1970 MFI enrichment circuit question

Cornpanzer · 12-23-2003, 10:51 AM

I had the MFI on my 70 "S" tuned up last year and it has run very good except that when it is really cold it runs extremely rich to the point that you cant even sit inthe car without smelling like a diesel truck. Until it warms up it fills the neighborhood with clouds of unburnt fuel. This cant be good for the rings. Is there any way to adjust the enrichment circuit without major recalibration? Thanks in advance

Mr9146 · 12-23-2003, 10:58 AM

There are two seperate racks inside the pump - main and idle. It sounds to me like you will need to lean out the idle circuit (on the front of the pump), but you need a special tool to do it.

It might be better to lean out the main circuit instead - leaning out the main rack will, by default, lean out the idle circuit - because you don't need any special tools. I think a flex-head 1/4 drive ratchet with a 4,5, or 6mm allen socket will do the trick - it's been a while so I dont' really remember. You're gonna want to count the clicks and be VERY careful when doing this.

The adjuster screw is under the cap on the back of the pump - I recommend that you leave this to a professional.

Cornpanzer · 12-23-2003, 11:07 AM

well, actually the warm idle is fine. The problem is when it is cold. It uses the timer device which squirts raw fuel into the velocity stacks. I need to do some reading on the system I guess.

Mr9146 · 12-23-2003, 11:51 AM

The cold start system is, in my opinion, a complete joke/waste of time. I modified mine to do away with the cold start and I've got it rigged up to a manual squirter ... requires a little finesse when cold, but after about 2 miles things gel.

If memory serves - which it rarely does

- the cold idle is directly related to these little discs that, when heated, expand and shut off the cold start squirters. I believe that they get their heat from either the exhaust or the warm intake air of the engine compartment - through the hose on that plugs onto the front of the pump. Basically, if this is not funcionting properly, logic would dictate that the squirters are not shutting off in a timely fashion ... they are shutting off only after the engine reaches operating temperature when they should be shutting off much sooner.

Lemme see if I can find a picture ... but don't hold your breath.

Mr9146 · 12-23-2003, 11:52 AM

Oh, the solution: fix the discs <-- may be a job for Pacific Fuel Injection.

Cornpanzer · 12-23-2003, 11:55 AM

the bi-metal discs seem to be working fine since the enricment circuit does lean out when warm. My problem is that it is WWWAYYY too rich when cold.
How did you do away with the cold start system?

Mr9146 · 12-23-2003, 11:58 AM

We got rid of the discs. We made a block-off plate with an adjuster screw permanently set to warm. Then we rigged a manual squirter system operated by a toggle switch on the dashboard. <-- I can definitely get pictures of this, but it'll be this weekend.

Cornpanzer · 12-23-2003, 12:08 PM

Yeah, I would like to see it. Thanks!
Dave
Any other suggestions welcome too.

Mr9146 · 12-23-2003, 12:13 PM

I found a link to the Bosch diagrams on how to assemble/disassemble the pump. Here is the link: http://www.drivewerks.com/tech/bosch_MFI.htm

You need to download the zipped file ... it's large but worth the wait. It'll give you a good idea of what the internals look like and how everything goes together. I'll see what I can do about a photo.

Mr9146 · 12-23-2003, 12:14 PM

Also, there has to be a way to adjust those discs ... I would start looking into that as a place to start.

Paul Thomas · 12-23-2003, 12:25 PM

Corn,
When you start your car cold, it uses a thermo-time switch to activate the cold start squirt system for start up. After that themostat (discs) gradually leans the pump out as the air in your heat exchangers heats up and spreads the dics apart.

There are two ways to adjust the pump. The main adjustment is behind a 5mm allen head bolt under the thermostat. The idle adjutment is a spring loaded hex-head screw under the stop solenoid. the main adjustment is rich-left/lean-right and the idle adjustment is the other way.

I wont go into too much adjustment detail cause it sounds like, to me, that your thermostat might be gummed up and isn't leaning it out quickly enough. Cleaning it is a snap with some carb cleaner and there is a great article on it in the tech section.

