Mid-70s 400sbc Bogging

[Renken2000Classic]

Thought I'd throw this issue out for consideration. It's an intermittent problem, happening usually under hard acceleration, and is sometimes right after a shift (though I've no idea what the trans would have to do with it). It seems like fuel starvation as it runs pretty well otherwise, and isn't misfiring when that happens, just losing power.

It's a 200k+ miles engine and 185k+ trans (400THM). Valve stem seals have needed replacing for a good while, and I think there are 3 plug extenders in there now. Last time I changed them they were fairly clean.

Here's what I've tried:

- Adjusting the secondary flappers to not open quite so soon/easily (set screw on the rear RH side), but that didn't fix it
- Changing the inline filter ahead of the fuel pump (it worked great right after that)
- Removing temporarily the filter at the carb

I haven't checked fuel pressure or flow, but my next thing will probably be to just replace the pump (it's a chain store one, maybe 5 years old). Other than that I'm wondering about maybe the carb needing done again (did that 5 years or so ago too), or some weird thing with the trans causing it that I've never heard of (lol).

 

[alldodge]

More info on motor

 

[Renken2000Classic]

Small block 6.6L (4bbl) that came out of a mid-70s Impala wagon. Went into a 1/2T GMC truck of the same era in the early 90s. Stock, never been into that I know of. 200k+ miles.

 

[Scott Danforth]

Start with basics. Compression?

Is this driven daily or does it sit. If it sits, go thru the fuel system

 

[alldodge]

It has a 4bbl but which 4bbl?

GM started using electronic ignition in 1975, and don't remember if the 400 came in HEI or points

Being intermittent in most cases that would lean me toward electrical

With it sometimes happen after a shift, look at the modulator and vacuum line

 

[ezbtr]

clogged carb jets? Accel pump? Fuel clean? Distributor miss?

 

[Renken2000Classic]

It gets driven every week or two; I almost always put 100% gas in it. It's a '75, so HEI; carb same year I'm sure (quadrajet).

Haven't checked compression in a long time. The shift points seem normal. I'll look at that vacuum line.

I did plugs, cap and rotor not that long ago. If it was misfiring, I'd do the wires too (might anyway). Might go ahead and put a $25 fuel pump on it tomorrow and see what happens.

 

[dolluper]

Three things to check..advance is free distributor shaft not seized on dist bushings...pickup coil under dist rotor wires and ignition module same location..4 vacuum advance