CIS mixture adjustment

JBO · 02-13-2007, 06:06 AM

OK, I've read all the archives, and have been adjusting my mixture for a few seasons after having it professionally set - richen a little in fall and lean a little in spring. My question - this past weekend when it was particularly cold - 20 in the morning - car was hesitating and backfiring slightly until warm (which to me indicates lean). Once when I started it warm it backfired a little, I "think" in the intake. It also has become harder to start - have to hold the key in start position longer, when previously it would always start immediately. However, once warm I had some slight popping from the exhaust on decel, and can't find any leaks, which indicates rich. When I went to Souks "field test" and lifted the sensor plate, the engine RPM's increased and the engine raced a little, which indicates lean. So, I think I need to richen it up a bit, despite the exhaust popping. FYI I have a cat by-pass and GHL muffler.

MotoSook · 07:18 AM

20 degrees is a big temperature change from when you set the mixture in the fall (I'm guessing at least a 40 degree diff). The WUR has to change the backpressure over a large range of cold start conditions. It's an aged system, and it doesn't react as well. Be patient or change your idle mixture ...as needed. Do it when warm. All of this is assuming you do not have other issues like a bad cold start injector. But let's not go there yet.

If you you think it's a PITA...have you ever tried running a carbureted car all year round? Ask the Weber and PMO guys what their start routine is like when it'sd cold. That's if they drive their car in the winter at all.

My '76 with a '78 3.0 in it has most of the stock CIS (but get this..it's got a later WUR!) .. and it's been sitting for almost 3 years. With conditioned gas in the tank. I go to start it when I feel like it or time allows, and it starts just fine everytime. If it doesn't start and I sense it's not getting start enrichment, so I lift the plate a little and prime it. Once that's done, things are back to normal and I can start it again and again warm or cold.

It's really not that complicated of a system. You seem to understand it well enough. So treat it like a tempermental mistress and you both will be happy (not that I know anything abotu a mistress). Otherwise trade up for a 3.2 with electronic fuel injection

81 911 SC · 02-13-2007, 07:22 AM

Well lets see since there is no choke for the PMOs I typically pump the pedal a few times, turn the battery on so I can hear the fuel pump pushing fuel towards the motor and crank.... continuing this process until the car starts.

Mine also comes off the road in December.