Runs rough once warmed up

[Rhinkel]

I picked up a project. 1969280 SL putaway 20years ago. I hauled it home and started thru the basics. tank re done, lines, fuel pump,cleaned and checked for flow, plugs wires, oil, oil filter, fuel filter, ......all the basic stuff.   And it started and ran ok.

I’ve done the split linkage adjustment. Idle in park 900,in gear 700, idles smooth. When it’s cold it runs PERFECTLY. The minute it reaches operating temp. It stumbles and sputters off idle to 2500 rpm. I am sure it’s a lean condition with the light backfiring. I have read way too many things on this board and want to take a step back,  Un-confuse myself and start with the simplest items.  Where would you have me begin.? Shim the BC? It starts great warm or cold.  It idles great warm or cold.

I know I could take it in but I’m having too much fun learning and want to give it a try.

Please give me a few suggestions.  I think it’s very simple to fix this.

[ja17]

Check to see if the WRD on the injection pump is stuck first.

[Cees Klumper]

Also, we assume you have done Joe Alexander's engine tune/linkage adjustment etc tour (on my car, I had forgotten to re-tighten one of the brackets that holds the linkage that goes across the valve cover after doing a valve adjustment, and the symptoms were as you describe. It can be little things like that):

https://www.sl113.org/wiki/TuneupOverview/Start

[Rhinkel]

Yesi will. Next weekend project. Both the WRD and BC. I checked the linkage and it’s all secure.

[teahead]

maybe running lean?

[Rhinkel]

Ok, a few things checked this morning. Pulled the small bolt from the side of the CSV when cold and when warmed up. No leakage.  Pulled the small air filter off the WRD and it had strong suction when cold and would stall if I plugged it. Once warm there is no suction and covering it led to no change.disconnected the linkage once warm and when I open the air slightly, immediate stumble.when I move the FIP rod, no change with slight movement but stumble when opened more..

I haven’t taken the BArometric sensor off yet to check it. That’s next but I’m off to another event. Any tips on getting it off? It’s pretty tight in there. What size is the fitting? I think I got a 19 mm on it but can’t loosen it. Not much room to move.  Gonna go buy a crows foot fitting, I think I can get it off with that.

Any suggestions on running so lean once warmed up? Appreciate.  Rob

[teahead]

maybe it's your ignition system

Still running points?  What's your dwell readings?

Set timing at all?

[Rhinkel]

1 mm is all it took. I was able to get the barometric compensator off using a crows foot 19 mm. Super easy not having to remove anything else.  It had a 2mm spacer on it.  I found a washer with the same OD and ID that was 1mm thick. Many on the board said a little makes a big difference.  Just came back from a 30 mile experiment and it runs perfectly now cold and hot. Truly a little goes a long way here. Thanks to all that have posted on this. It makes it so much easier for those that are just not familiar with all these little tricks.

[philmas]

Hi, Rhinkel.
In the  attempt of fixing what looks like a similar trouble (relatively rough running between 1000 and 2000rpm when warm) I was wondering how your intervention modified the air/fuel mixture.

If I understand the tech manual well, the  plunger inside the BC protrudes when altitude rises, i.e. when atmospheric pressure decreases, which means leaning the mixture.

Do you mean you replaced the original 2mm shim with a 1mm one, or you ADDED a 1mm shim over the existing one?
Besides, does this affect only the period between starting aids shutoff and end of warm up, or warm running condition itself ?

My car starts and runs perfectly when cold, until the starting aids shutoff.
Warm running is OK except in the 1000 to 2000 rpm range, where I experience bucking and slight backfiring when decelerating .

[teahead]

check the timing.

My car recently did this when I changed my distributor.

It got loose and became WAY retarded the more I drove it.

Re-set timing, tightened down the dizzy; no more issues.

[philmas]

Thanks for your advice.
My car has just been serviced, and ignition (new points fitted, timing adjusted) is 0K.
IMHO, this looks like a lean condition, as I can hear some back firing when decelerating.

