'78 Suburban 'chugging' on long grades

[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".

[RyanAK]

Thanks, Keith. Appreciate the response. Two quick things...

Truck ran strong and without incident all winter and spring. Seems to have become an issue with the onset of hot, humid weather. Is this something to consider, or just a red herring?

And...

When it does this, it

[Cliff Ruggles]

Couple of things I would do.

First order one of my SR rebuild kits and go completely through the carb.

My kit will have the larger solid (not windowed) .135" fuel inlet set in it.  I'll bet at some point the carb was kitted with over the counter parts and it has the tiny N/S assembly.  Even with the little 350 stock crate engine I've seen them suck the bowl low or dry with the smaller fuel inlet seats.

During the rebuild install my filter/spring w/o the check valve in it.  The check valve is not needed when you use the solid fuel inlet seat and it is very restrictive.

Check the power piston and polish it up with some 600 grit auto body sandpaper and brush the PP bore with the appropriate rifle brush. 

I'll supply a new correct PP spring with the kit.

Set the float to factory specs and check the power piston hanger arms to make sure they are level and exactly even.

Once the carb has the correct parts in it and correct settings it will take it out of the equation when troubleshooting.

If your fuel delivery condition still exists make sure all the rubber hoses on the supply side of the pump get replaced and clamps are tight. 

If all this fails to correct the issue I'd drop the tank and take a look at the pick-up inside the tank to see if it's cracked and sucking air or the "sock" is plugged up with debris.

It wouldn't hurt to move up a jet size with that carb number but if it was working fine then things went South it's going to be something else......Cliff

[von]

This sounds exactly like what my '73 Monte Carlo did in the mid 70's. The culprit was a clogged fuel filter "sock" (nylon mesh) on the fuel pickup in the tank. Going up hills or on level with heavy throttle it would bog then die if I pushed it. I could pull off the road, turn it off (if not already off), and wait a few minutes. Then it would start and run fine for a while. Rust and/or sediment would evidently get sucked onto the sock and clog it, then after sitting turned off for a while the crud would evidently fall off the sock and it would run fine til the cycle would start over. The fix was removing the sock from from the pickup (tank must be removed to get to it), cleaning out the tank, and installing an in-line fuel filter between the tank and fuel pump. No problems after that even when I was towing a Corvette on a trailer at times.

[RyanAK]

Great stuff. Thank you.

Ya know... When this first happened and I dove into it, I changed the filter to one without a check valve because that's the only one the parts place could get. The issue seemed to (seemed to) have gone away. Later I sourced one with a check valve and the issue returned. Could it be as simple as that?

Cliff - I'll call you to place an order for book, kit, jets, etc.

Dropping the tank will be my last move. It's a 40 gal tank and I'd need to cut the big tube steel hitch off the frame to remove the tank. Next time I'm low on gas I do intend to stick a borescope down the filler neck to try to get a look at the pick up and filter sock.

Much appreciated fellas. Now to find my spare no-check-valve filter...

R

[Cliff Ruggles]

If a slight restriction at the filter is causing fuel delivery issues I'm betting you have bigger issues elsewhere.  In any case I'd still do two things.  Get rid of the check valve in the filter and rebuild the carb with the larger N/S.  I'm betting your troubles will be gone just doing those two things.......

[RyanAK]

New developments...

Cliff, I got jammed up at work so didn

[von]

Trouble is with looking at the in-tank sock with a bore scope, is that when the engine isn't running under load the rust and sediment don't get sucked onto it and what was there has fallen off. Maybe you can see any stuff on the bottom of the tank though and general tank condition.

[RyanAK]

My posts keep getting clipped for some reason... here's the cliff notes version of what I had typed above. (pun!)

It's super hot here. And humid. Truck still acting the same. Yesterday on the way home from work I stopped at a gas station for a soda and a cigar. Came out, started up, didn't get past the gas pumps before the truck shut off. Wouldn't restart.

