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So my gasoline experience, so far, has been horrible, and I truly miss my Impco propane carburator... nothing to adjust!

My situation right now is that I can run the truck quite well at high RPMs, but the idle settings are a mess. I have not yet done all the homework, I still need to ensure I have no ignition issues, and I should time it appropriately. To start with this, what ignition coil is recommended? I have the petronix with civilian cap, wires and NKG plugs.

After I check good spark, I need to figure out why it will only run with the mixtures screws out at about 3 to 4 turns. Less than that and it will stall. Float levels ok, fuel pump new, carbs overnight in laquer thinner and rebuilt with new kits, new link rods and ball joints, shutoff valves click strong...
The engine will idle only when hot and run strong after about 2000 rpm, but needs 'pumping' to get it out of idle under some load. I have tried 4 to 5 turns out on the mixture screws and no luck.

Ideas?

Ideas: Ignition timing (and advance), valve adjustment, not necessarily in that order. You do still remember you have a choke, right?

Yes, choke working fine (a little stiff, wont go back to off always, sometimes needs help from the top hatch).
Timing is my next task... what coil can I use just in case mine is weak?

Many of us (me included) have used the Bosch Blue Coil ( http://www.amazon.com/Bosch-00012-Ignit ... +blue+coil ) successfully.

OK, so voltage is not an issue... ? 24 on the 12v coil?

OK, so voltage is not an issue... ? 24 on the 12v coil?

Voltage is not an issue; temperature is. If it gets too hot to touch, you might have problems, but I have not heard that any Pinz owner with the Bosch coil has had this problem.

Expect to measure roughly 17 volts at the "+" terminal of the coil, when running. If you have significantly (3-4) volts less, expect that the 4500 RPM module is failing. This makes for very weak spark. Check this first before changing the coil. If the voltage is low, try jumpering out the module: it is under the dash, about the size of a pack of cigarettes, with a six terminal connector (two rows of three). When you unplug the module, jumper the middle of one row with the middle of the other row (in the harness plug) and leave it unplugged from the module. This should bring the voltage of the "+" coil terminal up, and give you a much stronger spark.

Engine still a mess.
Engine ran on propane quite well. Now on carbs... ugly.

Engine runs ok over about 2000 rpm. Just ok, sometimes there is some popping....

But at low rpm's.. a mess.
It will only start with full choke. Still struggling to stay running if I don't pump the throttle, at least for the first minute.
Then it will run on about half choke and bit of pumping until finally when engine reaches temperature it will idle without choke. During all this process, backfiring, popping, banging and puffing happens.
But it idles... now, to drive it, it has no power at low rpm. As soon as I load the engine, it wants to die. I have to often use low gear to start in some situations.

Running pertronix - nkg bp6es plugs changed yesterday - blue bosch coil
Carburator rebuilt to the best of my skills and knowledge. New gaskets. Float level checked with the correct tool/technique with engine pump running.
New fuel pump.
New fuel lines. and transparent clean fuel filter.
Got a timing light gun and set it to my best guess of about 5 to 10 degrees advance (about 10 mm from block mark to pulley mark.)
Tested auto advance with RPM and it looks like it is working fairly well. Back and forth.
Idle screws are to whatever I had to let it idle and to my best skills, synched with an airflow meter as the one depicted in the Pinz manual.
Now THIS IS STRANGE: engine will only run without choke with mixture screws at at least 3, 5 to 4 turns out. When I changed the plugs yesterday, number 1 and 4 where dark rich. 2 and 3 not too bad, just a little on the rich side but not bad. Number 1 in particular, dark wet with texture due to carbon deposits.

Ideas that I am considering, but I need help in prioritizing and making sure I am not missing one.

First thought now is: vacuum leak somewhere behind the carburetor gaskets.. under the tin tunnel? But why would the truck run ok on propane? Maybe because it always ran very rich but because propane is clean, no side effects noticed?
Coil going bad. (Or that module that kills the coil voltage). Although at this point I suspect this is a low chance cause.
Valve settings.
What else?

Can someone please describe what a vacuum leak would sound/look/feel?
I am running out of time to fix the truck. Anyone knows of a mechanic in the GA area?
Thank you!

It is possible that the manifolds have cracked from carrying all the weight of the plenum (happened with my FI setup, initially) but I can't say why propane would work better - although the single plenum may have held the manifolds "together" with just small cracks, and removing it allowed the cracks to grow. DO take the tins off and carefully examine the manifolds and hope there is no problem there. You can do a quick check by getting the engine to idle (well warmed, in your case) and then spray carb cleaner, WD40, etc. around where you might have a leak or crack. The liquid both helps to very briefly clog the leaks and adds fuel so you would hear a change in the running of the engine if there are leaks.

He can also use propane. Also hook up a vacuum gauge, they tell all

In my approach to fixing the problem I never considered doing a compression test. Problem found... bad news.

My numbers are 145 - 155 - 20 - 85 psi. Obviously something wrong with valves, since there is absolutely no smoke coming out the exhaust, is this a fair assumption? Who can fix my cylinder heads? Should I do all of them or just 3 and 4?

Thank you
JC

I suggest you first try adjusting the valves. It's not to hard to do and just may fix your problem if you're lucky.

Jim LaGuardia wrote:If you find low compression in any cylinders, adjust the valves for that cylinder and test again.
At least 75% of trucks with a low cylinder that I have checked, had a tight exhaust or intake valve.
When the engine warms up, the miss gets worse due to metal expansion, resulting in less rocker arm clearance , and the valve not fully seating.

Then check that the fuel idle solenoids are functioning. I assume they would have been eliminated with the propane set up.

Who can fix my cylinder heads? Should I do all of them or just 3 and 4?

It may not be "just" the heads; you might have worn cylinders, broken rings, etc. But we can hope for the best and start with the valve adjustment, see how things go from there. Depending on what is found, the answer may change. If cylinders 3 & 4 have tight valve lash, and setting the lash correctly resolves the compression variance, then all is probably well. But if one or both have burnt valves that don't seal well, even with the proper valve adjustment, then you might suspect that the heads on 1 & 2 should be also rebuilt. Others may have suggestions for discriminating between bad rings/cylinders and poorly seating valves.

Thank you all for the advice, a little bit of hope!!
I am assuming cylinders/rings are good, because there is no oil smoke at all, is this a fair assumption? I could borrow a compressor fitting to blow the cylinders and check the rings.
If I adjust the valves, what would be the worst acceptable compression difference? Have not checked the manual for this spec.

Checked valves for cylinders 3 and 4, the ones with low compression values. Both had the exhaust valves tight. I have not yet started the engine... I will be back with the results tomorrow.