Quadrajet Problem Solving > Dialing in your rebuilt Quadrajet carburetor
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.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version