New to this car, but long time ago Fiat owner. My recently purchased 500 starts fine and runs fine for 1/2 of RPM. If you try to rev it (even manually), it stumbles and reduces RPM. Timing looks OK , new fuel filter also. What's up? Any ideas would be very helpful.

Hello and welcome to the forum!

Can you clarify a little more - does the engine start misfiring when the rpm increases?

Any popping noises through the exhaust or carburretor?

Or does it simply just refuse to rev freely?

Starts Ok ( a little tough in this cold weather). Idles OK. Will rev partially( Maybe 1/2 of the way to the shift markers on the speedometer). You can drive it as long as you shift before the stumble. If you try to get it to rev more, it will lose RPM unless you let up on the gas. Ironically, letting up on the gas actually makes it then run better. If I try to rev it manually by operating the throttle at the engine, it reaches about 1/2 revs then reduces revs until I give it less throttle.

This is very weird to me.

It may also be worth checking the advance and the plugs.
Being 2 cylinder 4 stroke engines, they are particularly sensitive to dirty spark plugs, though this usually manifests as difficult starting and rough running. Failure to rev is more like a problem with the advance.
Chris

I have looked at the advance using a timing light. Looks like it is advancing as it is supposed to. I just recently found (on this site), a way to check the static timing. I will do that next, although it does appear to be in the ballpark when I check the dynamic timing. I have not pulled the plugs - the wires are new so they are OK.

It would seem to be a spark problem as there is no coughing or popping as you would expect with a carb or fuel problem.

Any other ideas are most appreciated.

This sounds like fuel starvation - check your float level on the carb, it sounds like it may be set too low also check for air leaks - let the car idle and spray WD40 around all mating surfaces (carb to manifold/manifold to head) if theres an air leak the idle will pick up a little

I've had a similar problem with my 500 and then I realised that the engine was working with only one cilinder. But it can also be the regulation of the carb. When you rev up it lets in to much air and not enought petrol and so you get a poor air/fuel mixture

That sounds like a possibility to me. I will check it. Thanks.

Also check the points gap and dwell angle. Also the distributor rotor arm to HT pin timing/distances.

NOTE! I've not worked on a 500, only 600D so the 500 may not have a typical 4 cylinder distributor setup.

Also check the points capacitor (difficult other than by replacing)

Will do. Right now the car is stuck in the shed after the big snowfall. Hopefully I will be able to get it out by the weekend to work on it.