Linkage Split Test- help please

[jedwards]

Gentlemen,
There are several quite opposing and conflicting instructions to be found in our forum on how to use the Split Linkage test to determine Idle mixture. I fully understand the Fuel/Air Ratio concept and why the split test is so useful, but interpretation is where my confusion is coming from.

So, this is my difficulty.  At idle, if I disconnect the butterfly rod and manually move the butterfly forward say 5mm  to introduce more air and the RPM rise then I have found 3 differing views expressed on our forum :

View 1- If introducing more air causes a rise in RPM, - then it is RICH.  Lean the mixture off.

View 2-  If introducing more air causes a rise in RPM of  around 100 revs before stumbling, the mixture is OK.  Do nothing.

View 3 - If introducing more air causes a rise in RPM of  less than 300 rpm before stumbling, then its LEAN. Richen the mixture.

That’s my problem.

Can someone please tell me how far to the move the butterfly and if the mixture is correct, what should I expect to happen to RPM?   If I move it 10mm what should I expect?  15mm?  Is the distance and rpm to stumble used or not?

The same applies to working the  IP pump cam.  What should I expect and over what distance of manual movement? Most of the time, 5mm movement makes no perceivable RPM change on my car.

Any and all help on how to physically do the test properly would be hugely appreciated.

[ja17]

Hello Jedwards,

Not that conflicting. They  all say that an increase indicates a rich mixture. Specifically around 100 or less rpms increase is acceptable.  If a larger increase is noticed, then the mixture should be leaned. 

If no increase is noted, the mixture is lean and should be richened.

[jedwards]

Should I be seeing (hearing) these changes over very small movements of the butterfly or reasonably large ones?
How about the IP side of things? Anything other than quite large manual movements of the IP cam seem to make no appreciable difference to rpm in my car. It runs beautifully, so I know everything is  working properly, but I usually end up with air telling me its too rich and fuel telling me nothing or that its slightly lean.
Any suggestions on refining my technique?

Jeff

[garymand]

I usually disconnect the fuel because it is closest to the American driver side.  It doesn't take much movement, pushing down on the link,  4 or 5 mm?  It isn't rocket science.  You are just injecting more gas, but I do appreciate your analitics, but I don't understand your confusion about what should happen when you umbalance a balanced gas mixture.  May be that is how you should think about it.  When balanced, the gas supply should be just a little rich: and rpms = 750 +- 50.  Adding more gas should slow the motor more.  Only a little movement of the link is needed: no more than 5 mm, but it takes a second for the motor to respond, and its just the quick responce: the motor rpms drop.  That's all you want to know, rpms drop from 750 with slight enrichment. 

The air link is the opposite.  Gas is already rich, so adding just a little air raises the rpms but no more than 150rpm.  If more, you are wasting gas at idle.  Why idle too rich?  The pump should be set 2 or 3 clicks rich and you verify it by turning the air screw a 1/2 turn at a time until the rpms peak and then start searching (surging) then its too lean.  turn back 1/4 turn at a time to no search.  This rpm should be 100 to 150 rpm above rich idle.

Move the links slowly, like 1 second for the 5mm movement.  I don't disconnect my air side, I just open or close the air screw. 

[wwheeler]

I never disconnect the air either. To me, that side is a little different. Where as with the fuel rod at idle, all you do is add fuel. With the air side disconnected, you open the flap off the seat and it changes a few things. One is the vacuum, two is the distributor advance and then also the air screw becomes less of a factor. So because of that, I am not entirely sure you aren't changing something else other than just adding air with that side. Might be wrong but it makes sense to me.

5mm should be more than enough travel to register a change. If the engine is running well, I would listen to the fuel and not the air as described above.

I use the air screw to add air and the fuel rod to add fuel. One other thing to note from my experienece: When adjusting the idle screw on the IP, drive the car around after EVERY adjustment. Don't just rev the engine in the driveway. It won't completely change as much as it will with a drive around the block. You will over adjust if you don't take a drive. 

[garymand]

I second that, all of the adjustments on the fuel regulation screws and the fip rod.  They baffle me as to why you have to drive the car around for them to stablize.  I've found I sometimes have to drive a few miles to get new stable results.    

[jedwards]

Thank you all.
Using the air screw to alter the air side is a revelation. I had thought the butterfly was the only way of doing this, but as wwheeler says, doing so  also changes a lot of other factors as well.

So from your combined advice, I suspect I am not as far off as I had thought. 

