Hi All

With my seat-of-the-pants preferred mixture setting I was running at 4% CO. I re-adjusted engine to 1.5% CO on a Bosch analyser as per the upper end of the factory CO specs. I only need below 6% to pass my local emissions so I'm safe anyway. However I would like to set it right.

Now I find that
1) cold start after choke disengages and before engine fully warming up is very lean
2) the idling is rougher with very light misfires when warm
3) power is better but I cannot verify this
4) fuel consumpution is only marginally lower

The mechanic who let me use his machine says that it will help the engine head last longer with less carbon and fuel dilution. I read somewhere that to get the right ballpark CO setting one must get the smoothest idle and than lean it even further into the slightly rough range. That ties in with what I've done.

Anyone have any experience to share on CO setting please?


Regards
Oreo
1979 2 litre 123, 115 carbed engine

Hello,
The factory MAXIMUM for the 175CDT/U carb is 4.5% at idle for all types of M115 engines
IIRC, 1-2.5% is the range allowed for the M115 cars. If you have hesitations, run it slightly richer. Remember this, you drive the car everyday, not the mechanic :p
I doubt you are getting *better power*, a leaner mixture will give slightly better throttle response on initial acceleration. Fuel dilution of the engine oil is a factor when the choke is engaged, at that time CO is in the range of 8-10%
I bet that if you rechecked the CO% again on a wet day, you will get a different reading
Finally, after having owned and worked on many cars with carbs, let me share my experience with you. The factory published CO figure should be treated as a guide only. My Volvo 264GL(2.8 V-6 with SU)ran well at 0.5%, Volvo just said*maximum 3.5%*.
Have a good week.

__________________
Nachi11744

Nachi

I better recheck my specs again. I thought it was 0.4 to 1.5%. I think maybe 2% is better. I don't understand about the rainy day changes in CO% . I know that the jet is supposed to be temperature compensated as it is riding on a spring loaded device. Is this what you mean?

Maybe an extra quarter turn out won't harm.

Thanks
Oreo

Oreo:

If you have a carb, idle mixture and running mixture are differently determined, so unless you are driving on the idle circuit all the time (you are more often than you think, maybe!), running power and fuel consumption won't change.

If it rumbles more than you like, rich it up a bit. The specs for the 72 280 SE 4.5 is 1.0 to 3.0%. I set mine about 1%, but that was some time ago (and many "twiches" on the MAO -- probably ought to do it again).

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Oreo:

If you have a carb, idle mixture and running mixture are differently determined, so unless you are driving on the idle circuit all the time (you are more often than you think, maybe!), running power and fuel consumption won't change.

If it rumbles more than you like, rich it up a bit. The specs for the 72 280 SE 4.5 is 1.0 to 3.0%. I set mine about 1%, but that was some time ago (and many "twiches" on the MAP -- probably ought to do it again).

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

Hello,
On the Zenith/Stromberg/Pierburg 175 CD type carb, changing the mixture setting at idle causes it to change across the entire engine speed range, with the change being proportionally greater at the lower and partial load range, the main needle taper exerting more control as the full load mixture range is approached.
My experience with the Volvo B21A and B23A 175CD carbs is that a very small adjustment *tweak* can give a huge improvement in drivability, and that has held true with the Pierburg 175 CDT on my W115 200.

Oreo-----you have to look up the MBUSA CD section on the carb, there is a page with a lot of specs for CO% on dyno testing for all the W114/115 cars and around the last page for the 175CD engined testing it is stated that a MAXIMUM CO of 4.5% is acceptable to cure *splashing acceleration*, I think it means hesitations.
I am not saying that you need to set CO to 4.5%, just that there is a lot of latitude for the setting on this type of carbs. My current setting checks out at around 3-3.5% and the car starts without choke and runs smoothly(at sea level)in all traffic conditions, including the three hour long traffic jam in a thunderstorm last night(engine 175F, ambient 90F, a/c vent 36F, humidity 1000%.......) Your results may vary
Have a good weekend.

__________________
Nachi11744

Hi Nachi and psfred

I took it out about a quarter turn and all is smooth again. I estimate that I'm still within the 2% range. When I was at 4%, my mechanic said he could tell my engine was rich just by the smell from the exhaust. So come on guys - lets all train ourselves now by sniffing our own exhaust.
If you feel:
high = rich >4%
woozy = <4%
loopy = <2%

Regards
Oreo

That rich stink is terrible -- I've been known to change routes while driving to get away from a badly tuned "smoker" because of it.

You don't want to run that way, it burns too much gas, as well as asphixiating other drivers and birds.

Peter

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!

asphixiating other drivers
I don't have a problem with that............less traffic to deal with

__________________
Nachi11744