I have a dilemma, I have placed postings before under the name ClassicFiat5500 so potential buyers can see what problems my little”Ava” has or have not.
The problem I have I am in my mid 70s & live in a nice apartment with an underground car park. As little Ava is highly tuned she is rock solid on the road with very little suspension, she is LOUD when I turn off the ignition she emits an enormous “BANG” (as i said as I live in a apartment & this resonates through the whole building waking the almost dead, NOT GOOD!), she is FAST in great condition only got caught in the rain once in 2 years. I feel it is time we part company, My dilemma is I want another 500 that I can drive with comfort & not have to worry if when I turn her off I am not going to switch off some of my neighbours at the same time. I would like to find someone who has a 100% reliable 500 who is looking for a Fast rust free TUNED pocket rocket.
Ava has a Weber DCO. oil cooler, 123 ignition, discs all round (not deal) adjustable coil springs, Abarth 3.5ltr sump with lots more tuned internal parts. her body is in dark blue with no marks. I have a fully listed spec of the work carried out over the last 2 years. As I said if any one has a very good condition rh drive 500 & wants to swap a pocket rocket in great condition please email me.

It's a shame you can't just find a way to stop the backfire. If you can find someone to do this swap I'm sure you realise that you risk taking on a whole load of problems which will need to be sorted in any case.

Having said that, I wish you well with the quest and I suspect that the supreme driveability of a well sorted but fairly standard 500 will give you at least as much pleasure as a tuned, hard riding speedster.

Yes I think I will persevere with Little Ava I love the car & she looks great, but can’t seem to find anyone by me (Woburn) to tune it correctly, it looks as if I will have to find someone with a trailer to take it into a garage, as far as I know Little Barton is the closest unless anyone out there can recommend a good mechanic to sort her out, I am not competent enough to tackle her myself. If I could wave a magic wand aver her to change the tuned lump for a nice quiet sedate smooth engine it would be great. The previous owner drove a 911 as a daily car & I believe he was trying to turn it into another Porker spending mega money at it but just gave up before it was finished.:bang:

Maybe you can tame the beast a little? Either way you are going to need help by the sound of it. Could it be that you have a reshaped Panda 30 cylinder head on there. Is one of the cylinder head studs and nut missing?

Hi Toshi Thanks for your input, I wouldn’t know if it has 6 , 12 or 16 studs! how many are there on a 650 cc 127 lump?

There should be 8 cylinder head studs/nuts---the one nearest the rear of the car seems to be missing (I believe it may also hold the inlet manifold in place).
I accept that the previouse owner spent a great deal of money on the car, but why oh why do people insist on fitting electric fuel pumps--the original mechanical pump is perfectly capable of supplying all the fuel that your engine requires (that pump is also used on the '850'). Most fuel pumps run at too high a pressure for the '126' engine---I wonder if that is contributing towards your problem---unused fuel being ignited in the exhaust system? Would it be worthwhile re-installing the mechanical pump (it would seem to still be in place) and see if that helps the problem?
I know you didn't fit them, but the gauze covers on the inlet trumpets seem to be very crude---there are proper gauze covers, which much neater, available which are held in place by a specific rubber ring, and I can assure you that they hold the gauze very securely. If you look on "gauze covers for Weber inlet trumpets" you will find a number of suppliers for the gauze covers (they come with the rubber ring). All you have to do is ascertain WHICH carb has been fitted as there are different sizes.
Please don't take this as any critiscism of you---people don't buy 500s for going fast, they buy them to have fun with, so why spend humungeous amounts of money trying to make something go fast that isn't designed to go fast. I accept that the Italians DO make them go fast, but at enormous cost. In original-parts format (ie not fitting steel cranks etc) the 500/126 engine won't rev much more than 6,000rpm---you aren't going to get big BHP out of it no matter how much money you throw at it.
Let's help you sort your problem out, and then enjoy the car for what it is---a little bumdle of fun. As for setting the engine up; any comptent rolling road/dyno businees should be able to do it. Among their equipment they should have a gas-analyser and a vacuum-gauge, which is basically all you need to set it up. Keep the car, have fun.

Sorry, missed a word out---it should read" most ELECTRIC fuel pumpsrun at toohigh a pressure forthe 500/126 engines"

I second Tom's fuel pump comment. Electric fuel pumps on small engines can be an evil, especially if there's not a return line or pressure regulator of some sort.


If that's not the issue, it sounds like you have a great opportunity to play with the jetting, since your baby clearly is getting too much fuel at idle. An "open" float is another possible culprit. Maybe something as simple as adjusting the float level could cure the backfire?

All that said, I've never met a Weber side draft that couldn't be made to run spot on provided it's properly sized for the engine.What size is your carburetor? Big Weber side drafts on small engines sometimes gets the majority of the fuel from the idle circuit all of the time, since there's not enough air flow through the venturis to draw fuel from the main circuits. To compensate for that and avoid fuel starvation many people go way too rich on the idle jets. The car will run fine when the butterflies are open-ish, but gets too much fuel when you lift off the pedal completely, and the excess fuel combusts in the exhaust pipe. BANG! Neighbors unhappy. Don't ask how I know.

