Linkage length guidance

[jedwards]

Gentlemen,
I have an early 250SL auto and hope to get some guidance on the set up of the various accelerator linkage rods, not referred to in the Linkage Tour.
I have recently acquire a complete set of rods and sockets as my old set was fairly battered, which make fine tuning difficult.  So time to set it all up correctly.

The Linkage tour is really helpful, but focuses exclusively on the FIP rob length (233 mm between centers) and the need to set the throttle rod to a length to give a fully closed butterfly at idle.

There is no guidance provides for the other 3 rods, so I don’t know whether my start point is reasonable or not.

Can someone tell me what should determine the accelerator to the Linkage pivot point length? The tour shows this as the Regulating Rod, but no information  is provided on setup or length.  The same applies to the vertical rod between the lower accelerator linkage pivot point and to the Manifold lever. Is there a designated setup , length or procedure?  Then finally, the manifold lever length.

There must be some rationale behind what each of these lengths should be, so any help or guidance would be greatly appreciated.

[ja17]

Hello jedwards,

Just measuring linkage rods and installing them can cause real problems. The IP rod is the one critical length which must be set to length. The other rods are adjusted then installed so that components function properly. Setting the IP rod to 233mm automatically defines what the length of the long  venturi rod should be. With the IP linkage rod adjusted and at rest and on it's stop, the venturi rod is adjusted to allow the venturi to be at rest on it's stop. The other rods are not as critical, but must be adjusted to allow the engine to have full throttle when the accelerator is floored. If you have a firewall micro switch, it must be adjusted correctly as with the kick-down -switch for automatics. I rmeasured an original set of 280SL linkage rods some time ago. Be aware that only the 233mm measurement of the IP linkage rod is critical and the other rod lengths can may vary slightly. IP rod =233 mm, venturi rod = 295mm, block pivot to intake manifold pivot=140 mm, from cross-over-rod to intake manifold pivot = 210mm

[jedwards]

Thanks ja17
As always with your replies, exactly what I was looking for.
Jeff

[ja17]

Just realize that these lengths except for the 233 rod are all plus and minus. There is also a accelerator pedal height adjustment at the firewall in the engine compartment.

[jedwards]

Hi Ja17
Thanks for the numbers.  Its great to have a reference point, however in this case I am even more confused than before as those number don't seem to work on my early 250SL.

I unpacked my nice new rod set and set the IP rod to 233mm between centres, as suggested, and install it.  All looked good. See image 1. 

However, with the IP rod set at 233mm, my alignment hole and venturi ball on the primary rod mount are now at least 10mm out of alignment. See image 2.  The tech manual talks about both, but not which one is “more right”  than the other on the 250SL.

If I stick with the recommend IP rod length of 233mm, my next problem is the  venturi rod. I start by setting my new venturi rod set to 295mm length, as you suggested, and plan to adjust it so I have a fully closed venturi.  However,  this  rod length is at least 10mm too long for my car and my nice new rod cannot be shortened enough to fit onto my car. See image 3.  In fact I am going to have to cut it down to achieve the 285mm my car seem to want.

So now I am really confused.  If I set the IP rod length via the alignment hole method, then my venturi rod fits and can be close to the 295mm you suggested.
If I stick doggedly to 233mm IP length, then the alignment hole is out, the new venturi rod wont fit and my numbers are different from yours.

What’s a bloke supposed to do?

[ja17]

Are you measuring the 233mm linkage from the center of the ball to the center of the ball?  It does not look right.

[jedwards]

Hi Ja17,
Yes, definitely centre of ball to centre of ball. I even made a jig so it would be accurate to within 0.25mm.

When I look at various images of 250SL engines in Goggle image, I see the "arm" that the IP Rod connects to on the cross over rod is often at all sorts of angles, even on professionally restored cars, something its pointing slightly upwards, sometimes downwards. There seems to be limited consistency between engines.

Are you able to photograph what yours looks like both at the Pump end and the cross-over rod end?

[jedwards]

Just so I don't make a complete fool of myself, I am assuming that between centres is as per the attached picture?

[Fintail]

Hi
On the second picture the ball nipple should line up with the centering hole (10mm dia approx) 233mm measurment only for cars without the centering hole.

All the best John

[jedwards]

Thanks  John, but I don't believe it is quite that simple.
There are two quite different cross rod brackets with two quite different holes, and is it not entirely clear if these are meant for the same purpose.
In image 1, there is a 14mm hole, like I have on my 250SL. This is just a hole, and quite unlike the the clearly more complex alignment bore in image 2. The second is clearly designed to take a 10mm guide dowel, but mine is just a hole.   
As a result, it is not clear (to me anyway)  whether those of us with the plain 14mm hole should be using this to set the IP rod length, or whether we should be using the 233mm rod length instead.

Help please!

[Flim]

Jedwards,

I have the exact same alignment hole in my cross linkage bracket as you do, on my 250SL. My cross linkage  doesn't quite align with this hole either. I have my IP linkage set up to 233mm.

I have set my venturi control linkage from this point to the throttle stop, as described in the linkage tour. I am unsure of the length, just ensure that both at rest and at full throttle, both IP lever and venturi are on their respective stops.

My engine runs great with this setup. When experimenting with the alignment hole as a set up when replacing my linkages my engine ran hesitantly, misfiring on acceleration.

Charles

[Flim]

jedwards

I just noticed that you have a non factory hex bolt connecting your cross linkage bracket to the intake manifold. Are you sure that you still have the correct spacer washer under the cross linkage bracket (picture below)? I believe these should be present to correctly align the bracket.

