High Idle

[72targa]

We balanced the airflow in my MFI stacks and checked the timing in my '72. We did not adjust the valves. We cannot get the idle below ~1,100 RPM. What gives?

[Chuck Miller]

Peter,

You didn't mention if this was a new symptom after the timing was set and stacks balanced … or a symptom that was already there that you were trying to cure with your adjustments …

Anyway,

I'd check if ALL the linkage points are on there stops, check if there's something up with the hand throttle, and check if the vacuum advance and hose are all functioning properly at and in the distributor…

Hope this helps,

[72targa]

It was there before. I'll try your suggestions first!

[Zeke]

Very complicated issues at work here. But, it's all mechancal rather than hidden in some micro circuit. There is a lot of info beyond the CMA:
http://www.pelicanparts.com/techarti...MFI/TipMFI.htm
A couple of people have striven to collect a comprehensive list of thread and other info on the MFI over on Pelican. Maybe youi've seen some of the comments by Grady Clay.

In a nutshell, you probably have a problem somewhere in the cold start system. Off hand, I can't list the 2-3 sensors that are in series to let the MFI know to drop the idle. I had this problem too. If I got close to proper adjustment, I got the idle surge syndrome. My biggest problem was a tired engine and throttle bodies that needed rebuilding. You can't overcome those deficiencies.

But, there are simple (depending on your skill level and I'd rather pound on a bent fender than do these) tests for the various components to see if replacement is in order before you start fine tuning.

[72targa]

Looks like it's time to "go to school"...thanks for all the help so far!

[spydr32000]

As Chuck suggested, make sure your distributor is working correctly, and is not sticking in the advance mechanism. Also be sure the vaccuum hoses for the distributor are clear. These engines are VERY sensitive to the distributor!

Also do all the normal checks as Zeke mentioned along with checking your butterfly shafts and air bleed screws for the idle speed. The air bleed screws tend to block up over time.

[72targa]

As Chuck suggested, make sure your distributor is working correctly, and is not sticking in the advance mechanism.

The air bleed screws were just cleaned (thanks for the confirmation!). As a potentially dumb question, but since it may help others besides me, how does one "make sure your distributor is working correctly, and is not sticking in the advance mechanism"?

[304065]

Ok, so it sounds like you have been following check measure adjust. That's good, you can't reliably diagnose anything without it. Remember. . .

Any
Competent
Porsche
Driver
Tracks
Faster
If
Brakes
Come
Smoothly

Air filter
Compression test
Plugs
Dwell
Timing
Fuel pressure
Injectors
Belt (MFI pump timing)
Correlation
Smog


Now, that being said, you have balanced all the stacks to the same synchrometer reading. I assume you are using an STE BK synchrometer. What Kg/Hr reading do you have at 1,100? 3,000? The reason I ask is that if you are showing 11 Kg/hr at 1,100 RPM, something is way off.

The idle can be too high due to one of two reasons only. These are:

1. Too much ignition advance at low engine speed. When you say you checked the igntion, what did the curve look like? What was the advance at 1,100?

2. Too much airflow. Have you had the throttle bodies off lately for an inspection? When you hold them up to a light, how much light can you see around the plates when they are closed? Have you adjusted the throttle stops at all from their original positions? Carboned-up air correctors could be the cause of your problem if a hunk of carbon is keeping you from closing the screw all the way down, but in my experience the air correctors are for very fine adjustment, e.g. going from 5.0 -> 5.5 Kg/Hr, not 5 to 10. If you have had them off, were the gaskets replaced, both between the head and the TB and the TB and the stack? A missing or damaged gasket can cause an air leak in one that requires compensation with the other five.

Anyway, I would start with the advance and if that's not the problem, try making it idle with all the throttle rods removed. If the idle falls to 900 or 1000 even, that's your indication that not all the preload has been removed from the linkages, which is easy to remedy.

Zeke, the microswitch/speed switch combo only applies when the engine is above 1800 RPM and the throttle is closed e.g. microswitch circuit closed. That will signal the speedswitch to fire the solenoid, moving the rack and cutting off the fuel. For pure idle measurement it will not be a factor.

More fun stuff here: http://forums.pelicanparts.com/showt...ct+correlation

[304065]

More fun with the distributor. My example is an SWB car but the principles are the same. Make sure you have the right curve for your engine, it's in the factory manual.

http://forums.pelicanparts.com/showt...0&pagenumber=1