Paul

Cornpanzer · 12-23-2003, 12:35 PM

well, i dont want to adjust the mixture since I had it set using a CO meter only a year ago. Car runs great when warm, no probs with teh mixture. It is only when cold that it runs rich.
There has got to be a way to shim the thermostat stack os it doesnt richen it so much when cold. But Im just guessing here

Zeke · 12-23-2003, 01:16 PM

To eliminate the cold start circuit/timer from fault, remove the middle fuse in the engine electrical area. This disables the cold start system. It will start, takes twice as long. It will run, use the hand throttle. If you are still rich after half a minute, keep looking, it's not the timer or solenoid.

Oldporsche · 01:31 PM

The stock heat exchangers that are used with MFI have intergeral small separate heat exchanger to supply hot air via flexable tube to you metalic discs that are located on you fuel injection pump.

Is warm air reaching the bi-metalic discs? If they aren't getting heat you will run rich. They could have carbon in them or they might be dirty, etc.

I believe that there is some adjustment that can be made there.

Start there.

Good luck,
David Duffield

Cornpanzer · 12-23-2003, 01:29 PM

Quote:

Originally posted by old_porsche
Is warm air reaching the bi-metalic discs? If they aren't getting heat you will run rich. They could have carbon in them or they might be dirty, etc.

Start there.

Good luck,
David Duffield

Hehe, ok this is like an old Abbot and Costello routine...

One more time....When the engine warms up it runs great. No mixture problem when warm. Warm is good. Everyone happy when warm.

Only when engine is still cold is it too rich. The bi-metal thermostat IS WORKING. It is simply way too rich when the engine is cold. Usually takes about 10 minutes to warm up and lean out all the way.

Oldporsche · 01:43 PM

Yes, warm is good.

Is hot air being ducted to the stack of discs?

On page 27 of the Mechanical Fuel Injection Supplement, it warns when cleaning the "Warm-Up Thermostat" "don't slip the expansion elements and compensation discs off the shaft. If the expansion elements are mixed up, the warm up characteristics of the thermostat will be changed."

There is a clue here!

Good luck,
David Duffield

Mr9146 · 12-23-2003, 01:37 PM

Zeke (aka Milt) is on the right track ... that's the best method of isolating the problem.

Assuming that is in fact the source of the dilemma, how would one go about fixing the issue? I was looking for diagrams and instructions on the net but haven't found anything better than I posted above.

power · 05:15 PM

I would concentrate my efforts on the thermostat and the cold start enrichment circuit. I would try and eliminate the cold start solenoid as a problem first. You could put a voltmeter on it to see if it is working properly. On and off. Once I had done that I would start looking at the thermostat. As mentioned above it needs the hot air from your heat exchangers to expand the alluminum disks inside the thermostat which in turn slowly expands a shaft that leans out your mixture. Check the hose from H/E'ers to thermostat make sure no leaks. If hose is O.K. I'd next try as someone mentioned, removing thermostat and cleaning, be careful I always drop the disks all over the floor!Make sure you replace the discs correctly as they only go together in one pattern to allow for heat and expansion.
Warning:
As others have said, if your car is running fine while warm, I would not adjust the part load or idle adjustments! It is very easy to get lost trying to tune MFI adjusments correctly.....

Paul Thomas · 12-23-2003, 04:36 PM

My point was that if the thermostat is gunked up, it is not leaning out the pump as quickly as it should. Thats why it may run rich for so long after start up.

Paul

Cornpanzer · 05:37 PM

LOL.
Thanks for all the suggestions, but as I mentioned before, the thermostat is working fine. It is not slow to react. It leans out the mixture just fine as it warms up. The problem is that when it is cold the mixture is obcenely rich to start with. It is during this period of time (before the bimetal discs can start working) that I am most concerned with.

I do have a bit of experience with the MFI system on this car. The hose is in place and I cleaned the thermostat assy (carefully) a year ago) The problem I am faced with is not the typical MFI newbie tuning woes.
I will double check the cold start enrichment circuit and make sure that it shuts off after a few seconds, but I am pretty sure it does.