Okay guys well I just picked up an 82 F250 with a 351 Cleveland in it (yes...it is a 351C), 4spd, 4x4. The motor was rebuilt and has about 100 miles on it (previous owner had it rebuilt and never really went anywhere with it).

Anyways I'm having some issues with it, I noticed it was not running well and was getting some popping (sounded more like a piffff) out of the carb especially after shifting (RPM change?). Pulled some plug wires and realized it was not firing on cylinders 6 and 8. I found the plug wires were hooked up wrong on them.

Replaced distributor cap, plugs, wires, and reoriented them to the correct location. It has a Holley 670 Street Avenger carb on it, the accelerator pump was leaking so I removed the carb and replaced the diaphragm and reinstalled the carb.

After that the motor sounded stronger at idle but when I would give it gas it would bog and hesitate. I pulled the plug wire off Cylinder 6 while running and it didn't seem to make any change to the motor so I still don't think cylinder 6 is firing.

I got a timing light from Auto Zone and could not make it work at all, the light wouldn't flash, tried plug wires 1 and the coil wire and nothing. So loosened the distributor holding bolt and tried playing with it to see if I could get it to run better, idle picked up but I started hearing a tapping metallic sound come out of the carb (not loud, except one time was louder than the others). Killed the motor and said forget it and walked away.

I noticed the distributor top half could move left and right some, opened the cap and it looks like the little plastic piece inside that holds the bottom plastic part in place broke. The distributor itself has no play but the plastic bottom half that the cap snaps too can move left and right some. Can this be replaced individually or do I need a whole new distributor?

I'm stumped. The timing obviously is not quite right, I'm gonna try and get a different timing light and see if I can get that set. I still can't figure out why it would not be firing on cylinder 6.


I have not done much with old trucks in a while (well 8 years), I'm very mechanically inclined I fix and build custom motorcycles on the side. I'm in vet school and picked up this as a play toy/work truck and for nostalgia as I missed my first vehicle (78 bronco). I'm already banging my head on this one.


Any help/suggestions/things to try would be GREATLY appreciated.


Thanks,
Spencer

 

This thing? The distributor cap adapter?



(Image is representative, your particular unit may look different)

I imagine around $10 at your local auto parts store.

 

Once you get the adapter sorted and the initial timing set then check the color of the plugs. That should tell you if each cylinder is running, although just idling may not so a drive would help.

Then, if you still have problems, it is time for some testing. The typical approach would be a compression test, but I much prefer a leak-down test. That will not only determine if you have compression problems, but what is causing them if you do have leaks.

 

Okay guys, I had a guy that is pretty familiar with 351 motors come out and help me some today


Update:

Replaced the distributor cap adapter (thanks ctubitis)

We started by playing with the timing, tried it sat to various settings and still no huge improvement

Figured out that I accidentally left the spring out of the accelarator pump, didn't bog/pop as bad after that

The truck still does not run right, it will idle fine and if you accelerate slowly it's fine. But as soon as you mash the throttle it will pop out of the carb. It's not a backfiring its more of a sharper pop sound (no fireballs).

Vaccuum was strong at 20psi

We played around with different cams and air/fuel setting on my Holly Street Avenger 670 and it idled better but still would pop when giving it gas. It has a 6.5 power valve in it, should it be swapped to a 9.5? He said typically you want them to be half what your vaccuum is

We swapped on his 600 Summit Racing carb off his truck and it did the same thing and idled worse (probably not enough fuel/air).

The cylinder that was not firing was 6, we checked the plugs. All the plugs on all the cylinders are BLACK. We are getting spark on cylinder 6, compression is 130 (check 5 too and it was 130).

He says the motor sounds healthy in his opinion

Vaccuum was strong at 20psi

Oil pressure was good (sprayed out like crazy when turning it over with valve covers off)

We found one of the rockers on the right side had been assembled wrong (base/pedestal mounted the wrong way) and fixed that (no change it how it ran).

We tried his coil as mine is the original one it looks like and no improvement.

The cam going bad was mentioned but the motor sounds perfectly healthy other than under accelleration.


