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1972 911e MFI

Hi

I have a 72 911e with MFI

I have read CMA more times than i care to recall and searched endlessly for threads on raising the idle. Heaps on lowering it but none on increasing it.

The car has a re built motor (by me) and the thing runs like a treat the only issue is i cannot raise the idle from about 500rpm with hand throttle completely off.

Microswitch is disconnected but was working but the RPM transducer is not. This i beleive should not effect being able to raise the idle though.

stacks are synchronised as are the linkages all in spec but cannot seam to raise the air screws out any more to increase the idle. They are not blocked or carboned up they are clean and all vacuum is new also. Car has vacuum and advances when connected and retards when disconnected.

Timing and dwell spot on, big fat spark ...Mixture is pretty much spot on - i know that because i have an Innovate AFR gauge hooked up to the car with a bung in the exhaust.

Would a vacuum leak cause the issue ???

Any help appreciated without referring me to CMA - i've been through the thing in detail and the car, once accelerated with the hand throttle or foot pedal runs a treat, no hesitation not backfiring, JUST CANT RAISE THE IDLE

Adam
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Adam C
Bells Beach Australia
1972 911e
Old 11-19-2017, 03:59 PM
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If you are sure everything else is correct you can adjust all 6 throttle plate stop screws to slightly increase idle airflow, checking syncronization as you go. After you increase airflow you may have to erichen idle range mixture. Rob
Old 11-19-2017, 05:20 PM
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Bob

Thanks for your reply, the issue is i'm at 8 half turns on every screw or very close too - i.e: open them much more and they'll fall out !!!

CMA also says if you have to get past 8 half turns without increasing idle then the holes are carboned up and need to be cleaned but i can assure you they are as clean as a whistle.

The needles on the screws are also spotless and like knew shiny gold zinc since i had them re zinc plated when re building.

I also tried leaning up the mixture (in an attempt to increase the idle speed) but didn't seem to make a lot of (any) difference - have returned idle mixture screw to read spec (bang on 2.5% Euro) after the enrichment thermostat warms up which is definitely working since runs richer unit that kicks in, as it should.

Maybe i should lean it some more ??? Thing is with the hand throttle slightly up i can make it sit bang on 900 RPM and it will just sit there purring away nicely.

Maybe my accelerator / hand throttle linkage is out but for idle the throttle bodies should be sitting on the stops ?? and if i was to adjust the throttle cable to increase RPM that won't be the case... perhaps i need to nudge up the throttle butterfly stops so it rests, at idle in a slightly more open position ?

Its literally the last piece of the puzzle as the car is now running a treat. Nice brown furry plugs .........just cant idle

Driving me INSANE .....
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Adam C
Bells Beach Australia
1972 911e
Old 11-19-2017, 05:42 PM
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If you suspect idle mixture issue a good test is after everything is warmed up pop off the linkage rod from the pump, then slowly move the pump arm only to increase pump output. The idle should increase initially as you add more fuel then as you continue moving the pump arm the idle will decrease as the mixture is overly rich.
If the engine speed does not increase initially then your pump slow speed mixture is too rich.
Long ago I had a 72e. Best running long hood I ever owned. Rob
Old 11-19-2017, 06:08 PM
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Rob

Thanks (and sorry for calling you bob in my first reply!!) i'll keep playing with it.

I'm getting close and she sure does run a treat in the higher rev ranges. Its just the damn idle or lack there of

One thing i'm thinking is the throttle stops are letting the throttle bodies close just a smidge too much - potentially they are worn just a touch (although not showing any other signs of this)

The relationship between them opening and the pump arm seem to be spot on once you introduce some throttle - i.e.: part load mixture measurement is spot on and so is idle (albeit with some throttle) and it runs a treat with some revs on

If i wind up the throttle body stop position to stop them closing quite as much isn't this basically just having the effect of having a bit of throttle on and not changing the air / fuel equation between the pump and the throttle valves ????
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1972 911e
Old 11-19-2017, 09:38 PM
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Make sure timing with vacuum retard is spot on (5 degrees ATDC).

Then screw the idle adjustment screws back to where they are supposed to be.

Then adjust idle stops to give it a bit more throttle (air) at idle and synchronize.

Then adjust rod to MFI to match or enrich it with the idle enrichment screw.

Guaranteed to work.

Also. the hand throttle should give you 2k of RPMs. Make sure it is connected to the rod in tunnel correctly. In fact, I would do this first.

