Technical Engine won’t stay running.

[the hobbler]

I don't know what the rate of flow should be i'm afraid; I do for a classic smallframe Vespa, but doubt that'll be much help!
I've got a vague memory of a friend of mine having an issue with the pump on his 500, something to do with the travel on the pushrod that operates the diaphragm. He did a video on it, i'll see if I can track it down...
Aftermarket parts can be a minefield, not just in the Fiat world, but classics in general, as i'm sure you know! I'd consider trying one of the other pumps in any case, as an easy elimination route.

if the pump IS over-supplying, the needle-valve SHOULD stop the excess flow. The adjustment forthe fuel-pump operating rod is that it should extend out past the gaskets and spacer 1 to 1.5mm at the very BEGINNING of its movement. The easy way to measure that is to lay the correct thickness of feeler-strips alongside the tip of the rod, up against the last gasket. I must admit, I am still of the opinion that it is the carb that is over supplying fuel. Did you say that you have checkedthe float? If there is any fuel in the float (and if there is, it wil l'rattle'as you give it a vigorous shake), that will cause the engine to run rich.

 

[Arancia]

I don't know what the rate of flow should be i'm afraid; I do for a classic smallframe Vespa, but doubt that'll be much help!
I've got a vague memory of a friend of mine having an issue with the pump on his 500, something to do with the travel on the pushrod that operates the diaphragm. He did a video on it, i'll see if I can track it down...
Aftermarket parts can be a minefield, not just in the Fiat world, but classics in general, as i'm sure you know! I'd consider trying one of the other pumps in any case, as an easy elimination route.

I don't know what the rate of flow should be i'm afraid; I do for a classic smallframe Vespa, but doubt that'll be much help!
I've got a vague memory of a friend of mine having an issue with the pump on his 500, something to do with the travel on the pushrod that operates the diaphragm. He did a video on it, i'll see if I can track it down...
Aftermarket parts can be a minefield, not just in the Fiat world, but classics in general, as i'm sure you know! I'd consider trying one of the other pumps in any case, as an easy elimination route.

The Vespa sounds fun!
If you do manage to locate that video I’d be happy to see it for sure.

 

[Arancia]

if the pump IS over-supplying, the needle-valve SHOULD stop the excess flow. The adjustment forthe fuel-pump operating rod is that it should extend out past the gaskets and spacer 1 to 1.5mm at the very BEGINNING of its movement. The easy way to measure that is to lay the correct thickness of feeler-strips alongside the tip of the rod, up against the last gasket. I must admit, I am still of the opinion that it is the carb that is over supplying fuel. Did you say that you have checkedthe float? If there is any fuel in the float (and if there is, it wil l'rattle'as you give it a vigorous shake), that will cause the engine to run rich.

Thanks again Hobbler.
The float in the carb is good - checked it and set its levels. Hadn’t thought about the fuel pump set up. I simply put it on with the ‘black block’ sitting behind it. It has a new rod fitted - lost but then found the old one. Should I have more than just that black mounting block behind the fuel pump?

 

[the hobbler]

Thanks again Hobbler.
The float in the carb is good - checked it and set its levels. Hadn’t thought about the fuel pump set up. I simply put it on with the ‘black block’ sitting behind it. It has a new rod fitted - lost but then found the old one. Should I have more than just that black mounting block behind the fuel pump?

Yes, there should be more than "just the black box" (the 'black-box'is in fact a spacer). There should be a gasket between the crankcase and the spacer. The book says a 0.7mm gasket, but I just fit "a gasket" and then a gasket (the book says a 0.3mm gasket) between the pump and the spacer. You then adjust the total thickness of the spacer and the gaskets (so that you get the correct protruberance of the pump's operating pin) by adding gaskets between the spacer and the crankcase. But I still think that the fault lies in the carb.

 

[Arancia]

I’ll be checking what I did with the fuel pump gasketing today - I can’t recall if I added a gasket between the spacer and the pump.
Having spent a lot of time trying to ensure I had one ‘good carb’ from the two that came with the car, coupled with the small fortune I’ve spent on this project so far, I took the plunge and ordered a new Weber carb from Ricambio yesterday.
As for the air bubbles in the fuel line (pleased I used the original green transparent fuel line), I have tightened up the various joints between the tank and the carb. Only issue I could find was a small nick in the rubber gasket under the sender unit.
Let’s see what the weekend brings.
And thanks again.
Peter

 

[the hobbler]