It may be that your bracket is positioned incorrectly as these spacers may be missing, and this may be the reason for your venturi rod length to be different to Joe's reference….?

Were you replacing your linkages to improve engine running?

My linkage set up below shows that my cross linkage bracket hole is in a different position to yours.

Charles

[jedwards]

Interesting comments Charles. My bracket had countersunk holes and I posted a question some time ago on this excellent forum about their importance and was told that we must should use special bolts like speed bolts with angled heads to guarantee perfect location between the  bracket and the manifold. However I do not have any washers under mine. What thickness are you using?
The other interesting thing from your images is that you have alignment with the 14mm hole in your bracket =which I do not.  Is your FIP rod 233mm between centres or did you set it's length via the alignment method?

Jeff

[ja17]

The nuts that hold the linkage support have tapered cone-shaped sides. They look like common lock nuts so a lot of people install them backwards with the cones-side outward. This is incorrect. Both of the centering nuts must be installed with the  cone shape side down so that it centers nicely in the pocket of the linkage bracket. In this way the linkage cross-rod support  is precisely positioned.

There is always the chance that someone has altered the bracket position on the injection pump?

[Flim]

Jeff

My bracket has countersunk holes too, and as Joe points out there are special cone shaped spacers that allow the bolts mounting them to be centred precisely in position. When I dismantled my linkages for my engine rebuild, I discovered two washers/spacers underneath the cross linkage bracket (not the cone shaped washers, those were correctly above the bracket under the retaining bolts) - Joe is this incorrect to find flat spacers underneath the bracket?

I have my linkages set up with FIP rod at 233mm between centres (as you have indicated in your picture) and not by the centring hole. Although it looks as if it aligns with the centring hole, the venturi control rod on the cross linkage does not align exactly with the centring hole.  I have taken another picture with FIP on its idle stop. As you can see, my alignment hole does not centre with the venturi cross linkage ball pivot line up as well. The position of mine appears to be further towards the back of the bracket compared to yours though….

Charles

[jedwards]

To help me determine what a good start point for my car should be, can someone tell me what the relative positions of the Venturi and FIP rods should be at maximum throttle?
By this I mean, if I disconnected all the linkages other than the Venturi and FIP rods and then rotate to cross over bar as far as it will go, which Rod is supposed to hit is stop first, and how much additional "travel" should there be for the other? I seem to recollect that the FIP rod should hit its stop and leave a few millimetres of potential travel for the Venturi, if it was free to move.

The reason I ask is that if I start with both rods on their respective stops, and known which should hit its end stop first, and by how much,  that should give me  a good idea of how far out mine might be.
Can anyone help?

I've also ordered a Gunson Exhaust Gas Analyser, which should be here the weekend after next, andy I am hoping that will help me "see" how the linkage rations are actually changing  through the various rev ranges.

[mbzse]

.../...My bracket had countersunk holes and I posted a question some time ago on this excellent forum about their importance and was told that we must should use special bolts like speed bolts with angled heads to guarantee perfect location between the  bracket and the manifold../...

I attach an image of this special type M8 hex head screw
/hans in Sweden

.

[jedwards]

Thanks Hans,
I used M8 speed bolts which serve the same alignment function, because I could not find any countr sunk Hex bolts.
Do you have washers or spacers under your bracket to raise the bracket up from the  intake manifold, like Joes has?

[jedwards]

Sorry, I forget to attach the image. Getting too old.

[mbzse]

../....Do you have washers or spacers under your bracket to raise the bracket up from the  intake manifold, like Joes has?

Nope, never seen those in any car. I presume they were installed by a PO
/Hans in Sweden

.

[Cees Klumper]

No washers or spacers underneath the bracket on my car, either. Sounds like someone tinkered with exactly the issue you are now dealing with.

[ja17]

Yes, I don't think the spacers should be there.  This may be throwing all your linkages off.

[jedwards]

Hi ja17,
I dont have any spacers on my car. That was a suggestion of another member.

Where I am currently is that I have 233mm on the FIP rod, 286mm on the venturi rod. At idle both sit on their stops and at full deflection the FIP hits its stop fractionally before the butterfly. I have 6 degrees of advance at idle and 38 at 3,000 so I think I have things set up as best I can. I have a Gunson exhaust analyzer on order  so hope be able to check for mixture ratios across the entire rpm range next weekend.

All this is in the hope of resolving why my car has 4.2% co at idle, but the moment I put it into gear (auto trans) and the CSS restores RPM, the co drops to 1.8% and runs rough.
I have my fingers crossed that the exhaust analyzer will help unearth the issue(s)

[ja17]

 You can also do the split linkage test with the car in gear at idle but, obviously, do not try it without having a helper hold the brake when the car is in gear. This should give you a mixture reading for your rough idle in gear.  Strive for a smooth idle in gear. This is when a good idle is most noticeable.

[jedwards]

Yes ja17, that is the ideal, but it is the one area I just cannot seem to get in my head.

My car runs perfectly at idle, perfectly under power and at revs, but rough under CSS (idle in gear).  I've checked valve clearances, timing, ignition, advance and now Ive done a rebuild of all the linkages.  So this points to lean mixture at low RPM in my mind.
Compounding this, is that the car does not behave this way when cold. When under WRD influence, idle under CSS and in gear is perfect. It just gets rough when hot.
What would you try?