Throttles won't open fully. Why?

[Formerly Steve Wilkinson]

With new PMO carburetors installed on my modified '83 SC, I can't seem to get the throttle butterflies to open more than about 75-80 percent of full, no matter how I set the rods, cables and bellcranks between the accelerator and the carbs' actuating arms. I suspect I'm doing something very basic wrong, and that the positions of the two main bellcranks (one on the left side of the transmission housing and the other atop the engine just aft of the firewall) might be the key.

With the throttle fully closed, the two arms of the bellcrank attached to transmission are like the hands of a clock indicating ten after five. With the accelerator floored, they go to the "twenty after seven" position.

The bellcrank atop the engine is an inverted triangle with the ball ends of the linkage attaching to the two top corners. With the throttle fully closed, the triangle is leaning slightly toward the rear of the car. With the accelerator floored, the triangle is leaning strongly forward, its top edge exactly parallel to the vector of the rod leading down to the transmission bellcrank.

Does this make any sense? Can anybody visualize it? No matter what I do, and that includes cutting short the plastic leg under the accelerator pedal that serves as a stop, I can't get the butterflies to open fully. Help! There are about 40 hp hiding from me.

Stephan

[T Bird]

Stephan
Just a thought- how about changing the overall length of the accelerator rod linkage? I would think that you would need to lengthen the distance between the pedal and the accelerator lever and if necessary, shorten the length between the accelerator lever and the bellcrank in the engine compartment. If that makes sense, the first thing to do is adjust the length of the push rod that connects the pedal to the accelerator lever. In the case of the push rod, I would loosen the lock nut and thread the ball cup OUT to lengthen the rod. This should provide some additional pedal travel (i.e., move the pedal away from the floor board).

If that's not enough, I would next move to the ball cup that connects the accelerator lever to the accelerator rod that runs through the tunnel. There, loosen the lock nut and turn the ball cup IN as far as it can go, to shorten the overall length of the accelerator rod. If this doesn't provide enough travel, go to the connection under the car between the accelerator rod and the rod link, loosen the lock nut and turn the rod link IN all the way. Make similar adjustments, if needed, to the ball cup on the rod extension that connects the link to the transaxle bellcrank, and the ball cup that connects the accelerator pull rod to the bell crank in the engine compartment. I doubt that you will have to make all those adjustments, but you get the idea. There are a lot of places where you can make minor adjustments to the overall length of the accelerator linkage.

Other thoughts- if shortening is the ticket but the sum of these adjustments still provides insufficient pedal travel, you could shorten the rod extension (between accelerator link and transaxle bellcrank) by cutting off whatever length you deem appropriate and rethreading the end.

If all else fails, try speaking to Richard Parr at PMO. Over the years, he has always gladly helped me with cogent advice whenever I've had a problem with my 40 IDA 3Cs that I couldn't sort out myself.

Good luck.
John

[Ned nyna 11]

Stephan,
I had the same condition in my 87 Carrera. Took some real artistry to pop the ball joint down at the top of the engine. A flat bladed screwdriver did it.
I dinked with every part of the linkage to get it as good as possible, still not wide open when it is supposed to be that way.
Further complexities from a limit switch that opens at idle to energise the idle speed control doodad. A right pain adjusting that as the only way I could do it was to change out the 2 phillips head screws for little hex heads.
If you decide to lengthen or shorten the linkage to get the throttle to open, be quite sure you are adjusting in the right direction before starting.
bring it over on a 'warm' day next week if you wish and we can look at it in the garage, no heat there, see above under weather.
Merry Christmas, Ned Monaghan

[911_Dude]

I had the same problem when I switched to Webers on my 2.7. I did 3 things to get the throttles wide open:
- Screwed the accel pedal stop (plastic screw on floor) almost all the way in
- Ran the ball joint link on the accel pedal in (shorten) all the way. These 2 things give the pedal max travel from full up to full down.
- Next, where the linkage comes up from the front of the engine to the big cross linkage, I drilled a new hole for the ball stub about 5-6mm in toward the center on the cross link.
This is working well. Just make sure you adjust the pedal stop, so you dont try to open the throttles too much. Excessive stress on the linkage.

[Formerly Steve Wilkinson]

911Dude, wouldn't shortening "the ball joint link on the accel pedal" reduce the distance the accelerator pedal would travel? I'm not sure I understand. I should think you'd want to lengthen that link. (I'm assuming you're talking about the short one that runs from the pedal itself to the bellcrank that's part of the pedal cluster.)

Richard Parr at PMO also recommended drilling a new hole in "the big cross linkage," as you did.

As for the accelerator-pedal stop, my '83 SC simply has a plastic "leg" as an integral part of the bottom of pedal itself--no adjustable stop screw. I've hacksawed about half an inch of it already.

[911_Dude]

I just fixed the linkage this summer and Im fairly certain I shortened it. If you shorten the total length of the linkage with the carbs at idle stop the only thing left to give is for the gas pedal to raise up. You then get more "down" travel on the pedal in order to open the throttles more.
This would make it seem like you could just shorten all the links and be done with it, but at the bell crank on the transmission Im fairly certain the geometry doesn't allow it. Cant remember why.
Hope this helps.
And hope everyone gets some new Porsche toys/tools/parts for xmas!

[J P Stein]

I would start at the carbs and work my way forward.
Disconnect the crossbar to bell crank rod, rotate the crossbar and see if the butterflies open all the way. Clamp it there. Put a brick or sumthin' on the pedal. Does the aformentioned rod end reach it's desired ball? If not (and I doubt it will), this rod and probably the next rod upstream are too short. Should be about a 1/4 inch(WAG) total.

My linkage (914/6) is a bit different than yours, so I may be way off base here. I did my best "visualization" and came up with the above.

HTH

[Thomas Owen]

On the bellcranks you want to move the actuating rod (from pedal) pivot closer to the bellcrank axis and the rods going out to the carbs further from the crank axis for maximum travel/unit moved by actuating rod.
Good luck