The Stovebolt Site Forums The Shop Area Engine Shop rough running engine---I'm stumped

I have a '48 chevy 2 ton with a '54 235 engine. When I first started it it ran ok and a little better upon each improvement. Now however it runs rough at about 900 to 1000 rpm. In an effort to correct this I have replaced the coil, the points and condenser, the spark plugs and wires (the cap and rotor looked ok after some scrapping the build up on the copper. I've rebuilt the carb and checked for vaccum leaks.
The engine starts very easily and idles nice and smooth. Any suggestions for what might be the cause of the rough running?

Make sure the negative side of the coil is connected to the points. Also, if it is a 12 volt system, do you have a balast resistor between the ignition switch and the coil?

Dale

di1953gmc
Thanks for the response. Yes the coil is a new one that can work with an external resistor but it also has one built in and it is connected to the negative side.
Ben

Have you done a compression test and a leak down check lately? How about valve(s) adjustment? Solid lifters or hydraulic?
Does it do it under load ? Have you checked your vacumn advance is working properly?
How is the condition of your carb ?
I know.... so many question !!!!!!

Did it start after you replace the points?
If so you may not have installed the points return spring. This will cause point float when revved up.
Some points come with a separate strap spring that spoons the brass connector strap.

helilog 56,
No I haven't done a compression check or adjusted valves that was my next step. I believe they're hydraulic. I can't put it under a load test not quite yet. Yes the vaccum advance is working and I have kited the carv. Actually I think it ran better before the carb kit. The puzzling part is it idles great and for the first couple r's it runs very good and smooth.

I haven't done anything more than back the truck out of my garage and drive it back in. This rough running just appeared.

Pre'68 Dave
Most everything I listed in the first post was done as an effort to correct this roughness.

As I've stated before it starts very easily and runs very smooth at idle and for the first couple hundred r's. Then at about 900 and more RPM s it runs rough. Sounds like a flooded engine and I'm just waiting for it to clear up but it never does.

Thanks Gentelmen for all your help. I guess I need to pull the valve cover and check the valves.

If excessive fuel is getting in, pull the plugs and check if they are black. You are coming off the idle circuit when opening the throttle.... what type of carb?
Keep us posted !

'54 engine, I'll guess it a rochester "B' with a stuck down power piston.

Check that the spark plug order is correct. These engines can run with the plugs being out quite a few degrees. They can actually run almost well until you put the throttle to it...

Jeff

It does have a rochester carb just rebuilt. Not sure where the power piston is. My carb has an accelerator pump. I'll check that.
I've double checked the firing order. It's ok.
The spark plugs are black sooty but not wet.
Ben

The power piston is located next to the main metering jet. It is adjacent to the float. It is located in the top assembly of the carburetor, (part with the choke) while the accelerator pump is located in the middle assembly.

You should be able to grab the stem and move it up and down.

Kurt

The power piston is brass in a bore of alloy. It must be free in the bore. The idle port is below the throttle plate. You have stated that the engine runs good at idle. If the power piston is stuck on the power valve ball a rich fuel mixture is entering just off of idle. When you torque down the power piston assembly, make sure the piston is not in a bind. The manual image will put you in the right pew 'where the power piston is'. Good luck.
http://chevy.oldcarmanualproject.com/Rochester/BBC03.htm

Suggest you focus on the carb. If the spark plugs are sooty, black with carbon build up check the float level specs. This is a sign of a rich mixture.

Might want to check this post too.
https://www.stovebolt.com/ubbthreads/ubbthreads.php?ubb=showflat&Number=834008#Post834008

Power piston stuck, power ball in wrong position (goes above spring), gasket blocking off vacuum passage to power piston (credit to pre '68 Dave), will all cause a very rich mixture off idle.