Technical Popping exhaust

[Flock]

I've searched the forum, the internet but can't seem to find the reason my exhaust is popping on deceleration, except for the obvious...the sportexhaust.

Lately, thanks to this forum, I transformed from 'not touching the car' to 'i'm a mecanic.' (Avoid my wife, she is not happy with that.) What started with starting trouble, was fixed by changing the carb. As always, once I get going there's no stopping me so I basicly did everything I was afraid off before. Checked and set valve clearance, set timing (static with bulb, advanced with super fancy timing light), checked condensor, changed and learned how to read spark plugs, set breaker point gap, checked for air leaks carb etc etc.

The engine is running perfect. Starting on first try, after 20 seconds of choke I get it to idle steady with choke off, even at very low rpm if I want. The one thing that I can't get rid off is the popping (and sometimes a small bang) when letting of the gas. It never happens on idle or acceleration. This was driving me nuts because all seem to be ok. Than I started thinking.....could it be that the sport exhaust causes this and should be seen as a feature? (not one i like.)

From the moment I got it (6 months ago) i noticed some popping on decel from the back but being a nood, I never investigated it. Lately I discovered that I could manually rev the car from the engine compartment (duhhhh) so I can clearly hear it coming from the exhaust.

Am I searching for a problem that doesn't exist?

- Imb 28 carb
- 650 cc
- wasted spark

 

[varesecrazy]

Hi

Popping is generally the exhaust tubing "sucking air",
and letting unburnt fuel in the waste gasses
burn/explode with this newfound oxygen.. And heat .

Not really a major problem.. But you can try better sealing if you want

 

[RalphM]

Yep 99% sure it will be a air leak on the exhaust system.

 

[Flock]

Any smart way to check for exhaust leaks?

 

[varesecrazy]

Any smart way to check for exhaust leaks?

Hints of Soot on joins and welds..
It doesn't need much of a gap..

 

[bugsymike]

Any smart way to check for exhaust leaks?

Get a piece of thick cloth and hold it tight over the tail pipe, it will show if any leaks in exhaust system, which as others have mentioned is the usually reason for exhaust popping on the over run, if no leaks engine will stop

 

[Flock]

Did the test and engine kept running. Hard to tell where the leak is since the exhaust is wrapped in band. Anyway...now i do know why the pops are happening.

Thanks again everybody.

 

[bugsymike]

Did the test and engine kept running. Hard to tell where the leak is since the exhaust is wrapped in band. Anyway...now i do know why the pops are happening.

Thanks again everybody.

Try putting a little oil down the carb whilst running to make it smoke, then block exhaust with cloth.

 

[Flock]

Removed the heatwrap and blocked the exhaust...keeps running like nothing happened (just very quiet).. can even rev it without stalling. Felt with my hand, looked for smoke, listened, tried with a piece of toiletpaper to see if it moved somewhere....nothing to detect.

How do I add oil to the carb?

 

[bugsymike]

Removed the heatwrap and blocked the exhaust...keeps running like nothing happened (just very quiet).. can even rev it without stalling. Felt with my hand, looked for smoke, listened, tried with a piece of toiletpaper to see if it moved somewhere....nothing to detect.

How do I add oil to the carb?

With engine running, if you have a proper oil can with engine oil in it squirt a few drops in, otherwise get a small cap full of engine oil from a can and slowly tip it in the top of the carb after removing the air cleaner. If you do it too fast it may choke it, you may need to rev it a little.

 

[Flock]

I just felt with my hand around the exhaust studs (engine off) and at the back below the stud there was oil. So I guess that could indicate a blown head gasket?

 

[bugsymike]

I just felt with my hand around the exhaust studs (engine off) and at the back below the stud there was oil. So I guess that could indicate a blown head gasket?

View attachment 434766

That could make a spitting noise as you rev up if head gasket blown, but also if a leaking exhaust manifold gasket in same area.
If you can do it without burning your hands is it possible to hold the thick cloth just at the exhaust manifold part?
Bare in mind it may start to smoulder, says the voice of experience.
If you get to the point of removing the exhaust complete you may find a gasket missing or a warped flange causing the problem, it can also leave a stain mark as evidence so check carefully.

 

[Flock]

Yeah, will try that as soon as possible. Engine doesn't make any weird noises when running or revving up, it's just the popping after I rev it up hard and let go to idle quick. If I do it slow their is no popping/occasionally bang from the exhaust.

When I look better at the picture it does look like the inside of the hollow part is clean, and only the outside is dirty. I could be that I confused the black on my fingers for oil but it actually was root. When I wiped a it clean with a cloth it did look like root (dry)

 

[Flock]

Ok, almost burned my hand but I defenitly felt air popping between the elbow and exhaust pipe on the back. So I guess i will have to first change the gaskets over there before convincing myself to explore deeper in the engine.

 

[The Panda Nut]

I always found on old engines tining with a light was bad news. Too precise. With sloppy cam chains and worn csrb and distributors best resukts are setting withe the lamp and the doing up the clamp so you can still move the distributor then make final adjustment by ear. A tiny bit of retardation may help.
Same with the carb tunibg by ear is best.

 

[Flock]

I found out myself that setting timing with a bulb was quite hard because after rotating the engine 360/720 the timing seemed to change. I guess that had to do with my bulb because it was never on/off, but could also be dimmish. So now I set it with a voltage detector which is really on/off. Than i rev the car, see how it responds and check with a timing gun if all seems to be okish with the marks. Final check is actually driving the car and how it feels.

Anyway, car runs beautifull expect for the pops. I do not like the pops. It's a tiny car, it allready makes a lot of noise because of the non stock exhaust and all the popping and bangs just don't match the cars (and my own) age.

 

[Cornish Ian]

I found out myself that setting timing with a bulb was quite hard because after rotating the engine 360/720 the timing seemed to change. I guess that had to do with my bulb because it was never on/off, but could also be dimmish. So now I set it with a voltage detector which is really on/off. Than i rev the car, see how it responds and check with a timing gun if all seems to be okish with the marks. Final check is actually driving the car and how it feels.

Anyway, car runs beautifull expect for the pops. I do not like the pops. It's a tiny car, it allready makes a lot of noise because of the non stock exhaust and all the popping and bangs just don't match the cars (and my own) age.

Maybe your car thinks it’s a Lamborghini. Plenty of crackle and pops with them.

 

[Flock]

Lots of pops and bangs coming out of the owner also

 

[FR85]

I have similar issues, same set up except for the wasted spark.
I have not yet had a minute to examine the exhaust for evidence of a leak but what I did notice tonight while running the car at a fast idle using the hand throttle is that the exhaust manifolds and the headers actually glow red so I dont know if we will ever see evidence of carbon build up from a leak as it'll burn off with the head of the pipes.

Also, I have a small leak at the base of the carb, it's not drawing air in that I'm aware of but when I turn the car off I can see fuel bubble in the heat from the top of the head and base of the bakelite spacer.

I have new carb gaskets, bakelite spacer, exhaust gaskets ect there to go onto it but I've not had the time. I may hold off until closer to the summer as winters in Ireland are wet so driving will be minimal therefore I'll not really know if I have gained or lost the battle. I personally think my issue is with ignition timing, I have only set static timing and a mate of my dads who is old school is going to come over and time it by ear as was suggested above by Panda Nut.

 

[FR85]

Flock, how're you getting on with this?

I'm in the exact same boat as you buddy!