LM-1 and stock Dual Carbs

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Amskeptic
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LM-1 and stock Dual Carbs

Post by Amskeptic » 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
BobD - 1978 Bus . . . . . . . . . . .111,130 miles
Chloe - 1970 bus . . . . . . . . . . . 206,787 miles
Naranja - 1977 Westfalia . . . . . 93,996 miles
Pluck - 1973 Squareback . . . . . 55,510 miles
Alexus - 1991 Lexus LS400 . . . 72,113 miles

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Westy78
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Post by Westy78 » Mon Feb 23, 2009 5:57 pm

Interesting. I'd like to see what you find with the FI on the BobD. I'm sure Wayne wouldn't mind and neither would I, lending ours out to you to do some long term testing. I've found that a good long highway trip can tell you a lot with the LM-1. It only takes very small variations in road and wind conditions to see a difference in mixture with FI.
Chorizo, it's what's for breakfast.

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Amskeptic
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Post by Amskeptic » Mon Feb 23, 2009 7:12 pm

Westy78 wrote:Interesting. I'd like to see what you find with the FI on the BobD. I'm sure Wayne wouldn't mind and neither would I, lending ours out to you to do some long term testing. I've found that a good long highway trip can tell you a lot with the LM-1. It only takes very small variations in road and wind conditions to see a difference in mixture with FI.
That would be cool. I will be detailing and driving the BobD at least though May.
Colin
BobD - 1978 Bus . . . . . . . . . . .111,130 miles
Chloe - 1970 bus . . . . . . . . . . . 206,787 miles
Naranja - 1977 Westfalia . . . . . 93,996 miles
Pluck - 1973 Squareback . . . . . 55,510 miles
Alexus - 1991 Lexus LS400 . . . 72,113 miles

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satchmo
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Re: LM-1 and stock Dual Carbs

Post by satchmo » Tue Feb 24, 2009 2:00 pm

Amskeptic wrote: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?

Colin
So isn't the rich overrun condition THE reason why VW added the smog/air pump in 1973? Some throttle plates also have a small hole in them that might have been a help with this rich condition on overrun with barely open throttle. It would be interesting to LM-1 test a system with the smog pump and EGR system intact (Good luck finding that).

I also think it would be interesting to see how the LM-1 readings correlate with CHT readings. Or, if you don't have a CHT gauge (What? No CHT gauge??!! How can you drive?) perhaps you could check your spark plug electrodes after running a while with your new settings and see if they are in agreement with the new tuning adjustments.

Tim
By three methods we may learn wisdom:
First, by reflection, which is noblest;
second, by immitation, which is easiest;
and third, by experience, which is bitterest. -Confucius

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airkooledchris
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Post by airkooledchris » Tue Feb 24, 2009 2:54 pm

on my own LM-1 testings with FI, richness has made the CHT's lower, but so has much too lean readings.

there's a happy medium it seems where you have some wiggle room, like in the 12-16:1 range I don't see too much variation in the CHT's on short runs, but once it goes leaner than that it goes up and getting any richer just freaks out the LM-1 and it just starts reading 'too rich' with no numbers to go off of...

I recently setup the LM-1 on a friends camper special running duel solex's and once fully warmed up it was running the following:

3rd gear, 2500 rpm, slight incline:
11.5 - 12.5:1 AFR
340 CHT

4th gear, 4000 RPM's, flat surface, 65+ MPH
11-12 AFR with any throttle, 13-14 with almost zero throttle
370 CHT

since there seems to be plenty of wiggle room before the CHT's are in a bad range and as crazy rich as its running across the spectrum, he's going to try and lean it out a ways and see if he can find a happy medium.

that crazy motor still makes 15mpg in the city with those numbers, which is nuts, to me. it was the first time id driven one of those Raby motors and I have to say, it makes you want to tear yours down and start over from scratch. Even going up an incline in 4th gear the motor just feels like it wants MORE. it'll give you as much power as you could realistically need in an old bus.

