Idle Question - MFI

Ducman · 08-22-2011, 06:22 AM

Not sure this is going to be enough info - but I'll start here. First off complete race engine so no factory setting stuff applies. MFI race conversion with none of the street aids, twin plug distributor. Engine is pretty well tuned except:

Peculiar problem is that when allowed to come to an idle off of higher RPM it will stabilize at around 3000 RPM (no load) and run there all day. If I apply a small load (in gear, foot on brake, let out clutch a little) idle will drop to a beautiful 700 RPM tick and stay there all day as well. It is not an issue of the throttle linkage, in all cases it is not moving any further when a load is applied.

Any thoughts?

Thanks,
Don

flyingjay · flyingjay's Garage

Check to make sure that you're microswitch and mfi realy are working properly. When you let off of the throttle it closes the switch and will energize the fuel cutoff solenoid anywhere over 1500 rpm and will turn back on around 1000 rpm. That should help resolve your issue.

Ducman · 08-23-2011, 08:44 AM

Quote:

Originally Posted by flyingjay

Check to make sure that you're microswitch and mfi realy are working properly. When you let off of the throttle it closes the switch and will energize the fuel cutoff solenoid anywhere over 1500 rpm and will turn back on around 1000 rpm. That should help resolve your issue.

As mentioned - this is a race conversion. There is no microswitch. Thanks though

Don

Flieger · Flieger's Garage

Since your pump is stripped of all the devices meant to smooth out the running I think you will just have to live with the quirks. Or else ask Steve.

stownsen914 · 11:20 AM

Check the throttle bodies. If the shafts or bushings are worn, the throttle plates may not close all the way, or do so inconsistently. Not sure why a load would change that, but maybe worth checking. You could using a carb synchrometer to verify if the airflow changes for the two idle conditions you mention, if that is somehow possible.

I find the idle varies somewhat on my MFI race engine too. Mine tops out at 2000 RPM sometimes, so I just don't worry about it.

Scott

Jeff Higgins · 08-23-2011, 11:47 AM

If you have loaded it down to achieve the 700 rpm idle, then run it back up to 3,000 or more, does it then stay at that higher idle once again? Or, once it has settled back down to that 700 rpm idle, will it stay there even if you "blip" it, or run it up over 3,000?

With a twin plug distributor on a race car, I'll bet full advance comes in fairly quickly, like around 2,000-2,500 rpms. So, your high idle is occuring above the point at which the ignition would retard back to its idle setting. The full advance condition will keep the idle up a bit, but it shouldn't keep it up that much.

How much do you know about the MFI pump? It could be adding to the high idle by delivering more fuel than needed, making the motor rev a bit higher, which will begin to deflect the flyweights, adding even more fuel.

It may get into sort of an "upward spiral" of increasing advance and fuel delivery once it gets past a certain rpm. Below that threshold, it has not begun to advance the ignition and splay out the flyweights. Just my wild-assed guess...

Flieger · Flieger's Garage

His engine => The best engine Porsche never built

Ducman · 08-24-2011, 01:26 PM

Quote:

Originally Posted by Jeff Higgins

If you have loaded it down to achieve the 700 rpm idle, then run it back up to 3,000 or more, does it then stay at that higher idle once again? Or, once it has settled back down to that 700 rpm idle, will it stay there even if you "blip" it, or run it up over 3,000?

With a twin plug distributor on a race car, I'll bet full advance comes in fairly quickly, like around 2,000-2,500 rpms. So, your high idle is occuring above the point at which the ignition would retard back to its idle setting. The full advance condition will keep the idle up a bit, but it shouldn't keep it up that much.

Jeff,
Blipping: it will settle back down to 700 unless it is blipped above 3000. If blipped above 3000 it will settle back to the 3000 level.

Your theory on advance is interesting. I have also considered the spiralling up theory regarding the pump fly weights initiating increased fuel supply....problem is that both of these would suggest that the airflow at the 700rpm low idle is also enough to sustain 3000 rpm with only the addition of fuel or advance. Because the thottle body butterflies are most definitely not moving and at full closed position during both conditions. Well at least not the linkage - the plates themselves could be moving relative to linkage due to wear - but inspection does not show this and unlikely due to very low hours on complete rebuild.

Thanks,
Keep thinking

Don

Jeff Higgins · 08-25-2011, 10:42 AM

Looked at another way, maybe the air flow is sufficient to sustain a 3,000 rpm idle, but it settles down all the way to 700 rpm for a lack of fuel delivery.

It would be interesting to have both an exhaust gas analyzer and a timing light on it to see what it is doing at 700 and 3,000 rpm's.

How far out are the idle air screws? I know this is impossible to check while they are on the car, but are the butterflies really fully closing? Just because they rest on the stop screws does not mean they are fully "closed". It takes very little throttle opening to run an engine up to 3,000 rpm with no load on it.

How far out is the idle mixture screw, the one with the plunger through the nose of the governor assembly (accessable through the fan housing and the hole in she shroud)? You should be able to turn it in until it bottoms, counting clicks as you go, to see where it is at. It's a hex head screw behind the plunger, so six clicks to a full turn. If I recall correctly, there are no more than about six or seven turns between bottomed out and falling out, so don't go too far if you decide to try backing it out.

Maybe fiddling with the air bleed screws for less air, and the idle mixture screw for a bit more delivery at the desired idle, will keep it from reving up so much.

Ducman · 08-26-2011, 05:34 PM

Hey Jeff, wouldn't an increase in rpm without throttle input (for any reason)actually cause less fuel per stroke....due to fly weight translating cam but no rotation of cam?

Maybe it's actually too rich at idle. AFR doesn't indicate this but I don't really trust low rpm readings as I'm using a sensor stuck up the tailpipe.

Don