[jeffc280sl]

Philippe,

Is your 1971 SL a USA version?  I believe the emission system switches and fuel cut off devices on USA cars lean the mixture when decelerating to reduce emissions.   This stuff has been removed from my 1970 4-speed car and it seems very happy.  Just a thought

[philmas]

No, it’s a euro specs model .
I have had this issue since I bought the car some 25 years ago... I am quite confident with ignition as the car has been serviced regularly by trustful people.
Cold (or warm...) start  is always perfect, idle as well, but something is wrong between 1000 and 1800 rpm when warm (i.e from the end of the warm up sequence .) only under load.
The car would buckle and « shake » unless I am very precautious with the throttle, as lf I was trying to drive at 1500rpm in fourth gear...
This is weird, because this happens only in 1st or 2nd gear (specially in heavy trafic, or when cruising at low speeds), as if the inertia of the car would minimize (smoothen...) this erratic behavior in 3rd or 4th gear…
There is also some backfiring when releasing the throttle.…
I ‘m not sure tinkering with the BC would be the solution...what do you think ?

[jeffc280sl]

i did an write up on the BC some years ago with an explanation of failure modes with pictures.  another member did a more recent write up.  You can remove the BC and place in in home made negative pressure tank to see if it reacts correctly.  mine had a complete failure and did not work at all.  try starting there

[philmas]

I think I read your interesting post concerning BC and its failures; If I remember, bad news down there as the replacement part is pretty much unobtainium... though you gave some alternative solutions.
let’s hope mine is still in good condition!
I was driving on a mountain road last week at an altitude of ~ 1100m, without any noticeable change…but maybe this is not really significant to assume integrity of the compensator?

[Rhinkel]

Hey Philmas, sorry I didn’t see this. I had great cold running across all gears and rpm’s. Once warm, sputtering and stumbling with what I thought was a lean mix across all rpm. I added 1 mm shim to the 2 mim existing shim and I had success. I then went back and readjusted the air valve and the idle mix. Ran so much better but had a slight stumble upon takeoff. Wasn’t there when cold, so I assumed lean still. I took out the 1 mm and replaced it with a 2 mm. So now I have 4 mm total. I am close to sea level. Readjusted air valve and idle mix. Pretty happy with the result. Very smooth on all rpm range plus light brown plugs. Getting the BC off was a pain until I bought the right tool.  Good luck.

[Pawel66]

As for the tools to work with BC and FIP air filter - I bought 20-21mm spanner and cut it in half, so I have two short spanners - one for BC, one for the air filter.

[philmas]

What did the plunger look like when you got the BC out? Did you test it in a vacuum chamber prior to add the shims?
Besides, is it a 19mm or a 20mm nut?
Thanks!

[Rhinkel]

I didn’t do a vacuum check, I just compared it to a couple of pics posted here and pushed the end down and it would retract and return- spongy but very stiff. So I assumed it was ok. My BC required a 19 mm. Used a crows foot to get in the tight places. With a long extension and a torque wrench it broke free easily. Be careful when you unscrew and remove the unit as shims already there will fall into the engine area and go places you will curse!

[philmas]

Thanks for this information.
Next step is buying the right tool and get that BC out!

[Pawel66]

You may want to check what size of tool you need - there were various types (or shapes) of BCs and they may have various sizes of nuts. Mine was 20 or 21mm for sure (do not remember if the BC or the filter is 21mm). 19mm spanner is fairly popular, you probably have it - see if it fits your BC (the trouble is not to set the spanner on the nut, but the room to turn it - at least that was the trouble I had).

Truth is that if you buy 19mm crow foot and it will not fit the BC - you will have it for e.g. injectors holding when undoing fuel lines, no major loss there , will be used for something.

[philmas]

Thank you Pawel, might be the opportunity of buying a set of those, this is a relatively common situation when you just need THE tool you precisely don’t happen to own...☹️