Mixture: Almost but not quite

[Raymond]

In the elusive chase to find zen with spark, and mixture I'm getting closer but not there yet.  The low idle is still too low with an intermittent miss.  It's fine above 1500 RPM.  I've got the timing and dwell exactly right, and been through the linkage adjustment twice.  The distributor, FI pump and CSV are all recently rebuilt and I've re-checked for leaks.  I've used a vacuum guage to set the FI pump and bleed air.  I've checked the FI pump air filter and insured that there is a vacuum if I put my finger over the port.  I've checked the thermo switch for operation.  If I push down a bit on the FI pump linkage arm, the idle smooths out and increases slightly.
I think the only thing I haven't done is shims on the thermo switch. There are none there now.
So, what kind of shims should I ask for, and what dimensions or thickness?  Does adding shims lean the hot idle?

The other symptom that has me perplexed is smoke out the tail pipe.  When stoping for short stops and then accelerating hard, I get a small amount of light grey smoke.  If I sit at a long traffic light, say two minutes, and then accelerate hard it looks like James Bond's DB4 laying down a smoke screen.  The amount of smoke varies according to how long I sit at idle.  If I don't idle between stop and go or if I accelerate gently, I don't see the smoke.  Is this mixture related or do I have a valve seal problem?

Ray
'68 280SL 4-spd Coupe

[jeffc280sl]

Hi Ray,

Sounds like you have the car moving in the right direction.  You mentioned the vacuum at the FIP filter.  This vacuum should disappear as the engine warms up.  Does this happen?  I think the shims are a last resort kind of thing.  Can't help too much with the smoke. What do your spark plugs indicate?

Jeff C.
1970 280SL 4-speed

[George Davis]

Ray,

I agree with Jeff on the shims being a last resort, also about making sure the warmrunning device (WRD, proper name for what you called the thermo switch in your post) is shutting off completely  when the engine is fully warm.

Have you gone through the idle air/fuel mix adjustment procedure?  Assuming the WRD is ok, it sounds like the mixture is a bit lean.

Regarding smoke, my engine was rebuilt only a few thousand miles ago, and under hard acceleration it smokes.  I think that's pretty common on older engines without cats, and even on newer engines with cats.  The smoke at idle might be related to the poor idle, and once that is correct it may go away.

George Davis
'69 280 SL Euro manual

[Ricardo]

Ray
I wouldn't discount the valve seals, particularly if they haven't been done in some time....that certainly was the cure for my smoking issues which sound similar to yours....can you tell if the smoke is blue or grey?
On the low idle stumble....I think Dan has stated before that these cars are often difficult to get a perfect idle at low rpms ....unless your engine is completely rebuilt, it may not perform best at factory settings and a skilled mechanic will often know how to compensate for engine and distributor wear to obtain best performance...which is somewhat subjective too. Remember how muscle cars often sound terrible at idle, because they've been tuned to accelerate (and sometimes they have lumpy cams ) if you want best idle you will probably compromise acceleration a little etc.
If you've got a good CO2 reading at idle and it's lumpy, then you may have to richen the idle mix to smooth it and if you idle much, you may end up with quickly fouled plugs.....I think it takes a lot of persistence or experience to get these engines just right and I'm sure many here have spent considerable time to get there.....but that's half the fun eh!
Ricardo

[ja17]

Hello Raymond,
 If I understand correctly, all your other settings have been checked  and your idle rpm
is still too low. Try tuning the intake idle air screw to increase the idle. If this does not work richen the injection pump mixture  by turning the thumb screw on the back of the injection pump a few clicks clockwise. Now go back to the intake idle air screw and adjust the idle up. (Make sure the engine is off during this adjustment on the pump)

My reasoning here is the injection pump mixture control knob controls injection mixture up to 1700 rpms. Shims under the WRD will change the mixture at all speed ranges.

I usually do final idle mixture adjustment with the car in drive at idle (get someone to hold the brake while you are doing the adjustment.

Joe Alexander
Blacklick, Ohio

[Raymond]

Thanks everyone.  I only have about 600 miles since I did a bottom overhaul, and the plugs look even & light brown -- so far.  (The head was rebuilt about 2,000 miles ago.)   Fuel use checked at 12.9 MPG.  It was over 15 before the overhaul.  I have tried leaning out the FI pump.  I'll check the plugs again and maybe go back richer.  

Transmission is a manual but I'd like idle to support A/C compressor when I finally put a charge back in the system.

If I push down a bit on the FI pump linkage arm, the idle smooths out and increases slightly.  Does that indicate rich or lean?

I'll check the WRD again.  Isn't it supposed to draw air when warm?




Ray
'68 280SL 4-spd Coupe

[ja17]

Hello Raymond.The WRD should not be sucking air when the engine reaches normal operating temperature.

Your linkage test indicates your idle is lean. You could be leaking air through the WRD or you need to richen the pump idle up a bit (see previous post of mine).

Joe Alexander
Blacklick, Ohio

[Ben]

Are the linkages good and tight, or worn a little ?

Regards,
Ben in Ireland.
'64 230SL 4sp.