351W engine stumbles and backfires

Cougar Community Classic Cougar Maintenance Restoration and Repair
1

Hello guys.

I hope some gurus can shed some light on what might be wrong with my engine and why it stumbles and backfires through the carb.

I got a 1969 Mercury Cougar XR7 with a 351W (born 2 port but now has 4 port) and a Holley 4160 600 CFM carburator.

I am a very inexperienced engine guy and my father is a self-taught and experienced car guy who always had a passion for american classic cars, he has had them for 30+ years and knows his way around them. So I am trying to pass on his findings here as best as I can..

When we bought the car, it was running very high rpm at idle and running very rich mixture, and the electric choke was disconnected, to the point where some black residue was coming out the exhaust. But it seemed to not stumble on throttle. We then thought it was running too high so we started tuning the carb and idle screws but now it runs very poorly. After a lot of experimenting, it runs in idle but when touching the throttle the engine stumbles/hesitates, almost wants to stall, and if you give it some gas, it backfires with flames/pops through the intake/carb.

We have had the carburator apart, cleaned, new seals and new power valve (old didn’t seem to be bad though).

We have tried another carb (Edelbrock), same thing happens.

We tried playing with the ignition timing, setting it a bit later seemed to help a bit?

We tried to test for vacuum leaks but didn’t seem to find any?

Rotor does not seem 180 degrees off as we have read might happen (previous owner has had the engine apart at some point).

We measured around 8,5 bar (123 PSI) compression in all cylinders.

Could it be worn cam (lobes)? My dad thinks that the cam has a “longer duration”? Its hard for me to explain but that it was I got from my dad..

It seems in compression stroke that the intake valve closes late? Is it supposed to close at start of compression stroke?

In power stroke, exhaust valve opens half down, is that too early?

If it is the cam, how can it run OK at idle? Wouldn’t it have problems there too?

This engine has put my dad to the test, he has never had an carb/engine as difficult as this. Our next step might be to take the engine apart and check for any blown valves, lifters and checking the cam. But we just wanted to have your opinions.

Thanks in advance for any help or advice,

Kind regards,

Nicklas from Denmark

2

Hello Nicklas,
I would check the timing chain to see how much slack is in it. There are a couple ways to check it:
Always disconnect the battery first.
Remove the distributor cap.
Note where the rotor is pointing.

By hand, move the crankshaft clockwise until the rotor moves. Change direction of the crankshaft to counter clockwise. Record how many degrees the crankshaft rotates untill the rotor moves again. It can be felt when the slack is taken up. It should not move too much.
Another way is to remove the fuel pump and measure the slack in the chain with a ruler from tight to loose using the sharp edge of the timing cover as a reference point.

I would also confirm the firing order and check the spark plug wires for cracks, dry rot. Cross firing can cause this issue.

It seems like a timing issue. Having equal compression in all cylinders tells me the valves are sealing ok.

OR, the valves are sticking. Could there have been bad gas tank in it just before you bought it?
Was it a running car, on the road weekly?
Was it sold to you as a just found, barn find? Then the seller got running just before it was sold to you?
If that is the case, old gas tanks can seize up the valve train after the old varnish in the gas tank mixes with the new gas dissolving the varnish little by little and leaving it in the value guides. Question is, what was the condition of the fuel tank before you got it.
That’s all I can think of at the moment.

3

Pcv valve reversed or stuck open. Sounds like a huge vacuum leak