I have a 79 F-350 4x4 that I use for hulling my Lance 1161 truck camper, "4,600"Lbs. I am finding a vibration, strong enough to shake everything. I feel it mostly when under load with the camper, but it is still there with no load as well, just not as pronounced. I don't feel it in 1st, very little in 2nd, but when it shifts into 3rd, that's when it hits. It will go away for the most part if I drop it into 2nd or neutral and this starts to happen at 60mph and up.

I replaced the rear U joint, front is fine. There is a small amount of play in the slip joint on the drive shaft that I can feel by moving back and forth, but not enough to see it morving. There is a small amount of rear pinion play, when the drive shaft is moved back and forth. Tires are fairly new 35 inch. Truck has a 6 inch lift.

The amount of squat with the camper loaded and air in the air bags, is 2 inches. Transfer case and rear end is full with fresh oil and U joints pumped up with grease.

Any thoughts on this would be great. I know this is a separate problem, but I get back firing as well when this shaking is happening and giving it gas. Yes all electronic/electrical components and timing chain have been replace.

Thanks in advance.

 

I was going to go with pinion angle with a full load, but the backfiring makes this an engine focus before anything else.

What kind of timing are you running? How long since a carb rebuild? Ever do a compression test?

 

I would have my driveshaft balanced and replace and worn parts.

 

I have set it from 10 degrees to 18, it does not make any difference. The Back firing is still there, that said, more intense with 16 degrees and up. It's hard to tell, if it is all exhaust back firing. I know for sure a couple times with the carb, but mostly exhaust. The carb is a factory 2v that came from a remanufacturing company in Canada called Autoline, that rebuilds them for places like Car Quest/Raymac. The motor was rebuilt before I got it and I would say it's just over 8,000 miles on it. As for compression, 160Lbs on 6 cylinders and 145 and 150 on the other two. I had the heads off last year and lapped all the valves except two cylinders, should have did them all. I replaced all the gaskets since the rebuild was a short block rebuild and the person who put it together did not torque the heads and intake correctly I found it had been machined .030 and flat top pistons. So far, I have replaced the distributor, coil, wires, plugs and ignition module. The old distributor was a major problem and thought this problem was gone. By the way, It is a stock vacuum and mechanical advance on the distributor, it has not been recurved and made in China, but it works.
It is possible the heads have been over machined on the face and that changing the contact angle at the intake and head. I went with a Felpro printed gasket and also coated them with hi temp gasket maker goop.

 

It's the stock rear drive shaft and the rear U joint was just replace yesterday, front was still good. When the drive shaft was off, I could feel a small bit of movement in the slip joint, but once on the truck, I can't feel any play. The rear end is a Dana 60 and there is a small amount of lash, that can be felt back and forth with the pinion yoke, but no side to side or up and down movements of the yoke.

 

Did you by chance replace the pick up coil inside the distributor? Or have you tested it?
Mine went bad, I replaced with with a bwd from the local parts store. The truck never ran right.
It puked, backfired and bucked and while I could get it close it always acted out under a load..
I timed it so many times and at so many different degrees and nothing was working.
Eventually after many opinions on what to look at, someone suggested I try replacing the new part with another new part.
So I searched for a motorcraft pickup and when I installed that. My problems vanished.
The bwd ohm'd out correctly but it wouldn't work for chit in my truck. I have no idea if it was for a different weighted Dist or what the problem was.
Just a thought..

 

You might just have something there. It ran real bad, so bad the shaking and back firing, it was shaking the steering wheel and the hood of the truck. After changing everything, the only thing left was the distributor. I picked up one, seemed good quality, made in China from Car Quest dropped it in and it did make a world of difference. Thought the problem was gone, but found it came back some. I will try to see if I can get another pickup. Now to see If I can actually find a motorcraft pickup! Thanks

 

good luck it can get quite frustrating trying to troubleshoot stuff at times. And it gets really expensive just throwing parts at it.

 

Oh, I know the throwing money at it part! I will have a new truck by the time I'm finished!

 

https://th.bing.com/th/id/R1c0f59814...sl=&pid=ImgRaw Maybe cross fire? Are your plug wires routed correctly? Just a long shot. I think the firing order is the same for HO, 351/400.

 

That is the firing order of the 400 and yes it's orientated correctly.