Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
Fine tuning Q-jet
old cars:
continued
You have to use common sense applied to your particular engine to determine at what amount of load the engine should be switched from the lean economy AFR to the rich power AFR. The power valve opening point and the vacuum advance starting point (fully retarded vacuum amount) should usually be near the same intake vacuum.
A high compression engine that is close to stressing the available octane, such as a 10/1 mild-cam street engine on pump gas, like a muscle car engine in the late ‘60s, may need to be rich and retarded at 9 or 10 inches Hg.
A low compression engine may not need to richen up until much closer to WOT and may tolerate the vacuum advance not fully retarding until as low as 5 or 6 inches Hg.
Because lean mixtures burn slower they require the correct amount of additional timing, compared to rich WOT mixtures. The correct vacuum advance is a key to lean part-throttle tuning."
On the lean side of stoichiometric, as the AFR is made progressively leaner there is a larger percentage of unburned oxygen in the combustion gasses. As the mixture is leaned, the increasing amount of (now very hot) oxygen in the residual gasses improves combustion.
Generally, as you jet leaner, the small throttle opening area (light load, vacuum above about 8” or 10” hg.) some engines will get crisper and actually get more power with less throttle, up to 17/1 or so (indicated on a WBO2) before the ‘fishhook’ phenomenon of increased consumption occurs. Some engines with excellent distribution respond increasingly better as high as 18/1, while some engines with poor distribution are struggling at 16/1. The break point is when the leanest cylinder misfires. Before smog tuning began in the mid 60s, most old-school OE engines were tuned to run at about 16.5-17.0/1 up to 50%-75% load. The extent and characteristics of this phenomenon, of course, depend on the valve timing and headers, backpressure, etc, and varies from engine to engine.
old cars:
It will help to understand this about the sensors. On the lean side of stoichiometric there is an increasing amount of nitrogen oxides in the exhaust product. The O2 sensor is affected by the presence of the oxygen in the NOx and is reporting a leaner AFR than there actually is. This is not a fault of the sensor or sensor system of any manufacture. It is the physics of the catalysis and ion pump processes by which the sensor itself operates.
The clue here is rich idle, lean cruise, leaner (leanest, actually) part-throttle acceleration and rich WOT. The leanest is at mid-load, half-throttle or so.
The tuning goal is to find the leanest mixtures which the engine will tolerate without missing or surging in level road cruise and moderate to intermediate acceleration. As load is increased engines will tolerate leaner A/F. Flat level road load will nearly always need to be richer than ¼ or ½ throttle acceleration. In fact, the closer to WOT, the leaner an engine will run, although approximately 15% richer than 1λ A/F (12.5/1) is necessary for best power and engine safety.
The thing to understand is most engines respond to being leaner than stoichiometric at part-throttle because the lean exhaust gas has hot unburned oxygen, and hot oxygen improves combustion.
Using a WBO2 and a vacuum gauge to monitor this you will see improvement as you adjust the primary main jet to find the best A/F for moderate acceleration in the load range between a level road, perhaps 14-12 in. hg. and the point where the power valve opens (Metering rods)I, perhaps 8-6 in. hg. The engine's part-throttle acceleration will noticeably improve as the AFR is adjusted to the lean side of stoichiometric.
When the air bleeds are configured correctly the A/F will progressively become leaner (from the rich idle) as the throttle is opened, until reaching the low vacuum load point where the PV opens.
The key thing is, at moderate to mid load, engines will run lean and like it, and burn much less gas while doing so. They must be rich at idle and very low load, lean in the middle, and rich at WOT.
And if you think this does not relate to performance you will be mistaken.
old cars:
Thank you to "Tuner" for the above explanation.
Cliff Ruggles:
Wow! Long read but not sure it will help out much here.
Since I got into this hobby the trend has been to reduce timing at light engine load and throw MORE fuel at the engine to keep things happy.
Hence MSD sells butt-loads of billet distributors w/o vacuum advance and when you walk thru a car show with 250 carb present at least 235 of them have a gas sucking Holley or POS Edelbrock AFB clone sitting on them!....FWIW.
