1962 356B sudden occasional backfire.

[david johnston]

Hi folks, I own a 1962 356b and have been driving it quite a bit this summer (2000 miles). (I have 2 young children so I don't get tons of time) Carbs have always felt strong. However I recently went for an hourlong drive and noticed that my rpms dropped as I pulled up to a stop (normally it was fine). I also noticed that it backfired a little bit...The first 1/2 hour was great, then it started happening. I ran it for about 15 minutes tonight and it backfired again. I have a fuel filter on my fuel line and had the cap, rotor, coil, sparks and wires done about 3000m ago. I'm thinking about the timing and possibly a minor carb adjustment, however I would love any feedback from the experienced and technically inclined as to the order of which I should proceed with my diagnoses. Thank you so much, David Johnston

[JOEL JENSEN]

David,
When does it backfire, at idle, on acceleration, when you decelerate, flat ground, uphill or downhill ? Knowing the conditions under which the problem occurs can be key to detecting
the problem. Also does it backfire through the carburetors or the exhaust ?
Regards,
Joel

[david johnston]

It backfires when decelerating and my is off the gas..and I believe it's through the exhaust. though I haven't completely verified this.

[JOEL JENSEN]

Likely an exhaust leak or perhaps the points are gapped too tight. I'd check the points for pitting and correct gap, then timing and then exhaust leak. Have the valves been adjusted recently ?

[Mike Smith]

Usually an exhaust leak

Block the Tail Pipes (if you can) with your Hand / Hands protected by a rag - listen for where the gasses are hissing out

[Charles Foster]

I always start by looking at the J tubes for leaks.

Thanks,

Charles

--
Charles Foster
hc_foster@hotmail.com (hc_foster@hotmail.com)
http://www.northForkDesigns.net/Porsche356/

Subject: [356Talk] 1962 356B sudden occasional backfire.
From: davemich2000@yahoo.com
Date: Tue, 12 Aug 2008 19:51:20 -0700
To: 356talk@356registry.com

It backfires when decelerating and my is off the gas..and I believe it's through the exhaust. though I haven't completely verified this.

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[david johnston]

thank you for your help everyone. It seems there's also a consensus. J Tubes??

[JOEL JENSEN]

thank you for your help everyone. It seems there's also a consensus. J Tubes??

Exhaust can mean any point in the system. You need to isolate the leak, if in fact is isn't points, timing, valve too tight etc. Exhaust leaks can occur anywhere in the system and
a worn out j pipe is just one of them.

[PaulLear]

For posterity,

Which ARE the J tubes?

[JOEL JENSEN]

None of the above. They are the J shaped pipes that route exhaust from the 1 & 3 cylinders to the rear of the car.

[Guest]

David,
Sounds like an idle jet got plugged. Timing does not change
that fast. Try this test, when warmed up run the motor
slowly to 1800-2000 rpm and hold it there. If the motor
starts to backfire and pop or it sounds like it runs on just
3 cylinders, you have a plugged idle jet. Turn the gas off
and put a rag to collect the fuel from the carb and remove
the jets and blow them out and the carb side too.

Alan

david johnston wrote:

Hi folks, I own a 1962 356b and have been driving it quite a bit this
summer (2000 miles). (I have 2 young children so I don't get tons of
time) Carbs have always felt strong. However I recently went for an
hourlong drive and noticed that my rpms dropped as I pulled up to a stop
(normally it was fine). I also noticed that it backfired a little
bit...The first 1/2 hour was great, then it started happening. I ran it
for about 15 minutes tonight and it backfired again. I have a fuel
filter on my fuel line and had the cap, rotor, coil, sparks and wires
done about 3000m ago. I'm thinking about the timing and possibly a minor
carb adjustment, however I would love any feedback from the experienced
and technically inclined as to the order of which I should proceed with
my diagnoses. Thank you so much, David Johnston




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