Yet to be tested,
but. What if I do just as you described and the second I hook up the vacuum advance (or Rev it to 2500) it backfires, spits and stalls out, seemingly to the ATDC side. What needs replaced then?

*the whole truck* 😂

 

If the vac advance is giving you trouble just plug the hose and forget it for now. it makes your engine more efficient and lowers the EGT's but it's not necessary.

Once you have the mechanical advance set then you can get the vacuum advance working. it sounds like one of two things are going on with it, either it's pulling the plate in the wrong direction because of your smog set up, or it's pulling the plate and causing a short in the pickup coil or feed wire. either way it can be corrected.

 

^^^ 440Sixpack explained it well Assuming your vac lines are correct and carb setup is close enough. If it’s dying y when vac advance is applied, sounds like you’re probably ok. I like the idea of getting it running sans advance. You can rebuild or recurve that distributor later.

 

If the EGR plate and valve went along the way side when you put the edlebrock carb and weiland intake on the ol' girl then the distributor definitely needs to be recurved because the ignition curve was set up to run with it. You can send your distributor out to be recurved or do it yourself by following the links I'll leave. If you change out the emissions timing chain and gear set you'll pick up a bunch of low end torque.Wait, I forgot you have a 1977 truck motor, correct? When M-blocks were introduced in trucks in the 1977 model year, the truck engines went back to the original, non-retarded camshaft timing. So if your M-block is from a '77-up truck with the OEM timing set, it doesn't have the retarded camshaft timing.

Recurving your DuraSpark distributor: Duraspark_distributor_recurve_instructions_index
Recurving replacement springs : https://www.summitracing.com/parts/mrg-928g?rrec=true

 

Hey doubleD, welcome!
As usual I agree with what the others have already advised. But I have some questions and comments of my own to add.

Interesting. Not that I know all of the trucks, but I can't say I've ever seen a 2wd with the 351M/400 engine in it. Maybe that was the first year, since '77 was also the first year of the 335 series (your particular engine family) in the 4wd trucks too.
I like learning new things!

Did you remove the distributor to do the swap? Or did you wrangle everything out around it?
And it's been asked, but I didn't see where you mentioned removing the EGR valve.

Sounds like you've been all over it, and it sounds like you did remove the distributor for the swap. Correct? Otherwise there would have been no change in timing, and no need to mess with it (other than fine-tuning) even after the swap.
Nice introduction into old trucks and gas engines! Right?

Arrr arr, an ignition pun. Maybe new at this, but already punning with the best of them!

And I might as well add here, because I didn't notice anyone else having mentioned it yet, there is no such thing as a 400M.
Since you're new to these, you've probably missed most of the discussions over the years, so I'll give it a quick (hopefully) go.
Only the 351's got the letter suffix, due to there being at least three of them running around at the same time. This was the W (for Windsor), the C (for Cleveland) and the M (for?) I always called it for "Midland" but most people nowadays settled on "Modified" though I don't know what that actually stands for, since it's the 351M that is a modified (de-stroked) 400.
Anyway, just sayin' you don't need the suffix for the 400. Only the 351.

Good luck on the doggy behavior. Not sure what's going on, unless it started running super lean due to some part of the swap, OR there is something wrong with your distributor that was not wrong before the swap.
A vacuum advance is just that. An ADVANCE! Not supposed to be able to retard timing, except for those few that were boost-retard setups for turbo engines. Never seen one of ours act out backwards.
Hope yours gets sorted soon, and turns out to be an easy fix.

Paul

 

Oh, and I would think you will be able to post photos soon. It does take a little wait time, but it might be driven more by post count than by time. Or both perhaps?
Not sure, but it usually doesn't take too long. Hopefully we can see some images soon and maybe someone can point out something.

Paul

 

Interesting. My '78 Ranger 2WD rolled off the line with a 351M under the hood. Don't know for sure it was the same engine that's in it now, but it still has a 335 series.

Anyhow, I stumbled across this thread because I'm having a strangely similar problem with an eerily similar setup. Someone mentioned needing to recurve the distributor because of the carb swap. I can throw my support behind that idea because mine has a Carter AFB on top of an Edelbrock Performer 400 intake. Not identical to the AVS of the OP, but close enough to make it seem like these engines don't like that carb. I haven't hooked up my timing light yet, plan to do so tomorrow. Since my setup is so similar, maybe I can find a tweak that will help. And I live five miles from Summit Racing here... just in case I need the advance curve kit, or an MSD distributor.

 

Sounds like it was just my lack of paying attention to trucks back then. And not reading up on the more detailed in later years. I just always saw 302s and maybe 351s in the 2wd trucks.
it’s good to know. And anyway, like I still say fairly often, it’s always good to learn something new on an old subject!

 

Changing the carb and intake shouldn't have any effect on the timing, possibly a minor difference in the vacuum advance setting but that's all. how you supply the vacuum to the advance and the advance can itself is important though.

If you want to try some simple tests you can peak it out, which is always the best anyway. just keep advancing your timing a little at a time until you hear pinging ( detonation.) at WOT under load. then back it off a little until it goes away. this is your true maximum advance. now check and see where your settings are with a dial back. if it's all good you hit a home run. if it's not the work begins.