Plz help with sbc running ragged

350 Chevy zz4 crate with about 18k miles (according to prev owner). 350 automatic, HEI, good gas, clean fuel filters. Engine runs best when cold. Once it warms up, driving accelerating around 1700 - 2000 rpms it stumbles badly. If I back off the gas it smoothes out some. I can work the speed up accelerating till it stumbles then back off and accelerate more. If I rev the engine without a load it runs fine. It has an electric fuel pump by the tank.

What I have tried that DIDN'T work -
1. Changed carb to new holley 600 w/electric choke and vacuum secondaries. Old holley 750 w/vacuum secondaries had the same stumble.

2. Changed plugs and gapped to .035. Old plugs were gapped the same. Should I go to .045?

3. Changed distributer cap and rotor.

4. Rearranged plug wires. Wires are one year old.

5. Vacuum gauge when idleing shows 12 - 12.5.

6. New pvc valve.

7. Electric choke opens fully and seems to work correctly.

What should be the next step? Change the plug wires? Regap the plugs? Change something in the HEI? Change the fuel pump?

Thanks

 

Yeah, I know, I'm a FNG...... But, just new to this board. I can link to some 4x4 boards if you want my credentials.
I'm tossing out ideas here, been through lots of **** like this over the years.
1) Do you have a fuel pressure regulator in line? Is it plugged or turned down.
2) Does it have a fuel pressure gauge? What is the pressure? I don't remember what a carbed engine needs, off the top of my head I think it's around 5-7# max - but please don't quote me, someone will likely set me straight pretty quick around here.

3) Stick a vacuum pump on the distributor advance and see where it moves. It may have a hole in the diaphragm and not be advancing the timing, that is if you have a vacuum advance and not a mechanical.

4) This one may be obvious, but did you make sure BOTH carbs had their secondary floats set high enough? I ran my rock crawler w/ low floats just to keep bowl from overfilling when I was on my side. It didn't want to rocket down the highway that way.


Good luck, hopefully one of the gurus around here will chime in. I'm off to work in KS for a week, so I won't see this till next weekend (maybe).

 

Is the HEI wired directly to the fusebox to get 12.5V, minimum? A lot of harnesses have the votage step down with a ballast resistor to the dist/coil, but an HEI needs it all.

The symptoms are exactly as you describe them when it's done wrong...

~Scotch~

 

Timing is everything. Make sure the timing is true to the mark on the balancer. Nine out of ten times the problem is electrical...

 

On my crate I have Dart Pro 1s and they said to gap the plugs at 50. From what you have done I would make sure the fuel pump is ok. You could also find someone that has a disributer machine and see what the advance is comeing in at.
It either has to be fuel or ignition problem
You said you could let off the gas and then get into it again I would guess fuel pump.

 

Update -

Installed new Holley red electric fuel pump and new filter between tank and fuel pump. Pump is mounted near the bottom of the fuel tank. Tested car - same problem accelerating.

Unhooked wire to electric choke and rotated until the choke is "unchoked" all of the time.

Drove car until the engine warmed up and was having the same problem accelerating. Pulled over and checked voltage where the wire enters the hei cap - 14.2 volts.

I guess the next step is checking the vacuum advance. Change spark plug wires. Any other ideas?

Thanks

 

yeah why not swap the wires.. or ohm them out.. The resistance is higher when its hot. ALSO I would suspect maybe the module in the dizzy cause they may **** out when hot.. just a thought..

 

if it runs good when its cool and stumbleing under acceleration when warm i would go with timing.

 

OK, again as i've said, it's tough to fix em by remote control like this. I haven't read the responses, but i'll offer this. If it runs best cold, i'll guess it's a too rich condition, of which the motor will like when cold, like having the choke on. I hope this helps

 

I'd regap the plugs to .045. Especially with an HEI ignition.


jerry

 

That was it. . . sparkplug gap I changed them to .045 and the problem went away. Stupid I know but I a google search turned up .035 for a ZZ4. WOW go figure bad info on the internet.

I was going to change the carb anyway and the fuel pump needed to be replaced and moved to near the bottom of the tank. So, the engine running like **** was motivation. . .I guess.

So any ideas why a smaller gap would make the engine stumble so badly? Just not igniting big enough?

Thanks to all that answered. I appreciate it guys.

Bo