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Mon May 22, 2017 9:14 pm

I have a 1600 in my '67 convertible, and it is backfiring through the exhaust on deceleration.

My local shop has been over it extensively and can't track down the issue.

We just put in a freshly-rebuilt 205K distributor and a freshly-rebuilt german 30 Pict-1 carb. It wasn't until after these parts were replaced that the backfiring started.

Timing is spot-on, valves are adjusted, points are gapped correctly, all tune-up items replaced (cap, wires, plugs, points, etc.).

There does NOT appear to be an exhaust leak anywhere.

The engine runs great (best it has EVER been since I have had it, and that's been 25 years). The only issue is the backfiring through the exhaust on deceleration.

What else can I check?

Tue May 23, 2017 7:04 am

Assuming that you replaced only the distributor and carb, and that you still have the old ones, I would back-replace either the distributor or carb first, and see what happens. Then back-replace the other item and see what happens.

If back-replacing either solves the problem, then you've found the cause.

Otherwise, you may want to check the exhaust bolt tightnesses, as a guess...

Best,
Thom

Tue May 23, 2017 7:48 am

Are you running a stock exhaust? Years ago on my 1600 DP equipped 67 Bus I'd installed one of those header exhausts with the muffler that tucks up next to the valve cover. For some reason, whenever you'd let off the gas it'd go "POP POP BANG" and shoot a flame out the tailpipe! Admittedly, it was pretty useful/entertaining for getting tailgaters to back off, especially at night!! Once I replaced that muffler with a single quiet pack the backfiring went away.

Tue May 23, 2017 9:48 am

Try retarding the timing like 2-3 degrees. I had a similar issue once, and a small timing adjustment fixed it.

Or check your plugs or use a wideband. You may be a bit rich and on deceleration loading the exhaust with extra gas.

Just a few things to try.

Tue May 23, 2017 1:24 pm

Per Bently:

Exhaust backfire when car is overrunning the engine: idle mixture slightly weak. Enrich mixture at carburetor.

Your situation upon backfire is somewha different than this, but try enriching the mixture a little and see if it helps.

Tim

Tue May 23, 2017 8:23 pm

Thanks for the suggestions! I will try them.

It is a stock exhaust, just to answer the question above.

Tue May 23, 2017 9:23 pm

Exactly what "spot on" static timing was the distributor set at?

It's actually pretty rare for a shop to do this correctly. Now an 009, they can do all day long

It's also not unusual for the wrong vacuum canister to be installed. There's no such thing as a 205 K. The mate to the 30 PICT carburetor is a 113 905 205 K. There are three other "205 K" in the ACVW world so it's probably the above or at least a 113 K. The "205" is VW code for distributor= all of them.

Verify that your points dwell is between 40 and 50 degrees. Gapping them is not thurough enough. You need a dwell meter on it to verify.

Tue May 23, 2017 9:32 pm

The shop told me they verified the timing, but I didn't check it myself. I guess I will do that.

It is a 113 905 205 K.

Wed May 24, 2017 6:06 am

Well, the housing looks all nice and purty having been polished. They did not replace the o ring, shoulda coulda been done especialy since the polishing process wore it past its useful tolerance. There's a strong chance that it has the wrong vacuum canister on it. The originals had a star pattern on the back that yours lacks. There were some later replacement canisters that lacked the star pattern. You should have a canister that is delivering approximately 25 degrees of advance and should have been static timed at 7-8 degrees BTDC. That would have been the closest notch on the crankshaft pulley to BTDC.

Wed May 24, 2017 8:02 pm

Check for exhaust leaks. We recently had two bugs come through the shop that had stock engines that were backfiring under deceleration. Exhaust leaks were the issue in both cases. If you can't determine where it's leaking, call around to find a shop that has a smoke machine. It can pump smoke into exhaust or intake systems. It's also great to use for finding vacuum leaks in newer cars that use vacuum to control Heat and A/C. Good luck.

Fri May 26, 2017 10:57 am

My 68 beetle with a bone stock 1500cc engine used to pop/backfire on deceleration when the vehicle is over-running the engine. It is a Canadian car and as such was not equiped with a throttle positioner.

quote from Bentley:

"The purpose of the throttle positioner is to keep the throttle open in deceleration long enough to prevent emission of unburned rich fuel mixture into the atmosphere......Besides reducing the emission of the hydrocarbons from the tailpipe, the throttle positioner also prevents backfiring in the muffler."

I replaced the original 205K vacuum only distributor with a NOS German 009. The popping/back firing on decelleration is now gone. The the 009 does not retard the timing when the vehicle is over-running the engine, unlike the vacuum only distributor which would loose its vacuum signal with a closed throttle. I believe this retarding of the timing on deceleration with a vacuum only distributor contributes to backfiring on deceleration and is perfectly normal.

Fri May 26, 2017 12:25 pm

It hasn't been mentioned yet, but check to see if your brass accelerator pump nozzle is still in place. I had backfiring through the tailpipe of a 71 super and this turned out to be the issue. It worked its way out was sucked into the engine and being brass did not hear it happen nor did it damage the head or piston on the way through.

It was kind of fun though I could rev it up and it would shoot flame out the quiet pack, I was young and immature back then, now I am older.

Mon May 29, 2017 3:16 am

Mark's 68 wrote: My 68 beetle with a bone stock 1500cc engine used to pop/backfire on deceleration when the vehicle is over-running the engine. It is a Canadian car and as such was not equiped with a throttle positioner.

quote from Bentley:

"The purpose of the throttle positioner is to keep the throttle open in deceleration long enough to prevent emission of unburned rich fuel mixture into the atmosphere......Besides reducing the emission of the hydrocarbons from the tailpipe, the throttle positioner also prevents backfiring in the muffler."

I replaced the original 205K vacuum only distributor with a NOS German 009. The popping/back firing on decelleration is now gone. The the 009 does not retard the timing when the vehicle is over-running the engine, unlike the vacuum only distributor which would loose its vacuum signal with a closed throttle. I believe this retarding of the timing on deceleration with a vacuum only distributor contributes to backfiring on deceleration and is perfectly normal.

You do realize don't you, that you replaced a 009 with another 009? An example of why it's important to used full part numbers or at least not just use the 205 designation.

0 231 137 009/113 905 205 K

0 231 178 009/ no VW number because VW never installed one on any of it's cars.

Just sayin.