Ive been fine tuning my FI system for optimum power and economy and its still a slug with any sort of load and I get about 11-12 mpg in the city at best.
1979 California Transporter

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Gypsie
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Post by Gypsie » Wed Feb 25, 2009 11:19 am

airkooledchris wrote:Ive been fine tuning my FI system for optimum power and economy and its still a slug with any sort of load and I get about 11-12 mpg in the city at best.

Thank you... (tears streaming, big masculine hug) ...thank you...thank you...

I have been tweaking with my afm for months because in the city I was in the 10-13 mpg range and I was beginning tio question my own sanity.

People kept telling me I should be getting near 20....Phbphb...Riiiiight.

My last engine drop I replaced rings but not Pi$tons or Cylinder$ for obviou$ rea$on$.

That will be in the next drop, along with other stuff.

She runs smooth. Auto tranny and she feels (and sounds) like a George Jetson commuter...

Best thing to come out of it is increased confidence in making AFM adjustments.
So it all started when I wanted to get better gas mileage....

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Amskeptic
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Re: LM-1 and stock Dual Carbs

Post by Amskeptic » Wed Feb 25, 2009 12:04 pm

satchmo wrote:
Amskeptic wrote: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?

Colin
So isn't the rich overrun condition THE reason why VW added the smog/air pump in 1973? Some throttle plates also have a small hole in them that might have been a help with this rich condition on overrun with barely open throttle.
What I do not understand is why fuel starts gushing when the throttles are barely cracked open on overrun. I tried the acceleration runs from 1500 up to 4000 rpm at light/medium/heavy throttle openings, and in no instance did I drop below mid-12 and mostly was at 13.5 ± .3
Colin?
BobD - 1978 Bus . . . . . . . . . . .111,130 miles
Chloe - 1970 bus . . . . . . . . . . . 206,787 miles
Naranja - 1977 Westfalia . . . . . 93,996 miles
Pluck - 1973 Squareback . . . . . 55,510 miles
Alexus - 1991 Lexus LS400 . . . 72,113 miles

72Hardtop
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Re: LM-1 and stock Dual Carbs

Post by 72Hardtop » Tue Feb 12, 2013 2:32 am

One reason why I've leaned away from using an LM-1 on dual carb setups. The changes (humidity, temps, elevation etc...) seen during a day/trip can have a wide effect on LM-1 readings with carbs vs fuel injection. Remember carbs are essentially a controlled vac leak...jet it for where you live and live with it's short comings. Trying to use an LM-1 on carbs will leave you chasing your tail around like a dog :shaking2:
1972 Westy
2056cc T-4 - 7.8:1 CR
Weber 40mm Duals - 47.5idles, 125mains, F11 tubes, 200 Air corr., 28mm Vents
96mm AA Biral P/C's w/Hastings rings
42x36mm Heads (AMC- Headflow Masters) w/Porsche swivel adjusters
71mm Stroke
Web Cam 73 w/matched Web lifters
S&S 4-1 exhaust w/Walker 17862 quiet-pack
Bosch SVDA w/Pertronix module & flamethrower 40K coil (7* initial 28* total @3200+)
NGK B5ES @ .028
002 Trans
Hankook 185R14's

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Amskeptic
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Re: LM-1 and stock Dual Carbs

Post by Amskeptic » Wed Feb 13, 2013 10:31 am

72Hardtop wrote:One reason why I've leaned away from using an LM-1 on dual carb setups. The changes (humidity, temps, elevation etc...) seen during a day/trip can have a wide effect on LM-1 readings with carbs vs fuel injection. Remember carbs are essentially a controlled vac leak...jet it for where you live and live with it's short comings. Trying to use an LM-1 on carbs will leave you chasing your tail around like a dog :shaking2:
I am many miles down the pike and have lived with the LM-1 hooked up for thousands of miles and have come to a personal conclusion . . . the intuitive methods I have used *do work*. When you are asking your engine directly, it tells you directly.
Colin
BobD - 1978 Bus . . . . . . . . . . .111,130 miles
Chloe - 1970 bus . . . . . . . . . . . 206,787 miles
Naranja - 1977 Westfalia . . . . . 93,996 miles
Pluck - 1973 Squareback . . . . . 55,510 miles
Alexus - 1991 Lexus LS400 . . . 72,113 miles

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