Technical Engine won’t stay running.

[Arancia]

So, I have fully rebuilt the engine and surprise surprise it started first time before putting it back in the car. I only ran it once or twice for a few minutes but it seemed fine. It idled ok.
Now back in the car it will start but after a while - say a few minutes - it will stutter and stop.
Then it won’t start until it has cooled down.
I have cleaned the carb and fitted a full overhaul kit from Ricambio.
After first running it there was some fuel weeping from the top of the carb - replaced the fibre washer with a rubber washer and that has now stopped.
The float is good ( it’s not got a leak) and the needle valve it operates on seems ok - it operates smoothly. That said I can’t figure out how such a tiny device actually works!
The base of both the carb and the ‘plastic’ base on which it sits were both carefully smoothed flat using a flat surface and piece of abrasive paper.
Fuel pump is new.
Plug number 1 is black - haven’t yet looked at number 2.
It’s the fact that it will start and eventually idle when warm but then stutter and stop that I can’t figure out. Plus of course the plug is covered in black soot.
In total, as a newly rebuilt engine, I would say it hasn’t run for more than 15 minutes in total.
Can’t say I understand the carburettor (!)
It has been rebuilt twice now and I was most careful blow out all of its components with a compressed air line before and during reassembly.
It seems to me that it is a fuel supply issue when it finally stalls but at the same time it is running rich.
I would greatly appreciate any thoughts from the gathered wisdom of this forum. Before I give up and buy a new carb!!
It’s a standard 500cc engine in a ‘72 500L.
Many thanks in anticipation

Peter

 

[Toshi 975]

Hi Peter, recently another 500 owner had similar problems to you after a carb rebuild, he finally gave up with it and sent the carb to me for inspection. Turned out the rebuild kit he used was not good quality and in particular the main gasket was about half as thick as the originals but worse thing was the inlet needle valve was shorter than original, I assume skimping on brass, this made it impossible to set the float height properly leading to flooding. Might be worth checking.

 

[Arancia]

Hi Peter, recently another 500 owner had similar problems to you after a carb rebuild, he finally gave up with it and sent the carb to me for inspection. Turned out the rebuild kit he used was not good quality and in particular the main gasket was about half as thick as the originals but worse thing was the inlet needle valve was shorter than original, I assume skimping on brass, this made it impossible to set the float height properly leading to flooding. Might be worth checking.

Many thanks Toshi - I think I saw that post somewhere….
When you say the main gasket, do you mean the one between the top and the main body? I’m just wondering what difference a thicker gasket would make. I took the top off this morning (again) and there is around 10-12 mm of fuel in the bottom of the reservoir which makes me think that it might not be a lack of fuel that causes it to stutter and stall. Am now wondering if this is distributor and timing related….
Not being a carburettor expert do you mean the inlet needle valve is the one in the attached photo? I’m also wondering how to ‘set’ the float? I was guessing it was a fit and forget!
Finally, I wonder where you are. Sounds like it may be time for an expert to look at it.
Thanks again - really appreciate the time you took to respond to this.
Peter

 

[Toshi 975]

Many thanks Toshi - I think I saw that post somewhere….
When you say the main gasket, do you mean the one between the top and the main body? I’m just wondering what difference a thicker gasket would make. I took the top off this morning (again) and there is around 10-12 mm of fuel in the bottom of the reservoir which makes me think that it might not be a lack of fuel that causes it to stutter and stall. Am now wondering if this is distributor and timing related….
Not being a carburettor expert do you mean the inlet needle valve is the one in the attached photo? I’m also wondering how to ‘set’ the float? I was guessing it was a fit and forget!
Finally, I wonder where you are. Sounds like it may be time for an expert to look at it.
Thanks again - really appreciate the time you took to respond to this.
PeterView attachment 442700

Yes that gasket. Often the carb top cover will bow a little over time so a thinner gasket is less likely to make a seal.
This is the float setting diagram from the Haynes manual

The setting can be adjusted by bending the little tab, no 6 in the diagram but be careful not to damage the float.
I would not claim to be an expert but I would guess that I have worked on 50+ carbs for small Fiats and learnt that way.
I am located in Chesham HP51RR.
That is what is referred to as the needle valve you have shown there. Is that the old one you took out because the insert looks to be from a different Weber carb as it has a tiny groove at the top to retain a spring clip. The outer body should have 1.25 stamped on it.