I had actually given up on the butterfly air in preference for setting up  my warm idle mixture to give me good in-gear idle  when the  CSS was engaged.  I spend a lot more time sitting at traffic lights in gear than in neutral, so felt that a good idle in practical driving conditions was the better target to go for.
However, when I tested this setting by adding a little air via the butterfly, engine RPM would rise by around 100-150 rpm, which I took as being overly rich.  If I opened up the fuel slightly, to test the fuel side, then little or no change would occur until I opened it up quite a bit, which I took as being about right.  So I had smooth running telling me it was about right, fuel telling the same but air telling me its too rich.
From what Garymand is saying, I must have it a few clicks on the rich side, which is about where I need to be.  Wow! Perhaps not going crazy after all!

On the questions of drive the car around to stablize any new IP adjustments, could I achieve a similar outcome by manually manipulating the rack for and aft a couple of times using a bolt in the front rack port?  This should upset any previous “setting” and do much what a driver around the block would achieve?  Any thoughts?

Hugely grateful to everyone. Thanks
Jeff

[garymand]

I don't suspect off-putting the rod will help.  More likey it is similar to adjusting the screw on the rack.  There is a complicated set of leverarms linking the altimiter and cold start to movement to the 3D cam.  My late 280 FIP has some other inter-acting mechanism that (if it is working correctly, I think it is) stops the free movement of my rod when it is pushed from either end with the motor not running.  I adjusted my rod screw 2 clicks last night and had no efect on the cold idle that I could tell.  I put the car in nutral about 2 miles of driving and the idle was about right.  20 miles later, I put the clutch in and my idle had restabalized to 1100rpm.  20 miles later it is still at 1100.  At lunch I'm going to warm it up and see where the idle is.  Its been my experience, the idle will still be high and if so, I'll take out 7 clicks worth of idle gas: 1100-750= 350/50 = 7 (50 rpm's / click).  And it should stay stable.  The idle adjustment is not as radical to the FIP as the regulator screws or the rod screw, IMO. But we are all still learning about these cars.  I will come back in with results after I drive home -just to ensure some stabalization / repeatability. 

BTW, when the FIP in all is adjusted well, the motor starts right up.  I can let it sit for months, let the FP turn until I hear the pressure is up and pow, it starts like it was run yesterday. 

[wwheeler]

I think driving it is the only way. You can't put a load on the engine in the driveway very easily and that changes things in the pump. The I. pump is a dizzying array of levers and things that spin. From my experienece of doing both for several years now, the drive makes it final. You can get close in the driveway, but it will ALWAYS change some after you drive.

[garymand]

Took out 7 clicks and drove around the block: 750 again. 

[Benz Dr.]

I've found that every engine runs differently. Some respond more to certain inputs than others. I suspect that overall conditon has more to do with than anything else. So at best, we can only talk in general terms.
Any engine, like the ones we are talking about, will behave differently on the road under load than sitting in your driveway. There is no mystery or revellation about it. If you start the engine from cold, the plugs won't be at running temps and neither will the engine. The coolant may be at operating temp but not the oil. It can take a few miles even on a warm day to get the oil up to full operating temp. In the winter it can take many miles before that happens.
I find it very difficult to dial in a fresh engine during our cold winter months. The plugs always foul out because the engine is not hot enough or under any load until it's driven. You will find that driving away with a cold engine won't give you any fouling problems if everything is working OK because the engine is under load. This is the input that changes most everything. It isn't just the IP that's adjusting to new settings - it's the whole engine.

I open the throttle valve to test for a lean/rich mixture and will continue to do so. I will also open the injection pump but that's only to check for an extreme lean mixture. Since most of the time these engines are often running too rich I tend to look in that direction. If it's really rich then I would look at the usuall suspects - cold start valve, BC, WRD. Rich running can also be caused by incorrect spark plugs or carbon core wires - it's not just one thing in most cases.   

[jedwards]

A brief update.
With your help and advice, I seem to have the warm idle running reasonably well. I’ll continue with fine-tuning the mixtures over the weekend when I can drive the car between adjustments as recommended.

Overnight, I had a look at my cold-start mixture which has been a pain since I bought the car.  She starts well with all starting aids working correctly, but runs very rough and without any power for at least 30 seconds after starting.

I followed  Wwheller’s advice from another posting about WRD that suggested I should try cold starting with the IP rod disconnect and use ther throttle to add air, and then again the other way around  to determine if my poor cold running was due to too much or too little fuel.  That worked a treat and I now know for sure that I am way too rich until the WRD starts to warm.  I had already removed all the oval shims from under the WRD in an attempt to lean out the mix during early warm up, but clearly must now load more round shims onto the piston in an attempt to extend the reach of the WRD pin to lean off the mix. 

