LM-1 and stock Dual Carbs
Posted: Mon Feb 23, 2009 5:39 pm
RandyInMaine was gracious enough to loan me his LM-1 for a couple of days of testing out my factory Solex dual carbs.
The LM-1 is a portable oxygen sensing meter that reads either a Lamba output (base value 1) or the air fuel ratio (stochiometric is 14.7:1)
Many of my intuitive conclusions were borne out, and some of my pet armchair theories were rendered inconsequential.
The oxygen sensor was a very close fit in the curved tailpipe of a stock bus, but I managed to get it in the curve, clamp it with the thumbwheel, and it just sat there nicely. The sensor wiring was brought through the deck lid, run through the right carb's air horn clip and up through the access lid to the aisle between the seats where it could go no further. I used my laptop inverter power supply with a long custom power supply wire so I could both watch the meter while driving and bring the meter back to the engine for viewing while adjusting the carbs.
Without touching a screw on the carburetors, I started and ran the engine from cold up to temperature on surface streets. Not surprisingly, the dual carb engines are stinky dirty environment killing rich pigs when cold. I average 11.6-12.0 in the first couple of minutes.
At operating temperatures, the idle was 12.9, full throttle acceleration was 13.5 to 14.0, cruise varied between 13.5 to 14.7 and the weirdest damn thing, it richened like a pig when at barely open throttle during overrun conditions.
As mentioned at Air-Cooled.Net, 13.7 or so is the VW's sweet spot for power, and my dual carbs after 30 years and countless cleanings and tweaks were dialed in perfectly for all acceleration requirements and they behaved very acceptably at 75-80 mph as well as street driving.
How boring. So I started screwing around.
First, since the stochiometric ratio is the new religion, I thought to see how close I could tune for stochiometric. I have no control over higher speeds due to main jets and air correction being beyond the scope of "adjustment", so I stuck with playing with mixture screws and idle speed and retard and vacuum advance timing.
I first disconnected the central idling circuit and detached the vacuum retard to get a nice 800 rpm dual-carb-only idle. Whoa. Ran fine at 17. That is lean lean lean. Engine was happy. Richened to stochiometric and it still ran fine but it did not want the vacuum retard, bogged right down.
It would not accept a reconnect of the central idling without plunging the ratio to 10. So I leaned the central down to get an overall 14.7 and found that the engine would stall at stop lights.
Reverted to my original adjustments then I went back down on the other side i.e. I kept the central idling circuit rich and leaned the dual carbs way down to 20 (fresh air is 21.9) and ended up with an idle of 13.8 almost all brought by the central circuit. Engine ran beyooootifully, snapped through the shifts held an idle through all slowdowns, and maintained a perfect 13.7 through acceleration runs. Hmmmm. But when I disconnected the central idling circuit, the engine stalled immediately.
Interim conclusion: that central idling circuit is a very powerful player, the dual carbs are NOT designed to carry the idle half as much as I thought, and the reason it accelerated so beautifully was that the accelerator pumps finally had plenty of dual carb AIR to work with, and any stumbling I have had in the past under acceleration was because of FLOODING.
Reverted back to original adjustments. This time, I hit Interstate 35 outside of Austin with the central idling circuit disconnected and the vacuum retard disconnected. The reason for this was that I thought maybe the central idling circuit was responsible for sucking gas into the engine at that partial throttle over rich that I mentioned further up.
Nope. The mixture DIVED into rich at any speed if I held the throttles barely open, but if I closed the throttles completely as in coasting, I would get 15 to 16. WTF?
edit
The rich overrun at partial throttle has been a real mystery. I have stock jetting and everything is clean and functional. Could this have to do with calibration for the stock VW's air pump/exhaust afterburning system? It came supplied with a big gulp valve that used to dump in a huge blast of air every time you lifted off the accelerator to help prevent backfiring, and I wonder if the carbs were designed to dump in a blast of fuel to relight the afterburners as you reapplied the accelerator. Who is left in Wolfsburg to answer these questions?
Reverted back to original adjustments. This time I perched the meter in the back and went through my usual finger-on/finger-off method of mixture adjustments. With the procedure in my write up of making the dual-carb-only idle drop when you take your finger off the "reference hose", the mixture would go from 17 to total dilute, no matter what I did with the vacuum retard hose. I think there is too little airflow through the carbs alone and was getting pulse dilution at the sensor. So at any rate, I left the dual cabs-only at 17. When I snapped the central idling circuit back in with the retard hose, I got 12.8 result when adjusting the mixture to the slight increase of idle speed when you crack your finger off the "reference hose" as mentioned in the write-up.
Surprise surprise. . . . the engine gave me the "no change in idle speed" mixture at 13.7.
Final adjustments: I decided to avail myself of the above information to run the dual carbs leaner than usual because that helps the calibration of the accelerator pumps on my bus (yours might be different), and I am holding the central idling circuit closer to the "no-change" reference hose test. Otherwise, our intuitive method works just fine!
I wish I could have kept this LM-1 until the BobD is pulled out, so I could do a L-Jetronic test, but there are others who need it.
