Engine Pops/Backfires at Higher Speeds After Major Tune-Up

[sjmcbride]

Hey guys,
I’ve been working on restoring my 1973 VW Type 2 Automatic (Bay Window) Microbus with the 2.0L Type 4 engine (engine appears to be an 1976) and 003 automatic transmission. I’ve made some solid progress, but I’m hitting a wall and would love your insight.

Original Setup:
• Old Dual carbs (previous owner had them installed)
• Old vacuum modulator for the transmission
• Speedometer and wiring had issues
• Engine idled poorly, died within seconds, shifted rough, and struggled on hills, lots of engine and transmission leaks.

Recent Changes/Upgrades:
• Installed new Redline Weber 40 IDF dual carburetor kit
• Replaced the alternator (upgraded to 75A)
• Replaced the transmission vacuum modulator
• Cleaned and reinstalled vacuum lines (including transmission line)
• Redid all engine and transmission seals and gaskets
• Installed brand new battery and verified voltage
• Speedometer was stripped — I fixed it with epoxy and it works great now

Current Symptoms:
• Idles much better than before — now lasts ~10 minutes before dying (used to die in seconds)
• Shifting is smoother but still a bit hard
• The big issue:
When accelerating past ~30mph, the engine starts popping (like a backfire), and really struggles to climb speed.
The faster I try to go, the worse it gets — sounds almost like it’s misfiring or running lean under load.

Here’s a video of what’s happening (acceleration sound):

[youtube]
https://youtube.com/shorts/PsoT0R2IfRA?si=FFNapGyXpHu72jO_
[/youtube]

[sjmcbride]

My guesses so far:
• Timing might be off? I tuned the new carbs per the manual but might be doing it wrong.
• Possibly running too lean?
• Carbs need syncing or jetting tweaks?
• Weak spark or coil breakdown under load?

Would love to hear what you guys think. Appreciate any advice or things to check next! You guys have helped me a ton so far with this thing

[sjmcbride]

Forgot to add, when I put it in Drive or Reverse it still dies in seconds. When in Park or Neutral it lasts 10minutes or so

[67rustavenger]

Popping out of the carbs would be a lean condition.
Popping out the exhaust would be a rich condition.

The carbs sound like they are fairly well synced. But may need a little syncing after you get the jetting sorted, IMO.

What is the carb jetting?
What is the engine timed at?

An auto trans likely needs a little higher idle speed to keep it from dying when shifting into forward or reverse gears.
I have no experience with an auto trans in a bus. I'm just thinking out loud.
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[SGKent]

if the popping is under sustained load and not when shifting, I would vote for too lean, but if it is excess fuel hitting the air and burning then yes it could also be too rich. Your pump may not be keeping up with the flow needed, the floats may be too low, or the jetting may be wrong.

Last - make sure that you do not have a rev limiting rotor on it and that the ignition system is in good condition. It can also be valve float from weak valve springs if it is only happening at the higher RPMs.

With the low cost of air to fuel gauges, I would put one on and see what the mixture looks like. you can put a port in each bank and move the sender from side to side if you want to see what each side is doing.
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[sjmcbride]

Thanks guys!
I decided to check the spark plugs and realized I installed NGK BR5ES instead of B6ES spark plugs as seems to be recommended. So I ordered those and will pop those in and report back!

[Wildthings]

What timing spec are you using? With a blown out retard canister and/or aftermarket carbs you can NOT use the book timing specs, instead time your engine at 28° BTDC @3800+ rpms, with the hose(s) removed from the vacuum can. You probably do not want to reinstall the retard hose if you have one since your carbs are likely not set up to use the retard function.

[SGKent]

one heat range difference might not make that much of a difference, but if you combined a resistance plug, with resistance wire, with a resistance rotor, you could lose spark. Generally a 4 cyl VW engine does not produce enough RPM to get coil saturation.
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[Wildthings]

Make sure you have the wires on the cap in the correct order.

[mikedjames]

If it has a condenser or coil issue it might show similar rpm related issues.

Interesting also that the underlying dying problem persists even when changing the carburettors.. what remained common ?

