Fine tuning Q-jet

[quadrajam]

Is the power piston free? Does it have the correct spring?
Correct length rods? Hanger height normal?
Does the acc pump shoot a strong squirt the entire throttle opening?
Did you restore the air bleeds back to stock?

I'm running out of ideas. Must be a tough one.

[bob69]

Quadrajam , I'm  running out of ideas too. I hope someone can steer me to a possible solution.

To answer your questions: power piston is free and has the stock pp spring that's orginal to the carb. It's also the same size pp spring as found in the other 207s I have. Rods are stamped with B indicating the correct lenght. Hanger not damaged or modified. The accelerator pump shot looks strong when throttle is opened (but I can not tell if its ok while driving).

The MAB were increased from 0.050 to 0.070. This change is what caused the problems listed in the original post. The MAB were changed back to stock. This fixed those issues. The IAB were not modified.


For the lean condition, I  have an innovate lm2 to record the afr. Been using it for over 10 years on several q-jets on different cars. On each of the other q-jets, I see a momentary rich spike when opening the throttle. A spike that lasts about a second. Showing the effect of the accelerator pump shot.

So when i say I see a lean spike its for the second or so following the throttle opening. I suspect it's contributing to the weak acceleration I'm having. I've seen this before.

On one occasion I  found a lean condition when opening the throttle that turned out to be an out of adjustment pump rod. I adjusted the rod and the rich spike showed up. Acceleration improved.

On another occasion/car, the car had not been driven is several months. The afr showed a lean spike on opening the throttle. This time it was enough to cause a mild lean stumble. Turns out the blue accelerator pump cup shrunk and was not pumping. After about 20 minutes driving,  the cup expanded and sealed.  The lean stumble went away and the rich spike returned.

Old cars, I also see the lean condition on light acceleration. But it may be too lean. E10 runs leaner. The stoic afr for e10 is 14.1 not the 14.7 for gas.

If i understand it correctly,, Stoic is the theoretical ideal afr for the fuel. The lm2 uses 14.7 (gas) as the default ideal afr. But  stoic for e10 is 14.1. The lm2 readings must be adjusted for the the difference.  A 16.0 on the 14.7 scale  converts to 16.7 for e10. A little too lean.

The lm2 records lambda and converts to afr. Lambda is 1.0 for the ideal fuel/air ratio, regardless of fuel type. 1.0 is converted to 14.7 afr for dislpay. I would use lambda but it's a whole different language.   

[77cruiser]

If it's too rich & it stumbles it will show lean. Any misfire shows lean, because of unburned oxygen.

[quadrajam]

Maybe 77 cruise is onto something with the misfire idea.
Is the mechanical advance free and smooth?
Do you have another carb to swap out?
Another distributor?
Some crazy ideas yeah but we gotta whip this thing.

[old cars]

I will try to comment/explain without being offensive , so don't take this the wrong way.

First of all you started drilling an innocent carburetor.
Second you changed main jets to address idle fuel ?
Then you changed main jets To address WOT?
Now your explaining Lambada/AFR?

Can you feel this lean condition as a hesitation if you taped over the AFR gauge?

"There is no reason to run an engine at part-load any richer than as lean as it will run without misfire. Of course for each engine just what that AFR may be depends on the particular engine’s characteristics, cam, compression ratio, headers, mufflers, etc. The biggest factors are uniform AFR distribution to the individual cylinders and exhaust reversion through the valve overlap. At part-throttle, distribution is affected by throttle angle and other carb geometry as well as the manifold, manifold heat, fuel distillation curve, etc, etc.

Between about 10% to 20% and 70% to 80% load some engines will run well at part-throttle with 17/1 AFR or leaner and others start to turn bitchy at 15/1.

You need to realize that the leaner the AFR, the larger percentage of oxygen in the exhaust. When running at part-throttle the high intake vacuum is drawing hot exhaust back into the intake manifold. Leaner than stoichiometric the excess oxygen in the exhaust is returning to the cylinder in the reversion gasses and the hot oxygen improves combustion.

