Quadrajet Problem Solving > Diagnose a Quadrajet carburetor problem

'78 Suburban 'chugging' on long grades

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RyanAK:
Hey, gang. I've searched through just about every thread here and on 67-72 Chevy Trucks trying to find someone that's had a similar issue but came up short. So gotta post. Sorry if this is long winded. This is my daily driver and my alternate vehicle isn't available for a while, so I'm wound pretty tight about this. This has been going on since June. Appeared out of nowhere after driving all winter and spring... about 8,000 miles.

So... the truck: 1978 K10 Suburban pretty bone stock with a GM crate installed by the previous owner. I believe the 190hp version. About 18,000 miles on that motor now. Carter 4MV Quadrajet 1705821? ('78 GM Truck), TH350, NP203 (stock), 3.73 gears. The only ‘upgrade’ to the drivetrain would be 16” wagon wheels and 275/75R16 tires.

At highway speeds of 60-75mph (2500-3000rpm), at load (uphill or long periods of acceleration), the truck feels like it gets starved for fuel. It bogs and goes in quick time, chugging, and feels like it’s sucking more gas than what’s available.

I added a “diagnostic tool” in the way of a vacuum gauge to help figure this out. I’m good as long as I keep the load to where there’s 10” or more of vac. (Idles in P at 20" @750rpm). Unfortunately this can drop my speed to 50 mph or less on a grade... with traffic zinging by at 75. If I spend any time with my foot in it and vac is below 10”, eventually it will start stumbling. When it bogs, the vacuum drops. So if I’m climbing a hill and engine vacuum is at something like 7”, when it starts lugging the gauge will bounce in time with the lugging between 7” (power) and 4” (stumble). If I take my foot out of it it smooths out and we pull along. Just slowly. Right now I'm limited to about 1/4 throttle or less. I just got back from a 1,200 mile trip between Pennsylvania and Maine, so I've had a LOT of time and a LOT of hills to think about this issue.

This will happen on the level or even going down hill if I put my foot into it, too.

I also put the back glass down. When it stumbles the exhaust smells rich. Hard to tell if the rich condition CAUSES the stumble, or it smells rich from when it catches itself.

Engine temps remain steady. Ditto oil pressure. Idles fine with 20” at 750 rpm. It’s been hot and muggy. This truck came from Montana at 5,000ft and now it's on the East Coast at between sea level and 2,000ft.

TBC...

RyanAK:
Stuff I did on this past trip to try to improve things:

Air cleaner lid flip (to bypass the valve in the air cleaner)
Wired the choke open (thought the choke could be closing and choking the engine)
Insulated the hard fuel line between the fuel pump and carb where it sneaks between the block and alternator with rubber fuel line (hot fuel?)
Fiddled with the choke pull off - seems as though it won't pull in on its own when engine is first fired. I have to 'nudge' it to retract.

TBC...

RyanAK:
Not sure what's happening that my posts keep getting clipped...

RyanAK:
Stuff I've done in the last few months chasing this:

Complete ignition tuneup (plugs, wires, cap, coil, rotor, module)
Timing dialed in at 10* initial, 32* all in @2800rpm, 15* vac on manifold vacuum
Idle air/fuel mixture - screws responsive and set to max vac
New mechanical fuel pump
New fuel filter with check valve
New soft lines at pump - old were soft and kinking. looked like an artery pumping!
Confirmed clear fuel lines from tank by blowing air back to tank
New air cleaner
Moved vac advance from ported to manifold
Double shot of Sea Foam
Cleaned carb with carb cleaner without disassembly
New PCV valve (suspected sucking blow by and causing lean condition...)
New divorced choke spring

I have video of this chugging happening.

I feel pretty confident this is carb-related at this point. And I'm determined to fix it, educate myself, and have it reliable. I have faith. But I haven't rebuilt a carb since I was 10 and helping out my Pap. So any help with diagnostics and guidance to a fix is very welcome.

I wish I had a known good carb to put on just to keep my rig mobile so I could pull this and really dig into it...

Sincerely, Thank you for any help that comes my way.

Ryan

Kenth:
You are running lean not rich, most if not all hesitation is due to too lean A/F mixture
I would open the idle down channels from .049" to at least .052" and replace the std #63 jets with .066".

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