I've had this 77 F150 for 15 years and have always dealt with a low power issue, but have never really investigated it. After my father in law caught it on fire and burned most of the vacuum lines on the engine, I decided to get rid of it all and simplify with an alum intake and 4 bbl (1406 Edelbrock) Also installed a new distributor.

It idles fine and runs decent enough to run down the road. But any time you step on the throttle, it bogs and backfires through the carb when the accel pump hits. It completely falls on its face past that with any throttle input. I would chock it up to carb tuning, except that it did the same thing with the stock 2bbl.

Compression is between 110-120 lbs on all cylinders.

I initially set the ignition timing to 10* which gave me total of 45* all in. Vacuum reading was around 15" and bouncing a little at idle.

I played with timing and could get the vacuum to 17, but it took 35* of initial timing to get there. Ironically, the backfire and the bogging went away, at least with no load on it.

I'm suspecting maybe the cam timing is off or the chain has slipped.

Has anyone had issues with the ring turning on the balancer?

Just looking for a little feedback before I pull it apart.

 

Just comparing notes ... sounds like yours is lean. I would check again for vacuum leaks if it were mine. I have a '77 F150 4X4 with 351M / C-6 / 3.50 gearing. I've only had her since 1986, she only has 104-105K miles now. When we started out in 1986, she was stock except for some trim.

Early in our relationship I slipped a Crane cam and three way keyed timing set in her, she's set "straight up" now. Stock, her cam was somewhat retarded by the factory, as were about all 351M motors. I have a Edelbrock 1405 carb, Holley 12-803 Fuel pressure reg set at 5.5 (using gage and tee) on Edelbrock Performer 400 intake .... no EGR.

She runs great, no p[inging, really OK for a 8.0-1 motor with a bumpy cam. Idle about 16-17" vacuum at about 7-800 rpm.

I did buy a step up piston spring set and a new style alcohol resistant acc pump, and I use grease on my intake-carb base gaskets.

Timing .... Way back when I also installed a quick advance spring kit in the distributor. Her balancer is now marked for up to 40 degrees BTC every 5 degrees, seems to me I added a last couple lines. Anyway, to time her I pull and plug the vacuum hose, loosen the clamp bolt, start her up and holding timing light on pointer ..... bring rpms up until no more advance is left in distributor, I set that at 35 degrees, drop rpms, snug clamp bolt, rev and check setting. Then I hook vacuum up and check to make sure vacuum canister is good still.

The other end of the vacuum lead, I've had it either on ported vacuum where it comes in only above idle and I have used it on manifold vacuum with a slight turn of the idle stop screw to slow the engine back down. Currently, it's on ported vacuum, either way .... she runs well, no stumbles, no spitting once warmed up (she spits when cold if not enough choke though).

 

I did have some vacuum leaks along the top of the ports with the valley pan style intake gasket set. I swapped them out to fiber gaskets and that solved the problem. Don't seem to have any vacuum leaks that I can find.

In my experience with the Edelbrock carbs, they run on the rich side out of the box. And it really hasn't changed much at all between the stock carb and the 4bbl.

I'm assuming that the timing set has been changed at some point. It has quite a few miles on it and there is little play in the chain. Not sure if the cam is the stock grind. The truck belonged to a long time friend and mechanic that passed away so I'm really not sure of the mechanical history.

Thanks for the reply,
Jason

 

Yes the harmonic balancer can slip and the signs of it are the rubber between the 2 pieces begins to push outward .. Best to check with a piston stop for TDC .. I don't like to check/use the marks for that reason .. The distributor should be recurved if the EGR valve has been removed ..

 

Sorry about your old friend ..... that's tough.

A carb can be jetted OK and the engine still be running lean because of vacuum leaks is where I was going .... maybe you saw it that way? Anyway, with him being a mechanic making changes if and when he wanted as he wanted, you'll have a lot of unknowns to deal with.

Take Care .....

 

Yeah, I probably need to have the distributor recurved for non EGR. I have to find someone around this area to do it. The only one that I knew of retired several years ago.

I might pull #1 and check TDC before I do anything else. I didn't see any signs of the balancer moving or the rubber pushing out.

Thanks guys,
Jason

 

Here is a good guide on doing the recurve Duraspark_distributor_recurve_instructions_index

There is plenty written on the subject but if you want to send it out the author of the article is a good choice