'78 Suburban 'chugging' on long grades

[tayto]

the line that is hardest to get to is probably the problem, murphy's law. haha

[RyanAK]

Posts keep getting clipped...

Rigged up a soft line from the pump to carb with a clear plastic inline filter so I can see what

[RyanAK]

AAAARRRRGGGGHHHH!!!...

Rigged up a soft line from the pump to the carb with a clear plastic inline filter so I can see what’s happening. Seem to get good fuel on first start on a cool engine. When I shut it down, it immediately drains back. After a 10 min wait on a hot engine, the carb will suck the filter dry before the pump can push fuel to the filter. Maybe 10 seconds from when the engine fires until fuel starts arriving in the filter.

I have a return line on the pump. Are these mechanical pumps supposed to allow fuel to drain back? Could something in the carb not be closing/sealing to allow the fuel to drain?

[hiy_po]

I am trying how to understand how the carby can "suck" the filter dry
 I see a suck as a high pressure trying to get to a low pressure. so would need a pressurized line with a low in the carb to move fuel that way. Once pump has stopped and no pressure in line fuel shouldnt be going into carb unless you have heat causing a high pressure in the line. Is it possible your tank is vacuum locking and you are getting a huge low in the tank as the level drops causing movement of fuel back to the tank once pump is stopped?? This would also explain why it would need time to reprime. If you dont have an intank pump then you are rellying on gravity to get the fuel from tank pickup to first pump.

[RyanAK]

Thanks for chiming in, Hiy_Po. I

[RyanAK]

Thanks for chiming in, Hiy_Po. I’ll try to get video. The tank isn’t under vacuum. The vent is clear. It will do this with the gas cap removed... I checked. Pump is mechanical on the block. Not sure if it is supposed to have some sort of check in it to keep fuel from draining back... either through the supply line or the return line. But it drains as soon as the engine is cut. As for the fuel remaining in the filter after drain-back... after a few seconds after startup, it gets pulled from the filter to the carb. I’d guess the low vacuum in the intake manifold (20”) might be enough to pull the fuel from the filter to the carb. A few seconds after that the pump is pushing and refills the filter. If the engine were under load right after starting, the carb would get sucked dry before the pump could get fresh fuel to it and the engine would stall.

[Cliff Ruggles]

I tried calling yesterday, then saw this morning where you called again.

Was out of the office most of the afternoon running parts and working on my shop and driveway.

I'll try again in a couple of hours, orders don't ship out till around noon......

[tayto]

Posts keep getting clipped...

Rigged up a soft line from the pump to carb with a clear plastic inline filter so I can see what

trying pressing "Preview" button first then "Post"

[hiy_po]

lol I think I may have read it wrong. I read it as the carb was emptying the fuel filter after shut down. With no manifold vacum or pressure in fuel line I couldnt see how that was happenning. I get it that the carb is running dry before the filter is primimg on start up.

[RyanAK]

I'm thinking that hot fuel / vapor lock is contributing here. I'll check the fuel return line is functioning from the fuel pump and explore ways to keep the pump and lines cool, along with the carb rebuild. Anyone have thoughts?

[RyanAK]

Fuel comes out the return nipple on the pump at idle. Stops when revving the engine. I'll need to check back at the soft line sections at the between the pump and tank to make sure fuel is getting all the way back and not turning this into a deadhead system. Lines were blown all the way back to the tank when I first had this issue to make sure they were clear, so I'm thinking it's not a return line issue.

Rigged up a fuel pressure gauge after the temporary clear filter between pump and carb. 6.5psi at idle. 6.5psi at 2500rpms. No load, sitting in the yard. Engine up to temp.

Next to rig the gauge so I can see it while driving. (short drives... this is all hillbilly rigged together for diagnostics only!!)

The soft supply line going into the pump from the hard line on the frame was resting on a heater hose. Moved that and put a split piece of fuel line on it in that area as an insulator.

Fuel pump gets HOT. Now I'm thinking the heat on the pump may be the culprit. How do you keep a block mounted fuel pump cool?

