Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
1903 Won't Respond to Tip-In Procedure
Cliff Ruggles:
Basic tuning guidelines and I've covered this on here and other Forums a few times.
Start out with known "recipes" for N/A engine combo's. What we need to do first is to get close so the vehicle drives well without any detonation anyplace. Detonation, especially at heavy or full load will damage or destroy the best engine build out there.
Tune JET size on the primary side first. This assumes your "recipe" for idle/off idle fuel is where it needs to be.
Do this by climbing some steep grades/hills in higher gears w/o downshifting and without the secondaries.
Once you determine the most ideal jet size leave it alone, at least till much later in the equation and most likely you won't need to make any changes.
If you are also doing some distributor tuning tune TOTAL TIMING first. Make sure the engine does not ping under any type of heavy load as this can damage the rod bearings, piston rings, piston ring lands, etc.
I like to start out a bit "fat" on the secondary side but may lean that up later. Being a bit rich is always better than lean when it comes to heavy and WOT operation.
Anyhow, tune the APT system next. Initially you want to start out a bit lean or rods pretty low in the jets then sneak up on the most ideal setting. This part of the tuning may take several tanks of fuel and driving in various conditions to nail things down. I may do some vacuum advance tuning here as well in later stages, but initially get the best results that you can.
Realize when tuning in the "normal" driving range that superficially a lean setting may show a slight improvement in MPG's, but overall I've found being lean on the primary side of the carb requires more throttle angle in many driving scenarios than it would if things were a little richer and overall everything else suffers. Remember that the basic laws of physics apply here and it takes a given amount of energy to accomplish a specific task.
For WOT testing it is difficult if not near impossible to do it "by the seat of your pants". It's even worse with some vehicles if they have poor traction as spinning tires flaws all testing. It really helps here to use a dyno (engine initially) then the drag strip or some sort of timed/distance thing to real nail down the best tune. Weather, head wind, air quality (DA), slight uphill/downhill grade and even how much weight is in the vehicle effect this testing pretty dramatically so once you get close it may take some time to nail down the best settings.
Continued.....
Cliff Ruggles:
For the distributor you need to nail down initial and total timing. Some engines will not tolerate a lot of initial timing or they will "buck" the starter on a hot restart. So after you determine best total timing you may have to "adjust" the mechanical spark curve some if the initial ends up high enough it causes restart problems.
For spark curve I have never found to date that a super-quick mechanical curve getting ALL the timing in early works well for a well thought out engine build. Engines with optimum compression for pump gas with well chosen cams and good flowing heads typically don't like, want, need or respond well to a lot of timing right to start with, nor will they like a lot of it early in the RPM range.
For vacuum advance, with a well thought out efficient engine build I can't remember ever going past about 14 degrees VA timing and most are fine with a bit less. If you have a "low" compression engine and put a "hefty" cam in it forget the last statements, it's going to need a lot of timing everyplace to make up for poor dynamic compression and poor combustion efficiency.
Anyhow, this particular engine combo has a small cam in it and tight LSA. This will spike cylinder pressure pretty high and early in the RPM range. Unlikely you'll need to throw a lot of timing at it anyplace, even at light engine load.
For "basic" distributor numbers I usually start out with around 9-11 degrees of mechanical advance 18-22 at the crank. A good starting point for initial timing is usually around 10-12 degrees so we'll be in the 28-34 total range right off the bench. I usually set the VA up for around 10-14 and work from there. The mechanical advance needs to start right off idle, but NOTHING in at idle speed or idle tuning will be difficult if not near impossible.
I'll add here that most of these "new" shiny billet offshore HEI's are HORRIBLE for that sort of thing to a point where I refuse to work on them here in the shop. Many have low quality center cams/weights and the relationship between those parts has been "adjusted" to a point where by the time I put enough spring tension in them to keep some timing out at idle they delay too far in the RPM range for start and stop points with the advance curve. Not tying to put a big black cloud over all of those parts, but quite a few I've seen here are JUNK and if you have good luck with them I'm happy for you. I'll also note here that we don't know what old car bumpers the Chinese melted down any particular day to cast the gears from and I've seen enough of them chewed to nothing in 200 miles or so to NEVER want one in any of my engines!
One to better things. So some good basic numbers for N/A engines will be 10-14 initial timing. 18-22 more mechanical, and 10-14 from the vacuum advance. This puts total timing around 28-34 degrees and around 40-48 at cruise, or potentially that much depending on your exact cruise RPM and when the mechanical curve reaches full advance. I like to see 2800-3200rpms for full mechanical advance with most of these engines.
Continued......
Cliff Ruggles:
The above numbers have served me very well for decades. I'd add here that I tune for a living, and still open up a couple Saturdays a month for custom tuning. I've had vehicle brought here from as far as 2800 miles one way, but most are 200 or less. It's ALWAYS the same thing, and fixing the issues are to go into the carb and distributor, usually the distributor first. I yank out the POS MrGasket or other crappy spring/weight kit (EVERYONE buys that junk for some reason), get the distributor up to par then move on to the carb. I'm about 98 percent successful to date, the few I couldn't fix I had to "crutch" but still got them to work OK. The ones I couldn't nail down, guess what? They had short seat timing cams in them on pretty tight LSA's and to much static compression for the CID and cam choice.......FWIW.......
bry593:
Correct, Chinese distributors are junk. The weights have no stop and it will advance to the moon. I only use GM distributors and this one has the nice #41 weights.
Agreed, aftermarket curve kits are crap. Not sure why they didn't simply copy the original GM design including the correct size bushings. Anyway, I steer clear of aftermarket everything when possible.
My total current cruise timing is: 10 + 16 + 23 = 49. It can go a little higher since my mechanical is not "all-in" at 2400.
If I do encounter kick-back during hot start, a switch from manifold to ported vacuum would resolve it.
Cliff Ruggles:
There isn't enough vacuum cranking to overcome the spring tension in any VA I've ever seen, so NOT part of the tuning for "base" timing.
As for "aftermarket" distributors the readers should know that there is NOTHING waiting for you with any of them, aside from not having to replace points every 20,000 miles or so.
I've dyno'd MSD billet distributors back to back against my stock HEI with a factory "990" module and coil and they gain NOTHING anyplace. Matter of fact the runs showed a very slight advantage to the factory HEI. Most likely because of the very PRESISE fine points on the reluctor vs the wide (and usually all rusted and corroded up because they didn't plate them) ones on the MSD stuff.
I'd add that I hooked up a 6AL box on the MSD for those pulls, still NOTHING over a stock HEI.
At the drag strip on back to back runs we swapped in an MSD billet distributor to replace a stock points distributor I had done for a good friend about 30 years back. It was "locked out" and plate brazed down, we just used the points as a trigger.
To my surprise the car slowed down about .03-.04 seconds and LOST about .5 mph....hum?
An "old timer" later told me that the points provide a "natural" timing-retard at high RPM's and why we went a little quicker with them vs the MSD billet.'
Learn something new all the time.......
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