Thanks Randy! Will ship it back shortly.
Colin
The LM-1 is a portable oxygen sensing meter that reads either a Lamba output (base value 1) or the air fuel ratio (stochiometric is 14.7:1)
Many of my intuitive conclusions were borne out, and some of my pet armchair theories were rendered inconsequential.
The oxygen sensor was a very close fit in the curved tailpipe of a stock bus, but I managed to get it in the curve, clamp it with the thumbwheel, and it just sat there nicely. The sensor wiring was brought through the deck lid, run through the right carb's air horn clip and up through the access lid to the aisle between the seats where it could go no further. I used my laptop inverter power supply with a long custom power supply wire so I could both watch the meter while driving and bring the meter back to the engine for viewing while adjusting the carbs.
Without touching a screw on the carburetors, I started and ran the engine from cold up to temperature on surface streets. Not surprisingly, the dual carb engines are stinky dirty environment killing rich pigs when cold. I average 11.6-12.0 in the first couple of minutes.
At operating temperatures, the idle was 12.9, full throttle acceleration was 13.5 to 14.0, cruise varied between 13.5 to 14.7 and the weirdest damn thing, it richened like a pig when at barely open throttle during overrun conditions.
As mentioned at Air-Cooled.Net, 13.7 or so is the VW's sweet spot for power, and my dual carbs after 30 years and countless cleanings and tweaks were dialed in perfectly for all acceleration requirements and they behaved very acceptably at 75-80 mph as well as street driving.
How boring. So I started screwing around.
First, since the stochiometric ratio is the new religion, I thought to see how close I could tune for stochiometric. I have no control over higher speeds due to main jets and air correction being beyond the scope of "adjustment", so I stuck with playing with mixture screws and idle speed and retard and vacuum advance timing.
I first disconnected the central idling circuit and detached the vacuum retard to get a nice 800 rpm dual-carb-only idle. Whoa. Ran fine at 17. That is lean lean lean. Engine was happy. Richened to stochiometric and it still ran fine but it did not want the vacuum retard, bogged right down.
It would not accept a reconnect of the central idling without plunging the ratio to 10. So I leaned the central down to get an overall 14.7 and found that the engine would stall at stop lights.
Reverted to my original adjustments then I went back down on the other side i.e. I kept the central idling circuit rich and leaned the dual carbs way down to 20 (fresh air is 21.9) and ended up with an idle of 13.8 almost all brought by the central circuit. Engine ran beyooootifully, snapped through the shifts held an idle through all slowdowns, and maintained a perfect 13.7 through acceleration runs. Hmmmm. But when I disconnected the central idling circuit, the engine stalled immediately.
Interim conclusion: that central idling circuit is a very powerful player, the dual carbs are NOT designed to carry the idle half as much as I thought, and the reason it accelerated so beautifully was that the accelerator pumps finally had plenty of dual carb AIR to work with, and any stumbling I have had in the past under acceleration was because of FLOODING.
Reverted back to original adjustments. This time, I hit Interstate 35 outside of Austin with the central idling circuit disconnected and the vacuum retard disconnected. The reason for this was that I thought maybe the central idling circuit was responsible for sucking gas into the engine at that partial throttle over rich that I mentioned further up.
Nope. The mixture DIVED into rich at any speed if I held the throttles barely open, but if I closed the throttles completely as in coasting, I would get 15 to 16. WTF?
edit
The rich overrun at partial throttle has been a real mystery. I have stock jetting and everything is clean and functional. Could this have to do with calibration for the stock VW's air pump/exhaust afterburning system? It came supplied with a big gulp valve that used to dump in a huge blast of air every time you lifted off the accelerator to help prevent backfiring, and I wonder if the carbs were designed to dump in a blast of fuel to relight the afterburners as you reapplied the accelerator. Who is left in Wolfsburg to answer these questions?
Reverted back to original adjustments. This time I perched the meter in the back and went through my usual finger-on/finger-off method of mixture adjustments. With the procedure in my write up of making the dual-carb-only idle drop when you take your finger off the "reference hose", the mixture would go from 17 to total dilute, no matter what I did with the vacuum retard hose. I think there is too little airflow through the carbs alone and was getting pulse dilution at the sensor. So at any rate, I left the dual cabs-only at 17. When I snapped the central idling circuit back in with the retard hose, I got 12.8 result when adjusting the mixture to the slight increase of idle speed when you crack your finger off the "reference hose" as mentioned in the write-up.
Surprise surprise. . . . the engine gave me the "no change in idle speed" mixture at 13.7.
Final adjustments: I decided to avail myself of the above information to run the dual carbs leaner than usual because that helps the calibration of the accelerator pumps on my bus (yours might be different), and I am holding the central idling circuit closer to the "no-change" reference hose test. Otherwise, our intuitive method works just fine!
I wish I could have kept this LM-1 until the BobD is pulled out, so I could do a L-Jetronic test, but there are others who need it.
Thanks Randy! Will ship it back shortly.
Colin