Backfire

[ArdWrknTrk]

So about 85% the density at sea level.

Yeah it seems the test people aren't nearly as stringent or well trained there in Colorado.

These Ford's came with a restriction in the advance line. It kept the distributor from reacting too fast to throttle position.

Does your emissions sticker show a dashpot on the throttle linkage?
If so is it functional?

[ArdWrknTrk]

Maybe you should check the timing chain for slop while you're under the hood for wet compression?
It's not uncommon on these trucks and leads to over fuelling and sluggish performance.

[Sheener]

Truck had been modified for “performance” when it was out of state. Emission equipment remove and the carb installed. Wife followed me home from sellers place, and complained about how rich it was. Went through a lot of fuel in the 20 miles, maybe 5 mpg. Never gave the mechanical secondaries a thought as to part of the problem. Emission standards changed in the last year or so. Probably have not trained for the upgrade. Do not see that it ever had a dashpot.

[ArdWrknTrk]

Well can you post a pic of the diagram sticker you DO have?
Because it would show there, and the throttle stop helps keep the carb from choking.

You've got us all guessing at a cause without any info or background.
I get it now that it's new to you and you don't know who screwed with it and how.

It's still worth checking out the timing chain, if the way it runs is any evidence of it's "performance" modifications...
We need to find the bottom before we can climb out of this hole.

[Sheener]

The only information sticker I have is the vacuum diagram. It is the same as the one on the vacuum documentation for the 460 that is in the documentation page of this website. I have found nothing else in the engine bay. Mine looks the same as the one on the top right. Am I missing one?

Did another compression test both dry and with a couple ounces of oil in the cylinder. Pressure was in the 130 range on all cylinders with the oil, up from a low of 95. But is that a true test of ring failure? A liquid is not going to compress, and that is taking up some cylinder volume.

[ArdWrknTrk]

Is the calibration number on your sticker exactly the same?
Because there are hundreds of variations.

It would be a lot simpler to just see what you've got.

And this is why you never do a wet test with more than a teaspoon (2/10 Oz.) of straight 30 weight.

Did you get a chance to rock the crank back and forth while looking at the rotor?

[Sheener]

Okay, this is all new to me as you have probably guessed. Going to let the motor sit with the oil in the cylinders and let it see if it will leak by into the crankcase. May have too much in there. Did not read the directions carefully before trying it. My fault.
Tried to load a picture of the sticker in the message. Will try again.

[Sheener]

[85lebaront2]

Gee, that looks pretty familiar, sort of like this one that went with the vacuum line collection.

[ArdWrknTrk]

Certainly close but ADT is not the same as APE, so something is different.

At any rate do you have any other suggestions as to why this truck is running so rich, Bill?

[Sheener]

Timing chain has 2-3 degrees of slop in it.

[ArdWrknTrk]

That doesn't seem bad.
Perhaps someone has already been there and got rid of the plastic toothed cam gear.

[85lebaront2]

If the carburetor (a) doesn't have a blown power valve and (b) doesn't have a warped secondary metering body and (c) isn't flooding due to either a sunk float or overly high fuel level. I would check the evaporative system and bowl vents.

[ratdude747]

Found it. There is a slight difference around the "BPV" in the lower right. But it may be the same thing, just redrawn for clarity?

[85lebaront2]

Jim, ADT dimension for dashpot clearnce is in mm. APE dashpot dimension is in inches. Any bets the ADT label was for Canadian sales and APE was US sales. I know the APE was US as that is the label from Darth.

[ArdWrknTrk]

Bill, while I'm not certain that the 650cfm 4777 double pumper was even made pre- 92 the OP did say they had changed out the powervalve.
Body could still be warped or floats set too high. IDK.
The first three of the VIN would tell if the truck was made in Ontario but not it's intended market.

An aftermarket carb doesn't have provision for connecting the bowls to a charcoal canister. So I can't see that being an issue.
With the fouled plugs, 5 mpg and a compression reading of 90 I can't see the engine not having cylinder bad wash down and worn bores or rings.

Just hoping for some solid evidence before condemning this thing as NWR.
It's certainly a challenge to diagnose a vehicle over the internet.

[Sheener]

Put everything back together today and I had read that a lean condition led to backfires, so I increased the primary jet one size. Went for a drive and for 5 miles of city-country driving it was great. No backfire at any rpm shift, ran smooth, idled well and then it came back. Loud violent backfire. Came home and bumped the timing from 8 to 14 degrees. Will occasionally pop, but not as violent unless I accelerate hard. Will try for emission test and see how it goes. I wonder if the residual oil burning off kept it from going lean at the start? Did Ford make a two cycle?

[ArdWrknTrk]

A lean condition shouldn't lead to incredibly sooty plugs and 5 mpg...  

Are you sure don't have an intermittent ignition fault that kills spark when the engine torques on its mounts?
The tach doesn't die when you push in the clutch, does it?

If advancing the ignition timing helped it would seem that your issue is timing related.
But you say the cam chain is tight, so that shouldn't effect the distributor.
Still, it seems to me like the timing is incredibly late.

Can you check to see that TDC #1 matches the mark on the damper?
The timing marks will move when an old damper's rubber loses its grip on the sleeve.
I find that 14° BTDC will always give me a labored crank with the starter fighting against compression, especially when hot.

I'm just taking a stab in the dark at this point.
Who has any idea what the PO did (or didn't do) to this vehicle.
It's obviously far too late to set it back to baseline for a proper comparison.

[Sheener]

The 5mpg was when I first got the truck. I had re-jetted the carburetor. I was going to pull a plug today when I got home and forgot. I had not thought of the balancer being off, I will check that.

[85lebaront2]

Jim, I should have replied to this one earlier, Holley introduced the double pumper mechanical secondary carbs around 1969 or 1970, we sold a load of them.