I have an annoying problem after running my car for more than a couple of miles when i switch of the engine it nearly always responds with a loud gunshot from the exhaust. I have read this is due to unburnt fuel in the silencer. My 652cc engine is fitted with weber 40dcoe (not ideal) & high lift cam with 123 ignition. I also have an electric fuel pump with a cut off switch on under the dashboard. As i can’t seem to stop the backfire.
Could anyone advise if I were to switch off the fuel prior to turning off the ignition, 1st would it do any harm? 2nd is it likely to cure the problem? The main reason for trying to stop the backfire is we are moving to an apartment with an underground car park & I don’t want to upset our new neighbours before we have moved in.

The backfire even on standard spec cars is often associated with valve clearances.
My very experienced mechanic pal has removed the 40DCOE he had on his engine and replaced it with a period Weber 30DIC twin choke downdraught carb.

I agree with David (Toshi)--the problem could be as simple as valve clearances. If an exhaust tappet is too tight, it might not be fully shutting. I would also get the timing checked (10 deg BTDC at idle). I also agree that a big side-draught carb is more than a tad OTT on these engines. IF you can lay your hands on one, try a Dellorto FZD. But beware--there are what look like 'real bargains' on the market--they were NOT made by Dellorto and are not worth the money. The "Chinese" versions do not have the word "Dellorto" or the words "Made in Italy" cast in to the side of the float chamber. Look for a good 2nd hand one (any size, 30/24 or 30/26 or 32/28). The manifolds are still available 'over the counter' from a variety of suppliers.
If you have any Italian connections, be very nice to them, and ask them to look for one on the Italian e-bay sites. Set up well,I doubt if you will notice much,if any, drop in performance.

Thanks for the advise on the carb from both of you, I know you are correct. as a temp measure we hope to be moving within the next 3 weeks so I am hoping to sort this out imminently. If I switch off the fuel before turning off the ignition can it do any harm? but more than that should it stop the backfire. I know its not the correct way but i don’t want to upset the new neighbours.

Hi CF500, As Dave says check the valve clearances first and then check if you have excess fuel dripping out of the carb throat during tickover.
The 40 DCOE weber must be set up carefully with the venturi and jetting adjusted to give consistent running throughout the rev range. If you are happy with the venturi/jetting check the delivery pressure, it's possible that your pump maybe 'overcoming' the carb and causing it to dump extra fuel into the manifold on shutdown. It is possible that the carb is just running too rich on it's idle circuit.
while you are at it check the plugs - light brown??? good - sooty/black/wet??? - bad. If you have a tuned engine do you have cooler plugs??? Has the engine done this always? It seems to me that you should check the whole set up.
Ian.

ClassicFiat500 said:

Thanks for the advise on the carb from both of you, I know you are correct. as a temp measure we hope to be moving within the next 3 weeks so I am hoping to sort this out imminently. If I switch off the fuel before turning off the ignition can it do any harm? but more than that should it stop the backfire. I know its not the correct way but i don’t want to upset the new neighbours.

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Hi CF500, turning the fuel pump off will only starve the carb and it will take a few minutes to run it dry. You won't do any harm.
Ian

Hi, My 500L fitted with a 652cc fiat 126 engine used to do this every time I switched the engine off, It was well known amongst our club members for this, so much so it was known as Poppy. I often had people diving for cover in the street when I stopped the engine. After one run it "popped" so loudly it blew the rear exhaust pipe apart just after the elbow, a new exhaust system was then needed. After fitting the new sports exhaust system, AND MAKING NO OTHER ADJUSTMENTS to the engine the explosions stopped completely, quite sad actually as it was quite endearing. Obviously the exhaust system was the sole reason for the "gun shots". I wouldn't worry about it, mine was "Popping" merrily for about 8000 miles before it blew the exhaust off, just start saving for a new exhaust system now.

Re-reading your request for help, and Ian's answer, I realised that I had missed the fact that you have an electric fuel pump. I really don't know why people fit them. There is a good chance that it IS running at too high a pressure and overcoming the needle-valve in the carb. Abarth only ever used the original mechanical pump, even on the 695SS models---an electric fuel pump is NOT required on the 500, no matter how much they are tuned for road use. You have a pump cut-out--is it an automatic cut-out so that in the event of you (hopefully never) having a crash, and the engine stops, the fuel-pump stops as well? If you haven't, you should have. I would suggest that if possible, you go back to the standard mechanical pump which generally speaking, are very reliable.

Check for any leaks round the exhaust manifold and manifold to pipe joints.

:bang:Seem to have found a cure, but not ideal as it hasn’t actually cured it but it has stoped the Gunshot. I have a fuel cut off for the petrol pump under the dash. As I drive into the garage i cut off the fuel switch that serves the carb & obviously stops the engine “SILENTLY “ it takes a couple of minutes though, so it’s a case of timing it right.

