Read my plugs? This is too lean, right?

ARCOgraphite · May 7, 2025

I think my father had one of those debored 472 in 1980, Low compression, retarded cam. Was that factory 150 HP net?

Did you go with stock pistons, cam? Watch out for those smog crank gears pushing retard IIRC, But I don't know about what GM did with Caddy smoggers never worked on one.
On your plugs, I can't read then from the posted images, I read plugs and tuned for WOT under load. Then tuned for tip[ in and part throttle cruise drivability.
Good discussion here, not much for to add. Good move to manifold vacuum. GM is easier to dial in a dist. than FORD with timed vacuum advance and coolant vac temp post "switches" and miles of rubber hose. LOL.

- Arco

14Accent · May 7, 2025

97prizm said:
Did you pull the carb jets and make sure that they are clean, and jetted correctly? They don't look bad. I added a link to compare. Hemmings also has a spark plug chart.
Spark Plug Chart

I haven't pulled the carb apart at all, I got the car out of storage last week and I've got to be honest: she fired right up and runs great still.

Chris142 said:
Every Edelbrock carb I have ran had that stumble @ light cruise. Now you have to change jets or metering rod but you don't know which with out an oxygen senser in the exhaust.

Last one I had got replaced by a Holley after I got few up with it.

I'd love to get this Edelbrock off. I hate the fact that it requires an adapter which then raises the air cleaner assembly and throws other fittings out of whack. Ideally I'd like to get a properly tuned Q-jet back on there.

Unfortunately, a properly built Q will cost me at least $500, maybe more because I won't have a core to return. On top of that, I'd have to either source an OEM transmission kick-down switch (roughly $300) or fabricate something using the micro-switch that's currently triggered by the secondary linkage on the Edelbrock. It's just too much expense when it runs well enough as-is.

snarl · May 8, 2025

Holley 1475 is a spread bore.

14Accent · May 8, 2025

snarl said:
Holley 1475 is a spread bore.

If you meant 4175, that does look like another good option... but at $800 (plus solving the transmission kick-down issue all over again), it seems like going back to an OEM quad + kick-down would be easier for the same money and outcome.

SammyChevelleTypeS3 · May 8, 2025

Those plugs look exactly as expected from a good running vehicle to me.

Jeg's Performance has so many carburetors (Holly + Q-Jet replacements and others). One's eyes could fall out before making a selection.
There were so many GMC Q-Jet carbs made and installed thru the years that there will be rebuilt and even new ones available probably till the end of time. Q-Jets were not only used in GMC products but were special equipment on certain other manufacturers cars at times. Now they are even being made new by aftermarket companies out there.

snarl · May 9, 2025

@14Accent Yes i meant 4175. Maybe a good used one instead of new.

thastinger · May 9, 2025

Unless you are doing plug chops, reading plugs is very generalized and will not help you diagnose where in the RPM cycle it is rich or lean.

A stand alone wideband would help you narrow it down.

SammyChevelleTypeS3 · May 9, 2025

As far as carburetor replacements: ( Just a few I know of)
Many have very fair prices from what I can see for rebuilds or new. The prices have even dropped and there is so much more available since 2019 when I had to drop out of the repair/tools part of the car hobby due to spinal issues.