Regarding my backfire is it possible this would be due to overheating ? I have noticed the cooling vent is staying shut when the engine is hot. As the backfire only happens when the engine has been running for more than 15 minutes. Is it easy to change the thermostat ? but my question is could this be the cause of the backfire obviously I need to get it onto a rolling road & have the timing & carb set up correctly.
Or could I just fix the vent open permanently. tithe car is only used in fine hot weather.
All advise welcome, Barry

The most likely cause of your thermostat flap staying open is a faulty thermostat. The themostat housing has to be removed to relace it, but when the housing is off, it is anly a few minutes work to change the thermostat. I would NOT reccommend that you have the flap permanantly open---both electronic ignitions (of ALL types) and condensers have a common enemy---heat.. When you have the thermostat working correctly. it would be wise to have the timing and carbs set up properly. Thor's (nuovo500) comments regarding the jetting of the carb is a very pertinant comment and one very wise to take note of.

Thanks Tom I will follow your advise.
Barry

Can someone tell me if I hold the air flap open (bypassing the thermostat) should i be able to feel a flow of hot air blowing when the engine is running at around 1500rpm? theoretically I should but there is hardly any. Today I am going to remove & check the thermostat I assume its straight forward to take out, any advice would be welcome.

You should feel some air-flow,although to be honest, I have never tried to see HOW much!
Removing the thermostat housing is not difficult (but make sure that the engine is cold!)-- there are 4 fixings at the front---1 into the crankcase, 1 by the manifold, a small 1 under the manifold and the 'hollow' bolt ----this is duplicated at the rear of the engine. There is also a small bolt on top of the housing, by the rocker cover. You might have to take the distributor cap off to wriggle the housing off. When you have the housing off, you will see that the termostat is very easily removed/replaced. It would be worthwhile testing the thermostat that you remove to see if it IS working. Put it in a jug of boiling water and you should see it open.

Thanks Tom is it possible to remove the thermostat without taking all the housing off? i am a little nervous of removing the 123 ignition I can see there are 2 nuts & 1 central nut with a locking flange, is the one with the locking flange holding the thermostat?

The big nut at the top of the thermostat housing is the top retaining nut for the thermostat. That is easy to remove---the hard part, and the reason that you have take the housing of is that the flap is spring-loaded (to stay open in case of thermostat failure) and the difficult (or, rather, mighty awkward) part is to fit the rod between the thermostat and the flap whilst the housing is in situ. You might find that by being very careful, you CAN remove the housing with the distributor still in place

ClassicFiat500 said:

Thanks Tom is it possible to remove the thermostat without taking all the housing off? i am a little nervous of removing the 123 ignition I can see there are 2 nuts & 1 central nut with a locking flange, is the one with the locking flange holding the thermostat?

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Don't worry about taking off the 123. If we talk you through replacing it, you might find it runs better/cooler.

I will check the thermostat next week, but on another thought re the unburnt fuel explosion, one of my neighbours is an ex BA flight engineer, he was intrigued by the fact the bang happens only when the engine is at its hottest, hence he suggested fixing a 6” booster fan with a thermostat (set at 68º) into the air intake at the top of the engine behind inside the grille above the boot (trunk) opening this theoretically would force more cooled air onto the engine cooling fins then out through the existing thermostatically controlled flap. His theory is the existing fan is probably fine for a standard 500 but as the engine is highly tuned the cooling also needs to be upgraded.
Any comments are welcome.

In reality you do not know what is going on with that engine. If it has a missing cylinder head stud or studs then I would be worried about what else has been neglected. Probably no help to you but if I owned the car I would feel that the engine needed a total strip down by someone who knows what they are doing. Having said that I have known these engines to overheat if the big jumbo hose is not connected or the grommets that seal in the cooling air around the spark plugs are missing.

To the best of my knowledge, Abarth used a standard fan (and fuel pump) on ALL his road going permutations on the 500. Adding a 'cooling fan' isn't finding out the cause of the problem, just trying to find a way round it. I am stillvery suspicious of the carb set-up, and possibly the timing. You mention that you have the '123' ignition---is that a complete unit (doesn't look like a normal distributor) or a 'add-on' which still utilises a distibutor cap? If it is the 'complete unit' it would be interesting to see what programme it has been set to.

Looking in more detail at your images, the previous owner has clearly had many issues with heat. There are plenty of attempts at shielding from the heat, but the combination of that extra padding together with all the pipework and other paraphernalia leaves a reduced space around the engine which will be counterproductive to cooling.
Your alternator pulley looks non-standard and the impeller may not be going full speed.
A standard carburettor arrangement draws from the forced air in the cowling before it has been passed over the engine; therefore it is cool. A setup like yours draws engine compartment air, which in your case must be incredibly hot. It will also have a negative effect on the natural flow of hot air from the engine compartment.
If your spark plugs are glowing hot under all this heat pressure, the chances are that any residual fuel vapour left in the combustion chamber when you switch off will reignite and suck in a little bit more. Have you tried letting the engine idle for a few minutes before turning off?
I'm beginning to think that your original decision was the best one as the installation, although very expensiveand well intentioned, is flawed.