rsm688

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

rsm688

I can take some pictures of everything as it sits under the hood and get some videos as well if that would help (which I'm sure it will).

rsm688

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?

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



At TDC rotor is pointing at number 1

Any help would be GREATLY appreicated.


Thanks,
Spencer

Chris_1

iTrader: (1)

Rotor should be pointing at 1 when you're at 10 - 12 deg before TDC. That's when it fires.
Actually sounds a little more like a lifter/ cam lobe problem, possibly.
Or what comes to mind as a very strong possibility is the valve timing. Especially when you factor in the fact that whoever put that motor together put a rocker on backwards, it's very possible that they could have installed the chain a tooth or two out of line.
Would make it crap to drive - maybe that's why only 100 miles on it.
Also firing order will ultimately be determined by cam lobe order _ I'd pull both rockers and turn it by hand and watch the intake valve opening order, just to be sure about that one.
That's an expensive motor especially the parts, personally I'd be pulling a few parts off to check a couple of the basics before running it, just so she don't go bang over a stupid issue that might be simple to correct.

Chris_1

iTrader: (1)

Other thing that came to mind is the distributor may be off a few teeth.
If you manually rotate the motor, get the intake stroke then up to 12 deg BTDC on the compression stroke with the dist. cap off, the little bump on the distributor rotor shaft should be exactly lined up with the pickup module.
If you rotate the distributor so that it is, NOW the rotor should be pointing at #1. If it isn't you would have to pull the dist. and turn the shaft enough so that when reinstalled all those things line up.

I'm assuming you're following all that, it makes sense to me as I write it but if you're a little foggy on what I wrote let me know and I'll try to re-explain it.

rsm688

Update, finally talked to the guy that put the motor together, here is what he said:

Motor was fully rebuilt, machined, everything in the top and bottom end was replaced and it has 3000 miles on it and ran great. The guy that owned it let a buddy borrow it and he thinks he missed a gear and over-reved it and broke a valve and scarred a piston up. He said they had it all celaned, freshened, cleaned block replaced the head on that side (right/driver's side). New pistons, rings, rods, and mains were put in it.He said the crankshaft was pinned 0.10 and had 10 valves/rods (I don't know what this means). He said when it was built 3000 miles before it killed that piston that the valves, springs, rockets, etc. were replaced. He said the cam was not stock and was new when it was rebuilt the first time and had 3000 miles on it but was in good shape and reuused the second time. He also said he broke it in properly after the second rebuild when it was at his shop, additives for cam, ran at 2k rpm, etc.

I told him what he was doing and he said it sounded like a cam lobe to him.


Thanks,
Spencer