I've done all my research and haven't found a solution that fixes my issue yet, so I figured I'd ask the brilliant minds here.

My son just picked up a 1976 F-150 that has a 351W under the hood...that seems to have not been an option with this truck from the factory, but I have no idea what car this engine originally came out of. We bought it non-running from some kids who gave up on the project after throwing some parts at it, towed it home, and we have it running but it's still struggling. We have it running and idling now, it'll drive down the road, but it continues to backfire out of the carb. The backfire is pretty consistent and increases with the RPMs. Backfires both idling and under a load. Also, it struggles to run when the vacuum advance is connected...best I can get it to run is with that disconnected and the port on the carb plugged.

Details: It has a brand new 2-barrel carb from the previous owners (looks identical to the Motorcraft that was still in the cab, but without a brand printed on it). It has a Duraspark ignition in it, and the firing order seems to be correct (I tried a different order I found on this site and it wouldn't run at all, so I'm assuming it's the correct one - 1-3-7-2-6-5-4-8). I've checked and re-set the timing a bunch of times, though any numbers/lines on the tab above the dampener are long gone, so I'm basically guessing what 12* BTCD might be. Even with that, I've turned the distributor pretty significantly in both directions and the backfire never goes away as the RPMs change, so I'm starting to rule out timing.

I'm now leaning towards vacuum leak (my vacuum gauge will only get up to 15 at most, and I know it should be more like 20-21). Most of the vacuum system is capped off and any emissions stuff that used to be there is no longer there. I've replaced a few lines and worn out caps, but still can't get that number above 15 or the backfire to lessen. I know the brake booster is bad and I thought that might be causing the vacuum leak...removed the hose and capped it off at the intake...still the same. This morning I was spraying brake cleaner around any potential culprits and had some fairly consistent results in the RPMs rising when I sprayed where these two hard metal lines come up from the left side of the intake and lead back into the choke and a vacuum port on the back upper-plate of the carb (attaching a picture below).


One of my questions is how is this piece attached to the intake and can a leak there be fixed? If I pull those two bolts out, will that little plate come off with the hard lines intact, or is this a more permanent/fixed piece of the whole intake? Regarding the bigger picture, would a couple little vacuum leaks like this and a couple more old caps I found coming off where the brake booster connects to the intake cause such a significant leak that I could get 5psi of vacuum back? Even more important, would a 5psi lack of vacuum cause such consistent backfiring?

I know the engine is burning oil (especially on the left side) and I haven't pulled a valve cover yet to investigate that. The Haynes mentions a valve or valve-lash issue as a potential cause of backfiring, but I'm trying to rule out all the simpler stuff first.

I'm sure the truck has more issues, but this backfire issue is the top priority right now. Any advice or insight would be greatly appreciated!

 

those are just for the choke they won't have any effect on your problem.

The most common cause your problem would be your firing order being off . if you're positive it's correct it's probably a top end problem. be sure you're starting at #1 and using the correct firing order, remember Ford does it all a stupid way.

 

Vacuum leaks cause lean running leading to spitting and popping.

Your firing order is correct as far as I can see, 1-3-7-2-6-5-4-8 ... numbers starting on passenger front is 1, then 2, 3, 4 to firewall, then front driver side is 5 with 6, 7, 8 to the firewall.

If spraying around those two lines raises rpm, disconnect and plug them for a "test" try.

 

IIRC the 302 uses 2 different firing orders based on cams installed.
One for the normal 302 and other for the HO 302
I think the 351 uses the 302 HO order but not sure as I have a 300 six.

If someone swapped cams and installed a normal 302 the order would be different.
Look up the other firing order and try it as you have nothing to loose.
It has fixed many issues in the 80's truck area.
Dave ----

 

First off, are you sure it’s a 351 Windsor? I don’t remember any Windsor engines having that choke stove as part of the intake manifold.
Most of them of this vintage had that connected to the exhaust manifold itself. of course I could be wrong on that, but that’s what I remember.
Do you have an overall picture of the engine, especially around the distributor area?
Do you have a picture of the valve covers and how many bolts hold them down?

Even though all 351W engines had the 137 firing order, as mentioned it is the camshaft that dictates this. Nothing else in the engine other than the camshaft. And since 302 and 351 camshafts are the same physically, they can interchange.
So again, as said, you need to verify.

