FIAT 850 gets grumpy when warm

[lewey]

I have had this modified 850 Saloon for 4 months or so now and have been improving the electrics and generally trying to make the car reliable enough to use to go to work. It has the 903cc sport engine fitted with the twin barrel 30 DIC carb, tubular exhaust, etc.

Starting and running is fine from cold (for an old car) but when it has been warm for 5 minutes, it starts to missfire. This happens quite gradually, but will end with me unable to get over 30 MPH and eventually I'll stall. Once stalled, the car will not restart again, no matter what I try. After 20 mins of waiting it will start and I can go through the process again. All this means I can only go about 2 miles from my house if I want to go home again in a timely fashion.

Clearly a temperature related issue, but what? I just rebuilt the carb, put new manifold gaskets on her, adjusted valve timing, ignition timing and have new spark plugs, leads, rotor, cap and even a "boyer bransdon" box to help give a more potent spark, which it does.

 

[rallycinq]

Have you checked the fuel tank breather.

Once its stopped, take the tank cap off. If you get a great inrush of iar, the breather is blocked/inoperative.

8th part hole in the petrol cap usually does the trick.

Reasonable well known fault of years ago. but it takes an old fart to remember them.

Cheers

SPD

 

[lewey]

Thanks for the reply. Very few people who know about these things frequent this forum.

I will experiment at my earliest convenience and report back. Would be a lovely simple solution if true.

 

[lewey]

Unfortunately, it isn't that. I tried when it started to get bad and the cap released with no drama and the valve seems to work TBH. I only JUST got home. Stalled outside my house and I rolled in! Very frustrating. I have spent 2 months trying to sort this. I have an electronic pump and have a pressure gauge rigged up, so know it has fuel supply. It does seem to be temp related. If I potter about, it will go longer before trouble starts.

 

[rallycinq]

This electronic fuel pump has a regulator on it, I hope.

Cheers

SPD

 

[lewey]

I have an in line regulator adjusted using the gauge which has now become a permanent fixture. 2.5 PSI is what I've gone for, but in reality it is very forgiving as it makes no difference running at 1.5 or 4 PSI, at least in my short term tests.

 

[rallycinq]

Points or electronic ignition?

Cheers

SPD

 

[lewey]

Points. Original Marelli non-vacuum dizzy. The points connection goes into Boyer Branson black box that switches the coil with a transistor rather than relying on the points themselves to transmit the current, ergo creating a better intensity and timed spark and not wearing out the points. I bought this to help to try to resolve this current issue though, so it is there with or without the box, which is easy to exclude from the circuit completely if required as you can imagine.

 

[rallycinq]

Ah, the thot plickens.

Simple test, when the car stops and refuses to restart, do you still have a spark?

Cheers

SPD

 

[lewey]

Sure I must have tested, but can't honestly remember now. Could you tell me what you are thinking if either outcome is true?! What would cause no spark when warm? I have changed all ignition components.

It is a bit of an ordeal doing a test on one thing at a time as it could easily leave me stranded and I haven't got tax or an MOT. Very tricky situation.

 

[rallycinq]

You don't need to go anywhere, just run the car till it stops.

If you have a spark, then its fuel related. If you don't have a spark and the plug comes out wet, then you have an ignition problem.

You have replaced all the ignition components, so the theory goes that its not an ignition problem.

However, I have seen new ignition components break down with heat, so you could be assuming that because something is new it can't be at fault, which might be leading you up a blind alley.

Likewise, it might be a fuel problem, and you are still looking for an ignition problem.

Cheers

SPD

 

[lewey]

Thanks. I'll give it a go on the weekend. It takes a long while to warm up when not moving and it's damn loud, so I have been driving to save my reputation with the neighbours, but it probably is a disproportionate risk!

 

[rallycinq]

It should take less time to warm up when not moving, unless you've got a strong wind up the rear.

