Linkage "stop" for MFI?

[luckydynes]

Hi all,

Been going over the engine on my '73 911s that had a 3.0L conversion at some point in it's life. Should there be a stop on the main "throttle" bar that crosses between the two intake stacks? There is a screw and locknut on the bell crank that operates the left throttle body assembly that look as though they should contact a stop. There is also a brass insert in the left intake stack right by the needle bearing that nothing is screwed into.

Thanks,

Sean

[Flieger]

The screw contacts the microswitch which is part of the "Speed Switch" fuel shutoff. The RPM transducer sends power to the microswitch when the RPMs are right. When the microswitch closes (when you close the throttle) the power goes to the shut-off solenoid, pushing the injection pump rack past idle and stopping fuel from getting into the engine. This happens until you get on the throttle or the RPMs drop below 1300, at which time idle fuel returns.

The only stop is on the injection pump itself, where the main throttle rod attaches to the left side bell crank/plate.

[Flieger]

Is the 3.0 converted to MFI?

[luckydynes]

Yes a 3.0 with MFI ... that was a longer answer than what I was looking for ... I need to think about what's going on here or maybe you have a quick answer ... there is nothing on the switch.

There is a knob I pull to manually prime 'er ... something to do with this switch being gone?

Thanks for the fast response.

[Flieger]

Note the extra linkage for high-butterflies and the extra bearing:




Here is a different system:

[luckydynes]

Originally Posted by Flieger

The screw contacts the microswitch which is part of the "Speed Switch" fuel shutoff. The RPM transducer sends power to the microswitch when the RPMs are right. When the microswitch closes (when you close the throttle) the power goes to the shut-off solenoid, pushing the injection pump rack past idle and stopping fuel from getting into the engine. This happens until you get on the throttle or the RPMs drop below 1300, at which time idle fuel returns.

The only stop is on the injection pump itself, where the main throttle rod attaches to the left side bell crank/plate.

Thanks ... a fuel saving device/circuit? ... doesn't sound like anything to do with cold start.

I can read up/search to get basic answers on what everything does.

Thanks again,

Sean


edit: nice pics!!! ... let me absorb those.

[Flieger]

You need a mount for the microswitch, the microswitch, probably an RPM transducer, and maybe a shutoff solenoid, depending on how many parts are left. These may be hard to find.

You can run without it, there will just be less engine braking and a little worse fuel economy. I think my engine runs crisper with it.

I had to find an RPM transducer to replace my original which went missing with a previous owner. I did a write up on repairing it.

If you want to install the system, I and many other MFI experts here can help you get it running well.

It is up to you, though. Some don't even like the system.

[Flieger]

The hand throttle just holds the foot-throttle cable so you do not need to keep your foot on it. It makes the mixture richer for start-up.

Many people remove the thermostat (like when they have headers) and put a screw in its place. This is set to run medium-rich to balance start-up and warm running performance. The best alternative is to use a different solenoid (like the unique '69 MFI pump) and have a button to push for starting. This solenoid then pushes the rack rich only when you need it.

[luckydynes]

wow thanks again for those pics.

There were these beautiful cross drillings in my intake runners that had been plugged with bondo and a bunch of other features had been ground off ... now I see what WAS there ... I made some stainless inserts to go in there for the cold start 'cause the PO had the cold start spraying in the air box which seemed correct on one of the years' designs. I'm putting on some oval K&N's so I was planning on injecting cold start prime fuel back thru the cross drillings ... I thought someone had done the cross drilling "aftermarket" but I was amazed at the accuracy they had picked up the locations ... now it makes sense .

cheers.

[luckydynes]

Originally Posted by Flieger

Many people remove the thermostat (like when they have headers) and put a screw in its place. This is set to run medium-rich to balance start-up and warm running performance. The best alternative is to use a different solenoid (like the unique '69 MFI pump) and have a button to push for starting. This solenoid then pushes the rack rich only when you need it.

