Chevy 350 help

Subzilla

Subzilla

Test Driver
Joined
Jun 13, 2011
Location
Triad - Mt. Pleasant, Concord & Georgeville
My 75 K5 has 97 350 installed by PO. No computer. It had a Holley on it, was running not so good, so I put on an Edlebrock carb borrowed from a friend. Still running like crap....stumbles and falls right off idle but will somewhat clear with more pedal. Adjusted the timing, adjusted the carb, replaced plug wires. The weird thing is that it is not consistent, it seems to get worse at times. Plan to replace fuel pump and filter just to eliminate that.

I have the competence to replace parts but I'm just not a great troubleshooter of this old school set up. I don't want to keep throwing parts on it - the few there are. Guess I'm looking for someone who can help me figure this thing out. I am frustrated and willing to pay money! Any suggestions on who to take this thing to? I'm in Concord. I am willing to do whatever to get this thing running right.
 
Not a mechanic, but could be issue with vacuum advance, or vacuum leak. 97 model, would generally have been throttle body, & computer, so would the distributor & heads, be compatible with a carb?
 
If the filter is pretty old I would start with that. A clogged filer will cause the thing to choke on any heavy acceleration and make the thing run erratically. To me it definitely sounds like a fuel starvation issue. If the "new" carb has a bad accelerator pump it will fall flat on its face at the first stab of the pedal then eventually catch back up...gradual increases in throttle should not make it stumble.
 
Could be two bad carbs!? Happens.

Might try to plug all vacuum ports on the motor/carb to eliminate any suspect vacuum issues.

Something else to check. Pull the distributor cap and make sure there isnt any oil down inside the distributor. 70's HEI distributors were prone to oil leaking into the distributor and will make them really stumble and miss.

Also, wouldnt hurt to cap and rotor it while the cap is off. Eliminate the obvious tune up items first. You'll want all that stuff new anyway!

Coil strong? Wiring to the coil in good shape? Those will def cause an off idle stumble.

Electric fuel pump? Fuel pump isnt over powering float in the carb causing flooding. Will act the same as fuel starvation.



Worst case...

Are the lifters quiet? Idle sooth? As in no burnt valves, or collapsed lifters, or wiped cam? Bad vacuum signals will cause this as well.


Just some thoughts for ya!:)
 
Ol jeeps covered it! does sound like timing or vacuum. I had this problem on my 79 I replaced dizzy and all vac lines and fixed it I also did fuel pump and carb at same time! Holley carbs are very tunable in my experience but if it were me I'd put qjet back on it unless it's performance engine, Or maybe I'm old and thinking mpg out of my old 400
 
I would also check all ground wires make sure there tight and have a good connection
 
Id go over all the basics here. Im not a chevy guy but for the most part an engine is an engine. Id check the firing order, check for 12 volts to the coil and ohm the coil, and then set your timing to 10-14* initial. Then put a vacuum gauge on it and see what its doing. Make sure with the timing light that it advances as the rpms raise.

Yes you could have two bad carbs on your hands but the that 2nd carb ran fine on a buddys engines it should be fine unless you dropped it and the float level got jarred.
 
benXJ said:
You haven't checked/adjusted the timing? I'd start there.
Click to expand...
There's an old saying, "90% of carburetor problems are in the distributor". Bump the timing up and see if it helps the off idle bog.
 
First thing, make sure 0 mark on balancer lines up with #1 cylinder TDC. I've seen several where the outer cover slips on the actual pulley, moving the actual timing mark.

Pull the plugs, set timing mark at 0 on crank pulley, make sure #1 is at TDC.

Inspect plugs, take pictures and post up here of the plugs...

Put in new plugs and check the gap. Are you running a newer hei coil, or older coil? The newer stuff can tolerate more gap, while the older lower energy coils need a tighter gap.
Most newer vortec motors call for .055 to .060 gap, while older stuff is in the .035-.045 range. The higher the voltage coil, the more gap.

Next warm up the engine to operating temperature. 190-210.

Connect vacuum gauge to non ported vacuum port on carb or directly on intake.

Change idle mixture and set mixture for max vacuum.

Change timing and set to max vacuum.

Classic Inlines - Tuning with a Vacuum Gage

Post up your results and findings.
 
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