Technical Where to begin?

[dweebit]

1969 Fiat 500, going down the road about 30 mph, she suddenly quit. Coasted to curb, could not restart. Not overheated. Turns over fine.
Have checked spark and fuel. Distributor and points look normal, but not sure how to test. Any thoughts?

 

[bugsymike]

Check which you haven't got:- engine compression, a good spark at the right time and good fuel getting into the engine.
Sudden death sounds electrical, such as a coil or a broken wire or if you have electronic ignition then component failure, as original points and condensor set up usually gave a misfire before dying. A distributor drive failure could give similar symptoms.
If fuel it would have to be a total blockage to cause what you describe, maybe a blocked breather on petrol tank.
Most of these things can be quickly tested for to see what is not there, then sorting out the why, it is not there is next.

 

[dweebit]

Check which you haven't got:- engine compression, a good spark at the right time and good fuel getting into the engine.
Sudden death sounds electrical, such as a coil or a broken wire or if you have electronic ignition then component failure, as original points and condensor set up usually gave a misfire before dying. A distributor drive failure could give similar symptoms.
If fuel it would have to be a total blockage to cause what you describe, maybe a blocked breather on petrol tank.
Most of these things can be quickly tested for to see what is not there, then sorting out the why, it is not there is next.

Thanks Bugsymike. I'm getting good fuel volume to the carburetor, and good spark to the plugs. That leaves compression or timing to check next. No reason to suspect compression would die suddenly, so a timing slip seems most likey. I'll look at that after Christmas.

 

[dweebit]

Thanks Bugsymike. I'm getting good fuel volume to the carburetor, and good spark to the plugs. That leaves compression or timing to check next. No reason to suspect compression would die suddenly, so a timing slip seems most likey. I'll look at that after Christmas.

So, timing is good but compression is not. 155 psi in cylinder 1; 75 psi in cylinder 2. Because it happened suddenly, I presume either a blown head gasket or a valve or valve guide failure. Because 75 is not 0, what does that tell me that might be useful?

 

[bugsymike]

So, timing is good but compression is not. 155 psi in cylinder 1; 75 psi in cylinder 2. Because it happened suddenly, I presume either a blown head gasket or a valve or valve guide failure. Because 75 is not 0, what does that tell me that might be useful?

Can't see a valve guide affecting compression.
If you put a spoon of oil down the plug holes, leave for 20 minutes to drain down, then spin engine over with plugs out to get rid of the excess, you will need a rag over plug holes as can be messy. After that do a compression test again, if readings improve it is piston /rings area, if the same readings, then more top end, so head gasket with air cooled engine I would have thought it would sound a bit "spitty " like gas blowing out of a leaking exhaust manifold.
Generally if a valve fails as in burnt out you would have zero compression on that cylinder.
What are the valve clearances like?
I am surprised it isn't trying to run on one cylinder.
My knowledge is general motortrade over 50 odd years, there are several Forum members who have a lot of specific 500 knowledge on here.
The last 500 and the 600 Fiat with suicide doors I owned were in the 1970s.

 

[Toshi 975]

The worse burnt out exhaust valve I came across on a 500. Engine would run on one cylinder but amazingly would fire on both when the engine warmed up and the revs were up. The valve head had regressed up into the head as can be seen in the middle picture. The valve seat itself was fine.

 

[bugsymike]

The worse burnt out exhaust valve I came across on a 500. Engine would run on one cylinder but amazingly would fire on both when the engine warmed up and the revs were up. The valve head had regressed up into the head as can be seen in the middle picture. The valve seat itself was fine.
View attachment 443816View attachment 443817View attachment 443818

Pretty amazing to fire on that cylinder.