350 sb stumbles, timing is 51' retarded

I tried to figure out the list number on the carb from your photo but can't make it out. That might help some of us know exactly what carb you have.

Now to the problem at hand.

If you lean up over the front of the engine and look straight down behind the waterpump at the front of the timing cover do you see a timing mark almost in the center of it? It will probably also have what looks like a tube on one side of it. That is where a lot of the mid 70 and up timing marks are on 350's because of the accessories getting in the way on the driver's side of the engine.
I went out and looked and it looks like you have yours wired the same as I have mine on my 71. (79 engine with HEI) so that shouldn't be an issue.

The tube on the right side of the front metering block is or should be the ported vacuum and I'd switch the line from the distributor advance to that one and put the plug on the one on the base plate.

I'd suspect a vacuum leak and it may be in the spacer plate between the carb and intake if you don't have the correct spacer plate.

 

You say that the timing chain is sloppy, so fix that. Also you say it runs on the primary side, but poorly on the secondary side. Perhaps the spring in the secondary vacuum advance is opening up too soon. A 454 might tolerate this, but a 350 might get into a bog situation. Also ditch the open spacer deal. I've fought that battle and lost more then once. Use a four hole spacer/ adapter if needed. I also agree that .060 is too much gap, try .045.
Another issue that I have found is that sometimes those spacers are not flat, and they will leak if they are not. And any leak will have to be compensated by enriching the mixture to cover up the problem. Also the choke plate must be anchored in some way to keep it wide open when the engine is hot. And verify the float settings by pulling the site plugs. If they are set too high it will run too rich. And if your distributor is the original, swap out the pick up coil. The wires in and out of it will break over time from flexing.
And last but not least, it was running just fine when I pulled it, isn't much to bet the ranch on, unless you heard it running yourself.
Keep plugging away, and you will fix it, or swap it for a better one.

 

Does this engine have just a timing tube welded to the front cover, not a tad with marks? A lot of mid 70 engines did. That tube is for magnetic timing and it 10 degrees retarded from TDC. So if you are lining the mark on the balancer up in that tube, you are setting the timing at 10 deg after Top dead center.

 

fixed it, the timing was fully advanced at idle because I had the vacuum hose connected to the wrong port, once I switched this and re-gapped the plugs to .45 she ran like a raped ape. hit some troubles when I stomped it, but that is because I had no fuel going to the secondaries. Turned that out and she runs great. sitting at 13.5 degrees advance. the idle fuel circuit is about 3/4 turn out idles at 600rpm at temp. thanks for all of the advice so far, I am going to hook it up to a four gas and tune the shit outta it, re jet it and get a couple o holley books and see about tuning this monster unless ther are some online resources that I am not aware oif. Holley's forums were a bust, no mods to give me access are ever online been waiting for a month now. The truck runs but doesn't start, damned gm shimmed POS starters, the old one went out on me, I knew I shoulda had it rebuilt at ace alternator, but Noooo I bought another one from Auto Junk. The flex plate is warped, when the shims didn't work and I upgraded to alignment shims because the gear still wasn't touching the flexplate but nearly sat on the stop washer, I knew something was up. Starting Wed, I drop the tranny, and get to work on the flex plate. Thanks again Hambers.

Sgt Kevin A. Davies, USMC

 

Yep, EFE, EFE stands for "Early Fuel Evaporation". It's commonly referred to as a "heat riser". It aids emissions control by helping the engine warm up more quickly. Early models used a thermostatic coil spring and weight to actuate the valve. By the mid-80s, the valve was controlled by vacuum.