Last-minute MFI questions

[304065]

Well, I finally finished working my way through Check Measure Adjust, can anybody help with the following questions?

1) How do I adjust the throttle plates on cylinders 2 and 3? I tried a crow's foot wrench on a ratchet extension, it's very hard to get it in there. Has anyone improvised a tool to do this? I can't get the synchronization set correctly without doing so.

2) WHY is my IDLE at 2000 RPM? I must have the throttle plates open too far, right? When the engine is cold it idles around 650, but as soon as the thermostat heats up the idle kicks up to 2000+. This suggests to me that the idle mixture is way too rich, so when the thermostat discs expand and move the rod, leaning the mixture out, the idle speed increases. But the fact that the engine is idling at 2000 tells me I have the throttle plates open too far when the throttle pedal is released/all the way up.

3) You fellas with Gunson Gastesters, what RPM do you set the part-load mixture at? An on-road test is impractical so I'm hoping to set it at an elevated throttle setting, will that work at all?

THANKS in advance, Lime Rock Club race is this Friday/Saturday!

[jluetjen]

John;
Regarding item 2, have you confirmed that the first linkage rod is exactly 114 mm long? (If you don't know what I'm talking about, then you didn't check it!, If you checked it you'd know exactly what I'm talking about) Lengthening it will slow down the idle, shortening it will speed it up. I just played around with mine this past weekend and the difference of a couple of mm's can easily make a big difference in the idle speed.

John

[304065]

John:

114.0 +/- .2mm, it's right on.

The funny thing is, it was originally MUCH longer when I started CMA, like around 120mm. I had the same idle speed problem, however.

If the throttle rod is longer that makes the mixture richer. So if it's in tune properly at idle and then you richen it up, that should slow the idle, I agree.

I think I've got the throttle plates open too far: that, coupled with the excessively rich mixture, is causing the motor to idle at 2000, I think.

[72doug2,2S]

I have the same problem on mine. (Sorry, haven't CMA'd yet) That's the way my wrench set it up. He was adjusting the distributor while it was running and said he couldn't get it to idle cold so he adjusted the distributor so it would idle 650 cold and 1500 hot. Just a guess, but if you loosen and adjust your distributor slightly one way or another could you solve this problem.

[Grady Clay]

John C,

Yes, it takes some creative tools. Search on “MFI Tools” I think someone posted pictures there. Moreover, these may not come loose without having been loosened, cleaned and lubed with the throttle body(s) off the engine.


Once it is warm and idling too fast, rap it a couple of times and disconnect the two side links. Tell us what happens.

Are all the throttles on their stops?
Can you push the throttles farther closed?

With the links still off, slightly open the right side using the center link. It should pop but not speed up much. Repeat for left side.

With the side links on and balanced, pop the pump link off. What happens?
edit (omit: Slightly move the pump link.) Add: “With the ball socket centered on the ball, slightly move the pump link both forward and rearward from its normal position.” What happens?

If you feel you must, put the links back on and rap it or drive it. Repeat above process.

You are trying to tell if it is a mixture issue or the throttles are passing too much air. Each throttle body should have the same response.

Best,
Grady

[derek murray]

Originally posted by john_cramer

3) You fellas with Gunson Gastesters, what RPM do you set the part-load mixture at? An on-road test is impractical so I'm hoping to set it at an elevated throttle setting, will that work at all

John,

I have been measuring my part load at 3000 rpm (sitting in the driveway), versus the 2400 specified in CMA. Having said that, I have absolutely no reason for picking that number (felt right, I guess)... but my engine runs very well...

[jluetjen]

Doug;
Tweaking the distributor is moving the ignition timing across the board -- a fairly blunt tool. If that's all that you're wrench could come up with I'd get another one because he's not fixing the problem. I recently "bumped" my ignition timing back (retarded) almost imeasurably because my engine was pinging on anything less then 94 octane. When I checked it it was slightly advanced from the mark. I slightly retarded it (which slowed down the idle even more) and now it runs happily on 93 octane. I've had to retweak the MFI settings as a result which I've just about got dialed in. While I don't have the luxuary of an exhaust gas analyser like Cramer, just using my "seat of the pants" meter and some methodical adjustments has allowed me to dial it back in. It's just about spot on again.

