Hello, My name is Ethan and I have a 1974 F250 Highboy. I'm just gonna start with the problem, I was driving the furthest I've driven it like 10 miles but traffic filled miles. On the way there I could feel the brakes getting worse and worse (Mushier and less powerful) and finally around when I got there I was first at a stop light so of course I gunned it to hear the Bald Eagles fly. A loud backfire through the carb happened and from then on it got worse.

After I gunned it I was a mile away from stopping so I didn't have much time to realize differences but I did notice if I gave it more than 1/4 throttle it would misfire really bad a lot and if I kept going to like 1/2 throttle it would backfire again. I also noticed after the first backfire the brake pedal was instantly were SUPER stiff but engaged where it used to, it was just harder to get to where it would start to brake, almost unusable brakes.

I got to lunch and let it cool down and when I got back it misfired during starting but when it did start it drove. Same thing again I had to keep it under 1/4 throttle or else haha.

I got it home easily with the brakes getting stiffer and stiffer and parked it. The next day for some reason I was convinced into taking it even further AFTER it wasn't running well and I did.

I drove it 12 ish miles of highway and same thing with misfiring and backfiring and every time it would backfire the brakes would get stiffer. Finally, on the way back, the brakes were so stiff it was hard to keep at a stop light and the truck started to shake uncontrollably when we were stopped, the door panel shook half way off, exhaust was tapping my bed haha and so I put it in neutral and it stopped shaking even with brakes on. (It didn't shake when driving or with brakes off)

I recently had a similar problem besides the brakes and shaking, and it was a condenser. Possibly same thing but what could the brakes be, I do know the owner before me put CRAP distributor and coil and plug wires and all ignition parts in it. I have ideas and I'm not completely incompetent but I would like other peoples ideas. THANKS and I know THANKS THANKS THANKS

 

I would suggest bleeding the breaks. Depending on how the previous owner took care of it ( I doubt he did much maintaince if it wasn't his daily driver ). So I'm 100% sure there's gunk in the break lines. Hell one of them could not even be working ( like my back left drum on my highboy ). Also a lot of air. Fluids are a first on any old truck or second, third owner truck. Get a timing light/gun ( they are really cheap and your local auto parts store should have one ) Then check the timing. Don't know what engine you have but it should be around 650. But since it's second owner it's probably close to 1000 or higher. To high of timing can cause misfires due to to rich of fuel. Also check for vacuum leaks. Does the engine surge in idle?

 

Hey thanks for the reply and those all are good ideas and I have always been sceptical about the timing so Ill do that and bleed the brakes even though I just replaces the rears and bled but the backs were so bad the fronts are probably still really bad, all good ideas but none of it explains the extreme shaking.

 

In addition to what's already been stated, I would check low and high for vacuum leaks....carb to distributor, booster to check valve to intake...and anywhere else a vacuum line would be lurking. They tend to crack, get brittle and even discapear after limited or no PM.

Another thing to check is inside the distributor where the breaker plate is....check for movement when vacuum is applied and moves freely. Check for wire brittleness, too.

Definitely sounds like a vacuum leak

 

x2 on the vacuum leak. Likely in the line to the booster or the check valve. Alternately, the booster diaphragm may be busted.

 

The shaking could be a number of things, bad linkages, bad cab bushings, clutch came loose after so many years. Timing also can affect it when you shift into certain gears or are at certain speeds. Mine shook coming out of second and from 30-40 mph ( most of third gear ) fixing the timing made starting from second "shudder" disappear for me. And very little at 30-40 now

 

Thanks all, I will check timing, vacuum, and bleed brakes. Hopefully it stops and I can let her be free again.

 

X2 on the line to the booster - and hopefully not the booster itself... As far as vacuum leaks go, the booster and line are about the biggest you can have.

 

I replaced the brake booster and replaced the vacuum line going to it and my truck still runs like crap, I can tell that I fixed a huge vacuum problem because the truck doesn't shake anymore when in drive and we unplugged a vacuum line and the engine shook, the inlet where the line goes in on the booster was super cracked and broke off with a tap before I replaced it. So I fixed half the problem.

Now whenever I am driving and press the pedal fast, whether it's from 1/4 to 3/4 or from 0 to full throttle. It does fine rolling around parking lots and cruising downhill but uphill with gas or gunning it it sputters. I just fixed a big vacuum leak, timing was just checked.

HELP !

