Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor

1903 Won't Respond to Tip-In Procedure

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Cliff Ruggles:
Well said Kenth, we actually posted at exactly the same time so below I just left my comments on what you were trying to convey. 

Cliff doesn't like "rich", I tune every carb and distributor exactly for the application.

As for predicting fuel delivery with metering area, well, sadly it just doesn't work that way. 

Engine load, vacuum, airflow thru the carb/across the venturi areas, pressure differential above and below the throttle plates, fuel added by the transfer slot, idle mixture holes and LIAB plus the size of the idle and MAB's all have an impact on fuel delivery across the load/speed range.

I've NEVER once found success trying to use stronger distributor springs or VA cans with stronger springs in them (delay timing or have it fall out sooner) tuning one of these engines.  Superficially yes, but in the long haul other changes were needed to get the combo up to par and the most out of it.

The problem here is fundamental, too much cylinder pressure for the octane of fuel being used.  So you have to retard the total timing (mechanical) and limit the amount of vacuum advance added until it stops pinging.  A/F is such a small player there not really worth mentioning as we NEVER flood an engine with fuel to cure a detonation issue, that just leads to heavy fuel consumption and in most cases it pings anyhow.

Playing around with distributor springs and VA cans is more of a "crutch" fix than to cure the issues and get the best end results.

A cam change will be the best move, and you will also be nicely rewarded with a LOT more power with the right camshaft.  A small cam gets you in several ways, it does too good a job of cylinder filling early and with a tight LSA version you get a high spike in cylinder pressure early in the RPM range.  A small cam in a large engine KILLS power clear across the RPM range especially when you have to start pulling out timing to make things happy.

Change the cam, bigger and wider LSA and your tuning efforts will be nicely rewarded with a BUTTLOAD more power over a broader RPM range, and no detonation anyplace.......FWIW.....Cliff

bry593:
Is there a specific cam you would recommend for this application?  As I mentioned, I do have a ZZ4 cam.  And I can get creative with the springs and retainers to permit greater than .470 lift.  Does Melling or someplace like that make an "RV" roller cam?  That is basically what I need.  Would be great if I don't have to mess with the valve springs and retainers.

How about this one:  https://www.summitracing.com/parts/mel-22129/make/chevrolet

Cliff Ruggles:
I'm not even sure what's out there for hydraulic roller cams these days.  I got out of the engine building business about 4 years ago.  I've had very good success with factory roller cams dating clear back to when they started making roller blocks.  I've never built an older Chevy with a roller in it, only the 1987 and newer blocks.

Going from memory some of the late 80's/early 90 TBI engines had some really nice cams in them on wider LSA's.  There were pretty "low" lift too.  I used one once in a 350 truck engine with early Vortec heads and it was an absolute HOME RUN in that build.  I also used flat top pistons and thin head gasket, so the compression was up there some.  The 350 went into my dads 1991 3/4 ton truck backed by a NV4500 and 3.73 gears.  It pulled like a big block and got excellent fuel economy when he wasn't towing with it.  I'm sure they make something these days along those lines.  They didn't have a lot of lift and were easy on the valve train as well.

I'd recommend a cam with about 10 degrees more duration than you have now out on a 112.  I hate the word "RV" but a 204/214 @ .050" cam would work very well in the 383 with 9 to 1 compression.......CLiff

bry593:
I took Kenth's suggestion and put it to GM factory specs.  That means I've been driving around with the vacuum advance unplugged, only I did not install a 750 Holley.   ;)

I swapped out to colder plugs R43LTS ->  Still have detonation at WOT
I dropped static timing to 8 degrees -> Still have detonation at WOT

So, per Cliff's advice, have sought out a cam that does not suck.  Called Howard's Cams and they recommended a 180225-12:  .450/.465 207/213@.050 112 separation, idle - 4200.  It should be about as good as it gets when used with factory Vortec springs and retainers.

My initial tune will be similar to Cliff's original recommendation.  Hopefully reduced dynamic compression will permit the engine to use 87 pure gas, without WOT ping and engine run-on:

IT .036
UIAB (horn) .050 (1.25mm)
LIAB .064
50C needles
74 jets

I'm feeling optimistic about this......

tayto:
cliff gave me some guide lines to help select a cam for my "vortech" heads. these are the TBI heads that GM used from 87-95 before the "vortec" heads came out. Most consider them door stops but I did some pocket porting on them and put them on my 355. Flat tops with a tight 0.034" quench, i think compression worked out to 10.3:1. Cam was also a Howards cam 213/217 @ 0.050" 112LSA 0.485"/0.495" lift. Built motor for towing, seems to run fine on 91,haven't towed anything yet though.

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