 

[fiat500]

Yes that gasket. Often the carb top cover will bow a little over time so a thinner gasket is less likely to make a seal.
This is the float setting diagram from the Haynes manual
View attachment 442704
The setting can be adjusted by bending the little tab, no 6 in the diagram but be careful not to damage the float.
I would not claim to be an expert but I would guess that I have worked on 50+ carbs for small Fiats and learnt that way.
I am located in Chesham HP51RR.
That is what is referred to as the needle valve you have shown there. Is that the old one you took out because the insert looks to be from a different Weber carb as it has a tiny groove at the top to retain a spring clip. The outer body should have 1.25 stamped on it.

Everyone else would claim assert that Dave is an expert.

 

[Arancia]

Yes that gasket. Often the carb top cover will bow a little over time so a thinner gasket is less likely to make a seal.
This is the float setting diagram from the Haynes manual
View attachment 442704
The setting can be adjusted by bending the little tab, no 6 in the diagram but be careful not to damage the float.
I would not claim to be an expert but I would guess that I have worked on 50+ carbs for small Fiats and learnt that way.
I am located in Chesham HP51RR.
That is what is referred to as the needle valve you have shown there. Is that the old one you took out because the insert looks to be from a different Weber carb as it has a tiny groove at the top to retain a spring clip. The outer body should have 1.25 stamped on it.

Well, 50+ small Fiat carbs is roughly 49+ more than me. I’d say that makes you the expert
Thanks again so much for spending your Sunday answering my questions. I’ve had a go at adjusting the float valve per the diagram. It wasn’t far out to be honest but it’s a bit better now. Circa7.5mm and 15mm.
I think that is the needle valve that came with car 4 years ago. (Yes - it has 1.25 stamped on it.) I’ve been rebuilding it ever since. Interestingly the box of spares that came with it included two other needle valves, other jets, points and a condenser. I reckon I am chasing a long ago problem that was never solved given that the splutter then stall behaviour was evident when I first drove it!
The gasket, I now realise, although dry now, shows evidence of having been damp - I assume with petrol.
I’ll be checking for flatness tomorrow.
Off for my tea now - thanks again.
Peter

 

[Arancia]

Thanks again Toshi. Got it all back together, bolted it back on the car only for the mounting stud through the top of the carb to strip the thread. Took the top off the spare carb and put it on the car. No real change unfortunately- will start but after a few minutes will stutter and stall. Definitely running rich - plugs are black.
So - turning my attention to the ignition timing now. New points fitted and will set it all up today I hope. Bought a TDC tool - it screws into the spark plug hole. Or rather it would if I hadn’t allowed it to fall down into the tinware / fan! Beware the gap around the spark plug hole

 

[fiat500]

the ignition timing now. New points fitted and will set it all up today I hope. Bought a TDC tool - it screws into the spark plug hole. Or rather it would if I hadn’t allowed it to fall down into the tinware / fan! Beware the gap around the spark plug hole

Is the engine asembled? You should be able to trust the mark on the oil-filter cover when aligned with the arrow on the timing cover; both pistons are then at TDC. All you need then is to look at the valve rockers (presuming that everything is reasonably well adjusted). You should be able to rattle one pair up and down and at least one of the other pair will be tightly in contact with the valve stem and the spring(s) will be partially compressed.

The clackety pair are on the cylinder that is on the compression stroke.

 

[Arancia]

Is the engine asembled? You should be able to trust the mark on the oil-filter cover when aligned with the arrow on the timing cover; both pistons are then at TDC. All you need then is to look at the valve rockers (presuming that everything is reasonably well adjusted). You should be able to rattle one pair up and down and at least one of the other pair will be tightly in contact with the valve stem and the spring(s) will be partially compressed.

The clackety pair are on the cylinder that is on the compression stroke.