Pulled the fuel filter. Clean and dry. And the check valve seemed stuck to the rubber gasket. Did a 'gatorade bottle test' to see if I was getting fuel. Took about 10 seconds of cranking, but eventually a good flow of fuel came from the pump. Buttoned up without the filter and got on my way.

Same starvation issues as before even without a filter. If I keep my foot in it too long it chugs like it's out of gas.

Stopped at the parts store for an alternator. Came out, started truck, same thing. Didn't get out of the parking lot before it shut off. Another 'gatorade bottle test'. 10 seconds of cranking before fuel flow but then it was strong. Fuel in the bottle looks very clean. Buttoned it up, cranked for a bit, got it started and headed for home.

What the heck!? Is the delay in fuel from the pump to the carb normal? Is underhood heat an issue?

This truck drove so well all winter and spring. 15,000 trouble-free miles. Then all of a sudden this started in June. Heat? Summer fuel? Hmmm...

I'm definitely going to rebuild the carb. Not sure that's all that's going on though.

Cliff, I got tied up at work yesterday but I'll be calling you shortly for an order. Book, kit, jets, accelerator pump, whatever else I'll need to get this sorted and tuned.

I appreciate everyone's input. Von, that's a good point about the sock and the truck not being under load.

Here's a few photos....

[Cliff Ruggles]

Check all the lines and clamps before the mechanical fuel pump.  Also check the routing of those lines to see if any are too close to the exhaust system.

I've seen these issues plenty of times in really hot weather and it is often caused by a restriction in the system or a leak in the system prior to the fuel pump.  When the pump puts the fuel under suction and it's really hot outside and under the vehicle these problems car rear their ugly head.

I just went thru a LOT of "drama" with a customer that I built a carburetor for with similar issues.  His issues were even worse and his entire fuel line was draining all the way back to the tank so he had to crank it for long periods of time before fuel returned to the carb after long periods of being shut down.

He was convinced the carb was draining down and wanted to send it back.  I told him to check ALL the connections and replace all the rubber hoses before the fuel pump.  He insured me that all of it was new when the vehicle was being restored over the last couple of years and that it got a new fuel tank. 

So we go back and forth over a week and finally he goes under the vehicle and low and behold he finds a loose clamp on a metal line where it goes into the fuel pump on the engine.  It was allowing the fuel pump to suck air but not loose enough to show a fuel leak after shut down as the air coming in was letting all the fuel return to the tank and empty the line.

That loose clamp caused him a LOT of issues including the carb going empty on a hard run, empty fuel line after sitting and LOTS of cranking to re-fill the carb, etc. 

I've seen defected or old rubber hoses do the same thing, suck air but not leak fuel.......

[RyanAK]

Hi, Cliff. Thanks for taking the time to respond. I went through the fuel lines yesterday. Sucking air and breaking the vacuum created by the pump makes perfect sense as to why it would drain back or suck air while pumping (sucking). More sense than the new pump is wonky.

Supply and return soft lines at the pump are new and clamps are tight.

Hard line from pump to carb is clear and I put rubber line around it to insulate it where it was near the block. Temp on that line (meat thermometer taped to line) just before the carb is 140* after running for a few hours.

Factory heat shield is in place where the exhaust maneuvers around the transfer case and gets near the frame where the fuel lines are.

Soft lines mid-frame are in good shape and clamps are tight. I'll replace anyway.

I can't get to the soft lines at the tank... dropping this tank will be a huge PITA. 40 gallons and there's a massive hitch welded in around it. I'm gonna see what I can do to maneuver myself into a position to replace that line.

I had a thought to try to check all of these with a stethoscope with the engine running to see if I can hear a leak where it might be sucking air.

Are the Quadrajets susceptible to heat soak/fuel boiling issues like some of the Eddy carbs are?

And I'll still rebuild the carb. Left you a message yesterday.

And... I'm trying to not be one of those 'drama' guys. Ha!