I have a 72 F100 that I built a 77 400 for. I haven't been able to get it to run. I got disgusted and it has been sitting for about 3 years. I posted some questions at the time and tried everything suggested but nothing helped.

I am ready to find a running 351W and tran to drop in just to get it back on the road. What motor mounts will I have to look for to make this match? I know the 351W didn't come stock in 72. When I did the 400 swap, I thought someone told me that the frame was the same up to about 77. Is that correct?

On the 400 problem, it was bored .030, block decked, heads overhauled and shaved minimally. Crank turned. Everything new inside. "RV" cam. I don't have the specs here but can find them if needed. Timing set with the 3 keyways. I used the straight up setting. Headers with turbo mufflers and just enough pipe to clear the cab. C6 transmission.
I converted it to the Duraspark electronic distributor in the process.
It backfires through the carb. Timing, right?
I have taken it back down as far as pulling the cam to make sure they gave me the correct one. It is. I held my finger over the plug holes and turned the crank by hand. It is the correct firing order.
I removed the Edlebrock Performer 400 and carb, thinking I had a major vacuum leak, and gone all the way back to the stock intake and two barrell carb.

Still no luck. Everyone I talk to says it is timed wrong. I am at a total loss on this one.

 

I can venture no answer as to why your 400 won't run, but I'll bet its something simple, don't give up on it!!
Now as for installing a 351 W I'm assuming you have small block mount supports in your '72, just get mounts for a 302, they will fit perfectly & set right down on your supports,(I also have to assume that when you installed your 400 that you used mounts for/from a '77 to '79 Fseries.
Also understand that whatever transmission that is hooked up to your 400 WILL NOT bolt to the 351W. Hope all this helps!

 

You should be able to drop the 351w motor onto the same frame brackets your 400 sit on. As far as the tranny crossmember, that depends which tranny you have w/ the 351W. Is a C4 or a C6 (assuming automatic). You probably have a C6 behind the 400, if you are putting in a C4, you might have a shorter transmission and therefore need to relocate the transmission crossmember and get a longer driveshaft. Relocating the crossmember isn't a big deal, the frame probably already has the holes drilled in it for locating a C4 behind a 302 or 300. If you need a longer driveshaft, same story, the 302 or 300 w/ a C4 would probably have the proper length driveshaft. Do you have a manual? If so, do you have a bracket on the engine block or bellhousing for the inner side of the Z bar to pivot on? Think in terms of a 302, that block is the same length as a 351w and shares alot of dimensions. In not exactly positive, but I believe you can use the 302 application motor mounts as well on your 351W. One thing that it doesn't share is exhaust geometry and oil pans. You may need to check oil pan clearance. The 351W block has a higher deck heigth than a 302 and so the heads sit one inch higher and one inch wider than they do on a 302. Since a 351W never came stock in your truck, you will have to get creative w/ exhaust pipeing. Perhaps you will pay an exhaust shop to just fab you some new pipe, or maybe you can get the Y-pipe from a 351W in a car application to work? I'd really like to hear someone tell me that a header fits, because I've tried to find a listing in the header manufactures for this swap and not found anything including in there swap listings. I know it can be done, I've seen 351W in 70's era pickups. I think the main issue is going to be how you deal w/ the exhaust.

BBT

 

Sounds like you put alot of work into that 400, would be a shame to get rid of it for a timing problem.
Have you checked the static timing? Why not try turning the distributor to see if you can get it to run.
Of course you have probably tried all this, still seems like a waste to replace the engine and have to do so much modifications to the truck to get the new one to fit


Sparky

 

I don't really want to give up on the 400. Just at a loss as to what to try next.
Is it possible to have a problem with the electronic dist that might cause this? It is the only piece that I haven't tried to swap. It was a used unit and I got a new top and rotor. I also replaced the electronic ignition module that sits on the fender. That didn't help either.
It really sounds like a valve is open when it fires. That is why I say a timing problem. These have the non-adjustable rockers. Is there a way that they can be wrong and letting a valve stay open?

Thanks, Bob