Questions about the Lars Method of Q-jet tuning
11-04-2004, 12:55 PM
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Questions about the Lars Method of Q-jet tuning
I’m in the process of tuning my ’74 Q-Jet according to Lars’ paper and I have a few questions that you all may be able to help me out with. The carb previously ran well on an L-48 engine, and now is on a ZZ4. I installed an air/fuel monitor in place of the ashtray with the sensor mounted in the driver header collector.
1. For optimizing the main jets, the paper suggests to keep the metering area constant by changing primary rods along with the jets. The best jet size is then assessed at WOT with the secondaries locked out. However, all primary rods of this era have the same power tip diameter, so at WOT it shouldn’t matter which primary rods I have. Or does it?
2. The carb had size-75 jets in it when I started. I found that the best idle mixture was a little rich, then when I gave it a little throttle it would go lean, and then at WOT with secondaries open it would go way lean. So I locked out the secondaries by zip-tying the secondary lockout lever to the carb body (which still open a little bit at WOT, by the way… bad?), changed the jet size to 76, and now at cruising I am still running lean, but at WOT the needle buries itself on the rich side. Are these jets too big already?
3. I also have a lot of stumbling during acceleration. It was much worse with the smaller jets, but is still present with the 76’s. I’m tempted to try a larger jet size except for the fact that the 76’s are rich at WOT. Am I right in judging these variables by the air/fuel ratio, or should I go by performance instead?
Thanks all!
Dave
1. For optimizing the main jets, the paper suggests to keep the metering area constant by changing primary rods along with the jets. The best jet size is then assessed at WOT with the secondaries locked out. However, all primary rods of this era have the same power tip diameter, so at WOT it shouldn’t matter which primary rods I have. Or does it?
2. The carb had size-75 jets in it when I started. I found that the best idle mixture was a little rich, then when I gave it a little throttle it would go lean, and then at WOT with secondaries open it would go way lean. So I locked out the secondaries by zip-tying the secondary lockout lever to the carb body (which still open a little bit at WOT, by the way… bad?), changed the jet size to 76, and now at cruising I am still running lean, but at WOT the needle buries itself on the rich side. Are these jets too big already?
3. I also have a lot of stumbling during acceleration. It was much worse with the smaller jets, but is still present with the 76’s. I’m tempted to try a larger jet size except for the fact that the 76’s are rich at WOT. Am I right in judging these variables by the air/fuel ratio, or should I go by performance instead?
Thanks all!
Dave
11-04-2004, 01:26 PM
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Dave -
1. The reason for my recommendation on keeping the jet/rod area constant when changing the jets is so that you don't get an incorrect or invalid throttle response "feel" when doing the WOT evaluation. You are correct that all rods have the same power tip, so it does not matter what size you have for the WOT determination. However, let's say that you have a fair-running setup and want to optimize it. Your carb may have a 74/43 jet/rod combination that runs okay at idle and cruise, but you want to refine the WOT performance. If you simply swap over to a set of 77 jets and keep the 43 rods, you have now massively increased the idle/cruise mixture as well as the WOT mixture. When testing this, you will probably end up with a rich stumble as you mash into the throttle with a momentary rich condition that will make the car feel "mushy" at first. This may give a faulty impression of the 77 jets "not working right" when, in fact, the setup may be perfect once the car gets over the initial rich condition. For this reason, I advocate keeping the cruise metering area constant during the WOT evaluation: you only change one variable at a time that way (by keeping cruise metering area constant), and you can better evaluate how your car is responding.
2. It is quite possible that the 75/76 jets are rich. You can drop down slightly on the jets to solve to WOT mixture problem and then increase your cruise metering area by dropping down on the rod size. Finally, use the mixture screws to then tailor the idle mixture once the cruise mixture is where tyou want it. You can further fine-tune the cruise mixture by using the power piston stop adjustment screw - leave the plug out of the airhorn so you can adjust the mixture without pulling the carb apart.
3. If you were lean at part-throttle, this will cause a stumble. Solve the part-throttle lean issue wth smaller rods or by adjusting the power piston stop up a little higher.
