Backfire

[Sheener]

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.

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

[ArdWrknTrk]

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

That doesn't seem bad.

Perhaps someone has already been there and got rid of the plastic toothed cam gear.

[85lebaront2]

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?

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

[ratdude747]

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?

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]

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?

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]

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

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]

Do you have a good idea of how many miles are on this engine? (emissions records should show how many times the odometer rolled over)

Are you in California where you must use factory original parts?

Do you have any (even tiny) exhaust leaks at the manifolds?

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]

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?

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]

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.

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]

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.

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.