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85 G30 miss

I love my Chevy Chevette!
Posts: 1
Joined: 05/18
Posted: 05/18/18 04:12 PM

Hey guys, thanks in advance for any help you can offer. I'm trying to fix a friend's 85 Chevy G30 motor home with a 350/4bbl, engine VIN code M. He brought it to me with a stumble and miss that gets worse the hotter the engine gets. It feels like it's starving for fuel and goes flat on the secondaries even when cold but when hot it cuts out with any throttle and won't stay running, it doesn't feel like a spark miss. Owner replaced the carburetor and fuel filter, so it's got a new Quadrajet. What I've done so far:

*Disconnected supply-side hose from mechanical fuel pump and ran new hose into gas can to remove lines and tank/sump from circuit for testing... No change.

*Removed mechanical fuel pump and replaced; pushrod looks good, no obvious wear. No change.

*Replaced ignition coil and spark plugs, coil was oem dated 1985... 3 plugs coming out were oil-fouled, one severely, and two looked fuel-fouled. The other three looked normal. This made an improvement at first, still sounded flat when I got into the secondaries but there was no miss felt for a several mile test drive. After maybe 10 miles an intermittent hiccup showed up at 40mph cruise, which got worse and worse, eventually would not accelerate past 25mph and cut out with any throttle applied. If it stalled, it took starting fluid to restart.

*Checked fuel pressure in front of carb, the mechanical pump pulsed it too hard to get a usable reading but the needle bounced between 0 and 10 so it is getting some kind of pressure. On shutdown it immediately loses pressure, this doesn't seem normal.

I'm fixing to pull the new plugs and read them and do a compression test.... But when it's running fine and will maintain speed, it you get into the secondaries at all, it immediately goes flat and shakes like hell as soon as the butterflies open, but accelerating on the primaries there are no issues. But when it gets hot enough where it starts stumbling at a cruise, any throttle beyond about 10% results in a cut-out or stall.

Please let me know if I seem to be missing something here, I'm used to diagnosing fuel injection and this Qjet has me stumped!  

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