Welcome on the board! It sounds like the previous owner of your car thought he'd turn his Valiant into a zoom-zoom race car by tossing a 4-barrel on it. Bad move on his part; you
can make a slant-6 run well with a 4bbl, but it takes a great deal of twiddling and tuning and futzing around, and on an otherwise-stock engine it's way too much carburetor. You'll be very much happier (and wind up tearing out a
lot less of your hair) with a 2bbl rather than a 4bbl. Since you will have to remove the manifolds anyhow to correct the exhaust leak, it's a good time to swap on the factory 2bbl "Super Six" setup. These setups go by on a fairly regular basis both in the "Parts For Sale" section of this site and on eBay.
It will be worth your son's while to buy the three books mentioned in
this post. Once he's up to speed on general principles and repair techniques, the father/son project will go a lot smoother and yield more fun.
It is definitely worth getting the
good gaskets for the manifolds-to-head and the intake-to-exhaust (heat riser/hotbox) junctions. The parts store items you can buy locally don't seal well, especially at the intake-to-exhaust junction. Because your car is a '70, also check carefully atop the #5 (2nd to rearmost) exhaust manifold runner. Originally, an automatic choke thermostat was bolted there. This, on the '70-'72 manifolds, is a gasketed junction. It is likely the PO hacked or tried to remove the choke in some fashion during his 4bbl "conversion", so check for exhaust leaks here, too.
Additional upgrades? Well, you being a Chevy guy, you'll feel right at home with the see
HEI upgrade. Install a NAPA Echlin long-tip rotor # MO-3000 with Standard-Bluestreak CH-410X cap. Use NGK ZFR5N spark plugs, which have an extra-long nose that improves economy and driveability in the '60-'74 slant-6s. Stock number on this plug is 3459 in the US, 5699 in Canada. You will need a smaller 5/8"-hex spark plug socket to install these, rather than the 13/16"-hex item you use to remove the old regular plugs. Gap them to 0.045" and install them
without their metal ring washers. You'll want to replace the spark plug tube seals (rubber O-rings at the top of each aluminum spark plug tube) at the same time to eliminate a very common oil leak. B45384 will get you a spark plug tube seal at NAPA (you'll need to order six). Or, you can get a set of six from Fel-Pro under P/N ES12794.
The slant-6 in your '70, unless it has been replaced by an '81 or newer engine, has mechanical tappets that require periodic
adjustment.
And, once you get the basics taken care of and the engine's running well, the
fuel line mod improves cold and hot starting and running.
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Go and actually read the link I provided.
