General Category > Quadrajet Carb Talk and Tips

help with 17082224

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Cliff Ruggles:
Sounds like you've made progress.  I'd do some road testing and go back and make adjustments if/as needed.....

bruno:
that’s my plan, although i’m having some issues with tires and wheels!! can’t seem to get much up here. may have to run a used set for a bit. the ones in there now are 40+ years old!! they will turn to dust if i hit the throttle to hard!

cliff, do you have any suggestions on a timing curve to start with? from reading a ton of posts here, it seems you have a slightly different opinion that the “timing 101” article, so i figure i’d ask for your advice. think i have a bit too much centrifugal currently, and possibly a bit too much Vacuum advance as well.

from what i can gather from reading the internet(i know, don’t believe it), but it seems everyone wants as much initial as possible, 34-40 total, and around 10-15 from the vacuum. i am unsure how to determine the best initial. centrifugal i can figure out by driving the car, same for vacuum. just the best initial is confusing me. thx

Cliff Ruggles:
For most engine builds, at least those that are well thought out and good choices made for compression and camshaft events they will like around 10-12 degrees initial timing, about 10-11 (20-22 at the crank) from the mechanical advance, and another 10-14 from the vacuum advance.

I try to avoid all the manifold vs ported vacuum advance debates, but for most engines tuned here I do NOT add any additional timing at idle speed and use a ported source to the VA.  Of course saying that on most Forums is a good way to cause a bunch of folks to get their panties all wadded up.  Almost immediately you'll get a link to or copy/paste some LONG lengthy VA article in your face.  It will be followed by stories of how your engine will overheat if you don't use MVA, or it will use more fuel, not make as much power, or even detonate at high RPM's when the ported source applies the VA at WOT.

ALL of those responses are from very poorly informed sources with little to no experience at all doing this sort of thing, but that's the World we live in today and sadly WAY too many folks have opinions about topics based mostly if not all on Internet Google searches instead of actually doing it for a living or at least having a LOT of hands on experience with these things.

I've lost count as to how many of those aftermarket spring/weight "kits" I've removed from distributors over the years, then hooked the VA back up and to it's original ported source on the carb.  I'll go into the distributor and set up the advance curve so no timing is added at idle speed and it advances smooth and steady with increasing RPM's just like the factory set them up.

This "all-in" right off idle NEVER works well and not sure why despite the fact that it's now 2023 and we should be a LOT smarter these days that folks till tune with those JUNK parts.  The fact of the matter is here that the better you do with your choices for the engine build, optimum compression, cam events, tight squish, efficient combustion chambers, etc, the LESS timing and fuel the engine will like, want, need and respond well to once placed in service. 

The super-quick timing curves and "all-in" right off idle plus running initial timing WAY off the scale at idle are in most cases "crutch" fixes for other issues, like WAY too much duration, LSA too tight, compression ratio too low for the cam, or not nearly enough idle fuel available to the mixtures screws for the engine combo.........

Kenth:
It has to be added that most pre-1968 engines uses full manifold vacuum for the ignition vacuum advance. Are all of these engines poorly thought-out? Of course not. The timing set up is a natural part of the design and due to the principles of the combustion engines. For 1968 the goverment decided to reduce the amonts of certain emissions and to accomplish this the engineers reduced timing at idle speeds using a ported source for the vacuum advance. This heated up the cylinder heads for reduced emissions, but engine effiency suffered, dieseling (run-on) occured and idle-stop solenoids became mandotory, and most cars got a fan shroud. So would you use ported or full vacuum for the vacuumadvance? Use what the engine likes the best, there are really no absolute rules to be obeied, the only difference for the function is at idle speeds, and going from full to ported vacuum requires modifications (enrichening) of the carburetor low speed circuit to have an as complete combustion as possible, and possibly adding an idle solenoid.

And, "if everything else fails, follow the manual."

FWIW

Cliff Ruggles:
Yep, I forgot about folks reminding me that engines prior to the late 60's used MVA. 

I'd add here that most of if not all of those engines used very low initial timing numbers, then added some additional timing via manifold vacuum advance, but typically not very much.  The early engines also had pretty high static compression, and small cams in them on wide LSA.  They are making a LOT of vacuum without a lot of initial timing.

Those VA cans for the most part didn't add a lot of timing so they were NOT really using much at idle speed and didn't need to. 

This is where the water starts to get muddy with this topic.  Folks make POOR choices for these engine builds, following some stupid proverbial "brick wall" of not being able to run more than 9.5 to 1 compression on pump gas, and installing these "modern" profile whiz-bang short seat timing cams on tight LSA's to try to salvage some seat of the pants power.  Then they find that with "low" compression and all that overlap that the engine REQUIRES a lot of timing at idle speed.  So basically what happens is that they must tune with pretty high initial timing on many of these new builds because they didn't use enough compression, or bought into the BS that tighter LSA and more overlap is somehow going to work better.

Combine that with the resident Forum experts, guru's and "trolls" surfing around on the Internet finding long lengthy articles telling us that we MUST use manifold vacuum advance and butt-loads of initial timing or we're STOOPID.  So the water is muddied pretty quickly.

I've finish with a story, as I have lots of them.  Decades ago before I knew much about all of this I built a 406 SBC for my K-5 Blazer.  I ordered flat top pistons for it not knowing that the compression ratio would be well over 10 to 1 with the heads I was using.  I put a stock 350/300hp cam in it topped with stamped steel 1.6 ratio rocker arms.  That engine didn't like, want, need or respond well to any timing anyplace.  I spent months tuning it, varying the initial timing, total timing, how fast the mechanical curve came in, and varying how much VA was added and went back and forth between ported and manifold vacuum.  It HATED manifold vacuum and INSTANTLY developed a slight skip/miss in the idle note if you hooked it up that way.  I also had to lower the throttle plates some and mixture screws started to be unresponsive/lacking sensitivity (sound familiar).

When it was all said and done I ended up running zero degrees initial, 26 degrees mechanical and 10 degrees from the vacuum advance.  Yep, it ran flawlessly with very little timing added anyplace.  I set up the advance curve with pretty heavy springs so all-in was about 3400rpm's.

That engine made so much power I could ROAST all four tires right off the rims.  It pulled my race car and trailer like it wasn't even back there, and got WAY better fuel economy than the anemic dished piston 400 engine with crappy smog heads it replaced.  You couldn't pour enough timing to the original engine to get it to ping and it didn't make chit for power anyplace, just like all those mid to late 70's Chevy 350 and 400 engines GM put in their trucks and Blazers.

.......continued

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