Engine no wanna run :(

[LTZ C20]

So here we are! Truck is all tuned in and running like a champ. I borrowed a timing light from my girl friends brother and a vacuum gauge from a guy at work. Got timing set to just bout 4.5 deg BTDC. Then I removed all items plugged into vac ports on the carb and capped everything off. Brake booster, PCV valve, vac advance, even plugged the vac line to the HVAC control vacuum pod thingy on the firewall above the blower. That was after I discovered that the vac port on the mani was loose just a bit and the vac line to that HVAC pod was completely open!!!!

No wonder I'm having trouble! That's the FATEST vacuum leak in the history of mankind lol. I looked at the line and thought, "ok it's connected, wait what about the pod side?" Then I looked over at the line and went "SANTA MARIA!!" I'm an idiot!! Hahaha. Been trying to get it goin with my head up my butt this whole time.

So anyway, highest vac has been achieved, timing and curb idle set. I even adjusted the choke to run at 1500 at full on, 1000 at half on. Not high but elevated idle and it's been working great. Now park idle speed is about 700 and in gear idle speed is floating nicely at 550 or so. Allowing for fluctuation and the tach being not exactly a perfect number of course. Now she purrs like a kitten, sounds great, and runs AMAZING. I'm soo happy ad proud of myself lol.

I also, disconnected the tv cable for trans and then reinstalled it and readjusted it to make sure it was right. Truck is shifting into all gears, trans is working nicely, shifts are solid and positive, returns to idle every time, no hesitations, misses, or stumbling at all. Even got on it a little on my little test drive and she really goes. Open header makes a nice sound at 3 grand too.

Thanks everyone for your input and guidance. And thanks to Rich for being patient with me and explaining everything in technical and in easy examples. I have been well educated. Thanks again all! Love this site.

[rich weyand]

And I love posts that people got their truck to where they want it!

What was your peak vacuum, BTW?  I imagine you're pulling 18" or so with that cam and those heads, but I could be surprised.

[bake74]

     Glad to hear you got it running again, nice to have your truck running where you want it.

[LTZ C20]

Oh yes it's very nice.

Rich, with everything plugged and running at idle in gear it's about 8". I'de guess it's probably higher once the vac advance is plugged in. She runs soooooo good.

[rich weyand]

Vacuum will be highest at idle in Park.  Vacuum is an inverse measure of the load on an engine: more load, less vacuum.  Least load is idle in Park.

Your torque curve below 3000 is about the same as mine.  It really is sweet to have 420+ lbft on tap coming out of the hole!

I think the big issue, other than plugging your massive vacuum leak and tightening down the distributor, was to get the ignition dialed in, taking into account that massive vacuum advance.  At idle you are running 4.5 base plus 20 vacuum advance.  I'm running 17 base plus the stock HEI for 1978 light trucks, which is 7.5 degrees vacuum advance.  Notice anything about those numbers? 

[LTZ C20]

Oh ok. Did not know it was an opposite thing. I'm gonna run her for awhile like that and see how she does. Last night her overall performance was outstanding I would say. What I expect from a brand new engine.

Yes 420+ lbft is very nice.

I would say it sounds like our pulling less vac advance then I am. As for as I know, the GM Performance disti is not much different than the stock ones. They had 2 different ones in the catalog. Mine is basically a stock I think, except for the higher advance of course and maybe different weighs and springs on the mech advance. Also the aluminum housing is a little nicer.

The other I believe is the same inside, however the housing is a BILLET finish. Which I do not like and is way more $$$. If I did my current vac advance is to much I'll do a some adjustments like put a stock advance diaphragm or something like that.

[rich weyand]

It's not so much an opposite thing as it is the total.  You and I are both running 24.5 degrees advance at idle (base plus vacuum advance), we just get there a different way.

[LTZ C20]

Ahhh, I get it. Ok cool. So I should be good then. thanks Rich.

[rich weyand]

Ahhh, I get it. Ok cool. So I should be good then. thanks Rich.

Yeah, you're good.  That big advance is cool for the 9.5:1 compression ratio you are running.  When I get on it hard at low rpm, my vacuum advance goes away and I am running at the base timing of 17 degrees.  When you get on it hard at low rpm, your vacuum advance goes away and you are running at the base timing of 4.5 degrees.  Which means you are *never* going to get knocking, and won't need premium gas, whereas at 9.5:1 with my 17 degrees of base timing, you might.

[LTZ C20]

Sweet. Now I know what to expect when I jump on it. Haven't really seen what that motors got yet. Just a few tastes of a little power.

I'm running 91 octane anyway. I like it better with good fuel and should help keep insides a tad cleaner than regular juice.

[rich weyand]

Sweet. Now I know what to expect when I jump on it. Haven't really seen what that motors got yet. Just a few tastes of a little power.

I'm running 91 octane anyway. I like it better with good fuel and should help keep insides a tad cleaner than regular juice.

Not correct.  91 is not better, it's different.  It's slower burning.  I don't think it will do anything to help keep your engine clean, and it may cost you a bit in horsepower, torque, and mileage. 

I calculate your dynamic compression ratio at about 7.9:1, and anything lower than 8:1 is fine for 87 octane.  Particularly with 4.5 BTDC static timing.  Running premium is almost certainly not necessary.

Those heads with that cam and distributor, the more I look at it, is really a sweet setup.  I'd run regular and just keep your ears open for a while when you pull hard up a hill or anything like that, but I think you are pretty far away from the pre-detonation wall.

[LTZ C20]

Mmmmm. Ok. That's interesting. I was told with these heads I would need 91 octane. So let me ponder you this.

Would be ok to run on mid grade? 99.9% of the time I always use Arco gasoline. Knock on wood, I've never had a problem with the quality of fuel. So would it more beneficial to run regular 87 OR would it be better to switch to a better 87. Say slowly work her over into Chevron 87 and just stick with Chevron regular?

I haven't spent 8 grand on and engine and transmission to dump crappy fuel in her. I wanna use what's gonna be best.

[rich weyand]

I think with those heads you might need 91 octane if you were pushing the timing, but that distributor running base timing at 4.5 degrees because it has a 20 degree vacuum advance helps a lot.  I think that's why that distributor is set up that way.  That said, you should be paying attention when you go to low vacuum (high engine load) at low rpms -- slugging up a hill or whatever.  If you hear any knocking, go up one level, like to 89, and pay attention again.  So no loud stereo for a while, so you can hear the thing if it knocks.  With the stock dished pistons in there, though (You didn't change pistons, did you?  As I recall you left the bottom alone.) I would be surprised if you had problems running 87.  Flat-top pistons, different story.  91 octane all day long.

As for fuel, I always buy from a name-brand retailer.  I like BP, Shell, Mobil.  Cheap gas may not even meet spec, or may have other problems.  I think you are better off running 87 from a major than 91 from Lester's Bait, Beer & Gas.

[LTZ C20]

Ok so that means that 87 from Arco should be fine. They are name brand and I never get gas from scooter's stop n rob.

The block is a GM Performance fast burn 385 originally. Now it's bored 30 over, has all new Clevite bearings, the original crank to match the block, new rods and pistons are stock style, with 4 valve relief but larger diameter obviously for the over bore. They aren't stock made tho, they are Speed Pro pistons and rods, and the rings are either Clevite or Speed Pro, can't remember.

[LTZ C20]

And the block ha been completely hot tanked, bored honed and the main and cam bearing perchs and caps have been line honed I believe it's called. Wen they bolt it together and run the cutter tegu and make it all uniform and equal. I think the only thing that wasn't done was a blueprint and balance.