Cheers

SPD

 

[NottsCharger]

I have a tempramental 126 aircooled which prior to a rebuild used to disgrace itself in a similar fashion. It would start to play up when hot and not restart until it had cooled down. It would do this even though I had rebuilt the carb and fitted a new fuel pump and checked for tank pressure as has been previously quoted.

I started to work through the ignition replacing rotor, points, leads, plugs but fortunately a new Condensor cured it before I got to the expense of a new coil or disi.

I have other issues now, but there always are with old Fiats

May help!

 

[lewey]

It should take less time to warm up when not moving, unless you've got a strong wind up the rear.

Cheers

SPD

I can explain this one pretty easily. The gearing is so low that keeping up with traffic on the A road I live near requires 5000 RPM in top gear for 5 minutes at a time! This overcomes the rush of air whilst going along quite nicely as they are hardly famous for cooling efficiency with a rear rad anyway. If I pootle around down a back road, it takes a long while to warm up and the problem to begin.

No matter, I'll be brave and rev her up on the drive on the weekend.

 

[lewey]

I started to work through the ignition replacing rotor, points, leads, plugs but fortunately a new Condensor cured it before I got to the expense of a new coil or disi.

Thanks. I may remove my condensor when I conduct my testing as it is not actually required when using my Boyer Bransdon conductive discharge unit as the point are switching such low current.

 

[aquamarine]

Do you have a fuel filter fitted? Any chance it's debris in the fuel line blocking the jet? (old hoses etc, worse due to the heat)? Am sure you will already have looked at this but I've had similar problems in the past with an X1/9. Hope you get it sorted. David

 

[lewey]

Have spent many hours on her today. It is definitely an ignition problem. I have a spark tester that runs in line with a spark plug and after 20 mins of enthusiastic revving on my drive, I got it to missfire a bit and the tester is indeed missing the spark intermittantly. The light in the tester goes out completely for a second and then comes back strong for a little while and repeats the cycle. The off to on ratio simply gets higher as it gets warmer.

I don't know if the thermostat is broken or something else, but it takes forever to heat up in general when no moving. If I stop the engine to change something and I am lucky enough to start it again, it will take another 10 mins of seriously anti-social behaviour to get it warm enough to even start missfiring again. I tried swapping coil, leads, cap and checked spark plugs along with adjusting points and timing in an effort to improve things. There isn't anything else in the flippin' ignition circuit! I removed the condensor fairly early doors, with no effect. The plugs are dry, have a carbon depost ring round the edge, but middle is biscuit and seems spot on. The soot is likely down to the engine idling a lot of the time and this carb is known to be a bit rich on the idle.

The only change I could notice was when I swapped for a really old cap I have. It seemed to improve things very well and after going through a couple of fan cut ins and outs (has electric fan with auto fan switch), it was still running and revving fine. I then stopped it and started it as well a couple of times. I was sure it was fixed and very nearly booked the MOT. I thought I better go for a drive and sure enough after a couple of minutes of driving, it miss-fired so badly I couldn't get back home and I had to abandon in a lay-by. Before actually stopping, there were quite a few spit backs from the carb which seemed to be early indicators of trouble. I will never be convinced by an on-drive test again.

This has all left me confused. It really did seem OK, but it had been a long day and I was desperate for a cure.

Any insights or ideas based on what I have written?

 

[rallycinq]

Rotor arm?

When the thing misfires take the king lead off, fit any of the other leads and put a spark plug directly onto the coil.

If the spark returns, its not the coil or the points/ignition system, its the distribution of the spark.

Cheers

SPD

 

[timboengineering]

I had the same problem with my 900e camper- when warm it cuts out, leave it 5 mins or so, fine.(standard engine)
I swapped the radiator cap from my 127 to my camper and it cured it. It only registered on the water temp sensor as overheating after about the 20th time of cutting out......also check the hose clamps on the hoses that feed the windscreen heater- mine were both rusted through.