I’ll be checking what I did with the fuel pump gasketing today - I can’t recall if I added a gasket between the spacer and the pump.
Having spent a lot of time trying to ensure I had one ‘good carb’ from the two that came with the car, coupled with the small fortune I’ve spent on this project so far, I took the plunge and ordered a new Weber carb from Ricambio yesterday.
As for the air bubbles in the fuel line (pleased I used the original green transparent fuel line), I have tightened up the various joints between the tank and the carb. Only issue I could find was a small nick in the rubber gasket under the sender unit.
Let’s see what the weekend brings.
And thanks again.
Peter

Just for interest, do you know what size jets you havein your carburettor? What you should have is a "26 IMB 10" with a 112 main jet and a 45 idle jet. If the previous owner had been 'playing around;with the jets,it might be that you have too large an idle jet (or to be correct---slow running jet) The air correction jet (on top of the emulsion-tube)should be a '235'

 

[Arancia]

Just for interest, do you know what size jets you havein your carburettor? What you should have is a "26 IMB 10" with a 112 main jet and a 45 idle jet. If the previous owner had been 'playing around;with the jets,it might be that you have too large an idle jet (or to be correct---slow running jet) The air correction jet (on top of the emulsion-tube)should be a '235'

Now, that is interesting because I do have a number of different jets. Time for more fiddling….

 

[Arancia]

Now, that is interesting because I do have a number of different jets. Time for more fiddling….

Well, thanks for all of the various Fiat 500 brains who have responded to this so far.
Update -
New carb fitted.
The good news - having pulled one of the plugs the mixture looks good - nice tan colour instead of sooty black. Also, I travelled about 1.5 miles today……. Including up a steep hill!
The bad news… it then stalled ( I could feel it was about to happen as the accelerator input had no effect).
Managed to get it started again but only after it had cooled down. Thankfully the hill was in my favour on the return trip - but then it stalled again. Wouldn’t restart. Pushed it home
However, air bubbles in the fuel line continue- which is now my focus. All joints in the fuel line have been tighten (again). I think now however that the air bubbles happen only near (both sides) of the fuel pump - which is brand new.
Suspicion is that it’s now a fuel flow problem (an air lock in the pump maybe? Do such things happen?) so the pump is coming off and I shall endeavour to set it up properly with the correct shaft movement.
I hope this saga is helpful to others.
Will keep you posted.
And thanks again for the invaluable knowledge that the experts here gathered have shared!
Peter

 

[smart51]

Jubilee clips can let air into the fuel line because they're not completely round. They pinch the hose under the screw housing. On mine, it took hours of being switched off before it caused a problem. It was always fine when running, or restarting after it had stopped.

Are your valve clearances set right? The valves and push rods expand a bit when hot. You set 0.2 or 0.25mm of gap when it is cold and all that is taken up when it is hot. If your gap is too small when cold, the valves won't fully closed when the engine heats up.

 

[Arancia]

Jubilee clips can let air into the fuel line because they're not completely round. They pinch the hose under the screw housing. On mine, it took hours of being switched off before it caused a problem. It was always fine when running, or restarting after it had stopped.

Are your valve clearances set right? The valves and push rods expand a bit when hot. You set 0.2 or 0.25mm of gap when it is cold and all that is taken up when it is hot. If your gap is too small when cold, the valves won't fully closed when the engine heats up.

Thanks Smart51.
I have used fuel pump ‘clips’ rather then ordinary jubilee clips so I have, for now, discounted those joints as the problem.
Mm, yes - I am tempted to have another go at those valve clearances - was never entirely happy they were right - tricky little devils to get right.
Thanks again.
The new fuel pump seems to be much better than the old ones I have - pushing the activation cam does push the remaining fuel out and covering both inlet and outlet at the same time means the pump won’t move.
Disappointingly, the gaskets I fitted either side of the spacer seem correct - 1mm and 0.4mm.

 

[Arancia]

Ok, the saga continues but…. Maybe I am getting somewhere.
A recap - it starts, it travelled a mile or two but then stalls and won’t start until it has cooled down.
New carb fitted. Religiously tried to get air bubbles out of the fuel line.
Rechecked tappets.
Re set the ignition timing.
Started it from cold - it idles; but tonight, having got the engine up to temperature and trying to stop it from stalling again (by operating the throttle cable at the engine) I noticed a spark jumping between the HT lead and one of the low tension terminals on the coil. Stopped everything for a while and I could see the same again when I started it up.
It must be the coil, once at ‘running temperature - surely!
Any replacement coil recommendations would be gratefully received…. Ricambio as normal?
Thanks again
Peter

 

[smart51]

When the thermostat opens, the engine blows hot air at the coil (assuming yours is in the same place as mine). It sounds like your coil might be breaking down when it gets hot.

