Micro-switch

[BLD]

I think I possess all the various manuals for the early seventies cars and as far as I can see none detail the function of the micro-switch, only how to adjust it. Can someone tell me what it is for and how important is it to have the correct setting?

Bon weekend and thanks in advance

 

[pmjt]

I presume you mean the throttle overrun microswitch? As far as I understand it this is designed to close off the injectors when the throttle is closed (foot comes off!) and prevents excessive fuel being dumped into the back box and detonating - so it's adjustment is quite important []

There are many on here more technical than me (I can drive them ok though []) but my Carrera wouldcough and fart on the overrun - and then occaisionally 'BANG!' on the overrun - replacing the microswitch has made her a much more charming drive now! []

HTH

Philip

 

[jamescharnley]

Phil is correct. The microswitch there to cut off teh fuel on teh over-run. It should be adjusted so as the throttle is fully closed it just 'switches' the microswitch.

If it is not working or not activated you can get some pretty good back fires on teh over-run [8D]

 

[pmjt]

ORIGINAL: jamescharnley
If it is not working or not activated you can get some pretty good back fires on teh over-run [8D]

.........which is why James needs a new back box [][]

Philip

 

[BLD]

Understood. Many thanks Philip and James.

 

[ian cox]

Hi BLD,
I too had some pretty impressive back fires on my car when i first got it.
If you leave this unchecked it will effectually blow holes internally in your rear slincer and destroy it.
Anyway the system for this microswitch is pretty sophisticated and links throttle position, rev counter, engine rpm and a relay located in the lhs engine compartment under the black plastic cover, together.
The way to check if the system is working is to start the engine and get it to operating temperature running at tick over.
Then press the microswitch so that it clicks and then push the throttle linkage to rev the engine.
If the engine revs above the set limit of I think 1250 rpm and keeps going then the system is NOT working correctly.
If the system IS working the engine will rev but then fall back to tick over irrepsective on how far you push the throttle linkeage.
This means that if the system is working properly your could start the engine click the microswitch and push the throttle linkage by hand to its end stops and it would not buzz the engine.
If it does rev higher then the likely course is the relay switch and these are around £100 from Porsche and are a dealer only part.
In parts circles this is known as a speed switch.
I replaced this and back fires almost disappeared but the real difference came from having the MFI pump recalibrated and replacing the fuel injectors but that's another story.
However the microswitch may be at fault or need adjusting as pmjt has said.

If you need a further explanation please repaly.
Ian

 

[pmjt]

ORIGINAL: ian cox

I replaced this and back fires almost disappeared but the real difference came from having the MFI pump recalibrated and replacing the fuel injectors but that's another story.

Ian - that's a really useful summary - on my RS the switch has been replaced and it's still not right (although backfires are a thing of the past). I'm reckoning on having my MFI pump rebuilt next winter when the car comes off the road - a lot of guys on the Early 911S registry board rate a chap over on the west coast so it looks like it'll be going on a long haul holiday []

Perhaps I should also think about the injectors...[8|]

Philip

 

[ian cox]

Hi Philip,
I have already posted my expriences in recalibrating the MFI pump and replacing the injectors before.
Here it is again.

My MFI system was running fine when warm and cold starting was okish. What would happen is that the car would start immediatley and then die. I then had to crank the engine for 5/6 seconds and then it would start and run ok.
So what i did was first take off the MFI pump and send it to Tower Bridge Diesels in London. They are a diesel engine specialist but have all the kit to clean and recalibrate the MFI pump. Normally the only thing that is wrong with the pump is that the choke circuit gets full off muck and does not close off properly. These leads to a smell of petrol and rubbish fuel economy. Price for this work is around £300. The MFI pump came back as good as new.
The odometer said 80kms so i guessed at least 180kms. The reason for the strange starting problem was that the injectors could not close and seat properly to shut off the fuel when the engine was not running .Over several weeks of inactivity the fuel would leak out and air would get into the first few inches of the fuel lines. This led to instant start but instant stop as described above.
So I replaced all the injectors by buying the Bosch part code, much cheaper than Porsche, replaced the sealing washers as well and the car ran like a dream. Russel Lewis of RSR reset the pump and did tune up and the car ran perfectly.
With this and replacing the plug leads, spark plugs, rotor and points I got 219bhp on JZ Mactechs dyno. Steve McHale said that this was the best one he had seen.
So in my opinion get the pump cleaned and recalibrated, change the injectors get a proper tune up and the car will put like a train.
Oh and then I added SSi's but did not get the car re power tested before I sold it.

So you do not have to send your pump overseas just send it to Tower Bridge Diesel in London.
Please note if you lose your '019' pump a brand new one, if you can find one will cost you £2500.

Kind regards Ian

 

[pmjt]

Hi Ian,

ORIGINAL: ian cox

I have already posted my expriences in recalibrating the MFI pump and replacing the injectors before.
Here it is again.

I remember reading this a while ago and thinking.... must look into the pump etc - many thanks for this - do you fancy writing up your experiences into an article on the MFI for Porsche Post / Early 911 Register piece - would be really interesting for many other Early 911 owners.

ORIGINAL: ian cox
So you do not have to send your pump overseas just send it to Tower Bridge Diesel in London.
Please note if you lose your '019' pump a brand new one, if you can find one will cost you £2500.

