Before you do anything else make sure the engine is coming up to temperature. Id be looking at 190*ish or better. A cold carbureted engine is not going to be happy.
-In my experience, a lot of times when the carb is blamed, its really an engine issue, or an ignition issue. The carb is totally vacuum dependent. If the engine isnt making the vacuum that the carb was designed to operate at,it just wont work right. You would have to find and repair the vacuum problem, or re-engineer the carb to work with the new vacuum references.Of course the latter wouldnt help the power or the fuel economy.Now, Im not saying that your engine is toast;Just that before you spend hours tuning that carb, or throwing parts at it,maybe its time to step back and evaluate the engine. I read that the carb has been worked over 3 times already.And you also talked about an engine miss. And the 4.75 mpg. And falling flat on tip-in, and under load. Its just so many things all piled up.
-When this happens, I like to go back to the basics. The proper order of diagnostics is; A) compression, and B) Ignition, and C) carb.
-So start with a compression test and/or cylinder leak-down test.Lets see what we have to work with.
-BTW the terrible fuel consumption is a telling sign of a lazy engine or a really bad tune.
-If the engine is good, Id check the ignition system. Since the plugs are out, this is the time to read them and probably change them.The wires can be ohmed out and the cap and rotor inspected.If you have carbon-core resistance wires, the ohm test may not be totally accurate.If theyre baked hard or brittle I would recommend to change them. The cap must be checked for moisture,carbon tracking,terminal burn-off, and cracks.Moisture is an easy fix.Carbon tracks can be scraped off, but its a temporary fix as the tracks always come back.The last two make the cap scrap. Rotors seem to last forever.Sometimes with high-powered coils the tip burns off. I have a feeling that your dist. is not advancing properly. Pop the cap and give the rotor a twist. Be gentle. It should rotate about 1/4 inch to as much as 3/8 as measured at the metal point, and should snap back when you release it. If not its the wrong dist. for you or its seized somehow.
-If the plugs came out black, that tells a lot about the 4.75mpg.If they came out normal check all the white insulators for cracks. I have seen a cracked insulator not affect the idle, but as soon as a light load was placed on the engine it would miss.
-So now you have proved the compression is good,and you have brought the ignition up to speed. Finally,if the engine still has problems and the timing has been dialed in,its time to work the carb.
-But the timing systems have to be proved.
-If you follow these steps;Compression, Ignition,Carburation, I can practically guarantee you will find the problem(s)
-You may not be stuck yet. Theres still the old-fashioned way.
-Heres what you do; Go out and crank that dist. counter clockwise about 1/4 inch to advance it, then warm that baby up.While its warming up, pull the vacuum line off the carb and pull a good vacuum on it to the dist. While youre doing that note that the system is working and that the rpm rises. After the test, return the hose to its proper spark-port vacuum source, which has little to no vacuum at idle, but begins pulling vacuum as the throttle is opened. Once warm, take it for a spin, using part throttle only. Notice any improvement? If yes, give it a bit more rpm and increasing throttle. If you hear the dreaded engine rattle,also called ping, back off the throttle. If you cannot hear the engine over the road-noise or exhaust system, It may be better to wait for the better timing lite.But if no rattle appears, try a bit more advance and re-test.You can continue this until rattle does manifest, then retard it to the last no-rattle test.Now re-check the idle timing with the lite. If its less than 20* or maybe 24*, leave it there. Shut off the truck. Restart it several times to make sure the starter is well able to spin it over. If the starter has problems, back the timing off a couple of degrees. This will get you by until you can perform the proper tests with a good working lite. This is not the final set-up. And remember :
..................RATTLE IS NOT YOUR FRIEND.
-I know this is a long story and probably not what you wanted to hear, BUT; the tests are not hard, not expensive, not overly time consuming and will get results.
-Your engine may have 160 plus psi compression and 5% leakdown, and a perfectly functioning ignition system. But until you prove it,you or three other guys, could be just chasing you tails. Cheers AJ