I'm stumped and dishearted, and not sure where to go with this. Does this sound more carb related as in I should up my power valve and change my jets? Or does it sound more electrical?



Any help would be GREATLY appreicated.


Thanks,
Spencer

 

Oh and I was getting good spark on cylinder 6. I stuck a spare plug in the wire and when held against something metal I would get a spark. It didn't spark if it was just the plug wire and plug and not held against something metal.

Also, one thing that I just thought about. When I originally picked the truck up it could be revved up and did not make this popping out the carb. However plug wires 6 and 8 were reversed at this time.

Also, when I pulled the original plugs out all the cylinders except 6 and 8 looked like they had been firing. The 6 especially and 8 looked like they hadn't been very well. That was before everything like plugs, wires, cap, rotor, incorrect plug wire order, etc. were swapped


Thanks,
Spencer

 

If the plugs are black then the mix is rich. So there's no reason to change the power valve as you aren't likely to be getting the vacuum down to 6.5".

I would check the valve operation to determine if the cam is bad. Many people don't understand the need to break a cam in properly, so it is very possible this one has been ruined. The best way to check is to put a dial indicator on each rocker and turn the engine over slowly to determine the range of movement.

 

Thanks Gary, any suggestions on a good cheap dial indicator? Where exactly do I place it to on the rocker to check? Could it be done with a caliper or do I need a dial indicator?


Thanks,
Spencer

 

A dial caliper will work, but you need to measure at the top of the valve opening, and that is hard to find. So the dial indicator makes it much easier since you can turn the engine and watch the indicator, stopping at the max opening.

Harbor Freight has adequate indicators, and even ones in kits that have a magnetic holder, and that helps bunches.

 

Gary, does this one look adequate?

 

Yes, that looks very good. The assortment of tips is great. But, when you set the gauge up you have to get it on each rocker the same way and ensure that the stem of the gauge is running parallel with the pushrods. Otherwise the readings will not be accurate.

 

Thanks again Gary.

Is there any specific location on the rocker I should place the indicator at when measuring? Am I looking for the total range of motion (from the lowest point to highest point) or just seeing how far they open?


Thanks,
Spencer

 

Actually, it doesn't truly matter where you measure, although you want to use the same spot on each cylinder, as you are just comparing all of the cam lobes to each other. Or, maybe I should say intake to intake and exhaust to exhaust. It isn't likely that all of the lobes are damaged, and hopefully none, so you just need to see if they all lift the same amount. If so, the cam is good.

As for where to put the stem of the dial indicator, pick a spot that can be used on all of the cylinders. Sometimes there's a hole that oil squirts through, and that is a good spot as the tip of the indicator frequently stays in the hole if you use a pointed tip. And, again, keep the indicator's stem parallel with the push rod.

 

Also, I saw a post mentioning that some aftermarket cams have different firing orders and since this motor was just rebuilt. Very well could have an aftermarket cam. I'm going to try and call the shop that rebuilt it and get the details tomorrow.

I saw on another post (been searching online like crazy) that you can verify the firing order with the valve covers off by turning the motor over. What exactly should I be looking for when I do this to verify the firing order?


Sorry for some questions which may seem ignorant, I am very limited in my motor knowledge (well as far as truck motors, I know a whole lot more about motorcycles than trucks).


Thanks again for the help,
Spencer

 

Thanks again Gary, my dial indicator should be here wednesday and I can get to checking it then.

One more COMPLETELY motor ignorant question. What's the best way to know which is intake and which is exhaust? I'm guessing the exhaust valve/rocker aligns with the header tubes and the intake valve/rocker align with the intake ports on the intake body? ()

 

Yes, that's the way to find intake and exhaust. And, you can determine the firing order by monitoring the intake valves. I would mark the harmonic balancer every 90 degrees from TDC by measuring the circumference and dividing by 4. Pull the valve covers, bring the #1 piston up on compression, and as you rotate further you will see #1's intake open fully. Then, when you get to the next mark on the balancer, at 90 degrees, the intake valve on the next cylinder to fire will be opening. Continue on and you'll see all of them open in sequence.