Thanks Peter. Yep - it’s all assembled and I readjusted the valve clearances again last night. I reckon I put the distributor back incorrectly so I’m fiddling with that today. When the engine is tdc the points are a long way off opening. Here’s hoping eh?

 

[fiat500]

Thanks Peter. Yep - it’s all assembled and I readjusted the valve clearances again last night. I reckon I put the distributor back incorrectly so I’m fiddling with that today. When the engine is tdc the points are a long way off opening. Here’s hoping eh?

By TDC the points should be closed again, so that is correct.

 

[the hobbler]

By TDC the points should be closed again, so that is correct.

All the reports coming from this engine saga point to the engine staying "choke-rich" as it warms up when what it really needs is 'no choke' and the engine running leaner. Technically this carburettor does note have a "choke", but a "fuel-enrichment device" (which ,perversly, is known as "the choke"). Could it be that device is not be shutting off, thereby causing the engine to run far too rich as it warms up.

 

[Arancia]

All the reports coming from this engine saga point to the engine staying "choke-rich" as it warms up when what it really needs is 'no choke' and the engine running leaner. Technically this carburettor does note have a "choke", but a "fuel-enrichment device" (which ,perversly, is known as "the choke"). Could it be that device is not be shutting off, thereby causing the engine to run far too rich as it warms up.

Thanks for jumping in on this one Hobbler.
Well, I have rebuilt the carb for the umpteenth time and put it back on the car and have decided to look at the ignition side of things before trying again. Reading your note I do know what you mean - it does feel like that. Spent an age resetting the tappets and getting the distributor into the right spot. Refining that set up tomorrow and then let’s see what happens. (Had to fiddle about with the bottom part of the distributor to get it anywhere near right).
I will ‘wind in’ the enrichment device - that spring loaded brass screw bottom right on the carb if memory serves - although I did set it at 2 turns out as recommended.
Thanks again - this is such a good forum.
Will update tomorrow
Peter
Just read your post again - should the choke enrichment device shut itself down in some way?

 

[the hobbler]

Thanks for jumping in on this one Hobbler.
Well, I have rebuilt the carb for the umpteenth time and put it back on the car and have decided to look at the ignition side of things before trying again. Reading your note I do know what you mean - it does feel like that. Spent an age resetting the tappets and getting the distributor into the right spot. Refining that set up tomorrow and then let’s see what happens. (Had to fiddle about with the bottom part of the distributor to get it anywhere near right).
I will ‘wind in’ the enrichment device - that spring loaded brass screw bottom right on the carb if memory serves - although I did set it at 2 turns out as recommended.
Thanks again - this is such a good forum.
Will update tomorrow
Peter
Just read your post again - should the choke enrichment device shut itself down in some way?

The "spring-loaded brass screw" is NOT the "fuel enrichmant" device---this is the idle mixture enrichment device---starting with it at about 2 turns from fully (but gently) in, is about right for starting,and should be almost OK for running---you just have to screw it gently in until the engine idle falters, and then slowly screw it back out until the idle speed evens out. You balance this with the adjustment of the idle SPEED screw to get the correct, and even running when idling.
The "Fuel-enrichment" device (aka 'the choke') is operated by the left hand lever between the seats--up for 'choke' ON and down to turn the 'choke' OFF. When you push the lever down, is the device on the side of the carburettor going fully OFF? I have know it where the operating cable gets kinked,so when the choke-lever is pushed down, the cable just bends, and the the choke stays on. With the lever fully down, physically check at the carburettor that the enrichment device is fully off; the device's operating arm should be angling towards the rear of the car, and should have no more movement towards the rear.

 

[Arancia]

The "spring-loaded brass screw" is NOT the "fuel enrichmant" device---this is the idle mixture enrichment device---starting with it at about 2 turns from fully (but gently) in, is about right for starting,and should be almost OK for running---you just have to screw it gently in until the engine idle falters, and then slowly screw it back out until the idle speed evens out. You balance this with the adjustment of the idle SPEED screw to get the correct, and even running when idling.
The "Fuel-enrichment" device (aka 'the choke') is operated by the left hand lever between the seats--up for 'choke' ON and down to turn the 'choke' OFF. When you push the lever down, is the device on the side of the carburettor going fully OFF? I have know it where the operating cable gets kinked,so when the choke-lever is pushed down, the cable just bends, and the the choke stays on. With the lever fully down, physically check at the carburettor that the enrichment device is fully off; the device's operating arm should be angling towards the rear of the car, and should have no more movement towards the rear.

