Technical Vapor lock?

[FunFiat]

Hello all!

Just took delivery of ‘66 500f. After driving 5 or so miles, car will start “lurching”, and then stall…will not restart for 20-30 min, then ok for another short drive until issue recurs. Thankfully, I was near enough home that could I push it home (twice)! Suspect vapor lock, and if so, would placing an inline electric fuel pump near tank be best fix? Thx!!

 

[Pete O]

Have you checked for a dirty fuel filter?

 

[JumpJet]

What kind of of fuel cap is on the tank? Is it vented?

 

[Ant Knox]

The original mechanical fuel pump is perfectly suitable for the car's needs, usually.
Before changing the pump, eliminate other possibilties first; has the car been stood for a while? Has it got fresh fuel? Have you inspected the inside of the tank for rust? Are the factory fitted fuel filters in place? Are the fuel lines cracked (two soft pipes interconnected with a rigid one in the cabin tunnel. If you have transparent fuel lines, can you see air bubbles in them? Is the carb clean and do you have reliable ignition?
Hope that helps.

 

[dirkdeman]

I always try to diagnose first before switching random parts out. The first suspect would be dirty fuel or intermittent fuel delivery, as others stated. If that's not the case I would check the resistance of the coil. Coil OK? Is the spacer between the carburettor flange cracked? That could lead to fuel vaporization, too.

I'm not a huge fan of electric fuel pumps: there's a risk that they keep running in case of an accident or a fire. Modern cars have all kinds of fuel shutoff safety measures that these old fiats lack. The same goes for the little plastic transparent fuel filter you see all the time on these cars: if they fail (or if the fuel line slips off) you're dumping fuel on a hot engine which is a recipe for disaster. These fiats have fuel filters in the pickup tube as well as in the carburettor, there's no need for the inline fuel filter.

 

[the hobbler]

I always try to diagnose first before switching random parts out. The first suspect would be dirty fuel or intermittent fuel delivery, as others stated. If that's not the case I would check the resistance of the coil. Coil OK? Is the spacer between the carburettor flange cracked? That could lead to fuel vaporization, too.

I'm not a huge fan of electric fuel pumps: there's a risk that they keep running in case of an accident or a fire. Modern cars have all kinds of fuel shutoff safety measures that these old fiats lack. The same goes for the little plastic transparent fuel filter you see all the time on these cars: if they fail (or if the fuel line slips off) you're dumping fuel on a hot engine which is a recipe for disaster. These fiats have fuel filters in the pickup tube as well as in the carburettor, there's no need for the inline fuel filter.

A man after my own heart Dirk---one of my pet "hates" is the fitment of a plastic fuel filter between the fuel pump and the carburettor. As you say, a recipe for disaster. As well as on the end of the tank unit and the carburettor, there is also a filter in the top of the fuel pump---one does NOT need a 4th filter. Abarth used the standard 'mechanical' fuel pump on ALL of his variations on the 500---even the '695SS' had a mechanical pump. You are also correct Dirk about the pump continuing to run in an accident---most (if not nearly all) people forget to fit some kind of 'inertia cut-out'switch into the pump's electrical supply line.
Back to the problem in question---"FunFiat"---have you checked that the choke (or to be more exact--'fuel enrichment device') is shutting fully when you depress the choke lever (the lever nearest the driver in LHD cars) and it is fully moved down? The problem sounds very much like the choke is staying on, which would explain as to why the engine will start, and then run for a short time, after the engine has cooled down. How long has the car been sitting not being used? As others have suggested, check that ALL the fuel filters are clean, and that there is no loose muck, or rust, moving around in the fuel tank. If possible, disconnect the fuel line at both the tank and pump ends and blow down the fuel line with an air-line. If the car has been sitting for sometime,modern fuel goes "off" much quicker than that of a few years ago, and can leave deposits inside the carb which will hinder performance. Modern fuel also attacks solder; if the carburettor has the original float (in the carb) check that it has no fuel in it. A float with fuel in it will sink to the bottom of the float-chamber,and then not shut-off the fuel when the correct level is reached in the float-chamber---effectively running on 'choke' all the time (voice of experience here!).These suggestions should sort out any 'fuel problem'. If the fault continues,come back to the Forum and we can go through the possible electrical causes. But, whatever happens, do NOT fit an electrical fuel pump---fitment of an electrical fuel-pump will only hide the problem, not cure it.

 

[FunFiat]

Thanks all for great input!

Car is a recent restoration..did have 3 months “sitting” from time of purchase in Italy to receiving here in SC. I did top off with premium (from reading, still unsure of best gas +/- additive/lead substitute - would love to hear thoughts on this subject). Just put in new plugs.

