out have been ruled out. Nevertheless, a picture of your engine compartment (or several from different angles) might be useful in diagnosis.

If your timing and firing order are correct as recommended above, and you have ruled out vacuum leaks at the manifold, it sounds like a fuel delivery issue would be high on my list next. I would test fuel pressure and fuel volume being delivered by your pump. You may have a failing pump, or a clogged fuel line/filter. If any of the fuel lines are still the original, they may be dissolving internally and continuously fouling your fuel delivery circuits.
_________________
1976 Convertible Super Beetle, "June Bug".
FI-->34PICT3 carb conversion-->now dual 40 IDFs
Self-rebuilt 1904 (after totally botching a 1641 rebuild, cussing a lot, and throwing lots of cash in the fire)

[WojoBeetle]

[WojoBeetle]

[WojoBeetle]

[WojoBeetle]

[VW_Jimbo]

[ryohey]

yes, a pic of the engine would do a world of good to see what might be going on.
_________________
69 bug restoration thread http://www.thesamba.com/vw/forum/viewtopic.php?t=387230&highlight=

[ashman40]

Specifically, what model distributor(s) are you running (which do you currently have installed) and exactly how have you adjusted ignition timing? Ignition timing is based on the installed distributor model and not the engine or model year (unless your model year has the original distributor still installed). Different model distributors are timed differently. So you can only know how your ignition should be timed based on which distributor you are running.
If you are uncertain how your distributor should be timed, statically timing it to 5BTDC is generally safe for low rpms and just to get it idling. This even applies to DVDA distributors that should not be statically timed. Once you have the engine running/idling you can use a strobe timing lite to accurately set the timing based on the distributor model. Refer to models listed here:
http://www.type2.com/~keen/ignition.html

Check that your accelerator pump is squirting fuel as soon as the throttle arm is moved. If there is any play in the linkage or a delay in squirting fuel down the open carb throat you will experience hesitation on acceleration. Also check that the spray is good. You should see a strong stream of fuel coming out of the brass U-shaped nozzle. It should be aimed at the edge of the throttle plate so it goes into the intake.

Check for vacuum leaks with spray carb cleaner at idle. Put the red straw on the spray nozzle and focus your spray at possible leak points. If you hear any change in the idle rpms you have found a leak. Hit the gaskets and hoses between the carb and the heads.
Specifically focus on the throttle shaft openings in the carb body. These are are blocked by the throttle shaft flanges so you will need to carefully aim at the gap where the flange and carb body meet. Leaking throttle shaft bores are common for carbs with more than a few years on them (that means most OE carbs). Leaks are more common on cheaply made clone carbs.

Good luck!
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AshMan40
---------------------------
'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!}

[WojoBeetle]

Thank you for additional feedback. I have tried 3 separate distributors and caps, so if the dizzy were to blame, I’d see a difference between each of them, but there was absolutely none. Same behavior with all. Same goes for carbs. Not likely all 3 carbs would have a vacuum leak, especially that one is nearly brand new. I am still thinking there could be a vacuum leak because the engine does get up to higher rpm without a problem if I literally pump the throttle. I’ve just ordered new manifold boots and gaskets to eliminate that point of potential leak and will go from there. Pics of engine attached. Some components are disassembled as I’ve been working on the engine, but It has no bearing on the way it runs. All vacuum lines are capped and filter on, but none of it makes any difference. I will try to post a video later on, too.

[WojoBeetle]

[mukluk]

The see through distributor caps are typically garbage quality, so that's one potential concern. Your fuel line laying directly atop the left heat riser isn't doing you any favours either. The distributor vacuum advance hose is attached to the incorrect fitting on the carburetor; it should be connected to the left hand fitting near the adjustment screws. Might just be my eyes, but it appears the pivot pin in your fuel pump is receding/walking out towards the front of the car.

Have you checked your spark plugs yet?
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1960 Ragtop w/Semaphores "Inga"

[Xevin]

[Q-Dog]

The pictures of that carburetor tell me the choke is not adjusted correctly. The fast idle screw should never rest on the outside of the cam. And only in the coldest of weather should it even reach the highest notch on the cam.

The car can run like crap if the choke never fully opens up. Having the vacuum advance hose on the wrong port is doing you no favors either. It feels like lots of little mistakes adding up.
_________________
Brian

'69 Dune Buggy
'69 Beetle Convertible
'70 Beetle

[mukluk]

[vamram]

The vacuum advance hose connected to the wrong port combined w/the vacuum advance port left open on the carb are guaranteed to make it run like crap. I doubt you can time it properly w/those two items as-is. Plus everything else that's been mentioned.
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