I learn more every day! Yes, that was one of the first things I checked. The ‘choke’ cable, when the lever is fully down in the car does indeed mean the choke is closed at the carb. This all reminds me of tinkering with my Austin 1300 back in the late seventies
Let’s see where I get to today…..

 

[Arancia]

Ok, so, courtesy of my new strobe light and much fiddling with the distributor, I can say that at idle it fires bang on 10deg before tdc.
At least the ignition is correct.
Same issue overall however, it will idle ok (but it needs choke to do so) but after travelling 100 yards or so it will be eventually stall. Plugs are black.
I noticed that there is often a stream of bubbles in the fuel line before and after the (new) fuel pump. I’ll be searching for the cause later today.
Back to the carb and running rich….. how can I adjust it to run leaner?

Thanks in anticipation again. This really is a great forum with great contributors. ( One day I will do a write up on how my rebuild went that I hope will help others.)
Peter

 

[the hobbler]

Ok, so, courtesy of my new strobe light and much fiddling with the distributor, I can say that at idle it fires bang on 10deg before tdc.
At least the ignition is correct.
Same issue overall however, it will idle ok (but it needs choke to do so) but after travelling 100 yards or so it will be eventually stall. Plugs are black.
I noticed that there is often a stream of bubbles in the fuel line before and after the (new) fuel pump. I’ll be searching for the cause later today.
Back to the carb and running rich….. how can I adjust it to run leaner?

Thanks in anticipation again. This really is a great forum with great contributors. ( One day I will do a write up on how my rebuild went that I hope will help others.)
Peter

Peter;
Not doubting your ablities, but where do you live? I ask this because there may be a Forum member within your vicinity who can help you---2 brains are often better than one!

 

[Ant Knox]

Ok, so, courtesy of my new strobe light and much fiddling with the distributor, I can say that at idle it fires bang on 10deg before tdc.
At least the ignition is correct.
Same issue overall however, it will idle ok (but it needs choke to do so) but after travelling 100 yards or so it will be eventually stall. Plugs are black.
I noticed that there is often a stream of bubbles in the fuel line before and after the (new) fuel pump. I’ll be searching for the cause later today.
Back to the carb and running rich….. how can I adjust it to run leaner?

Thanks in anticipation again. This really is a great forum with great contributors. ( One day I will do a write up on how my rebuild went that I hope will help others.)
Peter

Arancia, you mentioned there that the fuel pump is new; have you checked that it is delivering the (correct) amount of fuel to the carb? I don't know if these are repro items, but there may be flaws in the manufacturing tolerances?

 

[Arancia]

Peter;
Not doubting your ablities, but where do you live? I ask this because there may be a Forum member within your vicinity who can help you---2 brains are often better than one!

I’m near to Rugby / Leamington Spa in Warwickshire. I’m getting close to buying a new carb!

 

[Arancia]

Arancia, you mentioned there that the fuel pump is new; have you checked that it is delivering the (correct) amount of fuel to the carb? I don't know if these are repro items, but there may be flaws in the manufacturing tolerances?

Mmm, after market part for sure…. and we all know what they can be like!
Actually there were two other fuel pumps that came with the car (and a second carb plus points plus condenser) so I suspect the previous owner was chasing the same issue. I put a new one in as a precaution! I’m wondering how to measure the fuel flow - any ideas?

 

[Ant Knox]

Mmm, after market part for sure…. and we all know what they can be like!
Actually there were two other fuel pumps that came with the car (and a second carb plus points plus condenser) so I suspect the previous owner was chasing the same issue. I put a new one in as a precaution! I’m wondering how to measure the fuel flow - any ideas?

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.