NONE of those folks complain in the least about idle quality, throttle response and if you stick around their car long enough you'll start to hear tails of how it ran 10's at the last track rental they attended even though I can look right at the combo and see that it would have a hard time getting into the 13's unless you dropped it out of a C-130!
Just some humor, but there is some truth there if you can read between the lines. I've followed this thread a bit and not understanding how one of the best set-up Q-jets to ever leave the factory (7029207) needs much help anyplace to work FLAWLESSLY on this engine combo. I've built more of those than anyone who will read this and rarely deviate much if any from the stock calibration. That simply happens because most of them end up on engines that have a small cam upgrade over stock and the 207 carburetors were calibrated for the 350/350hp engines which basically are the same thing but they were smart enough to INCREASE the compression ratio in conjunction with the larger camshaft to offset looses and efficiency from the added seat timing and overlap.
Typically when the 207 carb is used with flat top pistons and 64cc heads, even if the builder was smart enough to keep quench really tight (most are not) it does well enough that few if any additional modifications are needed to the idle system to keep thing happy. I will add here that not all 207's have the exact same set-up, so this changes things a bit when it comes to building and modifying one for any specific application. All 207's did come with smaller main jets and primary B series rods. This was done because the larger idle tubes and DCR's bring in MORE fuel as the throttle plates uncover more transfer slot contributing additional fuel at light engine load.
....continued
Cliff Ruggles:
Typically here, when I build one at most I'll go one jet size larger and slightly larger primary rods plus make the APT adjustable for full control of the primary side A/F from rich to lean. This is only done to make up for the lack of BTU's and lesser quality of this newer fuel compared to what those carbs were originally set up for. There is never a need to open up anything else for most engine combo's, so the IAB's, DCR's, MAB's etc stay at the stock sizes. It's a factory "hot-rod" carburetor so really no need to try to help it be any better, they are very, very good right out of the box.
IF you are having issues with one, and I've not ran into that problem to date, there may be other things at work here. Might be time to go back to basics and make sure that it doesn't have fundamental issues someplace. I'd start with making sure the secondary throttle plates are fully seated in the bores and little if any light showing around them. Make sure the baseplate is the correct part number. Not uncommon to see the wrong one in place as many got swapped out decades ago due to being loose at the primary shaft. Verify that the primary shaft isn't "twisted", very common problem if anyone ever tried to remove the choke parts from the end of the shaft without holding them with something to keep the shaft from twisting.
I'd also pressure test it to make sure it isn't leaking at a bottom plug or leaking from the main fuel bowl over into the passages below the DCR's. I've found a few that did this and it makes them IMPOSSIBLE to tune.
Double check the power piston hanger arm height and they MUST be exactly level and even. I get troubled carbs in here all the time and every single one has bent hanger arms and rods uneven in the jets even when the person building it has assured me that they are not.
Also make sure that there isn't a bunch of "monkey chit" (epoxy) smeared over the bottom plugs keeping the baseplate from fully sealing the gasket. I see this ALL THE TIME because every "guru" on every Forum, Youtube or blog out there tells folks working on Q-jets to smear a pound of JB Weld (useless for holding back fuel) over bottom plugs that were pressed/swaged in place and not leaking in the first place.
Once every thing is checked and found to be in perfect working order I'd put the carb back to stock, including the float height and start the testing/tuning process all over again. I'd also (taken from the lengthy thread above) incorporate vacuum advance tuning into the equation plus use your APT system for fine primary side metering changes as well.
If you can't dial it in at that point it may be time to scrap it and look for another unit. I've ran into a few carbs over the years that just weren't going to work no matter how much time we spent with them. Could have been something as simple as they got the wrong nozzles installed on the assembly line or they were driven too deep into the casting (have seen this a number of times over the years).
In any case when you use good parts, and everything checks out, and it doesn't work well despite your best efforts it may be time to jump off the sinking ship.......FWIW......Cliff
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