Re: My attempt at the HEI conversion
OK, have another look at "propane trick."
Test -- Make sure you can get liquid propane from the hose by holding the valve below the bottle - next step won't work unless it can.
Do the procedure where you run the long propane hose inside, while driving, hit the accelerator hard - make it burp or backfire. Do it a few times to make sure it still does it.
Then hold the small propane bottle upside down so you get liquid, not gas. Open the valve fast just as you mash the throttle, feeding liquid propane down the throat just before it backfires or stumbles bad.
See if it helps, if it's fuel starvation, the propane will supplement the lack of fuel and it won't fart.
If it still does, it's somewhere else rather than fuel mixture. (By doing that, you are imitating an accelerator pump on a carb -- yes, it works for those too.)
The MAP sensor should "see" the instantaneous drop in manifold vacuum when punching it, and the corresponding voltage change at the MAP should happen. The ECM "sees" the votage change, and instantly lengthens the injector pulse length, giving more fuel -- or that should happen.
Lots of things can cause fuel starvation - even charging system, MAP sensor hose too long or leaking, low fuel pressure etc.
But first it has to be determined if it's fuel or something else.
Lots of other things other than fuel can cause the same symptoms too. But most of them were left behind on the old engine.
Changing over to the diffent engine, then to the HEI only adds more potential trouble spots. I'm a firm believer in "one thing at a time!" I'd rather first determine the cause of the trouble before swapping in - shotgunning - parts.
Q - does it backfire out the TB or the exhaust???
Q- When you tested for vacuum leaks, you mean at idle - and when you used just a TINY amount of propane it dropped in speed? Lots of propane, too much, will always cause a drop in R's no matter what the speed -- called drowning!
Add a very little at a time - start by holding the bottle with the valve at the top - getting only gas. pen the valve slowly watching the reaction. If you need even more propane, close the valve, turn the bottle upside down, slowly open again, then you get liquid propane - as it comes out of the hose it expands into gas -- lots of it!
Idle usually upright works best, higher R's upside down.
Q - What happened at say 3000 RPM unloaded with propane?
Q - Can you snap the throttle - unloaded - and make it stumble or backfire?
Q - Does it do it with the vacuum to the distributor disconnected?
Q - What vacuum port are you using -- type of vacuum for the distributor? manfold or ported?
Q - Does it have an EGR valve? Is there a vacuum line running to it?
Q- What is your initial timing at idle set to -- without vacuum connected? What is it at idle with vacuum connected?
Q - The vacuum line to the MAP sensor -- it is direct to manifold vacuum, has no kinks, and is a small vacuum hose designed for vacuum, not fuel or washer fluid? No check valves, and not being shared with any other device. Simple, small, short hose direct to the intake manifold -- not a port on the TB -- right?
I'm no longer an enthusiast of Howell, they are harness builders and chip curvers, not automotive types. They don't seem to understand the basics like what an engine requires for ignition timing advance etc.. And they do things like call out sealing the big bolt holes on the TB adapter to the manifold with silicone -- any automotive person knows that silicone will not seal when gasoline is around. Leak city!
But it's just a simple GM system - millions in use today, nearly foolproof once installed correctly.
He He -- the one thing at a time thing -- This weekend I had to replace our water heater. Of course the outlets are way different, all new plumbing to it had to be put in. While I had the water off, my wife decided to replace an outside faucet that had been leaking slightly, AND she put a new float valve in the toilet. And she did something to the washing machine - don't know what yet, but she said it should fill faster, but now it won't fill at all!
When I turned the water back on, leaks everywhere! I not only had the water heater leaks, but a flood outside (she broke the plastic pipe, and the toilet was flooding all over! (Crossthreaded the hose, requiring a new one.)
ONE THING AT A TIME PLEASE!