[304065]

Interesting. I'm actually going to bump the other way, following Grady's recommendation in another thread, to try and get about 35 degrees total advance at 6000. (Will have to use a mixture of 93 pump and 100 leaded aviation gas, however.)

The spec calls for up to 4 degrees retard at idle, but I'm running about straight-up, that might be part of the problem, but I'd much prefer the advance at 6000 and the idle wherever it wants.

Grady, will follow those suggestions exactly and report back.

[Grady Clay]

John,

Where is your timing at 6000 set now?
What sparkplugs are you using?


In my list of questions above, I said:
“Slightly move the pump link. What happens?”

I would like to clarify one sentence:
“With the ball socket centered on the ball, slightly move the pump link both forward and rearward from its normal position. What happens?”

The general principal here is if the mixture if approximately correct, when you force the system to go lean, it will pop in the intake and/or exhaust. If one side is too rich, the other side will pop. If everything is too rich, it will pick up speed before it pops.
This is a good way to easily experience the “feel” of the mixture settings without resetting anything.

Best,
Grady

[jluetjen]

John (Cramer);
I retarded my ignition because I got tired of the hassle and expense of mixing toulene or 100 octane into the tank. The 100 octane is an easy mix, but there's only one station around here selling it and it's kind of out of my way. Toulene I've confirmed (as others have pointed out) needs to be premixed with about 5 gallons of gas before you pour it into the tank. Otherwise it sits at the bottom and you have quarter of a tank of 100 octane, and the balance of 93.

[304065]

Grady,

Timing is set at 30 degrees total advance at 6000 rpm. Spec book says 29-31 degrees.

Plugs are NGK BP7ES. With the NGK's, lower is hotter. I think I have some BP8ES in the spares box, these colder plugs might be better. BP8ES was stock for the 911E and S per the factory.

I understand your point about moving the pump link with the ball to the cross-shaft disconnected.

As it stands now, when the throttle plates are closed and the cross-shaft is at the stop, the pump link is exactly the correct length to fit between the ball on the cross-shaft and the ball on the pump lever, when the lever is all the way toward the rear of the car, e.g. at the "idle" setting. This would suggest that the pump is at the leanest end of its operating range (pushing the pump rod toward the front of the car richens the mixture).

I'll incorporate your suggestions into my tuning procedure.

John, thanks for the suggestion on Toluene. I have the benefit of 100LL Avgas less than a mile away so I'm sticking with that for now. It is only about a quarter a gallon more expensive than pump gas!

[Grady Clay]

John,

It sounds like you are pretty close. On track, err on the rich side even though the lean limit gives slightly more power. You don’t want to bring home a wounded Pelican. The plugs and exhaust should be a light gray when you come off track.

Are you driving to the event or trailer?

Yes, on track use the colder BP8ES (I use these in warm weather.) You might bring a couple of spare sets, ready to install. Perhaps even one set of BP9ES (usually too cold for your engine.) If it is raining or drizzling at 100% humidity you can use the BP7ES.
If you have a cool, high atmospheric pressure day you might not want to run the full 35 degrees. Do you know how to inspect a sparkplug for signs of high speed (not audible) detonation? The plug color changes slightly to a “gun-metal gray” color and there are signs of micro spherical balls on the perimeter of the plug and on the insulator. With my lame eyes, I need a Radio Shack hand microscope these days.

Best,
Grady

[72doug2,2S]

John,

You're right on about getting a new wrench. As much as I want to be able to do this on my own, can anyone recommend an MFI guy?

[Grady Clay]

Doug,

John is possibly right about your mechanic. Giving the mechanic the benefit of the doubt, he may have just found a reasonable compromise without spending another five hours labor on your nickel.

Don’t worry about your cold idle. That is what the hand throttle is for. Spend the effort on warm-hot idle and the transition to just off idle.

A critical issue for proper running is the timing at 6000 RPM. Get that right and see what the static and idle timing is. The distributor advance mechanism must function correctly.

Don’t sell yourself short. With the help of all on this Forum and some fun tools you too can DIY.

Best,
Grady