Yep the thermostat bypass has been performed on mine.

Is my cold start completely "improvised"? There's a knob I pull. There is a fuel line runs directly from a solenoid on top of the filter assembly to a couple of nipples on the air box housing on top of each intake stack. Seemed like a sure way to set a car on fire but this is the first time in years I actually had to analyze what was going on back there (gotta luv that ).

[Flieger]

Cold start system is separate.

On the Magnesium, earlier stacks, the cold start injectors were located in the big plenum chamber on top of the engine. When the ignition key was turned, the fuel pump would run and a quick gush of fuel would spray into the plenum above each stack.

On the plastic stacks, like mine, from the later 2.4 engines, there are little "T" fittings in each individual intake runner, just above the throttle butterfly. You can see them in the phot just below the microswitch/screw assembly. When the ignition key is turned, the solenoid on the fuel filter this time send the fuel to the Ts on the intake runners for your cold start.

The early version had a habit of catching fire and was often disconnected or updated. The later version really helps start the car, even in the mild California climate I actually need it in order to start.

Yours may have been the early version, then disconnected, then converted to 3.0, then hooked up with modified squirters and a hand pull. The hand pull is aftermarket. Kind of nice cause you can repeat the squirt if it needs a little more. The ignition key is the "proper" trigger for it, though.

As you see in the photos, RSRs often ran "simplified" MFI because they did not have to worry about starting performance and low-down drivability, just race performance and reliability. Less to fiddle with. They also ran the MFI pump in a self-contained oil supply.

You will have fun delving into the secret world of MFI system tuning. Do you know about Check, Measure, Adjust (CMA)?

[Flieger]

Some good threads:

Here is where I talk about RPM transducer repair: For MFI Gurus. . .and friends of Warren . . .


Ultimate MFI resources thread

MFI Pump - Open Heart Surgery

Finally getting around to CMA on my MFI


MFI Open Heart Surgery II - The Fuel Pump

Mechanical fuel injection resource index

[tobluforu]

Do you have this attached to your MFI pump, or a form there of?

[Grady Clay]

Sean,

The only stop on the throttle linkage is the plastic screw stop behind the accelerator pedal. The linkage should not hit any stop at the engine when the pedal is operated full travel.

In fact, when the pedal is hard against the pedal stop (Why won’t this engine go faster? force), the butterflies and pump should still be free to move a bit more. (That ‘bit more’ won’t get you any more power.)

If you have been stressing the linkage at full throttle, inspect the accelerator cushion just ahead of the throttle linkage transmission bell crank. It is possible that the cushion can be broken, the ‘safety housing’ providing the link and someone readjusted for full throttle. (The accelerator cushion is a long-term service item. Change it if old.)

Part of CMA is to check the throttle ‘correlation’ with the Factory protractors. This will tell you if anyone has inappropriately modified the cross-bar linkage or other. Specs are in the CMA booklet.

At the other end of the throttle operation, it is important for all six butterflies and the MFI pump come off their respective stops at exactly the same time.



The micro-switch at the left end of the throttle cross-bar is for the over-run shut-of system using the RPM Transducer and the shut-off solenoid on the MFI pump. While this is primarily for emissions and fuel economy, it gives better trailing throttle control. I keep it functioning for ‘sporting’ driving.

There are two cold start / cold running systems.
The cold start squirts raw fuel in the intake during cranking and with time and temperature constraints.

The cold running thermostat is on the rearward face of the MFI pump and turns off by hot air from the left heat exchanger.

There are a lot of good threads for diagnosing and adjusting these systems.

The most important admonition is to NOT Adjust anything without knowing all of the system (including engine basics) are functioning correctly. This is what the Check & Measure part of CMA is all about. "Check & Measure first, then Adjust."

Best,
Grady

[luckydynes]

Thanks guys for the replies.

Here's a couple pics of the pump ... doesn't look like that bracket is on it but I'm not sure I'm going to address that system this time around ... unless you guys convince me otherwise.