So today, using the thermal tan at work and a venier, I was able to determine that the maximum WRD pin extension at 67C  while fully shutting off airflow was 13.9mm or 29.2 from base to pin tip.
My WRD striker plate inside  the IP seems to have a maximum travel of 32mm before it “bottoms out” and stops head, suggesting that at  67C  the WRD influence on the IP rack still has about 2.8mm of travel available. 

Does anyone know what travel distance the IP’s WRD striker must move through before it reaches the neutral or fully running position and no longer increases richness?

[jedwards]

Gentlemen,
Following the advice and guidance from Mrfatboy, I successfully got my WRD functioning to specification and my cold start issues resolved,  which allowed me to tackle the warm idle mixture setting using the IP/ Idle screw split test technique, which you all kindly shared with me last week.

However results were a little mixed, so some further advice would be appreciated.

Following the forums’ combined advice,  I fully warmed the car and opened the idle air screw for maximum rpm and found that the current IP fuel setting supported a peak of around 1100rpm.  Using Garymand’s formula  of  1 click per 50rpm,  (1100-750=350/50=7 clicks) I leaned out the mix 7 clicks.
On re-starting the motor would not run. It would fire OK but could not sustain an idle. I added 2 clicks of richness, with some improvement. 2 more make things better and 2 more again made it run well, and one more (returning all 7 clicks) had it idling perfectly.   I drove it around the block a few times to settle the IP adjustments and tried again.  On redoing the split test, the RPM increase again indicated 7 clicks too rich. This time I leaned it off 2 clicks at a time. Same result. 2 clicks lean made it run badly, 4 clicks made it unstartable.
The ignition is good, plugs new, timing spot on.

Any suggestions?

[Benz Dr.]

There's a short answer and long answer.

The short answer; quit playing around with it.


The long answer; same as above. Set it where the idle is a least stable so you can drive your car. Like I said earlier, every engine will run a bit differently and that's the way your engine runs right now.
All you really need is for it to start, warm up enough to come to a decent idle, and then off you go. If you start your car and then drive away down the street the engine will go through the warm up cycle as you drive. Aside from actual starting problems, I really can't see all of the concern for having everything perfect.


Trust me. If you can't accept a small amount of wrongness about your car, you will never really enjoy it. 

[garymand]

My sentiments exactly.  My car has never been perfect and if I think I can get it there, I am the problem.  That said, finding the max idle rpm with the air screw is not correct.  Its too lean.  My motor is unstable and searches +-50 to 100 rpms at that point.  Yes find it a few times until you know where it is, but then turn the air down for a smooth stable idle.  The rpms should slow gradually as you turn the air back a turn or so, then drop  more rapidly with only a partial turn.  I have to guess at the numbers because I've done it so many times.  I stay at the high side of the knee and want that to be at about 700 to 800 rpm.  Its a ball park thing.  Just get it in the park.  and as Dr. Benz says. expect it to vary.  I think the manual says never reset it after a freeway run.  Or expect it to be high.  The reaction of the FIP is only aproximates the motor requirements, a compromise that gets close most of the time.  At some point you should be able to tollerate the small variations of the normal operation.

[jedwards]

Sounds like great advice. 
I now have it running nicely, but not entirely "by the book" .
I've have new throttle bushes and ball joints coming so will replace everything, then enjoy driving it and see how it "settles".

Many thanks to you all.
Greatly appreciated.

[Benz Dr.]

Yup. It takes almost as long to figure out when something is wrong as it does to figure out when it's right enough. Knowing when something is working within its normal range takes a bit of expirience. Don't worry too much about getting perfect.

Some cars I see seem to run without any effort. I lay my hands upon that motor, close my eyes, mutter a few incantations, and they work prefectly.

Others I just swear at ( yeah, I swear a lot, big %$@*^%$ deal, eh? ) and they refuse to co-operate. So I take out my 10 lb sledge hammer and threaten mechanical harm and .................... they smarten up.......... sometimes. 



If it's any consolation, I find 230SL's to be harder to tune in. Maybe it's beacuse they're older and the IP and other systems aren't as refined as a 280SL.

[garymand]

Oh man, I'm driving my sl right now because the 560 won't start and the 944 blew the head gasket.  And my wife won't let me drive her car...  I have spent so much time troubleshooting the 560, I thought ignoring it for a few months might help.  Because once i pull out the big hammer...