Author Topic: Need help tuning the new carb  (Read 1220 times)

agav

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Need help tuning the new carb
« on: 30 July 2016, 05:22 PM »
Anyone here feel like walking me through carburetor tuning for my new Weber on the M110? Some might have followed my conversion thread, now I am trying to dial it in.

I have a timing light.
I have a vacuum gauge hooked up to intake vacuum.
I have warmed the engine up.
I have set initial timing to the white mark (around 7 degrees). I couldn't even go that far with the warped Solex without killing the engine which I think is a good sign.

I do not currently know exact RPM because my Petronix installation disabled my tachometer (different problem).

Weber is a dual barrel carb, so two idle screws which I turned all the way in and then backed out one turn as first setting.

The turned them back until vacuum dropped on one side, back out a little, move to other side and so on. So far so good, but I hear a clear "lean plop".

Also water out of the exhaust... What might be going on there?

Thanks!
1975 W116 280S in Topaz Brown - my project

agav

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Re: Need help tuning the new carb
« Reply #1 on: 30 July 2016, 05:26 PM »
Also forgot to metion. I can barely get her over 17in vaccum in idle, which according the the gauge scale is considered 'late timing'. But that is generic, so I take that with a grain of salt of course.
1975 W116 280S in Topaz Brown - my project

rumb

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Re: Need help tuning the new carb
« Reply #2 on: 01 August 2016, 08:10 AM »
I know nothing about webers, but with any dual carb you need an airflow meter to set them the same.

One way to set timing is to put it where the vacuum is the highest.

dont know your elevation, but vac is lower at altitudes.

Helpful link
http://www.gregsengine.com/using-a-vacuum-gauge.html

clear water out exhaust is just good combustion
'68 250S, '77 6.9 euro, '91 300SE, '98 SL500 '14 CLS550

agav

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Re: Need help tuning the new carb
« Reply #3 on: 01 August 2016, 09:39 AM »
Good advice on the airflow meter. I will try and improvise something, I have struggled balancing things out since one idle screw turned out to provide double the idle fuel with the other one turned in will still provide decent idle. Maybe I'll rig something up with two PC fans and a multimeter...

Good to know about the water! I got worried there for a second.

Only thing that confuses me is timing by vacuum. The manual calls for 7 degrees advanced. The engine will idle at higher vaccum if I advance more, all the way to 20-25. I think that is normal? One idle screw setting works better there than another, so I just assumed: Make sure RMP are right, set timing to spec, then idle mixture, then tiny adjustments to timing vy vacuum?
1975 W116 280S in Topaz Brown - my project

rumb

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Re: Need help tuning the new carb
« Reply #4 on: 01 August 2016, 11:43 AM »
This explains the process. What I like about it is that it compensates for engine wear, altitude, gasoline, etc.

http://automotivemileposts.com/garage/v2n8.html
'68 250S, '77 6.9 euro, '91 300SE, '98 SL500 '14 CLS550

agav

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Re: Need help tuning the new carb
« Reply #5 on: 01 August 2016, 11:48 AM »
Great link! I have read a lot of these in the past, but really wasn't sure about timing. My engine runs a ton better when setting timing at 20-25 degrees, which worries me a little. Or is this normal? I'll try and find out if the disk might have slipped and the readings are off.
1975 W116 280S in Topaz Brown - my project

rumb

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Re: Need help tuning the new carb
« Reply #6 on: 01 August 2016, 02:21 PM »
After you set the timing at idle,check the total timing - with all the vacuum lines hooked up.  34-36 degrees total and all in by 2500-2800 should be a good starting point.

Someone smarter than me may have a Mercedes spec for total timing.
'68 250S, '77 6.9 euro, '91 300SE, '98 SL500 '14 CLS550

agav

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Re: Need help tuning the new carb
« Reply #7 on: 01 August 2016, 02:27 PM »
Great - gives me an indication about the timing marks being correct or not as well. So will accelerate to 2500 and read where we end up then.

My distributor doesn't have vacuum. It's centrifugal I believe.
1975 W116 280S in Topaz Brown - my project

70sBenzGuy

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Re: Need help tuning the new carb
« Reply #8 on: 31 July 2019, 11:13 AM »
Its been a while, how did the tuning work out?  My 1975 280s is still sitting in the garage collecting dust.  I have the weber carb installed but have not tried to tune it yet.  Can you show where you connected the vacuum advance ports on the weber since the distributors on our cars have no vacuum pod?

raueda1

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Re: Need help tuning the new carb
« Reply #9 on: 01 August 2019, 04:04 PM »
I'm not sure about your Webers, but if you have true multi-barrel carbs you'll never get it right without the airflow meter.  If I remember right some of the MB Weber carbs weren't really multibarrel, more like "progressive" with another barrel coming into play under certain conditions.

Anyway, I used to have a 1966 BMW 1800ti with 4 side-draft carbs and an insane linkage mechanism (google!  Really fun little car and exact opposite of the big benz's in about every way).  Carb synchronization went out of whack from just looking at them.  Also a MB 220S (I think) with dual Weber/Zenith carbs, similar issues just fewer barrels.  The answer was this:
https://www.summitracing.com/parts/edl-4025?seid=srese1&gclid=CjwKCAjwm4rqBRBUEiwAwaWjjBTOa6_Wrud10roXas10TLvOZHihSwFYdcQcYJ5xIM4t-YJfNi4MfBoCLDMQAvD_BwE

Worked great and made it pretty easy. 
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0