Popping on overrun

[triumphelan]

My car has just started popping on overrun .I have checked for exhaust leaks nothing is obvious,the manifold nuts I could tighten a little .Anything else to check?

[rt6810]

I'd look at air leaking into the inlet manifold, down wind of the carbs. 'O' rings perished? Loose nuts? Don't over-tighten though - the carbs are supposed to move a little to prevent fuel frothing. Good luck.

[nebogipfel]

Sorry you will probably have thought of this but I don't suppose you have recently fitted a new silencer with less back pressure?

[type26owner]

It takes extra fuel which makes it's way into the exhaust to get it a popping. The airflow is limited by the closed throttles so any extra fuel that does not get burned will get swept into the exhaust. If you've got Webers then very likely the float bowl level is set too high or a needle float valve is leaking. The Lotus manual sets the fuel level within 1 mm of it spilling over into the passageway leading to the throat. That's a problem because #1&3 cylinders will splosh fuel under overrun deceleration. Lower the level another millimeter so the level as measured from the top of the carb body is 25mm down into the 'well'. A properly tuned twinkcam will NEVER pop or backfire.

Hey, wait a minute you have been fiddling with the Webers! A leaking needle valve could cause the excessive fuel consumption that you've reported. Are all the idle mixture screws open by the same amount? What kind of fuel pump are you using?

[type26owner]

On overruns the mechanical fuel pump is at it's highest output but there is very little fuel being removed from the float bowls at the same time. This is when a leaking needle valve(s) will be a momentary problem. A lot of fuel can get dumped into the throats so much so #1 & 3 spark plugs can wet foul while the engine is spooling down. Once the flooding stops the plugs will relight again like nothing is wrong.

The other problem to look for is the wear on the pivot pin of the floats. If that is excessive then this sort problem is likely because the effective pivot axis shifts due to the inertia of the fuel. It's a bit too much for me to describe in great detail but the end result is the floats are forced upstream and the valves are opened up while this happens since the lever arm moment is reduced. Fussy business resizing the hole in the floats back down to nearly perfect to stop the shifting. The pins are still available put the early brass floats are not.

[type26owner]

The other suspect is the cold start units. If the piston is lifted up by just 1mm from being fully closed then the fuel passageway to feed in fuel is uncovered. The induction system vacuum on overrun can shoot up to nearly 28 inches of mercury and having the a cold start unit hung open is a problem. The idling vacuum ranges between 18 in-hg and zero as it pulsates and this the time the cold start unit is designed to provide an overly fat mixture. Given a higher vacuum level which does not fall all the way to zero means the fuel is being sucked in at the highest flow capacity possible. Even if the piston is fully closed these units are subject to fretting wear and will develop a fuel leak no matter what after a couple of hundred thousand miles. Had to plug off mine recently due to leaks.

[purplepete]

Hi,at risk of everyone calling me a twit,before you start looking too deeply for solutions to your problem,you might consider sticking a timing light on it.Its all too easy to get bogged down looking for ever increasingly technical problems to blame-start with the most basic things first.
good luck.peter

[type26owner]

Seems too obvious but it also requires enough oxygen for any extra burning of fuel in the exhaust to take place and make a popping noise. An airleak in the exhaust system is highly likely too. Any chuffing sound at an idle is a dead giveaway.

[elansprint71]

.... before you start redesigning the carburettors, have you put in some different fuel? Then the timing and other SIMPLE stuff.

[purplepete]

....Hes already said that he checked the exhaust!!
What are the parameters here?Is it a daily driver?Conventional ignition or electronic??Was the popping on a long journey,or a nip to the local shop??Was the choke pushed in fully???
my first suggestion would be that you give your elan a couple of hours of tlc and check all the simple things in the process-nice sunday morning job!!,then a good excuse to take it for a blat in the afternoon.

[bill308]

Popping on overrun with Webers is ususally a sign of a rich idle mixture.

What do you spark plugs look like?

If they are a little on the dark side, see if closing them a flat or two alliviates the issue or at least improves it. This is relatively easy to do. Check also that the air flows, throat to throat, are also equal.

Bill

[type26owner]

Hes already said that he checked the exhaust!!

Gee golly, I read that already but the facts lead me to believe it's not true. The only 100% reliable way to leakcheck an exhaust system is to fill it with a positive pressure from an automotive smoke diagnotic machine. What are the chances he's used the correct diagnostic tool? The stock exhaust is a total bugger to get a tight seal at the collector.

Why don't you guys do a google search on 'overrun popping' and prove I'm wrong. What does the timing have to do with this? Please find me a single technical article written by a professional that describes the timing causing popping please. You will find references that claim it's caused by the mixture going too lean but that's highly unlikely with a Weber unless you've got the mother-of-all flatspots.

[type26owner]

I would feel somewhat safe to believe this reference resource.
http://www.mikuni.com/tg_backfires_in_exhaust.html
Perhaps with a 22:1 mixture when the idle mixture is set to the Lean Best Idle this is possible on the twinkcam. You might try and open up the idle mixture screws another 1/4 turn if it's pair or early carbies. This would also account for a tight exhaust system. Unfortunately this stuff is not this simple most of the time.

If you set the idle mixture screws to the halfway position between causing a lean or fat misfire then it will be right around 12:1. That's what I've set mine to. On overruns the mixture will climb to no higher than 17:1. John, you might try this and see if the Mikuni advice works for you.

[types26/36]

I tend to agree with Keith that if nothing else has changed, carbs have not been tampered with,no other work has been carried out and the popping just started out of the blue then the most likely cause would be an exhaust leak followed by a air leak i.e. vac. pipes, carb O rings etc.
Exh. leaks can sometimes be hard to spot, I've had leaks at the manifold to head that could only be detected when the engine is first started (cold) as the engine heats up the components expand and the leak is reduced.
Brian

[triumphelan]

Found it ,The weld has gone where the pipe enters the box ,not completley just enough to show some telltale black marks .As the box is rathered battered I have ordered another from CN .