MFI Correlation Questions

[orange911]

First off, some details:
72 911T
Stock throttle bodies, stock cam, stock heads, and stock pump
Engine is approximately 90,000 miles on it.
No indication of any rebuild on the MFI

I have just followed about every thread I can find on throttle bodies and John and Grady posted half of the posts. So I felt pretty confident in following their advice about using a synchrometer (BK, not the desired SK) to make all 6 breathe equally. So here is my step by step approach with results:

Cleaned the bejesus out of those dirty nasty scoundrels with BK-12 ad elbow grease. I could not get the cleaning tools in certain areas, but I got 95% or more of it removed. I used a miniature wire brush on a dremel to remove the sticky carbon inside the throats. Throttle number (cylinder) 3 was corroded at the head side ad had some internal pitting as well. More on that later. All others were in good shape, otherwise.

The brass vacuum ports were completely clogged with carbon. I had to drill it all out ad rinse with a little B-12

I pulled the air balance screws all the way out and polished up the tips and ran a drill bit by hand through the ports. Not too bad, so I think this could have been gone through by my mechanic. The only thing odd was that the screws were really tight and difficult to remove, so maybe not. I also cleaned them from the inside out, ad rinsed with B-12. I also realized that they were backed out way more than the limited 8 half turns; or was that 8 full turns. The other thing I noticed that cylinder #1 had a much longer spring, preventing closing the bypass screw as far as the others. No problem though, I just didn't have to open it far to get to 3.

I loosened the lock nuts on the throttle plates and backed the set screws all the way out until the stop no longer touched the linkage and the plate was touching the throat. I then used the vacuum attached on the bottom and the synchro on top and adjusted the set screw until the plates opened enough to change the flow. I felt this was less subjective than the touchy feely way. The bypass screws were closed during this test and the synchro readings for the break points ranged from 1.5 to 3.5 kg/hr. All but one was 2.5 or less, only one port, throttle #3, was 3.5. This bothered me knowing that I would not be able to get to 3.

Since I wanted to shoot for 3, I stuck with that number and did not adjust the 3.5 one with the bypass screw. I fear that if I change all to 3.5, then the idle may be too high. After I checked and triple checked, I was confident that 5 were at 3, and one was at 3.5.

Next, I pulled all the linkages, cleaned the cups and put in a light oil (trans oil) to set the lengths correctly. Rechecked the flow and all was still the same. I noticed when I was adjusting these, that if I went too far either way, it would open one throttle plate, not both. With the threads well lubed, I was able to feel the "in-between" point where both were closed.

Now I need to put grease in the cups, order some new gaskets and decide from your input if I need to change them all to 3.5 kg/hr or leave what I have. How much does it matter on the one cylinder (#3) has 17% more air flow at idle? I would expect this number to converge to a much lower percentage at WOT. Neither of which I spend a lot of time with.

Another question is, What else should I do while all this stuff is out before I put it back on? I am planing to clean the shroud. I thought about taking the pump off and changing the belt and gaskets, not knowing if they are leaking or not. But this seems like more work than I need to do since the belt was changed 3 years (5, 000 miles ago). How much of an opportunity am I giving up here.

Special thanks to John and Grady for all of your previous posts.

[orange911]

Just read this article by Lee Rice that I found on the MFI Tech article here on Pelica:
http://www.pelicanparts.com/techarticles/911_MFI/About_MFI_Lee_Rice.pdf

It indicated that you refine the air adjustment with the butterflies, not the air bypass screws, after you set the initial butterfly position. This contradicts what I read and did. Comments/feedback please.

[Zeke]

I don't think one "refines" the air adjustment with the butterflies. They should be set just off touching the bodies. There are 2 good reasons I can think of why this is important. One, they tend to get slightly stuck making just off idle foot control difficult and the second is they tend to bounce because of stand off and wear into the soft throttle body shaft. Having them adjusted just off tight will prevent both. The air adjustment screws are to 'trim' the air flow for equal flow. Worn butterfly shaft bushings can make all of this a nightmare.

Now to address the rest of your question, setting the length of the multiple linkages. After setting the pump linkage length, you should set the linkages between the throttle shafts so that any slack is in favor of all opening at the same time. This takes a little observation. You want to set the lenght just so they are snug against the springs. Then go to the push rods and set them the same way.

There is no way your rod ends are in perfect condition. There will be slop however slight that it may seem. If you can, get the system more balanced for good idle characteristics.

I'd have to agree with you that WOT settings are not super critical as long as everything is opening up properly.

[orange911]

Milt, thanks for chiming in. So I assume that I have made my adjustments correctly and should contiue to plan on refining with bypass screws. Any idea on the differences of the air flow readings for throttle number 3? Leave it or adjust all others. I have time to decide since I just ordered the gaskets tonight from our host and will get them in time for Friday Night Lights.

Didn't get any responses for "While you're in there", so I decided to button it up the way I took it off.

I must say this was a very fun learning project and I was excited to see the tightness of the throttle bodies until I got to #3 - and it was THE LAST ONE I CHECKED. It really let a lot of air out of my balloon, but I should have expected worse.

[RSTarga]

I thought one balanced the stacks at higher rpm?2-3000rpm

[orange911]

3000 rpm by the CMA, but I figured that you needed the plates/screws balanced for idle also.