My car only has 59k miles from new - so the pump us far from worn out - so it'd be hand delivered and hand collected! [] I've seen them changing hands on the Esses for BIG money - as so many other Early parts are now []

Philip

 

[38F3E1]

As we're talking MFI pumps I thought I would mention that my engine has a modified cold start mechanism. I'll try and describe it as accurately as I can but don't have the engine here at the moment.

The standard cold start circuit has been removed. Where the pipe from the heat exchange normally goes into the MFI unit this has been removed and a threaded hole tapped through the end into which a bolt is screwed. This must keep the circuit closed (ie normal mixture). In essence any enrichment mechanism has been disabled.

A solenoid that releases fuel directly into the six intake stacks has been wired to a push button fitted to the dashboard.

To start the car I set the hand throttle open some and then use the push button to inject the extra fuel whilst cranking the car until it fires. Then set the hand throttle to a fast idle (if you set it too slow the car backfires a little) until warm.

With this mod I have never once had a problem starting the car whatever the temperature. I also know the car will never be running too rich due to a blocked choke mechanism.

Ian.

 

[jamescharnley]

ORIGINAL: ian_uk

I also know the car will never be running too rich due to a blocked choke mechanism.

Ian.

This is possibly the problem I have at the moment.


My car has the full original MFI set up & always starts within the 1st few seconds, even after being sat in the garage for over 8 weeks untouched.

 

[ian cox]

Hi James,
yes the cars will run ok if the choke is full of muck and not shutting off correctly.
But the problem is that MFI cars POUR fuel in to get the outstanding power that they deliver.
So if the car overfuels it will back fire, not so bad just annoying, and in the very worse case the excess fuel will start an engine fire.
If the cars run rich the fuel econmy is really and I mean really rubbish
I have also heard that MFI racing cars remove the microswitch and fuel over run circuit so that extra fuel 'cools' the engine under braking.
Finally the youngest of these cars are 33 years old so lack of use can casue problems.
I believe an MFI car to be an oustanding system and gives mega power from relatively small engine capacities and ok course they sound great at 5000rpm upwards.
I am just a guy who wants his engine to perform as well as the day it left the factory.
The only case of an MFI pump breakage was when the governer bobweights inside broke. Other than that they are bullet proof.
Ian

 

[ian cox]

Hi Philip,
One more thing.
If you want me to put an article together please get in touch.
Ian

 

[jamescharnley]

Mine is pulling like a train, but the hydrocarbons are sky high whilst the Co emissions are normal. Other Porsche drivers following me complaign of 'stinging eyes' . For me obvious starting point to check is the thermostat, then possible airleaks around the intake manifolds & then injectors

 

[jamescharnley]

just been out in teh garage & removed teh discs from the thermostat ... they were clean, but I cleaned them anyway and refitted + checked all my warm air hoses ... everything seems OK

 

[38F3E1]

ORIGINAL: ian cox

I have also heard that MFI racing cars remove the microswitch and fuel over run circuit so that extra fuel 'cools' the engine under braking.

You also run them rich on circuit for exactly the same reason. Fuel ecconomy on a car set up like this is about 8 to the gallon :-o

I believe an MFI car to be an oustanding system and gives mega power from relatively small engine capacities and ok course they sound great at 5000rpm upwards.

Absolutely. Always wondered how they compare to a good carb setup such as Webber or PMO.

Ian.

 

[BLD]

I'm glad I stated this thread as the discussion has become very interesting. My 2.7 and I have an extremely sour relationship. It has now decided not to start at all after it's last short run of a couple of miles. It shot off like a dose of salts only to start misfiring and finally dying outside a pub with what appeared to be non-existant fuel pressure. The car was recently fitted with a replacement fuel pump and filter from the Swedish and German car parts people and the system cleaned thus far. I suppose the MFI is at fault and needs removing. Can this job be completed without engine removal?
I would really like to re-establish a loving relationship with this car as it is the only vehicle I have ever owned which seems to have permanent menopause.

 

[38F3E1]

ORIGINAL: BLD

I suppose the MFI is at fault and needs removing.

Not convinced that is the right diagnosis, not that I can offer a better one. Sounds more like a fuel supply problem than an MFI problem. Have you thought about taking your car to Bob Watson (near Oxford) to look at. I am sure he could stick it on his rolling road and diagnose the problem once and for all (he's rather good with these MFI engines).

Ian.

 

[ian cox]

Hi BLD,
Yes you can remove the MFI pump without taking the engine out but you will need mirror ro peer round that back of the engine where the camshaft drive, belt and gear assembly is. Note the pump runs half engine speed.
This will be essential when you put it back as you will need to 'time' the pump up with the rest of the engine.
There are marks on the various gears to help yu with this.
But as ian_uk has said it may be something else like fuel pressure
Go back to basics with the fault diagnosis.
Note 80% of car failures are electrical.
Ian

 

[ian cox]

Hi Ian,
IMHO the MFI systems is better as it compensates for altitude as well as being a fully sequential fuel system.
It was very advanced in its day and Porsche used on the 1983 SCRS cars to get the power as well as the turbo Le Mans cars.
Its major down fall in the end was when fuel economy satrted to be introduced in motor racing.
It is the best fuelling system bar none.
People tend to put PMO carbs on later SC and 3.2 Carrera engines to get the horse power they need as the standard fuel injection systems are built to meet the then world emsiision standards.
MFI was introduced to get power and any other considerarion was forgotten.
Ian