Sorry long read, and TIA.

So I’ve got a rebuilt 351w with less than 300mi since it was rebuilt, baby died on me coming home from work about a month ago, had to be towed back to the house. At the time it was acting like a massive vacuum leak (didn’t want to start, when started it would barely idle and only with the pedal nearly floored for a few seconds), and the front seal on the intake manifold had a small leak prior to it giving up on me, I just hadn’t gotten to re-sealing the manifold again, I thought this was the cause that it had just catastrophically failed and this was the problem.
so I resealed my manifold let it cure, tried to start and same problem persists, double and triple checked all my vacuum connections and lines, then went into the ignition side of it, I had a spare coil, no change, checked and found 6 bad plugs (OL on the meter) replaced all 8, checked plug wires well within manufactures nib range, swapped to a spare cap and rotor, grabbed a new ignition control module, truck fires up for a second then slow bogs down to dead, can only get it to idle for a few secs at a time with the pedal halfway to the floor, but now it occasionally backfires through the carb.
So I rebuilt the Holley carb (it had sat for several months while I rebuilt the engine but not the carb), still no change, so I pull the valve covers check the valve lash and see all the valves seem to be opening and closing no change after that, so decide f it, really seems to be a carb issue, and go buy a edelbrock carb, install, and turn key runs great for about 8 seconds then slowly bogs down and dies.

my next planned steps are to disconnect the headers from the exhaust and see if it runs and something just collapsed or clogged up a muffler and that’s why it’s building back pressure and stalling out, if no change it’s going to be pull the headers and run a compression test, and try to figure out which cylinder/s are causing problems and why.

If anyone has any ideas or other possibilities I’d love some help troubleshooting this.

-Theo

 

Fuel pressure and supply? Check tank and pickup for debris? Filters? Old rubber lines can suck air if cracked. Check those and any connection that might suck air.

 

When it dies, take the aircleaner off, climb up in the engine bay and look down the carb while you open the throttle. See if you have two strong streams of gas squirting into the engine when the throttle is moved.

 

So it seams to run the same after each part replaced or work done and 2 different carbs I think you are on the right track on removing the pipes from the headers and see what happens. 3 bolts at each flange and its down.
How long was the truck down for the rebuild and where did it sit, inside or outside?

Easy test would be run a hose from a can set up on the fender well to the fuel pump and see if it runs.
Even easier when it dies pop off the air cleaner, open the choke and looking down inside move the throttle and see if it squirts fuel.
No fuel squirt then a fuel issue and try the gas can on fender to see if it is a tank / pickup issue.
Dave ----

 

Fuel delivery isn’t an issue, a few cranks was enough to fill up the carbs after rebuild/swap, it seems to as if the motor suddenly developed too much back pressure as it will start run for a few seconds and not start back unless you let it rest for a good 20-30min. My dad (who’s been trying to help me come up with a why to the problem) and uncle got to talking last night and came to a bit of a consensus of the back pressure due to a potentially faulty muffler, or something having crawled into the tail pipe, or a stuck valve. The last to make no sense to me as for the something crawling into the pipe is a no go to me as I made it a good 12-15 min from work before she gave up, the valves don’t make sense to me as they all seemed to have opened/closed when I went back and checked valve lash yesterday after the ignition module didn’t change anything, so unless they moved by hand turning the engine but not by engine turning itself it makes no sense as valves should stick in an open position and the springs are all fine no cracks breaks etc. so they shouldn’t be stuck in a closed position.
Thanks for the help and suggestions, I mean fuel delivery is still a possibility if I’m just not feeding enough pressure from the mechanical pump. I’ll get a inline fuel pressure gauge and see what it reads. Should be somewhere between 7-10psi if I remember right.

If someone wants a used Holley 18650 I have one, I like that edelbrock! lol


p.s. I did check the fuel filter and it’s good, wasn’t dirty but cleaned it out anyway, the other thing truck sat in my garage out of the elements while the engine was torn down and rebuilt and I waited/fished for parts.(truck motor gave up on me week before thanksgiving of 2020, motor was finished/installed and truck drivable come august of this year including break-in of the motor, and a brand new exhaust system) so almost a year while I scrounged up money and then waited for parts to come in rinse repeat and doing the work on Saturdays since the wife made me promise not to spend the entirety of the weekend working on it.

 

Careful with your line of thinking. Go where the clues lead you. Don’t try to reach a predetermined conclusion, as if you were a climate change researcher.

Lots of good advice so far. Just be open to all possibilities. Spring the $20 for a fuel pressure tester and verify what’s really happening, versus what you think it might be.

Same with a compression test. Just do it as part of basic diagnostics.

Verify the timing marks are accurate. Put #1 cylinder at TDC (use a straw through the spark plug hole) and make sure the dampener hasn’t slipped.