[RyanAK]

Well that was enlightening! Took the truck for a drive. 82* here today. Truck up to temp. Fuel pressure at idle and lazy acceleration stayed at 6.5psi. When I pulled onto the highway and accelerated the fuel pressure slowly and steadily dropped to under 2.0psi. Then the stumble and chug routine. Let off the throttle, pressure stayed low for quite some time until it began to rise again.

When the engine is turned off, pressure bleeds off immediately. I would have thought with the check valves in the pump that some pressure would hold in the line from the pump to the carb...

Pump is HOT.

So... Heat? Restriction? Wonky pump?

Hmmmm....

[tayto]

is it starving from fuel or the new fuel pump defective? I think it's time to drop the tank, replace rubber hoses, inspect hardlines on sending unit and install a new pickup/sock.

[RyanAK]

Alright, fellas. For any of you still reading this saga, thanks for sticking around!

Rebuilt the carb with Cliff's kit - my first solo rebuild. Went very well. This thing was pretty damn clean inside. Just a few issues that I ran into...

As a reminder - 4MV 17058213 on a '78 Suburban, 350 GM 190hp crate, TH350, NP203, 3.73 gears, stock, stock, stock.

This 4MV doesn't have an "Idle Vent Valve" as shown on the instruction sheet. Not sure if this is an issue...

Also... the "Idle Compensator" on the back of the carb is missing.

I didn't get the APT screw exposed because I wanted to get this buttoned up... but I will eventually do that.

Also didn't replace the Secondary Air Valve Cam because I wasn't sure how to remove the staked screws without buggering something up.

As I said, this was pretty clean and everything seemed to be correct. The accelerator pump didn't have the skirt spring in it, and the Secondary Air Valve tension spring was a bit loos, but everything seemed "right" otherwise.

Some specs that are a mystery to me and I could use some input on:
Primary Rods - 40B
Jets - 64C
Power Piston - arms level and even, Cliff's orange spring
Secondary Rods - DP
Hanger - C
N/S - Cliff's .135" non-window, no clip on needle
Accelerator Pump - Cliff's
Float - was A67, now Cliff's A61 set to stock 15/32"
All linkage adjustments in spec.

Need to re-tune idle rpms, idle f/a mixture, etc. Now that the carb isn't an issue, I can go back through my timing. I had it slightly retarded to 8* initial to limit the pinging that apparently was my carb going lean from fuel starvation and not a detonation issue.

It was fun! My only worry for future rebuilds now is that Cliff will retire and the good parts won't be around!

All that said... the carb rebuild didn't fix my issue. Same starvation/chug when under load.

What DID fix my issue after everything I went through... was replacing the 2-month-old NAPA fuel pump! Ordered in a Delphi unit from NAPA and after some struggles getting the fuel line to seat and not leak, I have fuel pressure and flow to the carb. The NAPA pump fell apart in my hands when I pulled it off the block. Spring fell out, lever fell out, and there was a black rubber seal of some sort chewed up and jammed near the pump piston. UGH!!

New pump... pressure seems a bit high, but no flooding yet and no starved condition. At hard acceleration I was getting <2psi out of the pump. Now at WOT it holds a steady at 6psi with the new pump. Only thing that makes me nervous is at idle and cruise the pressure can be 10psi or a smidge higher. Am I looking at trouble with that pressure and this particular pump? As I said, zero issues so far. And no starvation/stumble/chug.

Appreciate any further comments on the pump pressure or the way the carb is set up. I learned a lot and had fun doing it!

[Cliff Ruggles]

That carb can  handle higher pressure w/o issues with the correct float setting.

The "staked" portion of the screws for the flaps need to be removed before attempting to remove them to replace the plastic cam.  You can change the spring w/o removing the shaft, but I recommend taking the extra time to replace the cam as well.  They get worn down and loose on the shaft and cause issues with the secondaries that are difficult to troubleshoot.

A working APT system is a nice feature and worth the extra time to remove the factory screw and install the custom APT screw I make here.

Not surprised at all about the failed pump, I see this ALL the time with newly purchased mechanical fuel pumps.  I believe Airtex (probably made in Mexico, Korea or China) supplies a lot of mechanical fuel pumps these days.  It is difficult to obtain on OEM pump, the prices for them on Ebay are really getting up there on them........Cliff