Very interesting---Does your carb have a support bracket John? I noticed that it is bolted solidly to the inlet manifold, which I find strange. The side-draught carbs can suffer from foaming of the fuel if the carb is allowed to vibrate, which is why most side-draught carbs (Weber, Solex, Dellorto) have flexible spacer-mounts between the carb and the inlet manilfold, and the carb then has a support bracket which is affixed to the carb between the carb-body and the inlet trunpets and down onto the base of the fan's top cover. Being that the 500/126 engine is a fairly vibratery twin, this can only exacerbate the foaming problem

the hobbler said:

Very interesting---Does your carb have a support bracket John? I noticed that it is bolted solidly to the inlet manifold, which I find strange. The side-draught carbs can suffer from foaming of the fuel if the carb is allowed to vibrate, which is why most side-draught carbs (Weber, Solex, Dellorto) have flexible spacer-mounts between the carb and the inlet manilfold, and the carb then has a support bracket which is affixed to the carb between the carb-body and the inlet trunpets and down onto the base of the fan's top cover. Being that the 500/126 engine is a fairly vibratery twin, this can only exacerbate the foaming problem

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Hi Ian There is NO support bracket Carb just bolts straight to the inlet manifold. It all seems to work but tick over is quite fast (above 1000rpm) any slower its very lumpy & tends to stall, I think that maybe due to the very lumpy cam. The guy I bought the car from spent a fortune on it and just wanted the fastest 500 he could have & left it to a garage to do their best, I should underline THEIR best & not the ideal set up.
All the best Barry

May I suggest a couple of things to do that might (a) help to produce a better idle and (b) reduce the 'popping and banging' when you turn the engine off.
(1) Fit rubber 'spacer-mounts' between the carbs and the inlet manifold along with a support for the carb. The mount will fit between the carb's body and the inlet trumpets, and bolt on to the flanges on either side of the fan's top cover and (2) have a word with you local 'rolling-road' and organise for your car to have a 'set-up' session on it.
The big 'side-draught twins' need careful setting up, vis-a-vie the choke sizes and the jets. This is NOT a 'by the ear' job---you need somebody who knows what the're doing and a gas anyliser.
These jobs should not reduce the power of the engine (a properly set up carb might even improve it) but will probably make it a much more pleasant car to drive.

:stupid:
Hi Ian SORRY Went to click “LIKED” & clicked disliked by mistake Whoever designed the website should move them further apart, I have seen other members do the same.
100% agree with what you say, the biggest problem down here is finding someone who knows what their doing. Shame your not near Bedford. (in one way it’s probably a good thing otherwise the car would be with you more than me)
All the best & sorry again for the dislike mistake.
Barry

Try "Cartune"---they are based in Cuffley (Herts) but will come out to you. They are experts tuning the Weber DCOE range. Give them a ring--they be the people you need. But first, fit the 'spacer-mounts' and the support brackets in order that the carbs are properly mounted for the tuners.

berkeleyman said:

Hi, My 500L fitted with a 652cc fiat 126 engine used to do this every time I switched the engine off, It was well known amongst our club members for this, so much so it was known as Poppy. I often had people diving for cover in the street when I stopped the engine. After one run it "popped" so loudly it blew the rear exhaust pipe apart just after the elbow, a new exhaust system was then needed. After fitting the new sports exhaust system, AND MAKING NO OTHER ADJUSTMENTS to the engine the explosions stopped completely, quite sad actually as it was quite endearing. Obviously the exhaust system was the sole reason for the "gun shots". I wouldn't worry about it, mine was "Popping" merrily for about 8000 miles before it blew the exhaust off, just start saving for a new exhaust system now.

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Hi Berkleyman, when you changed your exhaust do you remember what make you bought or where it was from?

sorry it was quite some time ago now, all I remember is that it was described as a sports exhaust, might have been bought from Moto Bambino but not sure

Hi berkleyman some time ago you responded to my question re popping when I turn turn off my engine, you said after the you fitted a new silencer your gunshot cured itself. I have now moved into an apartment with an underground car park, when I switch off the engine it sounds like a cannon going off as it resonates through the whole building, I haven’t used the car for three months as I have been waiting for my weber 40 to be serviced, now just re-fitted it, I have noticed the manifolds go into the centre of the silencer on my car with one large outlet from one end. When you bought your new silencer do you remember where it was from? Also where do the pipes go into the box I.E from the opposite end of the exit pipe.
I am trying to find as quiet exhaust as poss without fitting the standard silencer with the oval exit pipe.
Valves are all set in correct positions .
Carb overhauled .
Looking for the best place the pipes go in to the silencer & best place to exit , single or twin pipes!
Hope all that makes sense. Barry Classic Fiat 500

Hi Barry; I bought my 'sports' exhaust from Middle Barton Garage about 8 years ago---both pipes go down to the centre of the silencer and it has a single 'out' pipe. It is fruity, but most importantly, still in excelent condition.

Hi Barry, I feel your pain, so much that I have been through my archived receipts and found the details. Mine was an ebay purchase made in Jan 2015, It is a Fiat 126 Michael Speeder Sport Exhaust and cost £67.50 total. I cant get the hang of adding photos to this post but if you PM me your email address I will send them. I was disappointed when I lost my POP, it was hilarious, on one occasion I had two elderly ladies diving for cover hugging each other, they were laughing though. On one Micro Maniacs run out we pulled into a pub car park en mass, my passenger and I waited for the bang on switch off but it never came? on finding the pub closed we returned to the cars some 15 minutes later, as soon as I switched the ignition on there was a massive CRACK and several club members hit the deck, what fun. good luck, Eric