Also a pic of the cold start nozzles I'm going to put back into the individual runners. Two of the bondo plugs had fallen out and it looks like "a mechanic" balanced everything by tightening down the bypass screws on those cylinders ... whatever works I guess .

Also a pic of the parts I'm going to make up to attach oval air cleaners right to the top of the plastic runners.

Right now my goal is to just get everything re-balanced after plugging up the vacum leaks on the 2 cylinders. I thought I needed to make up an idle throttle stop up but you guys have answered that (microswitch should be there) ... I need to check what's going on at full throttle. I recall before tearing 'er down the stop on the MFI pump was the hard stop so I'll look at what's going on at the pedal.

Also attached a pic of the condition of the connecting linkage ... any good drawings or documentation on the protractor tools? I made up a new connecting link that was fatigued and broke when I touched it ... interesting that some of the links are aluminum and the links that are stressed are steel ... or was that another repair that happened at some point?

Also see pic of the solenoid on top of the fuel filter that activates my cold start enrichment/prime ... what usually activates this or is it an addition?

Thanks,

Sean

[luckydynes]

Looks like the protractor dim's are in the CMA booklet. I'll check that out.

Thanks again,

Sean

[Grady Clay]

The cold start enrichment (squirts into intake runners) is controlled by two things:
the starter being engaged (cranking) and continuity through the “Thermo-Time Switch”.

The thermo-time switch goes in the crankcase breather.
It is missing on your engine (green arrow).




There are two terminals on the thermo-time switch.
One gets power from the starter (yellow wire) and heats a resistance element inside the switch.
The longer you crank, the hotter it gets.

The other terminal makes contact to ground depending on the temperature of the device.
It has a bi-metal strip that is influenced by both the crankcase (outside) temperature
and the heat generated from the resistance element during cranking.

The net result is the cold start operates when the engine is cold.
It quits working if you crank for a long time.

Many substitute a ‘momentary-on’ pushbutton on the dash for the thermo-time switch.
You need to be careful to not ‘flood’ the engine.

Best,
Grady

[Flieger]

The lengths of the linkage rod from the MFI pump to the cross-bar bell crank must be 119 mm measured center to center of the cups. The other rods are then adjusted to fit on the balls while the throttle butterflies are flowing evenly and are NOT all the way closed. This prevents the butterflies wearing grooves in the Magnesium throttle body bores with intake reversion due to the high-overlap cams.

The MFI pump's main bell crank has a stop in the "off" direction. You would adjust the microswitch to trigger at the same time as this stop is reached.

You need the shut-off solenoid plus the microswitch and RPM transducer. Someone removed it.

You might look for a "core" MFI system to scrounge parts off of. They turn up somewhat frequently here on the Early 911S Registry: Early 911S Registry Bulletin Board - Powered by vBulletin

[Flieger]

I am stumped as to what that first linkage thing is that you posted. You have that part or is it a file photo?

I guess it could be for an RSR system with slide valves or high-butterflies. I do not think it is for the RSR tensioner rod.

[luckydynes]

Thanks for the clarification.

The magnesuim throttle bodies are painted black from the factory? Unfortunately PO had been over enthusiastic with spray paint so I stripped them and I'm trying to decide what to do with them. I'd luv to throw 'em in the bead blaster ... would anyone consider doing this while the throttle shafts are still in? I'm not taking them apart to do that but I'd like to freshen 'em up somehow. I blasted a larger throttle body once and masked critical features but glass bead has a way of getting into everything.

What tool should I buy for sync'g? At one time I thought the cold start ports could be used for throttle sync like on motorcycles with multiple manometers that you read simultaneously ... so then I thought about using an EFI map sensor with a scope to sync. Thoughts ... is a flow measurement tool the preferred method? Then I must ask what brand? I read on a thread somewhere a part fell out of one of 'em luckily not into a motor while it was running .

Thanks all,

Sean