Hey all. Done some work on the 352 lately. I replaced all the lifters. To do this, I pulled the intake manifold as I needed a new set of gaskets there anyway. I have torqued to spec, have no leaks, timed the motor, and have it in a place where she has smooth idle...

When the accelerator is pressed load she hesitates, then increases RPM before backfiring through the carburetor.

My first thoughts were: Fuel delivery, Timing/Ignition, and Vacuum.

I replaced my fuel filters, both the one on the fuel pump and an inline filter between the carb and the pump.

I replaced the vacuum line between the distributor and the carb.

I replaced all spark plugs. (all old plugs looked great- sortof a whitish/tan with no oil, carbon or other build up.)

I bought new points and condenser to find that the PO replaced them with a Petronix "Ignitor" module... a black box-looking thing.

I have inspected the rotor and cap, which both look acceptable.

Other related(?) issues...
Wants to stall upon take off
Idle slows when put in gear / engine is under load
RPMS decrease when brake pedal is pushed / demand for vacuum increases
when partially choked, pushing the accelerator arm causes smooth RPM increase

ANY HELP??

 

Hook a vacuum gage up to a constant vacuum source on the intake and look for a vacuum leak. You didn't say if you changed the carb vacuum caps. A vacuum leak producing the back fire since you are describing it only happens when you increase RPM. I would also bet that the engine will stall if you try to drive it.

 

I will try that... yes, it does stall when I try to drive it... particularly when the brake (power-vacuum boost) is employed...

Where would you recommend I hook up the vacuum gauge, exactly?

 

To the carb... look around at the base of it, there should be a port.

 

I cleaned the manifold as best I could with a wirebrush, picks, and a plastic scraper. Some of that carbon/burned-up-whatever didn't want to budge. I would have had it tanked, but was on a tight time budget... The pushrod from the earlier photo was STUCK! had to tap it out and then brush out the hole.

 

So I was able to adjust the carb to the point where the truck (grudgingly) made it 60 miles from my uncle's garage to my apt in DC. It still isn't right, but it is home. I will be tinkering with it more in the next week or two and post my findings back to this thread... thanks for the input so far.

btw, I'm AMAZED that I was able to get that manifold back on (two guys, no hoist) and seated without gasket issues in one shot. You folks were dead on about the weight and pain in the assitude of this task... now to nurse my sore back......

 

You might have timing chain issues. The chain and gears may have worn to the point that the slop is affecting the cam/ignition timing relationship.
The gears and the chain wear and stretch over use. The gears installed by the factory, if aluminum with plastic teeth, were soft, designed to eliminate noises. The teeth often fell off, and the aluminum wore down, retarding the camshaft timing.

The chain accumulates all this wear. The following is an easy check to determine the amount of slack, slop or wear, in your timing chain.

It is not necessary to remove any components as long as you can see the timing marks clearly. It can be done with one person, but a second person can be helpful if the distributor cannot be easily seen while turning the crankshaft from the front of the engine. This test cannot tell if a chain has jumped; it can, however, tell you if there is enough slop in the chain to have let it jumped.

Tools: Breaker Bar with socket to fit the front crankshaft bolt

1. Mark the TDC or 0 mark on the damper clearly. Ensure the rest of the timing marks can be clearly seen; clean them if necessary. You may find it useful to scrape a little chalk over them to highlight them.
2. Remove the distributor cap and place it out of the way. You must be able to see the rotor while turning the engine with the breaker bar. It usually is not necessary to remove the spark plugs, but some high compression engines may require it to get an accurate reading.
3. Turn the engine in its normal rotational direction until the TDC or 0 mark is lined up with the pointer.
4. IMPORTANT: Do NOT turn the crank backwards just to line it up. Doing so will give erroneous results. You MUST rotate the crank in ONE direction only. If you overshoot, just go around again. This is where it may be helpful to remove the plugs.
5. Once the TDC and 0 marks are lined up and while watching the rotor, rotate the crank in the opposite direction. When the rotor begins to turn, STOP. It may help to have a helper push against the rotor's direction of rotation. (Remember, it will be going backwards at this point.) They can feel it begin to move more accurately than you can see it begin to move.
6. Note the value of the timing mark the pointer is indicating. It will be BTC, because you went in reverse of normal rotation.
7. This value is the amount of slop your timing chain has in degrees of rotation. Anything less than 10 is safe, but performance begins to fall off after 5.
8. If you have anything over 25, STOP driving that motor and get the chain and gears replaced!
Post back with what you find, and I hope this helps.

 

I'll look into that this weekend, Banjo. Thanks.

 

Hey Sammy,
Nice F250, and you're making good progress on getting it up to snuff!

In following your progress I noticed...??...what is that gizmo located just ahead of your master brake cylinder....some sort of aftermarket power brake booster system?

How do you like your sliding rear window?

BarnieTrk

 

Was it backfiring before you did the work? Is it possible that they gave you the wrong lifters? Just asking.

 

DMAN- No, it wasn't backfiring prior to the work, and the lifters were correct. I cross reffed the part numbers once they arrived, prior to installation.

BARNIE- Thanks, I like the sliding window. It was installed by the PO. I find it to be just a bit "cheap" feeling, but it doesn't leak air or water when closed, and with how hot it will get here in DC in the summer, I'm sure that I'll appreciate having it. It came from LMC truck (one of the site's sponsors?) according to the PO.
That is indeed a power brake booster. I've never seen anything quite like it. I figured that it was original, but maybe the PO put it in? I have a hard time believing that, since I would think he would have opted for a more modern one... I plan to replace it (way) down the road with the more recognizeable modern unit... which begs the question: which units (new or from a different year truck) will bolt on in?

 

Sorry, but I don't have the specifics on which models / years of FoMoCo trucks will bolt in, however,
I suspect that power brake boosters from the '67-72 F100/F250 trucks may likely fit well.
I'd suggest you do a site search here on the subject and then file away the information and then just keep an eye out for the parts as you go.....

BarnieT rk

 

i tend to agree with the rest of these fellers, i would check the vacuume, maybe the module on the distributor is not working. this will cause the backfire when accelerating, you could also do a compression test, maybe you have a intake valve not closing all the way or leaking which would also cause a lack of power and backfire because combustion is getting into the intake. this will tell you whether or not all your valves are working correctly. dutch

 

not sure but if it wasnt doing this before you replaced the lifters it sound like a vacuum leak. possibly that heavy manifold pinched a gasket? mine did the same thing for years and i never figured it out. i installed a 4barrel intake and holley carb and no more issues. i sold the carb on this site hoping the guy that bought it could make it work right. he hasnt let me know yet.