Obviously, the leaner it is, the larger the proportion of unburned oxygen in the exhaust will be, and up to a point (unique to each engine) the lean running usually noticeably improves the part-throttle combustion.

Depending on your particular engine’s nature, you will be surprised how much part-throttle torque improves as it is operated progressively leaner on the lean side of stoichiometric. The limit is usually reached when the leanest cylinder misfires.

Because lean part-load mixtures have a slower combustion rate, they require additional spark advance, compared to rich maximum power WOT mixtures.

Correct vacuum advance tuning is as important as carb tuning. If the timing isn’t correct, you can chase the carb tuning into a box where a rich cruising AFR is wasting gas, because, without vacuum advance, a faster-burning richer AFR compensates for retarded timing.

[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

[bob69]

Wow old cars, lots of good information to digest.

I agree each engine combination requires it's own tune. I have four different engines, each built for a different purpose. The tunes are different. Each is tuned to give the best idle, cruise and wot performance.

The first rule for carb tuning is to get the distributor timing correct.  Before any carb adjustment I'll record the distributors timing from idle to 4000 rpm in 500 increments. I'm recording total advance (initial and mechanical) without the vacuum advance hooked up. I start with 10° initial and see what the total is. If total is greater than 36° the distributor is modified to get total to 36° or less. If timing advance starts before 1000 the advance weight springs are stiffened up. I only use  vacuum cans with a 15° limit. All cars have the vacuum advance hooked up.

Would you use 16.0 afr target with e85? I think not. Calif has e10 but adds a boat load of oxygenators. As much as 10 - 15%. About four years ago calif mandated yet another fuel formula change. All my cars started running rough. More oxygenators less gas.

I had to retune all the carbs. For example, it took me two months and many tanks of "gas" to get the set up correct on one. I tested jets from 71 to 75. Settled  on 74s as the performance was noticeably best. Even retested 73s and 75s to confirm 74s were the best. The point being that I use seat of the pants testing and not afr to tune a carb. Interestingly,  the 74s gave the best overall afr's.

I agree that misfires will increase the o2 levels the afr testor see. I do not hear any pings but its possible. Will test with 91 to see if that helps. By the way, 91 goes for $7.00 a gallon around here. Consider yourself lucky you're not in calif.

Regarding your opening comments. I do not do things willy-nilly. The mab were changed following reciipe #1 on page 111. The idle circuit was not altered, based  on comments found on this site. When I tried the large jets it was a last chance to test the set up before making any permanent changes elsewhere. Also,  jets in the 71 - 73 range for 0.070 mab are recommended on recipe #1 on page 111. And finally,  the oversized  jets privided the best idle quality. Very smooth and afrs in the low 14s. With the mab and jet changes, the afr had to be 13.0 to get close to the same idle quality. Still not as smooth.

I still  do not see any explanation why I see a momentary 1 second lean spike when the throttle is first opened. My other carbs show a momentary 1 second rich spike. Perhaps Cliffs comment that this carb may be a pos and use another carb is right. Just don't like writing it off without an explanation.

[Kenth]

Carburetors are "stone-age" technology and need to be treated as such.
A/F meters or Lambda devices are too sensitive to evaluate what happens in a carb due to no computers for fuel/timing control.
What controls a carbs ability to deliver a proper combustable mixture due to atmospheric pressure into the void in intake are the various openings in each circuit inside the carb to mixture air and fuel adjusted to the current vacuum signal strenght. There will always be a delay, larger or smaller, depending on speed of throttle openings that the A/F meter/Lamba device will notice that in reality mean little or nothing for the performance of the stone-age carb.