 

[Arancia]

When the thermostat opens, the engine blows hot air at the coil (assuming yours is in the same place as mine). It sounds like your coil might be breaking down when it gets hot.

Yep - that’s what I think also. Any coil recommendations?

 

[smart51]

Yep - that’s what I think also. Any coil recommendations?

I wouldn't know a good one from a bad one.

 

[Arancia]

I wouldn't know a good one from a bad one.

 

[fiat500]

Yep - that’s what I think also. Any coil recommendations?

This company know the spark business and won't sell a bad one. I have used the "Bosch blue" coil succesfully, but I see that they now recommend the Beru blue coil.

Classic Car Ignition Coil - Powerspark Ignition

Replace your ignition coil with a brand new one from Powerspark Ignition. Choose from Genuine Lucas, our own Powerspark Ignition Coils. Order online now.

simonbbc.com

 

[Arancia]

This company know the spark business and won't sell a bad one. I have used the "Bosch blue" coil succesfully, but I see that they now recommend the Beru blue coil.

Classic Car Ignition Coil - Powerspark Ignition

Replace your ignition coil with a brand new one from Powerspark Ignition. Choose from Genuine Lucas, our own Powerspark Ignition Coils. Order online now.

simonbbc.com

Excellent. Many thanks - I shall get in touch with them today.
Peter

 

[Arancia]

I may be tempting fate here but……
Went to Powerspark yesterday and picked up one of their Viper resin filled coils.
My test run, up the hill and back; 3 miles, successfully negotiated.
It starts on minimal choke, yes, there are still air bubbles in the fuel line (how do they get there?), plugs look good…. I may have got this sorted - finally.
Thanks for all of your contributions again.
Just for reference, the original coil looked brand new (although not branded) and it was the only thing in the ignition system that had not been replaced. Upon unscrewing the HT contact on the top, there was evidence of a very slight oil leakage - enough it would seem for it to track across to one of the LT connections.
And the Powerspark coil cost just £28 and is resin filled so should perform better than the oil filled equivalents. I could probably have saved myself the expense of a new carburettor if I’d addressed ignition issues before fuel flow. Still, you live and learn…..

Now to get the front wheel alignment and tow In sorted so the steering feels less like a rally car on a loose gravel stage!
Any thoughts on how to get this approximately right would be gratefully received.

Peter

 

[the hobbler]

I may be tempting fate here but……
Went to Powerspark yesterday and picked up one of their Viper resin filled coils.
My test run, up the hill and back; 3 miles, successfully negotiated.
It starts on minimal choke, yes, there are still air bubbles in the fuel line (how do they get there?), plugs look good…. I may have got this sorted - finally.
Thanks for all of your contributions again.
Just for reference, the original coil looked brand new (although not branded) and it was the only thing in the ignition system that had not been replaced. Upon unscrewing the HT contact on the top, there was evidence of a very slight oil leakage - enough it would seem for it to track across to one of the LT connections.
And the Powerspark coil cost just £28 and is resin filled so should perform better than the oil filled equivalents. I could probably have saved myself the expense of a new carburettor if I’d addressed ignition issues before fuel flow. Still, you live and learn…..

Now to get the front wheel alignment and tow In sorted so the steering feels less like a rally car on a loose gravel stage!
Any thoughts on how to get this approximately right would be gratefully received.

Peter

Set the toe-in to ZERO with a fair bit of weight in the front of the car (sand-bags, cement-bags, flags etc) so that when adjusting the toe-in, the car is basically in the same condition as when being driven. The most important fact to remember is that the track rods MUST end up the SAME length as each other. I have seen 'experts' just adjust a single side to get the toe-in correct---very ignorant! If the track-rods are of differeing lengths, there is a good chance that the "Ackerman effect" will be effected, leading to tyre squeal when cornering, squirly steering and differing steering-lock side to side.

 

[Goldnrust]

Excellent news that you've solved () the problem and been able to get out and about for a drive

The bubbles in the fuel line, I suspect, will mean there's a small leak in the fuel line somewhere. Remember the line from the tank to the pump is under suction, so will draw air in through any time gap it can find. As long as the pump is supplying enough fuel to the carb any air that is delivered wit the fuel will just bleed off through the vent in the float bowl of the carb and will cause no issues. I can't remember who it was, but another poster on here also chased air bubbles I the fuel lines as a potential cause of running issues and ultimately found it was a red herring.

Regarding the alignment, as mentioned above, toe of 0 would be a good place to start. Also check the rear toe, the bracket that holds the front arm of the rear wishbones has slotted holes and sliding it back and forth will give you some adjustment.

Also remember a 1960's lightweight rear engined micro car is never going to feel stable at speed like a modern!