Technical Air bubbles flowing in return fuel line - 650 Fiat 126 engine in 500. IMB28

[Goldnrust]

I've set the timing with a strobe gun to 18 degrees at about 3k rpm, using the adjustment on the crank sensor, and been able to drive it around, which is positive.

The timing when checking with a strobe at full advance should be 28 degrees, this is 10 degrees base timing + 18 degrees of centrifugal advance from the distributer. Presuming that's not a typo, then 10 degrees extra advance will certainly make it peppier!

Intermittent power problems are harder to trace. To be it's more likely fuel/spark related than something inside the engine. I'd give the wiring all a good wiggle, look for anything that could be loose or shorting. Plug leads or dodgy earths is a classic for issues like this. You've been through the car recently, so it's less likely, but check to make sure there's no loose flakes of rust or anything in the carb that could intermittently block a jet.

It could be sticking valve, thats a classic for an intermittent problem. But I've not really heard of many people having problems with that on a 500, and you had good compression recently, so put that one on the 'possible but not likely' list.

 

[Bianca500]

Okay that's good. I have the first type and a ballast fitted using existing captive bolts down on the left, so hopefully that shouldn't be a contributing factor here

Re the 18degs, admittedly I was referencing the (decade old, mind you) thread here, and set 18 on the timing light and it was broadly lining up with the mark:

Help setting ignition timing on Fiat 500 F | FIAT 500 (Classic)

Could someone help me on giving me some detailed instructions on how to set the ignition timing on my 1967 fiat 500F? I have a bulb light with pos and neg cables for testing; just don't know where to attach it and don’t know which spark plug chamber to check to see whether the piston is TDC...

www.fiatforum.com

Setting 18 deg on the timing light makes the light flash 18 deg early, so the TDC mark on the pulley will line up with the mark on the case if the timing is correctly set. You can do the same check at idle by setting 10 deg on your light, however in my experience it is best to get it right at the higher RPM and live with whatever it turns out to be at idle.

That said, this is the first time I'm using a timing light so not sure fully what to expect. Videos online aren't representative due to framerate, but is it expected to be absolutely unwavering? IE the mark not 'moving around' at all - appearing absolutely fixed in place? My experience was that it was 'more often than not' in the right place, but did move around a bit, whilst trying to keep a steady rpm.

I'll see what happens at 28deg, however I don't know there's enough movement in the crank sensor. Perhaps 8-10mm in total movement?



Thanks for the other suggestion, gives me a few things to try. Sticking valve is an interesting one, might explain why occasionally it sounds as though it's spitting back through the carb, mentioned by bugsymike earlier on.

 

[Goldnrust]

My apologies, if the pulley and case are marked to 10 degrees BTDC then 18 would be correct. If you think there isn't enough movement in the sensor adjustment, that suggests you were right the first time.

On a standard points set-up on a 500 engine the case is marked for TDC, but I cannot be sure what a 650cc pulley and case are marked.

 

[Bianca500]

Just to conclude the thread in summary - the air bubbles were causing some misfiring and underpowered running, which was traced back to a crack in the fuel hose where it goes into the fuel sender in the fuel tank. This was resolved by replacing the fuel hose end to end. I made up a new copper section for the engine bay for good measure.

Unfortunately, I committed the troubleshooting sin of changing two things at once... I realise now that I fitted the ballast resistor to the coil after changing the fuel hose, but before actually test driving the car.

Since the wiring was such that the ignition live went to the coil and piggybacked from there into the ECU, it meant that where I fitted the balast in-line to the coil, the ECU was also receiving the reduced voltage ...

So all the subsequent troubleshooting I was trying to do was a little in vain. Once I eventually realised and correctly powered the ECU with full voltage,
the car was instantly driving nicely.

Lesson learned... only change one thing at a time and test again!


From here I will be able to fine tune the mixture and timing to optimise, and monitor the coil situation (coil with ballast have reports of being difficult to start sometimes, which is rectified on electric-switched starters by temporarily bypassing resistor when key barrel is turned. Since I have (and want to keep) manual cable-operated starter, that might not be an option. And getting a mounting bracket from the late 126p ELX for the smaller 12v non-ballasted coil seems to be a challenge)

Thanks again for all your suggestions and knowledge.

 

[Toshi 975]

Is it this type of coil? I made this bracket from a piece of thin angle iron.

 

[Bianca500]

Yes, that's the one I was looking to put in. Nice solution!

Although my understanding from what I've read was that that smaller type of coil didn't need a ballast, so seeing your picture has thrown me. How have you found it? Do you have the ballast bypassed on startup?


I guess if that still needs a ballast then I don't know what I would gain by changing to that smaller type of coil? Other than maybe cheaper / more available to replace?

 

[Toshi 975]

Yes, that's the one I was looking to put in. Nice solution!

Although my understanding from what I've read was that that smaller type of coil didn't need a ballast, so seeing your picture has thrown me. How have you found it? Do you have the ballast bypassed on startup?


I guess if that still needs a ballast then I don't know what I would gain by changing to that smaller type of coil? Other than maybe cheaper / more available to replace?

Well I was not taking chances after burning out two new of the larger type original coils at £45 a time but I have only tested the coil on the bench.

 

[Kiwi126]

Just to conclude the thread in summary - the air bubbles were causing some misfiring and underpowered running, which was traced back to a crack in the fuel hose where it goes into the fuel sender in the fuel tank. This was resolved by replacing the fuel hose end to end. I made up a new copper section for the engine bay for good measure.

Unfortunately, I committed the troubleshooting sin of changing two things at once... I realise now that I fitted the ballast resistor to the coil after changing the fuel hose, but before actually test driving the car.

Since the wiring was such that the ignition live went to the coil and piggybacked from there into the ECU, it meant that where I fitted the balast in-line to the coil, the ECU was also receiving the reduced voltage ...

So all the subsequent troubleshooting I was trying to do was a little in vain. Once I eventually realised and correctly powered the ECU with full voltage,
the car was instantly driving nicely.

Lesson learned... only change one thing at a time and test again!


From here I will be able to fine tune the mixture and timing to optimise, and monitor the coil situation (coil with ballast have reports of being difficult to start sometimes, which is rectified on electric-switched starters by temporarily bypassing resistor when key barrel is turned. Since I have (and want to keep) manual cable-operated starter, that might not be an option. And getting a mounting bracket from the late 126p ELX for the smaller 12v non-ballasted coil seems to be a challenge)

Thanks again for all your suggestions and knowledge.

It's been great to follow this thread and read that you reached a great outcome! I'm soon to get some 126 engines for transfer into 500's so understanding how you've overcome challenges with exactly that has been great to read. Enjoy your fine tuning and I hope it brings you plenty of smiles.

 

[Goldnrust]

Glad to hear you got to the bottom of the problem.