Fuel mixture questions

[wwheeler]

I have two questions regarding fuel mixture.

1) Does the M180 engine tend to burn leaner in the winter than the summer? With cold air being more dense, it seems like the engine could burn more fuel and therefore leaner. This of course assumes that all adjustments remains equal from summer to winter.

The reason for asking is that my plugs are the cleanest I have ever seen them and I really haven't changed any settings since it was 100F outside. This is the first year that I have zeroed in on fuel mixture adjustments and things have remained unchanged from summer to winter. If the mixture will go back a little richer this summer, I don't want to change the overall mixture adjustment. Just wondering if this is a normal occurrence.

2) My M180 engine will run at lower rpms (700) and lean AFTER the WRD air valve closes at 160F (confirmed). However, when I drive the car for 5 more minutes (fully warm), the idle speed goes to 850 and the mixture is perfect and stays that way. It is only the time between the WRD closing and 5 minutes of driving that the engine runs lean and low rpm.

The cold running is normally rich at 900 rpm, then climbs to 1000 rpm and slightly rich as the engine warms up. The engine then slows down a bit to 850 while running slightly lean just BEFORE the WRD closes. Again,once the WRD does close, it runs leaner and slower until I drive it for awhile. What adjustment would I need to do to correct that lean running after the wrd closes but not affect the fully warmed mixture?

[jacovdw]

Hello Wallace,

Do you not perhaps mean the M130 engine as I see that you have a W108 280SE?

Since mechanically injected cars do not measure air density directly, it is possible that a change in temperature can affect the mixture strength.
Temperature (engine coolant and not ambient air) is measured by the WRD and translated to a change in the mixture via the pin in the WRD. With an increase in coolant temperature the pin extends and hence leans the mixture appropriately. Remember that the WRD shuts off at a coolant temperature of about 65 - 68°C (different for US spec cars as of model year 1970 where it has changed to 50 - 55°C) and that is before the engine reaches full operating temperature of about 80°C.

Now, it is possible that your engine might still experience a lean condition during that time that it takes for the coolant temperature to change from 65°C to 80°C due to the fact that the engine is relatively colder (winter) than what you experience during the summer months.

There are two sets of shims to adjust the WRD. The smaller round shims under the thermostat element and the oval shims under the WRD housing. The smaller round shims are responsible for adjusting the shutoff point whilst the oval shims are responsible for the effect of the WRD on the overall mixture.

Adding an oval shim under the WRD would therefore enrich the mixture over the entire range. If I remember correctly, there is a section in the BBB (section 07-14 page 13 - 14 Service Manual Passenger Cars starting 1968) that describes the testing of the WRD in terms of the shutoff point and mixture during warm-up.

There are other factors that influence the efficiency combustion as well, namely relative ambient humidity, altitude above sea level and ambient air temperature. Our cars can only compensate for altitude above sea level and that is a whole other topic on its own...

[wwheeler]

Jaco,

Thanks for the info! You are right, it is a M130 in a W111 though. Summer will be here soon enough in Texas. I'll wait and see if it changes when it heats up.

[jacovdw]

Oops, I guess I wrongly assumed you have a W108...   

If it does not bother you that much, I would well enough leave it as is. The problem with adding an oval shim under the WRD housing is that it will have an effect on the entire mixture range and may require you to tweak your settings again.

Another possibility would be to check and see if your barometric compensator is working properly. There are quite a few threads on this forum where it has been discussed at length by a few members, one being jeffc280sl, February 2010 if I remember correctly.