New PMO's - Touchy Throttle Response

[OZCarrera3]

Hi,

Well I finally solved my CIS issues - I bought some PMOs.

What a transformation! The car drives absolutely beautifully smooth at any speed and has absolutely fantastic throttle response (maybe too much as we'll see) and the looks and sound of these things is amazing!



Actually, the new found throttle response leads to some of this...!



Now to the problem. The actuation of the throttle is not linear. I seem to get most of the open throttle within the first 1/4-1/2 throttle and then it seems to taper off for the rest. This makes small throttle adjustments and getting back on the gas at the mid-to-exit of corners on the track very, very difficult. It tends to make the car 'jump' forward, which can make things interesting as you can imagine on the track.

I can't see to adjust the throttle linkages to make it more linear. Is this normal for PMOs or Carbs of any sort? Yes, I've done some searching, but no luck.

Oh, and I do also have the 'correct' early car bell crank linkage fitted on the gearbox as well.

Can anyone offer any suggestions?

[timmy2]

I would look into the jetting sizes, sounds like it's providing enough fuel as you apply 1/2 throttle but cannot continue to supply the full demand when you open her up.

[JP911]

If you haven't already, check the left-right balance at a range of RPMs using a synchrometer. It's possible to have even vacuum at part throttle and then have it go uneven as you open it up. If it's off, you need to make gradual adjustments to the position of the cross bar.

As far as throttle sensitivity, left-right balance could have something to do with it, but one way to decrease the sensitivity a bit is to add another spring into the linkage. The easiest place is at the bell crank on the trans. You can use a small piece of strap steel under one of the nuts on the side of the trans and then add a spring between the strap and the bell crank.

[Joe Bob]

Proper set up ensures proper operation. Sync test and jetting as discussed above.

[lonewolf]

YIKES! Is that one of those" sure to leave you stranded somewhere" silver coils?
The silver matches everything nicely but those coils are prone to failure without warning. Get the black bosch one for your car.
just a suggestion.

[RWebb]

it might

BTW - is the throttle link arm the same for carbs as for CIS? IIRC, they are different. I posted some comparison pics here long ago (if you can find them)....

[rfuerst911sc]

I have 46mm PMO's on my 83SC and the throttle response is very good and linear. Sounds like you have some fine tuning/adjustments to make. Once set correctly you rarely have to touch them. Good luck.

[VFR750]

I needed to play with the linkage to get it right. I think that is your problem too. I had to change the link at the pedal to be able to change all the other link lengths to get it right.

[OZCarrera3]

Originally Posted by VFR750

I had to change the link at the pedal to be able to change all the other link lengths to get it right.

Do you mean adjust or did you actually replace and change something.

I think the tuning may still be the problem as mentioned by a few people above or at least part as the short rods on each carb are quite different leangths.

[VFR750]

I changed the lenght of the long rod in the center tunnel near the pedal. This changes the angle of the bellcrank on the transmission AND the carb mounted bellcrank. Those angles affect how much a given pedal motion moves the links at the carbs. Its a system. So if someone sits in the car and presses the pedal you can watch what happens. Both butterflies should move in unison. Adjusting all links, mounts, and rod ends to get this is your goal.

Once that is done, then worry about jetting....

[Tippy]

Wonder if you are lean?

[lonewolf]

Originally Posted by OZCarrera3

Do you mean adjust or did you actually replace and change something.

I think the tuning may still be the problem as mentioned by a few people above or at least part as the short rods on each carb are quite different lengths.

Yes the rods at different lengths needs to be fixed.
It takes some time to properly set up the linkages. There can be no play in the ball/socket on any linkage. The way the crossbar is mounted to the manifolds is important.
The triangle part of the bell crank on the left carb manifold should be level. adjust the long rod that attaches to crossbar from bellcrank and the two short rods on the carbs all together .The crossbar brackets to manifold bolts should be snug but not tight.You may have to tap on one of the brackets to get all the ball/sockets snug.
Also to start, disconnect the rods from the carbs and make sure both idle screws are set so butterflies are just slightly open equally.Car should idle fine with rods disconnected .
work on the manifold bellcrank to carb linkage first to get full opening and then work on the gas pedal to bellcrank on manifold.if needed.

When you ordered the carbs I'll assume you told them your engine size and would think they were jetted accordingly. That said I think your problem is just in the setup.

Once your linkage is done a proper tune with a syncrometer must be done if not done already.