I’m not so sure about the possibility of a plugged exhaust. Even if blocked, any trapped pressure should bleed off almost instantly once the engine stops. You should be able to restart the engine, even if only momentarily. Regardless, it should be easy to rule out as previously discussed by disconnecting the exhaust.

Another trick is to keep a can of starting fluid handy. When the engine dies, quickly spray about a 3 second burst into the carb inlet. You’re not necessarily trying to restart the engine, but merely seeing if it will respond to a known-good fuel supply. If the engine comes to life for an instant, until the starting fluid is consumed, then you know the ignition is good and the fuel delivery is suspect. If no response to starting fluid, the ignition is the likely culprit.

 

Actually thanks to you guys I had a possible epiphany in the shower! So I pulled a spare plug was going to go ahead and run a compression test earlier today but the headers were in the way of the connection and wouldn’t fit so said I’ll leave that to after checking from the headers to mufflers, anyway even though the spark plugs are new they seemed to have a lean running appearance to them compared to the olds having run wear on the rich side of things(not overly rich but a bit rich) that popped into my head followed by the thought that if the fuel pump is say putting out 5psi instead of say the 7 it ought to be it could be the cause of the sudden lean run on the plug and why it will only idle at half throttle for a short period of time! That perfect storm so to speak! Thank you guys definitely starting with a fuel pressure gauge and going from there.

 

Check your ignition timing, then pull & inspect the distributor drive gear. If the roll pin holding that gear in place breaks (say, a piece of debris got caught in the gears) and that gear becomes loose on the shaft, you'll experience all sorts of random & bizarre symptoms (ask me how I know this).

A magnetic oil drain plug will help catch those small pieces (assuming they're metal).

 

How do you know this?

 

I’ve actually got a nemodium magnetic drain plug just to make sure I catch and pull any metal bits out instead of letting them flow through and destroy the engine. That being said I’ll work my way into the distributor and check it, but it’s a b**** to get back into place on level ground, and since it’s currently in the driveway which is sloped I really don’t want to pull it yet. But I’ll work my way there eventually. If I have too…. It’s really not going to be fun to get in on a slope.

 

I had similar symptoms on my 400, I think this had been going on for about a year.

For the longest time (several months), my primary symptom was erratic idle speed... 850 - 1500 RPM, it would bounce around. My first thoughts were that the carb linkage is getting held up, and/or the return spring isn't powerful enough. I eventually changed out the Holley Truck Avenger to an Edelbrock that had come from a Corvette, but that didn't fix anything.

If you remember, I wrote into here a few months ago trying to diagnose what was going on, and I at that time had eventually determined that the distributor had come loose and the timing got maladjusted as a result. Setting the timing back to 10-12 or so BTDC seemed to fix the problem - and it did, for a while.

But just about a month ago the same old symptoms of extremely low vacuum, occasional backfiring through the carb (especially over 2500 RPM), low power, needing to (sometimes) hold the gas pedal down almost all the way for it to stay running, this stuff came back. And then it would disappear, go away for a while.

An engine-builder friend of mine (the guy who assembled it when it was rebuilt) came over to help diagnose, and it wasn't long before he started suspecting the distributor. So, we pulled it out and, lo & behold, that roll pin had sheared and that gear was loose! It would turn on the shaft and then become "stuck" in a fixed position for an indeterminate amount of time, until it would move again.

We rebuilt that engine (400) in 2006 or 7, but it has less than 10,000 miles on it since then. I'm glad it was a relatively easy fix and not something like a slipped timing chain...

 

Does the acronym TMI mean anything to you? I think you could have safely stopped after saying you had an epiphany, and omitted any further details.

Anyhow, I’m not convinced your present symptoms are caused by fuel pressure slightly out of specifications. I have a fuel pressure gauge on my truck. IIRC, stock specs are 6-8 PSI, but I’ve seen from 4-10 with no change in performance.

I’ve suggested checking fuel pressure as a basic step for preliminary troubleshooting. My thoughts are you might see very low (or nonexistent) pressure. Such a fuel starvation issue would point towards a problem with the tank pickup, fuel lines, or pump.

In addition to checking pressure, you should check volume, too. The manual has details. You disconnect the line at the carb inlet and add a hose to direct the fuel into a bucket. You crank the starter for a certain amount of time and should have a specified amount of fuel in the bucket.

 

I didn’t think it was TMI but, will refrain in the future.
I didn’t think volume might be a problem vs pressure, as you have to x volume to be able to create and sustain x pressure, but that’s a good idea to check as well, and I’ll add it to the diagnostics.
By manual I assume you mean a Haynes or Chiltons manual? I’ll have to check mine before doing said test, thanks for the tips!

 

I think you can eliminate plugged exhaust because at idle there's usually not enough pressure developed for it to make much difference. It's when you try to accelerate that a plugged exhaust will interfere.

 

Unless it is almost closed off.
I also don't think it will idle or is that someone else 🤔
Dave ----