w11 other cars w108 idle

[jim 56]

My 1972 W108 sedan starts fine but takes about 15 minutes to warm up. After it is warmed up completely it drives fine. If I try to drive it (very slowly) before it is totally warmed up the car will slightly backfire like it is too lean. Can I add oval shims to the WRD I think that will make it richer to get rid of the backfire or should I just wait until it completely warms up as I am trying to save some gas and time.
Thanks Jim

[wwheeler]

I am assuming this is a six and not a V-8. Starting and cold running are two different systems. It can start fine and then run bad when cold. Sounds like your running too lean during the warm up phase. Easy to check with the split linkage method found here.

Yes, add oval shims as  needed to make cold running have the correct mixture. I never let my engine run more than a minute after first start up before driving off. Better for the engine and warms up much faster. Warming the engine just at idle will take forever. 

While you are at it, you might remove the IP air filter and check when and if the WRD shuts completely off. 

[Benz Dr.]

Sounds like the slide valve in the WRD might be stuck closed. This will give a lean running condition until the engine warms. A lean backfire is a sure sign.

[wwheeler]

Good point on the WRD. Wouldn't be the first time that has happened. Easy way to test that is to see if there is suction at the filter port when running cold. No suction, slide valve is stuck.

[jim 56]

Thanks for replies. I will add  oval shims. The WRD does shut off air (although some air does get thru but when I block it idle is not affected) Glad about driving off before total warm up I was afraid I was going to damage something.

[ja17]

As mentioned, make sure that the slide valve in the WRD is able to move during the warm-up period. If it is stuck adding shims will not help completely.