Popping and banging

Captaintremendous · Post by **Captaintremendous** » Wed Sep 09, 2015 11:19 am

Gents
As the title suggests, my 1969 2litre has suddenly started popping and banging not only on deceleration but also when trying to maintain a steady speed. No problem under load and acceleration though. It is something that has developed literally overnight. I came back from a drive on which the car had performed in it's usual manner, however,as I pulled into my place I noticed it was suddenly, literally as I turned into the drive, sounding a bit "off". Next time I drove it, it was a little harder to start than usual and sounded rough whilst warming up, which it never used to. Once driving it was popping and banging as mentioned to the extent it was embarrassing. I've had a cursory look round, checked exhaust connections are tight, cleaned plugs etc, but don't want to start blindly attacking things till I've had some feedback from the experts on here. Carb settings are untouched from when the car was running well.
This is a sudden development as it was running sweet as a nut and has never had any popping and banging issues before even under hard deceleration on the track.
Any ideas?

KS · Post by KS » Wed Sep 09, 2015 11:36 am

Blocked idle jets. Have you run it low on fuel recently? Check fuel filter, and for junk in the bottom of the carb float bowls.

impmad2000 · Post by **impmad2000** » Wed Sep 09, 2015 12:24 pm

Is this running the Zenith carbs ? These have a rubber pipe that pops off and let's air in. You may be able to hear the vacuum sucking ?
Have a good look and listen for vacuum leaks.

Captaintremendous · Post by **Captaintremendous** » Wed Sep 09, 2015 3:07 pm

The car runs on Webers. I have just cleaned all jets, fuel filter and float bowls, which to be fair were all pretty clean anyway. Problem still persists. Any more for any more?

911hillclimber · Post by **911hillclimber** » Wed Sep 09, 2015 9:49 pm

Get a can of carb cleaner.
Start engine and warm.
Spray the cleaner accurately around the carb to manifold area and if the engine note does not change, drown the manifold to head area by cylinder.
If the engine note changes when flooding around a particular cylinder their is an air leak being filled temporarily by the liquid cleaner.
As that cylinder ingests the cleaner vapour the air gap will re appear and the pops and bangs will return.
Spray again and see if the note changes etc.

This has worked for me 2 times.

Lightweight_911 · Post by **Lightweight_911** » Thu Sep 10, 2015 8:47 am

Could be a faulty ignition condenser - also worth checking the distributor cap for hairline cracks ...

911hillclimber · Post by **911hillclimber** » Thu Sep 10, 2015 11:38 am

This was a sudden arrival, so pipe off etc as suggested?
It's gone quiet, so maybe the engine is too!

Captaintremendous · Post by **Captaintremendous** » Thu Sep 10, 2015 1:19 pm

Sadly not. Engine still doing the same thing. I am beginning to suspect something ignition related, but dizzy cap looks OK, as do points and gaps. Previous experience with condensers is they either work or they don't, there is no period of deterioration before hand. What about HT leads starting to break down?

Captaintremendous · Post by **Captaintremendous** » Thu Sep 10, 2015 2:13 pm

Not sure what 911hillclimber means in the first line of his last post?

jb · Post by jb » Thu Sep 10, 2015 4:50 pm

Dodgy fuel?
Did it start shortly after a fuel upload?

Captaintremendous · Post by **Captaintremendous** » Thu Sep 10, 2015 5:18 pm

No. As I said it was at the end of a good long drive during which the car was running great. I am interested in the comment about a vacuum pipe coming adrift with Zeniths. Is there a similar problem with Webers? No pipes adrift that I can see, but I had a similar problem with my Healey which runs triple SU's and was due to a loose vacuum pipe. Drove me insane till I eventually nailed it, having been everywhere else first!
Still none the wiser I'm afraid guys.

Bruce M · Post by **Bruce M** » Thu Sep 10, 2015 6:03 pm

If it is obvious while the car is static (ie engine running on the driveway) then pull each HT lead in turn until you find the cylinder which isn't making a difference. Narrows down the location.

Captaintremendous · Post by **Captaintremendous** » Thu Sep 10, 2015 8:32 pm

Good call. Will try it and see what happens.

alanjf · Post by **alanjf** » Thu Sep 10, 2015 8:49 pm

You are certainly getting the right advice. With my experiences over the past few years:
My land rover did exactly the same thing. Tried pulling all the plugs and no difference - blocked jet at low revs. i had to really check carefully as I had already cleaned the one and only jet. This time i used a small wire to make sure.
911 2.0, same again. This time I cleaned the distributor cap which made it better before it got worse again. Put on a new rotor and cap and back to normal. Do you have a used rotor and cap you can try?

Alan

Captaintremendous · Post by **Captaintremendous** » Fri Sep 11, 2015 4:56 am

Alas not Alan. May have to take the plunge and risk buying a few bits on spec and hope for the best. Talking of which, anyone care to recommend a reasonably priced supplier?

DDK - Die Deutschen Klassiker