Hi. My 74 1.8, 123ignition with single Weber 32/36 has developed a new sound. A buddy pointed it out and now I cannot unhear it. It seems to have started after a 15mi drive (long for me - I am still getting everything dialed in).
At all speeds under 2500 I get a steady tapping or knocking sound at the speed of the engine. It sounds louder on the passenger side but with the stethoscope #2 the valve tappety tapping sounds a bit louder than the others (-scope on the intake nuts). Not sure if it is a normal sound for a T4 engine.
Valves checked last weekend at .006. Timing 7.5 BTDC at idle. 123 increases advance linearly +22 between 1000 and 3200RPM (total advance 30). Plugs gapped. Carb tuned.
Compression at 1=120, 2=120, 3=100, 4=140.
I tried snugging up the exhaust manifold bolts (#2 was a bit loose) but no change.
Power is fine but it does backfire on deceleration.
Pic of plugs (1-4 bottom to top) below. 4 has some carbon.
I’d appreciate some insight. Different diagnostics? Ready for tear down? Nothing to worry about?
Thanks a ton.

I just remembered that this started after a gas fill up. Is there any chance that could cause this?
I really appreciate any expert advice you can share.

Back fire could be an exhaust leak.

Thank you! I will double check the exhaust with the rubber hose trick tomorrow.

I read that detonation knock is not likely at idle. Is that true?

This link should go to a YouTube video of the engine at idle:
Engine at idle.

Yes. Detonation typically occurs under high heat & high load conditions - not at idle.

I listened to the video but don't have anything conclusive for you. I think I hear exhaust leak but cannot tell with any degree of certainty. I don't hear anything catastrophic.

3 plugs appear very white (lean) and 1 is black (very rich). It may be that the white plugs are really more of a gray (OK) but just look white to me due to lighting and/or by comparison the the black plug. You've got to address why that cylinder that is so rich that before worrrying too much about noises.

Given that you're running a single carb setup, it is very odd to have one cylinder running so rich. I'd be checking out spark plug wire to that cylinder. Is it sparking properly - every time? Maybe not lighting off combustion reliably in that cylinder? Something odd going on.

Imbalanced combustion events can cause some odd noises in air cooled engines where you pretty much hear everything. Likewise that rich cylinder dumping unburned hydrocarbons into the exhaust could very well be the source of your backfiring when it lights off in the muffler.

Check your valve lash first.
Once that is ruled out or fixed start looking for a exhaust leak
With the engine running (idle) have buddy place a rag over the tail pipe to create pressure in the system. Listen with stethoscope or Hose to ear to isolate the leak. Focus on the exhaust connection to the heads and head gaskets, OR air injection tubes (for late heads)

Back fires/ on deceleration is very typical with leaking exhaust

Lean pops out the carbs

Thanks Superhawk996 and rfinegan. This is really helpful. I especially like that nothing sounds catastrophic! Not sure how I could get by without this forum...

So here is the plan:
White plugs = they are much more white than gray (except for #4). I think the answer to that (to get it to run more rich) is rejet the carb - either the idle jet and/or primary. Does that sound right? Then running it and re-reading the plugs. Need to do some homework on the rejetting.
Black plug = deal with this first: the #4 wire came out of the connector as I was removing it to do the compression test. So perhaps was loose? First check for spark. Then make sure that the HT connections are all really tight and hopefully that solves it.
Valve lash = I checked them before my drive last weekend and all were at .006. I can check again. Out of interest, was this suggested because of the clackety-clack in the audio recording?
Exhaust leak = perform the tube trick as described. Address any leakage found. I may be back if/when I get to that point!

I am not sure I will get to all of this before the weekend but will report back when I do.

Again - many thanks!!

For a single outlet exh just press a piece of 2x4 over the outlet. It will load up and stumble if the system is sealed. You'll hear a leak when it's running.

UPDATE***

I tinkered with the car this past week and got some driving time in today.

#4 plug, which was black when the other three were white, did not have a good connection. I tightened up all the HT wires and after a good run and stop without letting it idle and now #4 looks like the others (see below). Easy answer.

#4 does have higher resistance than the other three. 5.5 KOhm vs 4.8. Not sure if that much of a difference could also have contributed.

I did the hose trick to find exhaust leaks and found nothing. And generally things sound smoother/ better. The knocking I was hearing may have been the inconsistent firing of #4!?

Plugs 1-3 looked pretty white - running rich. I opened the idle mixture screw and ran, as above, before reading plugs. All plugs are now a nice light tan. Maybe still a bit lean so have ordered some jets to see if it can be dialed in better.

The guidance I got on this one was really great. Thanks again for helping out.

Click to view attachment

Nice Job. Glad to hear its running better.

Yes not having consistent combustion in #4 can cause piston slap noise. Both from the uneven firing, as well as the piston not getting hot and having excessive piston skirt to cylinder clearance.

Just to keep things square . . . white plugs indicate engine running lean (not enough fuel) and the black plug was rich (too much fuel). Opening the idle mixture screw will make things richer. The blackness in your case on #4 was probably coming from a cold combustion chamber and intermittent spark.

Drive the car for a bit and then read the plugs again.

I don't think the difference on spark plug wire resistance will matter but re-reading the plugs after a few more good drives will tell you for sure.

I am checking back in on this one - most issues are sorted thanks to this Forum.
Fixed loose HT wire to #4
Torqued all plugs to 25 ftlb
Rejetted carb - up by 10 for both primary and secondary
Listened for exhaust leak but did not find any
Ran 15mi today. Plugs have a bit of brown now - may go a bit further ...
It ran cooler, with more power and and much less backfiring
Only backfires when decelerating in gear (foot off gas)
All in all a great outcome and it's more fun to drive to boot!

Sounds like you're making progress.

Post pictures of the plugs again when you get a chance. Light brown is where they should be.

If you're getting backfire on decel and you're sure you don't have exahust leaks, I'd turn the idle mix screw out 1/4 - 1/2 turn and see if that helps without compromising idle too much.

Hate to even bring it up. Loose valve seat?

Update - re-read of plugs (pic below) after carb re-jet show 2-4 more on the brown side but 1 still looking lean. Planning to bump them up another size to shoot for a consistent brown.

Regarding dropped valve seat - I sure hope not!!!

Click to view attachment

UPDATE
I upped the main carb jets by 10 to 160 primary and 170 secondary.
Ran it today and read the plugs. I think they are looking much better ... but #1 (on the left) still looks a bit lean
Thoughts on whether to bump them up again?

Click to view attachment