A problem has recenly developed and I can't figure it out. The car starts up easy and on the first crank. I let it warm up a few minutes and drive....It drives fine as it's warming up.
Then after about 15 or 20 minutes the engine starts developing a shudder. Idles fine but and can run ok under low load but under some load the #4 cylinder starts popping and firing up through the carb. feels like it's running 3 cylinders and wasting gas.

It's been suggested I do a compression test but I'm thinking if the engine runs great cold and while warmin up; the compression should be ok.

Is there a case where comprression is ok when cold but low when hot? Can't figure out why car runs great when starts f'n up.

1.7 with dual weber 40 IDFs

Sounds the opposite of a properly setup carb engine...usually they run like crap till they're fully warmed. Maybe it needs to be tuned so it runs best at warmed up temps.

I have no idea how my 356 mechanic does it, but it runs like a top when it's warmed up.

Did a compression test tonight.... 105 / 105 / 105 / 0

#4 is dead...what's the next step? take off the valve cover?

Squirt some oil in the plug hole then retest compression. If it remains at zero look to valves.

I'll try this today. How could this help? Stuck valve?

Sounds like a tight valve ( held open) adjust and try again. If it gets tight again (stays open) you may have a stretching valve or seat problem

Look at #3 intake as that is the one that is popping through the carb. But its not hard to check them all

Not perfect but an old trick, if it's rings the oil will seal enough to give you a bit of compression during the test. if numbers go up it's rings, if it doesn't likely valves.

That said 0 compression I'd suspect a tight valve or dropping valve seat.
With a 1.7 I really expect a dropping valve seat....sorry.

Head gasket I'd expect a couple of pounds and a hiss/whistle noise.

Leakdown

Leakdown will tell where the issue is, rings, intake, or exhaust.

Off work today and in the garage. I suspect an issue with the #3 valve as described so going to take the valve cover off and have a look.

Can i jack up one side of the car and not have to drain the oil?

You don't need to drain the oil to remove the valve covers.

? you do not have to drain the oil to take a valve cover off.

Leakdown test only works if you have SOME compression.

Waiting for 0 to leakdown to 0 isnt very useful.

Do a leakdown test. It will tell you where the leak is. Air goes in the cylinder, air comes out of the cylinder. You just have to find where.

+2
I've run my car on a lift before with the valve covers off looking for a valve train problem, but thats a different story.

Look at the valve tips when each cylinder is on TDC. If one of the valves on #3 is sitting with the tip closer to the head, it's a dropped valve seat.

With zero compression, the problem is almost certainly serious enough that to fix it you will need to drop the engine and remove the head. At a minimum.

--DD

Several things can cause low compression.
Not many things will cause NO compression.

Worn out rings won't give you 0 compression.
A tight valve won't give you 0 compression.
Carboned up, poor seating valves won't give you 0 compression.

Dropped valve seat will.
Badly burned valves will.
Hole in a piston will.

Leakdown can help you find where the leak is and then you can figure out what happened.

Jim

It needs to come out and anything you do will be easier once it is out. Don't waste your time, just drop the engine.