1. The reason for my recommendation on keeping the jet/rod area constant when changing the jets is so that you don't get an incorrect or invalid throttle response "feel" when doing the WOT evaluation. You are correct that all rods have the same power tip, so it does not matter what size you have for the WOT determination. However, let's say that you have a fair-running setup and want to optimize it. Your carb may have a 74/43 jet/rod combination that runs okay at idle and cruise, but you want to refine the WOT performance. If you simply swap over to a set of 77 jets and keep the 43 rods, you have now massively increased the idle/cruise mixture as well as the WOT mixture. When testing this, you will probably end up with a rich stumble as you mash into the throttle with a momentary rich condition that will make the car feel "mushy" at first. This may give a faulty impression of the 77 jets "not working right" when, in fact, the setup may be perfect once the car gets over the initial rich condition. For this reason, I advocate keeping the cruise metering area constant during the WOT evaluation: you only change one variable at a time that way (by keeping cruise metering area constant), and you can better evaluate how your car is responding.
2. It is quite possible that the 75/76 jets are rich. You can drop down slightly on the jets to solve to WOT mixture problem and then increase your cruise metering area by dropping down on the rod size. Finally, use the mixture screws to then tailor the idle mixture once the cruise mixture is where tyou want it. You can further fine-tune the cruise mixture by using the power piston stop adjustment screw - leave the plug out of the airhorn so you can adjust the mixture without pulling the carb apart.
3. If you were lean at part-throttle, this will cause a stumble. Solve the part-throttle lean issue wth smaller rods or by adjusting the power piston stop up a little higher.
11-04-2004, 02:42 PM
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A timely reply from the author himself... you're the man, Lars. 
Thanks for the feedback - it definitely makes sense about the primary rods causing the throttle response. I don't think I'm getting a "mushy" feeling when I hit the pedal, it's more like jerking when the car is trying to accelerate. Occasionally the tires squeal after it hesitates and catches again. One final question if you're still around or for anyone else...
If the car still stumbles and runs rich once the car is securely into WOT (after a few seconds with the pedal floored), that seems like a smoking gun for having jets that are too large. Could any other parameter involved in carb tuning affect this? Does it make sense that a 355 HP 350 may need smaller jets than a 190 HP 350 with the same carb?
Dave
Thanks for the feedback - it definitely makes sense about the primary rods causing the throttle response. I don't think I'm getting a "mushy" feeling when I hit the pedal, it's more like jerking when the car is trying to accelerate. Occasionally the tires squeal after it hesitates and catches again. One final question if you're still around or for anyone else...
If the car still stumbles and runs rich once the car is securely into WOT (after a few seconds with the pedal floored), that seems like a smoking gun for having jets that are too large. Could any other parameter involved in carb tuning affect this? Does it make sense that a 355 HP 350 may need smaller jets than a 190 HP 350 with the same carb?
Dave
11-04-2004, 10:15 PM
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Man u learn fast Dave. I can tell u'll be a Qjet expert soon. Just a heads up that since u don't have a heated crossover manifold - well no exh supply from heads to manifold - below about 50*F u may have stumble due to lack of fuel vaporization in manifold until manifold gets nice and toasty. Fuel tends to drop out'a mixture and puddle on floor of manifold. In colder areas carb icing can occur below carb throot. Early morning could be a bad time for testing until throughly warm. Yea what i'm saying is get the engine nice and warm before playing with it. 
Hey, nice idea with the A/F monitor.
U'll have to report how well they work (or don't) and/or help tuning. BTW what kind did u install? Edelbrock? LEDs or gauge with needle? cardo0 
Hey, nice idea with the A/F monitor.
U'll have to report how well they work (or don't) and/or help tuning. BTW what kind did u install? Edelbrock? LEDs or gauge with needle? cardo0
11-04-2004, 10:42 PM
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Originally Posted by cardo0
Man u learn fast Dave. I can tell u'll be a Qjet expert soon.
This sucker is my only car. Gotta love those public buses.Yeah, I figured as much with the warm engine. My last engine always stumbled a lot before it was warm, but I didn't the know reason behind it until your post. Makes sense.
I went with a Nordskogg analog air/fuel gauge (NRD-K7020B) and oxygen sensor (NRD-S8942), both from Summit ($140 total). The gauge looked like it was the right size to replace my dysfunctional clock (it isn't), but I was able to file down the edges and squeeze it into the ashtray cutout on my center console. That way I have it in the cockpit, but can cover it up with the ashtray cover to keep it out of sight. Ginger is a non-smoking Vette anyway
I mounted the sensor in the driver header collector, which I had welded on with the exhaust pipes after I finally got the new engine started. (that was a fun drive to the exhaust shop... no exhaust pipes or mufflers)
So far so good with the air/fuel gauge, so I'll be sure to let you all know how it works out. Have a good one.
Dave