It does have the clear filter between motor and carb…

Gonna do some investigation of choke, tank, filters, lines, carb and seals/gaskets,etc

Will hold off on electric pump and investigate filters!!

 

[the hobbler]

Thanks all for great input!

Car is a recent restoration..did have 3 months “sitting” from time of purchase in Italy to receiving here in SC. I did top off with premium (from reading, still unsure of best gas +/- additive/lead substitute - would love to hear thoughts on this subject). Just put in new plugs.

It does have the clear filter between motor and carb…

Gonna do some investigation of choke, tank, filters, lines, carb and seals/gaskets,etc

Will hold off on electric pump and investigate filters!!

Over here in the UK one can still buy petrol with a very low (5%) Ethanol content. However, according to the ESSO web-site, their "99+ super" is, despite showing '5%'on the pump, Ethanol-free in the bulk of England, so that is what I try to use exclusively in my '126' based engine. A number of "other classic" owners that I know now use that fuel, and say that their engines are running much better on it. My advice on your side of "the big pond" is to find the fuel (gas) with the lowest Ethanol content that you can---if is is "Ethanol-free", so much the better. Just to make double sure--a touch of "belt AND braces"--- I also add a good quality "Ethanol-absorber"in the recommended ratio-quantity. Remove that filter between the fuel-pump and the carburettor!

 

[BarryH]

Hi Dirk - is it possible the fuel filler cap is not venting so the manual fuel pump is continuing to suck but creating a vacuum so after a while it chokes. I believe I have the original fuel filler cap on my 1966 500F which has a little valve on it - maybe worth a look, just a thought

 

[dirkdeman]

Hi Dirk - is it possible the fuel filler cap is not venting so the manual fuel pump is continuing to suck but creating a vacuum so after a while it chokes. I believe I have the original fuel filler cap on my 1966 500F which has a little valve on it - maybe worth a look, just a thought

Possible... it should be easy to spot. The fuel cap has a pinhole vent in it that's closed off by a rubber gasket. My gut feeling is still that it has something to do with vaporization. When you start it again after letting it cool down, does it crank for a while? Or does it start inmediately?

 

[FunFiat]

Possible... it should be easy to spot. The fuel cap has a pinhole vent in it that's closed off by a rubber gasket. My gut feeling is still that it has something to do with vaporization. When you start it again after letting it cool down, does it crank for a while? Or does it start inmediately?

Fuel cap vented, can look right through it.

Car starts right up after cooling…

Tank looks pretty new, and looking in I don’t see debris. Interestingly, I don’t see a filter in tank upon removing sensor/fuel line cap.

There is a clear filter between mechanical pump and carb (which I plan on removing this week) - after last stall and pushing home, noticed top quarter of filter housing filled with “air”….after later restart, filter refilled with fuel, and has stayed full during brief garage startup. I may let it run for a bit and see if I can identify vapor bubbles in lines/filter

 

[dirkdeman]

Fuel cap vented, can look right through it.

Car starts right up after cooling…

Tank looks pretty new, and looking in I don’t see debris. Interestingly, I don’t see a filter in tank upon removing sensor/fuel line cap.

There is a clear filter between mechanical pump and carb (which I plan on removing this week) - after last stall and pushing home, noticed top quarter of filter housing filled with “air”….after later restart, filter refilled with fuel, and has stayed full during brief garage startup. I may let it run for a bit and see if I can identify vapor bubbles in lines/filter

All right, we're eliminating things right now! The filter in the tank is a mesh strainer and there's a mesh screen in the carb as well. It's OK to see the external fuel filter half empty (or full, if you're that kind of person).

It starts right up after cooling, so the carburettor is our next most likely suspect. However, it might be wise to check the coil first as this is very simple to do as explained here and doesn't require disassembly of parts.

If the coil seems fine it's time to check the carburettor. I don't know how mechanically inclined you are, but it's not as daunting as it seems to take the carb out and take it apart. But first: check if the choke lever is'nt stuck. Have someone sit in the car and pull the choke lever on and off. Do you see the arm on the carburettor side moving? It should.

If the arm is moving it's time to take the carb off. Check the bakelite spacer (that connect the carburettor to the engine) for cracks or warpage. Check for gunk in the fuel bowl and strainer (solution: clean it). In fact, it might be wise to clean the carburettor as a whole. (Here's a YouTube video of that). Check if the float (brass or plastic, not sure what version you have) is not leaking. If it is leaking it has fuel in it, making it heavier.