CO Readings on Carb'ed Engines

[ctaylor738]

I am still working on the carbs and timing of my 1967 200 fintail. I have moved the timing up to the max of 15 BTDC. The vacuum advance is working. The idle is perfect at 800 RPM with about a 4.5% CO reading. The car still has a minor hesitation on acceleration from low RPM and on up-shifting at lower RPMs like when driving in traffic.

I checked the CO at 3000 RPM and got a reading of 3% more or less. The only benchmark that I have is for the 230SL, where the recommendation is 4%. The plugs are snow white. So I am thinking the car is running lean and that may be the cause of the hestitation. There are larger main and air correction jets available for the carbs (Solex PDSI or PDSJ depending on which book you read).


Any thoughts? Arthur, are you listening?

[Mike D]

Chuck,
I think you have two different problems going on.

The hesitation at low rpms is probably due to the accelleration pumps not engaging soon enough or the duration of them is too short. (Lordy, I HATE dual carburetion!). This causes the "bog" until the increased air flow can "pull" the fuel from the bowl. Too low of a float level can excaberate this problem. Too low of a supplied fuel rate can also cause it as the pumps will drain the float bowl until the fuel pump can "catch up".

3% is fine for "newer" cars (1972 or above, hah!) but the older cars liked to run "fat". Keep in mind if you have a retard mechanism on the distributor then it is doing what it is supposed to by retarding the spark and giving you a cleaner burn once the engine vacuum comes back to normal levels. So, the 3% might be fine.

The "white" plugs are a different matter. Can you twist the tops of the carbs? I've run into problems where the base plates are actually loose at the bowl assemblies (I don't remember the actual set-up of the Solex POS, err, I mean , PDS carbs). I've also seen the throttle plate bushings worn in an egg shape which allowed a vacuum leak when the throttle reached about half open.

You could put in an O2 sensor and use a multimeter to monitor the actual readings as you drive. They are handier than a sniffer for actual driving conditions.

[ctaylor738]

Good points to ponder, thanks. I do think that 3% is too low - combustion is combustion, and if 4% @ 3000 is a good number for MFI, then it should be good for a carb'ed engine.

The distributor hooks to ported vacuum, so if I understand this correctly, it is pulled toward advance by the air moving over the opened throttle, not buy a reduction in vacuum.

I will take a look at the throttle shafts for any sort of wear. I will also look at the float level.

I did look at the pumps and they engage immediately upon throttle movement, they seem to put out enough fuel and the tubes are aimed correctly. The fuel pump is new Mercedes, courtesy of the PO.

When I drove it this morning, I had the manual choke about 2/3 on for a few minutes, and it really zipped along, better than warmed-up with the choke off.

[Arthur Dalton]

I alway block off the brake booster hose when looking for a/f mixture concerns....