You can follow the action of the valve train, or a simple way without removing the valve cover is to verify which cylinder comes up next. When the compression stroke of cylinder 1 is arriving, determine which cylinder is next. Is it five or three?
You can do this with a finger over the spark plug hole, among other methods.

 

If you are confident in the firing order, I would do a compression test on all cylinders. If you are way out on one or more cylinders, it'll be almost impossible to get the carb tune and timing right. Time for a rebuild or at least rehone cyclinders and get new rings.

 

Make sure the carb style gasket is correct for the carb, 2 styles, 2100 and 2150. It will leave you with a leak. spray behind carb base. As others have said a lot of performance cams left you with 302 firing order.

 

I think it would be easier to just swap the 2 plug wires and see what happens.
Cant be any worst then what you have now.
Dave ----

 

The 302 HO used the same firing order as the 351 (not the other way around), because Ford pulled a 351W Marine cam off the shelf to create the HO engine in '82. There was no other performance reason for the different 302 firing order besides economics, but I digress.

This kind of consistent carb backfiring is a timing issue, and since you've tried a whole bunch of things without any improvement, I would consider the timing chain may have jumped time on the gears, making a mess of everything. There's nothing on the outside you're going to adjust or change to fix that. You may need to pull the front end open and replace the timing set and start there.

 

Another scenario is a wiped out cam shaft.
Especially likely if the previous owners installed a new flat tappet cam, and weren’t able to give it a proper break-in.
Because the engine wouldn’t run, if It’s just been cranking on the starter, idling occasionally, and cranking on the starter, a standard non-roller camshaft will be very unlikely to live a full life under those conditions. and at some point in the Where cycle, can easily cause your symptoms. I personally experienced that with a friends Chevy 454.

But either a timing chain or wiped cam can be verified pretty easily. Well, easy for us! Since you’ll be doing all the work and removing valve covers and such.
You can check for the action of the valves at that point to verify the firing order, and you can measure lift and even look down to see what the condition of the lobe is.

 

A misfire that is contant across the rpm range would point to one bad cylinder (maybe two). You could try pull a plug wire off one at a time and listen for a change in the engine. Another option is to spray water on each cylinder exhaust at a time looking for one that doesn't burn off instantly. Any back sucking at the end of the tail pipe might lead to a burnt exhaust valve.

 

Driving since 1970, have twice had a cam lobe or two wiped. One was a Chevy 305 in a '79 Malibu, one was a Plymouth 360 in a '80 Gran Fury police car, both were bone stock from the factory. The popping got worse with increased throttle and power fell off as well .... as both had exhaust lobes go flat and when that intake valve opened, the previous failed exhaust pulse pushed back through the intake.

Both cases, new cam, lifters, oil change, filter .... but the Plymouth still ate a rod bearing ... but the Malibu did great.

 

Thank you all for the help...here's the update as of today:

Finally ran a compression test and got between 110-123 on six of the cylinders (which I think is on the low side, right?) Numbers 2 and 8 came back completely dead - absolutely 0 compression. I'm gonna go ahead and guess that is the major source of the backfiring issue. It seems to me that 2 cylinders with zero compression won't fire and that fuel is gonna get burned off somewhere like the intake/carb.

Is there anything with the valve train that might cause zero compression, or am I officially at the "this engine is toast" phase and it's time to replace or rebuild?

 

Well, at least part of the engine is toast! But no, it doesn’t automatically need a full rebuild yet. Yet…
The scenarios for a dead cylinder are many.
Could be stuck valves. stuck open, or stuck closed.
Having four stuck valves is unlikely, as is even two. But neither is impossible.
There could be bent or dislodge push rods. Meaning the valves are all stuck closed, netting you the same results.
Could be a hole in the piston. That’s not very likely, but is still possible.
Could be cracked cylinder walls. Also not as likely, but also very possible.
Also could be one of the previously mentioned scenarios, and flat cam lobes.

So it’s at least time to start pulling a valve cover off and seeing what kind of activity you see at the valve train on the two dead cylinders.

 

Oh, and while those other compression numbers might not be stellar, they are not out of the ordinary either.
Depends on the engine, but they could be perfectly satisfactory for your particular engine.
Yes, it looks mo betta and more impressive on a dial to be in the 150 to 170 range, but for some engines, your numbers are perfectly normal.