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Old 03-17-2011, 01:43 PM
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Trying to tune dual carbs using afr meter

Trying to tune solex 35pdsit carbs on a vw 1600 type 1 motor. What Im looking for is a general idea of what to expect for afr of carburators in general. I know 14.7 is the target for afr tuning, but my issue is the 1964 bus will bog just off closed throttle if Im cruising for a while at same throttle position, let off the gas, and when I get back on it, the bog occures for a brief moment. When this moment occures, the afr meter shows 20 or somethines slightly higher. This lean afr reading only stays for a spit second then drops back down to reasonable readings of 17 or less(usually less) Is it reasonable to expect the afr not to excede 15 or 16 at all? Ive messed with the accelerator pumps, rebushed the throttle shafts, ect and the bus runs well for the most part, but occasionally has that cough then go symptom. Now if I start from a stop and accelerate moderately through all the gears...no problems there. What Im lacking is the logic to rule out whats not causing this and attack what is. The carbs are brand new and I would think they should be able to run right. I am running a balance tube, 3/8s or 1/2 inch if I remember correctly. Fuel pessures 1.5(recommended 2 or less with these carbs). If the brand new distributor was at fault, wouldnt that show rich if it was not advancing quickly enough? I already have posed on thesamba.com(quartermilecamel is my user name on there) about this and am looking for any new ideas that might help as everyone over there is out of ideas. I figured there would be dual carb guys out there that might have some tips on hotrodders.com. What really throws me is It only does it when I have been driving, stay steady in the gear Im in, get off the gas then back on. Not too sure how to check if a accellerator pump is bleeding down. Im at my witts end with these carbs and are right about ready to just cut my 600 dollar losses with them.

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Old 03-17-2011, 01:53 PM
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If its when you let off and then get back on where it goes lean it almost has to be an accelerator pump issue, is it getting a pump shot too late, or is it perhaps not aggressive enough? A bus under acceleration si a high load condition where it will need an AF ratio closer to 12:1. At cruise your AF ratio shoudl be around 16:1, maybe a hair richer (slightly lower, like 15.5), under light acceleration it should be about 13:1 and under hard acceleration it should be about 12.5:1.

The only time it should ever read 20 is if you leave it in gear while on the brakes rather than the gas pedal.

14.7 is the stoicimetricly perfect ratio, and for your situation relatively useless.
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Old 03-17-2011, 02:11 PM
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What kind of AFR meter are you using? A narrow band will not be accurate. I have one hooked up just because I had it but it always reads about 18 and up and I tuned my truck with an LC-1 so I know the reading is incorrect. My cruise is a little over 15. Overall it is very similar to what ap72 said for the LC-1 readings.
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Old 03-17-2011, 11:39 PM
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It is actually a "build it your self unit" Yes, it is a wide band one that uses a bosch 17053 wideband sensor. The unit is called JAW. Just Another Wide band. If you were to youtube jaw you would see mine.
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Old 03-18-2011, 12:10 AM
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Yes, hard acceleration is about 12.5. I have seen 16 and 15 on light cruise at times, but usually around 13 to 14, so I guess light cruise is barely on the gas? Idle is about 12.5 on up to 13. To give you an idea of the gear ranges and rpm. 4 grand is roughly 65mph in 4th gear, 45mph in 3rd gear.
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Old 03-18-2011, 12:21 AM
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Heres the carb pictures.
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Old 03-18-2011, 12:28 AM
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I didnt put on the nylon nut on the shaft to the left of that spring. Thats how you adjust the accelerator pumps. Im just not sure how or what the concept is here. Screw the nut in gives you longer pump shot? Or screw the nut in gives you stronger pump shot? The first time I ran the bus last week, I had the nut unscrewed pretty far and I only saw 18 or less. This time around, I screwed in the nut some and now its worse. I could try to unscrew the nut more but theres not much more threaded shaft.
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Old 03-18-2011, 01:48 PM
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It sounds like your are about right. You can try leaning out the idle mixture screws but you don't spend a lot of time there so the 13 is no big deal. I believe mine is a little rich at idle as well. Light cruise at 15 is good. Had accelloration at 12.5 is really good.
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Old 03-18-2011, 11:40 PM
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Quote:
Originally Posted by ap72
If its when you let off and then get back on where it goes lean it almost has to be an accelerator pump issue, is it getting a pump shot too late, or is it perhaps not aggressive enough? A bus under acceleration si a high load condition where it will need an AF ratio closer to 12:1. At cruise your AF ratio shoudl be around 16:1, maybe a hair richer (slightly lower, like 15.5), under light acceleration it should be about 13:1 and under hard acceleration it should be about 12.5:1.

The only time it should ever read 20 is if you leave it in gear while on the brakes rather than the gas pedal.

14.7 is the stoicimetricly perfect ratio, and for your situation relatively useless.
Yes I believe its getting a pump shot too late, or its delayed. Just cant figure out why. I think I will shim that long shaft spring more.
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Old 03-19-2011, 11:38 AM
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I've been tuning my the Edelbrock 1406 on my SBC with an AFR, and this calibration seems to work.

Idle - about 13-13.5 (can't seem to get it any leaner)
Cruise - about 16
Light acceleration from cruise - 14-14.5 (accelerator pump may not kick in)
Hard acceleration from cruise - about 12.5
Choke on, cold engine - 12.0-12.5

With these AFRs it seems to run very smoothly.

Bruce
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Old 03-20-2011, 11:49 PM
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Ok heres what I did. Bus sat for 5 hours....go to hit the throttle while standing there watching left carb. Well the first time I did that slowly.......it did not squirt for half of the accellerator pump travel. When I pumped several times.......then I got instant squirt. So I took the carb off, took the 2 check balls out and checked for dirt, and found none. So I took the check ball out of the squirter side and then plugged that with my thumb and tried to see if the float bowl check ball was seating......not very good at all!!!! So I then took a small rod and tapped the check ball so it would seat itself in the carb body. The result is much better. When I first took the top of the carb off and checked the float bowl ball, I could make it leak down (hearing air and liquid sounds) Well now I can plug off the squirter and apply pressure to the pump lever and it will very slowly leak down. Question is: Are these check balls supposed to be totally sealing? Both of them still leak a little.
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