[dipso]

There are 2 mounting holes on the crossbar for the male ball socket pin. Looks like you're in the lower of the 2. The lower one will pull faster and easier. The upper one will pull a little slower and might give you more travel.
Maybe you are not getting full throttle. I think mine is in the upper of the 2 positions.

[1QuickS]

Since the PMOs are new the drop links and 8mm ball studs on the throttle arms should not have wear issues which would further cause trouble in setting side-to-side air flow balance. Therefore you could remove both drop links and adjust them to have the same overall length (dial calipers would work well here or be creative in establishing a comparative gauge setup) and this would be the first step in adjusting side-to-side balance. The second step would be to remove the cross bar and set the tube level on a flat surface, by level I mean parallel and this may be achieved by setting the tube ends to rest on a couple of hex nuts of the same hex size; the flat surface needs to be flat like a table saw top or a glass coffee table top. The continuing of the second step is to check to see that the two 8mm ball studs at the ends of the stamped lever arms are equidistant from the same flat surface the cross tube is resting on. You can gauge this with another pair of hex nuts of the same size, rest one of the ball studs on one hex nut and check for clearance/interference with the ball stud with the second hex nut. The idea is to have the 8mm ball studs coplanar with the axis of the cross tube. Correcting the alignment is easy, just take an adjustable wrench, set the jaws to grip the end of the lever arm just behind the 8mm ball stud and twist the lever arm about its long axis (easy way to twist it) and recheck alignment as before. The third step is to install all components and nominally set drop links to just slip onto the 8mm ball studs on the PMO throttle lever arms and tighten the end support brackets to hold everything in place. Start the engine and using either the bell crank at the forward, driver's side intake manifold or an assistant to operate the throttle pedal; adjust engine RPM to 3000 and check air flow for side-to-side balance. Use the side support brackets as the method for setting flow balance; the bolt hole clearance will allow a fairly large amount of adjustment.

If you have different length drop links, or alignment errors in your cross bar or operate the throttles in a fashion different than how they are actuated while driving then you will have unequal throttle arm rotations for a given throttle pedal position.

[OZCarrera3]

Hi guys,

Thanks for the responses. Based on the above I decided to let someone someone who knows what they are doing have a look at the car.

There were some carbie issues (idle mixtures and balance), but the biggest change from what he told me was the linkages. He ended up drilling a new hole between the two existing ones on the bellcrank at the back of the carbs (near where the linkage comes up through the tinware) and he also put a return spring on the bellcrank on the gearbox. Now the accelerator (gas) pedal travel is linear and perfectly weighted. The car is an absolute dream to drive.

PMO carbs may not be as 'efficient' as ITBs, but they are cheaper and do a great job all the same. I am one very happy 911 owner now. Especially after 3yrs of pain with the CIS and bills totalling nearly double the cost of the PMOs!

[Fishcop]

Great to hear Sheldon. When can I borrow it

[Vin-barrett]

Originally Posted by OZCarrera3

He ended up drilling a new hole between the two existing ones on the bellcrank at the back of the carbs (near where the linkage comes up through the tinware) and he also put a return spring on the bellcrank on the gearbox.

Hi Sheldon

Could you be kind enough to post a picture of your modified bellcrank and new return spring?
This would be really helpful for me as I am about to do this as well

Thanks
Vinny

[OZCarrera3]

Sure John, any time! :-)

Will get some photos over the weekend Vinny

[OZCarrera3]

Vinny, as promised, here are the photos of the linkage modifications.

First the engine bay bellcrank. This was simply done by adding a third hole between the two provided by PMO. PMO advise to use the bottom hole for the "tall manifolds" that my car has. Trouble is, neither the mechanic who fixed the linkages, nor I could adjust the linkages adequately to get a full throttle opening on this setting. You can jhust see the bottom (original) hole in this pic...



As for the gearbox bellcrank. He added an extra helper spring on the gearbox by drilling a small hole in the cooling fin and then adding the spring. Simple, but it works amazingly well. The throttle now has perfect weight, as it was a bit too heavy/dead feeling before.



I hope these photos help. As I said earlier, the car is a revelation to drive now in comparison to how it drove with the CIS. In years to come when C3s gain a bit value I will have the CIS completely rebuilt from top-to-bottom, but at this stage I am more than happy to live with the PMOs and will be a very strong advocate for them.

[Vin-barrett]

Many thanks Sheldon.. Excellent work!