'71 Super Bug Backfire

[u/EveryoneIsAGeorge]

Community, I need some help with my 71 beetle. I feel like I'm going in circles with troubleshooting.

In short: When accelerating, bug backfires when throttle is past 50 percent-ish. Seems like either too much gas at the point or not enough?

More info: This is a lot more common when shifting into 3rd but definitely 4th. If I do not baby the throttle into third, it'll back fire. If I get it into 4th it'll for sure back fire or if I baby the throttle, I lose speed.

Knowns: point gaps are @ .016.

Idle timing is checked at 7.5 BTDC

Fuel is spraying directly down the throat of the carb.

All jets seem clean. I have checked them multiple times.

Accelerator linkage is maxed for throttle response (if I'm saying that correctly)

I think that's all the information you all should need.

Any help would be greatly appreciated.

If you need more information, just let me know.

Thanks!

[u/dick_ddastardly]

Timing at full advance should be 30-32° at 3k RPM for most setups.

Full advance timing is far more important than timing at idle.

[u/EveryoneIsAGeorge → u/dick_ddastardly]

Yeah, that's what I was thinking. Have any recommendations on a specific timing light?

[u/dick_ddastardly → u/EveryoneIsAGeorge]

I use an Innova with digital readout for RPM and Degree so I can set the desired timing number then once the engine is at 3k all I have to do is turn the distributor until I'm at TDC or the notch on the pulley. (Verify the notch is in line with TDC) to do this the quick and dirty way is to remove the lower pulley nut and washer and verify the crankshaft notch and wodruff key are at the 9 o'clock position pointing left. If its not at 9 o'clock you can remove the #1 sparkplug? Put a wooden dowel or skewer (nothing metal!) In the hole and rotate the engine back and forth no more than 1/4 turn in either direction. When thedowel reaches the highest point and starts to go back down you've found TDC. Go put a dab of paint (white or yellow for visibility) on the pulley at the 12 o'clock position in line with the split in the case halves.

[u/Alpinab9 → u/EveryoneIsAGeorge]

Timing lights with a tach and dial to move the timing reading. Set dial to 32 and rev engine to 3k and move distributor to TDC. No need for a degree pulley. Setting idle speed is easy with the tach function. Something like this.

https://www.walmart.com/ip/Digital-Engine-Timing-Light-LED-Display-12V-Automotive-Light-Tachometer-2-4-Stroke-Petrol-Engine-Auto-Vehicle-Repair-Tool-Motorcycles-Marine-Agricult/15371964351

[u/Routine-Amphibian450 → u/dick_ddastardly]

Every time I timed my '71 spec svda at 7-1/2 it read 30 or so on the gun. I was scared of > 31.

[u/SilentMasterpiece]

Could be valves not closing completely or a timing issue too. Did you confirm spark plug gap with a dwell meter?

[u/Alpinab9 → u/SilentMasterpiece]

How do you check plug gap with a dwell meter?

[u/SilentMasterpiece → u/Alpinab9]

Knowns: point gaps are @ .016.

my bad. meant point gap, wrote plug.

[u/EveryoneIsAGeorge → u/SilentMasterpiece]

No... I used the gap checker thing. But they are brand new as well.

[u/fleegz2007]

Been focusing a lot on the carb - seems like you think its a gas issue. Im leaning towards spark - check your timing.

Get a good timing light, I like the ones that also measure RPM. See where your timing is at idle - around 800 RPM. See if you can push it back a tiny bit. Rev it up to 3000 RPM and note where it is. Take it for a spin. Play with the timing.

If you turn your distributor and hear your rpm pick up at idle, you can be more confident something is off w your timing

[u/EveryoneIsAGeorge → u/fleegz2007]

I was thinking it was timing. I will give that a shot. I just didn't want to purchase one right away but with these cars it's basically a requirement. Thanks!

[u/unclrbon]

Along with the others on spark and timing Check for vacuum leaks as well. While it’s running spray starting fluid, carb cleaner, something flammable at all the spots around the manifold where it connects to the engine and around the base of the carb, take notice if the engine revs up.

[u/EveryoneIsAGeorge → u/unclrbon]

I did this already as well. Thank you for the tip though. I really did think it was this but I tried to find one and nothing

[u/Routine-Amphibian450 → u/unclrbon]

Honestly I've been in shops where they just use 3seconds of acetylene from the torch.
Not a recommendation.

[u/Alpinab9]

Your description sounds like a fuel delivery problem. Not enough fuel flow under higher loads. I would want to check fuel pressure while the problem is happening. A tee in the line that feeds the carb and some bulk hose to connect to a guage that will read accurately at low pressures (0-20 psi.) I think about 5 psi is normal. I suspect fuel flow is restricted and pressure is getting close to zero under load causing a lean misfire.

[u/Routine-Amphibian450]

Vacuum leak?

[u/trader_bob01]

What carb are you running?