 

Your initial description sounded exactly like my yellow truck once. It backfired and blew out the check valve on the brake booster. No power brakes, and I had to keep the RPMs up to keep it running.

Check for additional vacuum leaks. Check your vacuum tree, all vacuum hoses and caps. The yellow truck ran like crap for a month before I found the crack in a vacuum cap.

 

Glad to hear you're sorting out the problems. Here's another vacuum check fer ya, 'cause it sounds like there's still a vacuum gremlin sucking around:

Check the vacuum advance can's diaphragm - yank the vacuum line off the carb, apply suction to the can's nipple. If you feel/can draw a breath, and no suction holds vacuum to your tongue or lip, the can's pooched. Same test, different peek-see: apply vacuum/suction to the nipple....with the distributor cap off....and look to see if the breaker plate and the vacuum can's arm moves.

If it doesn't move , the can is junk. If it does move, let us know the test results and we'll go from there.

 

I didn't seen if you mentioned it, but how long have you had the truck and what else have you done to it?
What about the engine in general? Was it recently rebuilt by any chance? Reason I ask is that the symptoms that match a vacuum leak also mimic a bad valvetrain issue. Such as a flat cam lobe or two from improper cam break-in or using modern oil with a flat-tappet cam without some sort of zinc additive.
The latter may not be so dire if it's an older well worn-in cam, but the break-in thing can be huge.

Or if you just recently messed with the heads or just the valves, it might be something valve related after all.
But definitely check for vacuum leaks. Are you familiar with some of the ways to test for these?

Good luck.

Paul
Hoping it'

 

Well I got the truck, and changed the oil with modern oil, "high mileage" I forgot specs but I just replaced water pump, valve covers, brake booster, and I took it to a shop and they replaced oil because gas got in it, and I replaced everything gas related, tank, pump, sending unit, filter. The shop replaced carb.

Now that you've scared me with valvetrain failure IM HOPING its vacuum, even though vacuum leaks are so annoying and hard to find without risking catching on fire haha. The canister is kind of of iffy, and the line going to it was really loose and when we disconnected the line we got really intensified symptoms of exactly what we were having, so I think that shows it is vacuum but if you're experienced with valvetrain failure inform me!

Could this be low vacuum instead of a leak? The line going to the canister felt weak, what should it feel like?

 

Which canister? You talking about the charcoal canister or the brake booster or one of the vacuum reservoirs?
If the evaporative charcoal canister, it's not really manifold vacuum, so will definitely be weak if even perceptible. It's just plumbed into the air cleaner in our earlier years. Not until computer controls came into play more recently did they employ full vacuum to the charcoal canister.

If you mean the booster, then that's definitely a problem. Should have massive vacuum straight from the intake manifold. Usually just from one intake runner in the back via the vacuum "tree" fitting.

The reservoir(s) will have smaller tubing, so not as much a vacuum signal to the finger tip and the idle quality, but still should be able to feel it pretty good.

A new brake booster is not necessarily a "good" brake booster. New overseas sourced stuff can leak right out of the box. But I would think you could detect that, or easily test for it. Maybe someone will know a good procedure for checking that.

I would not worry overmuch about carburetor or brake cleaner for testing. Yes, they're supposedly very flammable, so just don't spray it right on the coil or spark plug wires! Assuming your wires are good and not arcing that is. In which case don't go near the whole engine with that stuff!
But no, it's not that dangerous unless you let the fumes and liquid build up, or shoot it right on to an ignition source. I've used it forever and more often than not when you find a leak the idle goes down instead of up like you would expect.

Now, with some people recommending propane or, even worse(!) acetylene(!) I would say pass on that and go to Plan-B.
Even something benign like WD40 or water will still find a vacuum leak. Some use water for it's obviously neutral and non-damaging properties, but even WD 40 doesn't hurt paint or usually anything else unless the paint is brand new.
I use Chemtool to good effect. Never had a fire and never failed to show any leaks. I just never spray too much (you don't need to) and never let it build up and never sprayed it in testing on something in an enclosed space. Always in the greater wide open spaces.

I know I'm recommending something potentially dangerous to someone I don't know. But if you follow some simple common sense rules you can use just about any thin liquid in existence safely.

Good luck.

Paul

 

Oh, and by the way. What PCV setup are you using, and how is it set up? Did you use the valve specified for your engine, or substitute some other?
The are different and some can really make an engine run lean, while others don't have any negative effect.

Paul