Ignition system ?
How good is the compression?
Is the cam good?
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Ancient vehicles and vessels

1974 VW T2 : Devon Eurovette camper with 1641 DP T1 engine, Progressive carb, full flow oil cooler, EDIS crank timed ignition.
Engine 1: 40k miles (rocker shaft clip fell off), Engine 2: 30k miles (rebuild, dropped valve). Engine 3: a JK Preservation Parts "new" engine, aluminium case: 26k miles: new top end.
Gearbox rebuild 2021 by Bears.

1979 Westerly GK24 24 foot racer/cruiser yacht Forethought of Gosport.
1973 wooden Pacer sailing dinghy

[W1K1]

I can tell you one thing, you need to fix the linkage install first and foremost.
That’s not even close to correct alignment
Post a picture of that……
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1965 squareback 1500E-sold
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[timvw7476]

Check the pockets where the plug wires plug into the towers on the distributor cap. For graphite powder from breaking-down plug wires. It can act as an insulator & mess up spark delivery. Same for a failing condenser, a weak breaker point spring, a worn-out carbon button under the distributor cap.
Put it in a dark garage a rev it, you may see sparks from the wires or a failing condenser.........it acts like this is mis-fire from the high-tension side of the ignition system.

[airschooled]

I haven’t seen any mention of a fuel pump, before or after the carburetor swap. Fine idle/low speed with problems under load/acceleration sounds like classic fuel starvation.

A fuel pressure test would be my next move, ideally it T’s in safely so a helper can watch/videotape the gauge while driving. You want 3psi when running, and it should change much at all with driving.

Another great data point would be a fuel volume test: at either of the carb fuel inlets, running the pump into a catch container should give you. Stock spec is something like 13oz per minute, so if it can’t fill up an empty PBR you could have a weak pump or blockage in the fuel system somewhere. (Filter, tank outlet, vacuum-sealed tank that needs venting, etc.)
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[W1K1]

This right here is crazy, pulling upside down, arms aren’t at the same level on the bar, linkage is not vertical………
How would you expect that to pull the carb’s evenly and stay balanced throughout the throttle travel?

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1973 super 2110cc
1965 squareback 1500E-sold
1971 bay window westy- EJ2.5 subi swap

[lil-jinx]

seems to be running rough and blowing black smoke when it back fires ,check the firing order.looks to be a porsche engine,don,t know if that would make a difference.
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[mikedjames]

Probably worth tweaking the idle speed upwards as automatics like more like 1000 rpm, along with their using a modified distributor curve. - so if the distributor is a generic aftermarket that will not help.

Also its quite likely that the entire "design" of the automatic transmission and its throttle control really needs to be altered with carburettors with different characteristics- maybe need to at least make sure your new linkage matches the original in terms of how far the throttle cable travels for WOT.


You have moved away from a system solution - original carbs, linkages and springs, to a new permutation of parts.

Maybe spending the effort and money in restoring original carburettors that are matched to the transmission and its linkages may be a better way out..
_________________
Ancient vehicles and vessels

1974 VW T2 : Devon Eurovette camper with 1641 DP T1 engine, Progressive carb, full flow oil cooler, EDIS crank timed ignition.
Engine 1: 40k miles (rocker shaft clip fell off), Engine 2: 30k miles (rebuild, dropped valve). Engine 3: a JK Preservation Parts "new" engine, aluminium case: 26k miles: new top end.
Gearbox rebuild 2021 by Bears.

1979 Westerly GK24 24 foot racer/cruiser yacht Forethought of Gosport.
1973 wooden Pacer sailing dinghy

[wagen19]

As "wildthings" already has mentioned, have you already really checked the firing order?
Also the spark plug connectors can have a too high resistance.
Should have 1 kOhm or 5 kOhm. Are some single sparks black?
Does the engine run on 4 cylinders?

[sjmcbride]

[Wildthings]

The engine doesn't stop in the same position each time, so what you are seeing is absolutely normal, so long as you can pick out the 1-4-3-2 snippet you are fine

[sjmcbride]