Why does split and vacuum tests produce such different results?

[jedwards]

Gentlemen,
My early 1967 250SL is a wonderful thing and starts and runs beautifully when cold and right through the temperature range,  but once the heat fully soaks in, say after the third journey of the day, my idle becomes lumpy. Naturally, one suspects  lean running, the solution is to enrich the FIP.

What confuses me is why the Split Test method tells me one thing and the Mixture method set out in the in technical manual tells me something quite different. If I set the hot idle mixture using the Split test method it is 20 “clips” richer than indicted by the  vacuum/air/fuel/ idle speed  mixture setup procedure set out in the Tech Manual (http://www.sl113.org/wiki/Restricted/Linkage-tour#Idle) .

Both methods make perfect theoretical sense, but the results are quite different.
Naturally, I have doubled checked for mechanical faults including timing, advance, linkage setup, WRD, BC, FIP and vacuum leaks.

Any views on the relative validity of one method over the other?  Should fully warned to fully hot be making such a difference? (engine temp is to spec)

[stickandrudderman]

I have never had the need to read the split linkage tour on here since I've been doing it for longer than this site has existed.
However, once the linkage is set up correctly and verified to be free of any play then you should not be splitting it to set the idle speed or mixture.
If you encounter problems after trying everything you can think of I thoroughly recommend fitting a wide band O2 sensor. It will tell you so much more than you realised.

[mdsalemi]

If I may interject some reality here--

Any wide-band 02 sensor testing measures oxygen level in the exhaust, and adjusting things against some standard.
The split linkage test (a/k/a the Blacklick test after Joe Alexander's domicile) is testing against runnability, with all things taken into consideration.

Where or why do they differ? Well, I would think that if we were to turn back the clock, and find a right off the assembly line new Pagoda, and run the tests, they would yield the same basic result.
Today, decades later, there are a myriad of issues, from some worn but serviceable and functional parts; worn and or slightly misadjusted linkages, and more. Think about all the things in our complex cars with their fuel/air systems and think about all the possibilities of potentially out of adjustment or slightly worn parts.

That's my theory. Taken another way, get a group of any Pagodas together, particularly ones that start easily, idle properly, and run well through the range. I'd guess that the 02 sensor results would not be 100% consistent.

[garymand]

jedwards, By ‘clips’, do you mean clicks?  Clicks on the idle gas screw? 
My lady is in the paint shop, but she has a vacuum and A/F gage.  She should be back on the road next week.  My to-do list includes resetting the WRD, verifying the resulting change on the rack (rack clicks), and attempting the vacuum method of setting the idle.  At that point I will be able to compare the vacuum peak method to the A/F ratio method.

A couple of things that I think would affect the difference:  A) my engine has 100K mi on it and a little (small amount) blow-by.  B) The O2 sensor (only one) is in my passenger-side down pipe.  C) In lieu of the split, with the motor running, you can adjust the idle gas mixture, quite effectively, by adjusting the rack.

A)   The blow-by pressure is pretty consistent in pulses to the intake, indicated by the leak down similarity and watching it come out of the valve cover port with the hose off.  None the less, on a properly adjusted throttle, it doesn't have much effect on the idle until you split the A/F and crack open the butterfly.  The motor gets a gulp of rich mixture from the air filter canister and the valve cover.  Then it leans out as fresh air mixes in.  (I am thinking.)  I have noticed odd RPM responses when doing this to check idle mixture. 

B) The sensor smells the exhaust of only 3 of 6 cylinders.  But the leak down on all 6 are very close, indicating the smell in one down pipe should be comparable to the other, on my motor.  Not necessarily so on a motor with one or more leaky chambers.


B) One day about 6 months ago,  while totally frustrated with my WRD/FIP/Altimiter calibration, I decided to use the rack instead of the split because the spilt was giving me odd responses.  I had gotten used to adjusting the rack for top full load, pulled over to the side of the road with the motor idling because I didn't want to do a restart and possible change the calculus.  And I have easy access to the rack screw adjustment hole through a rubber temporary plug.  I easily found it was about 12 clicks too rich on the rack and I could bring the idle rpm in perfectly as I also turned in the air screw.  I thought it was pretty slick.  Then I put the 12 clicks back on the rack and took 12 clicks off the idle screw (w/motor off).  This is mt mwemory of 6 moths back and I think it was how many shims I had under the WRD at the time.  That is also when I discovered that at least on my motor, the WRD was effecting my mixture up at operating temps, by a lot.  To find that relationship is also on my to do list.  I’m told that at some point the WRD rod extension maxes how much it effects the FIP.

I might put a mic head on the WRD mount and measure the effect.