Sticky throttle on 3.0CS

taylorcom

taylorcom

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Hi all,

I've had my '72 3.0CS coupe since '85 and from time to time as I drive, the throttle has refused to fall back to idle speed when I release the gas pedal. It tends to stick at the 2K-plus rpm level, UNLESS I quickly gun the motor to about 3K rpms. After I gun the motor, the rpm' s fall back to where they should be (around 1K at idle).

This symptom has occurred intermittently over the years and the solution has always been to spray carburator/throttle cleaner all over the carburator linkage.

Unfortunately, this easy fix has stopped working and I've been forced to attach a strong spring to the throttle linkage (on the driver's side of the motor) to keep the revs down. This solution works fairly well (I used the same technique on a '71 Triumph TR-6 that had a similar issue). But the spring makes the coupe's gas pedal very stiff and is not a permanent fix.

Does anyone have ideas on fixing this symptom, or any prior experience?

I'd rather not rebuild the carburetors, of course, unless I have to. The car runs smoothly, btw.

Thanks,

Steve
 
Steve,

You should be able to easily pop off both the horizontal linkage from firewall to spring plate on engine, the vertical linkage up to the main carb linkage, and each of the small carb linkages and then test/move each part of the system to find out where it is binding. Clean all the balls and cups while off and re-grease.

I think there is also a spring from the accelerator pedal arm to the steering column. Is that still there?

good luck,
 
Springs, etc.

Thanks, all.

Well ... the spring from the gas pedal to the steering column is in place (it's inside the driver's compartment).

I see no springs in the vicinity of the oil filter, tho. My '70 2800 CS doesn't have a spring near the filter, either, tho it has a smaller spring plate than the 3.0 CS does.

The 3.0 CS has a much larger spring plate (on the driver's side of the engine), with holes at each end thru which a spring could be attached ...

I've lubricated all the linkage pretty well, I think.

It's interesting that this seems to be a pretty common issue with the classic BMWs.
 
Sticky Carb Throttle linkage

The oil filter, top right bolt, should havea 1/2" x 2" rectangular sheetmetal by which a spring hooks on the bell crank ( plate that bolts on the driver side of the block connecting the throttle rods.) That plate also has a bushing behind it.

First would be to see if the dimensions of the main two rods are in spec and the longer one up to the first carb bracket is not slightly bent since it takes the brunt of the torque on over center.

Second- the rod connecting both carbs- set the lateral play( nearly zero) as the nylon bushes wear and the rod above wants to exert force upward rather than twist. The bushes are still available for $3. Look closely for the pin with ball and make sure it's stable; they tend to work loose and no they're not available. Repin the pin( ball) so to speak by restaking it in the bracket.


Now for the most common mistake on carb linkage- everyone grabs the back bracket when checking. Don't. It's opposite the side that operates. Press the accelerator and check for equal movement and use the back carb adjustment rod slightly as the front carb wants to move first which is opposite if you lift the back bracket and the back carb linkage gets adjusted the wrong way and sticks open slightly. There's a reason for the screw under the front bracket- geometry.



P.S. has to be done when sync'ing too.
 
Last edited:
No sheetmetal on filter bolt

This is good info.

I'm confused a bit, tho, because none of the oil filter's bolts have a 1/2" x 2" rectangular sheetmetal for a spring to hook on to. Also, the oil filter is on the driver's side of the engine, not passenger side.

I do see the nylon bushes, tho, and the rod.
 
Clarification

Yep, oil bracket on left or driver's side. My mistake- got in a hurry.

It's just a sheetmetal bracket with a 1/8" hole that catches one end of the return spring. Looks just like a short carb spring for a Holley, etc. The bolt allows the sheetmal piece to set the spring tension and hold it when tight.

I can stand with the door open and operate the accelerator pedal and check the carb linkage. That's how I knew what the manual didn't say when adjusting and why there's a wheel adjustment on the rear carb.

It's also a good way to see how far the throttle linkages open and the secondary linkage.

If your carbs are still hanging up- push down on each linkage at each carb. But usually, it's the main rod racking/ twisitng up on one side or the other.

The other thing is wear on the ball joints- sometimes you have to take that into account. But the rods should be tight in the closed/ return position just ever so slightly.
 
Throttle linkage cont'd

http://www.e9-driven.com/Public/Library/BMW-E9-Manual/pages/en/35410900.html#refertoc

Where rod A connects to the pivot bracket- there should be a small hole below the connection whereby the return spring hooks. The opposite end of the spring attaches to the sheet metal that the top right oil filter bracket bolt holds in place.

Ideally clean the pivot bracket, but in a pinch spray lubricant on the backside where the bushing is located. I just figured it hasn't been touched in years.

Rod A typically is bent a bit too and always check that the carbs are opening fully which is mainly dependent on the length of that rod.
 
Found it ...

Thanks for the great diagrams.

After fiddling around with throttle, and oiling everything (in place, not taken apart), I think I've found the problem area: the front carb is not closing fully when the gas pedal is released. The rear carb seems to be closing fine.

After doing a lot of lubrication, which improved things somewhat but not completely, I finally connected a lighter-gauge spring from the front end of the spring plate to a gizmo on the front carb.

This works better than before, but of course is not a complete solution.

It could be that the connection bar (E in the diagram) has, as you say, gotten bent with the years. That's hard to tell w/o removing it, I think.

Anyway, something is binding in the front-carb linkage, but not in the rear-carb linkage. The bar with the nylon fittings at each end seems to be tight, not loose.
 
Throttle rod adjustment

Turn the wheel on the back carb linkage; slowly, deliberately, and carefully. It acts as the fine tune balance of the rod and can take up a little slack on the front by preloading the back..
 
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