There is seldom if ever needed to alter the main airbleeds.
Adjust if needed the main jetting from the main airbleed sizes.
Since you have Cliffs book take a look at pages 97-98 "Primary Main Fuel System" and you´ll get an idea.
Also, on page 99 Cliff describes the "tip in" procedure. Have you done this?

FWIW

[old cars]

bob69

Answer this question. When you see the " lean spike" do you feel a hesitation / stumble

[old cars]

Carburetors are "stone-age" technology and need to be treated as such.
A/F meters or Lambda devices are too sensitive to evaluate what happens in a carb due to no computers for fuel/timing control.
What controls a carbs ability to deliver a proper combustable mixture due to atmospheric pressure into the void in intake are the various openings in each circuit inside the carb to mixture air and fuel adjusted to the current vacuum signal strenght. There will always be a delay, larger or smaller, depending on speed of throttle openings that the A/F meter/Lamba device will notice that in reality mean little or nothing for the performance of the stone-age carb.

There is seldom if ever needed to alter the main airbleeds.
Adjust if needed the main jetting from the main airbleed sizes.
Since you have Cliffs book take a look at pages 97-98 "Primary Main Fuel System" and you´ll get an idea.
Also, on page 99 Cliff describes the "tip in" procedure. Have you done this?

FWIW

Kenth
Not sure I am understanding your first paragraph. Can you elaborate

[Cliff Ruggles]

I completely agree with Kenth about using A/F meters to tune these engines using carburetors and conventional distributors with VA.

Before you get your bricks out and start throwing them at me hear this out.

In any and ALL cases we tune for best results in every area.  Idle tuning comes first, but it is done in conjunction with finding the ideal initial or base timing.

It's super easy to dial in the ideal idle mixture the engine wants, and you don't need a vacuum gauge or hand held tach.  Just listen to the engine.  Back the screws out till noticeably rich but not "blubbering".  Turn them in until you just notice a speed change or the engine starts to become slightly "rough" or unhappy, then gently back out till it cleans up nicely and smooth again.  Do both sides, "balance" them if/as needed, DONE with idle mixture.  You can gander at your A/F meter at this point, but no matter what it says the reading is what your engine wants, likes, and responds best to.

I like to tune the primary main jet next.  This may take a few tanks of fuel as described above but we need to NAIL DOWN the most ideal jet size for heavy throttle without the secondaries. 

Move on to the primary metering rods.  IF you have the idle fuel system nailed down and your "recipe" includes metering rods providing full control of A/F from rich to lean, which it should, start out lean (APT down/metering rods deep in the jets) and start driving the vehicle.  Realize that the vacuum advance and mechanical advance are "players" here so running leaner mixtures will require the ideal spark lead to effectively burn them.  I've spent days and even weeks nailing down the ideal settings for timing and part throttle APT settings.

I was always thinking that a lean mixture would net the best fuel economy, but found that with these things it's difficult, if not near impossible to get past the basic laws of physics.  It's simply going to take a certain amount of energy to move a given mass a given distance.  So going noticeably lean has NEVER once resulted in the best fuel economy combined with the best throttle response, acceleration, and power for all "normal" driving scenarios.  I'll add here that I've installed A/F monitoring devices and was actually quite surprised by the readings I was seeing compared to the actual test results.  This is where monitoring becomes difficult.  Long-lengthy reads on the subject, following resident "experts" on Forums, and others who do testing in these areas throw monkey wrenches right into the middle of that deal. 

What is my "target" A/F is the first thing you need to ask yourself?  Am I looking for a "fixed" number, like 12.5 to 1 for WOT?  Or will my very efficient and well thought out engine build be happier with 13.2 to 1?  Or 12.8?  How about part throttle A/F, is it a magic number like 13.5, or 13.7, or even 14.2 to 1? 

I was seeing a WIDE range of A/F in the "normal" driving range at times going clear up over 17 to 1, and my engine still operated FLAWLESSLY.  Throw that number into most of these conversations if you want get the natives all stirred up!

continued....