Chris
73 E.
Old 11-21-2017, 05:34 PM
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Thanks Chris will give that a try
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Adam C
Bells Beach Australia
1972 911e
Old 11-22-2017, 12:14 AM
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This is pretty much what Chris wrote, but more detailed. I did not write this, but added to it a tad. Its what I did to get my original stacks to idle perfectly......
Setting idle mixture & throttle body maintenance
It is assumed that your throttle bodies have been cleaned and intakes (throttle plate, air mixture and vacuum) are “surgically” clean. Regarding idle mixture tune-up, first turn the air correction screws on the throttle bodies fully closed, then back out exactly 21/2 turns. Then adjust the throttle stops by removing all the linkages and, one at a time, loosening each throttle bolt locking nut. Lightly push the throttle plate towards the fully closed position. Turn the throttle stop bolt to allow the throttle plate to close completely. When you get to the point where you have almost removed the bolt, the throttle plate will stick shut. Now screw the throttle bolt back to the point where the throttle plate no longer sticks shut. (This is rather critical as the throttle plate can be closed too far and stick, as engine temps increase and alter clearances). Now that you have the throttle stop bolt set to just keep the throttle plate from sticking, screw the throttle stop bolt another 3/4 turn in the direction that opens the plate. Check for free play by pushing lightly toward the stop. If all is OK, secure with the locking nut and repeat on remaining 5 throttles.
At this point the engine should start and run, but idle will be slow. Run for a while (until engine oil temp. is about 180 degrees +), then shut down and wait 15 mins for heat expansion, then restart engine.
Check each throttle with an air flow synchronizer to find the throttle with the lowest air flow. Adjust throttle stop bolt on this throttle to allow more air through throttle plate. Note: NEVER adjust the throttle plate for less air (closing the plate)!! Then adjust all six throttles up to identical air flow. Idle speed should now be close to spec. (circa 900rpm). Note that bad plugs, timing, MFI pump adjustment will affect these settings, so make sure all is well before attempting this balancing.
Next, with engine still running, adjust and reinstall the four throttle valve connecting levers to just keep the throttle arms on the stop bolts without changing the idle rpm. L & R throttle valve push rods should be as close as possible to 149.5mm and reinstalled. Car should now idle but idle mixture should be now checked.
Normally if the idle speed is low, you must determine whether the engine needs more fuel or air. There is an easy way to check this. Remove the injection pump control rod at the cross shaft end (the large metal tube linking both throttle bodies). Leave the other end attached to the MFI pump. Check all throttles are still at their idle stops, and then very slightly push the control rod open. This will inject more fuel into the engine.
Does the engine speed up? This means the pump is set too lean. Does the engine slow down and puff a little black smoke? This means the pump is set too rich. At perfect adjustment, pushing slightly on the control rod will cause the engine to initially increase revs a little then rapidly decrease. The MFI pump is adjusted @ 1000rpm for basic rich/lean with the main enrichment. Idle speed is adjusted with the idle enrichment and is only effective below 1000rpm.
Fine tuning can be achieved by turning all six air correction screws very slightly (1/16 turn for example) then rechecking with your air synchronizer to be sure they are all the same. Then recheck all linkages are lightly tight and ball sockets have Bosch ignition grease in them. Finally, check throttle pedal goes all the way down together with the MFI pump being at full throttle.
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Although it is done at the moment, it will never be finished.
Old 11-22-2017, 03:00 AM
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Thanks for the post Tobluforu - I'm going to have a go at it this weekend. Will let you know how i go
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Bells Beach Australia
1972 911e
Old 11-23-2017, 12:16 PM
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low idle

Ok you have a setup that obviously been played with to make it work.Micro switch and transducer are very important.Best transducer is the one for a 911T.Label is black & yellow and ends in 001 for the Bosch number.Jbell had one for sale here about a month ago.When you drive and close the throttle the microswitch tells the RPM Transducer to push the enrichment solenoid back to lean out the mixture.When you step back on the gas the rack enriches.You can keep your current setup as it will run pretty fair but you will shorten your new rebuild as under closed throttle excess fuel is being injected and washing your rings.The transducer prevents that.I had Porsche Fuel Injection school in 1971.I have always felt the Porsche engineers were smarter than I would ever be.Another plus is your mileage will improve for sure.Tough part is the re set up with the transducer working.On your idle circuit of the pump take the 8mm socket adjustment and turn clockwise one click at a time until it stops.Count the clicks and write it down.Then turn counter clockwise 24 clicks and stop.That is factory Bosch pre set within a click to either way.Then adjust main rack with the small screwdriver to get your CO% in range.Then your hard work will last.Fred
Old 11-24-2017, 09:30 AM
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Start with basic checks

Also rod from stacks to pump should be about 114mm but I am in C.R and my original training manuals are in the states.Do what Porsche says and you will be happy.
Old 11-24-2017, 09:34 AM
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What CDI is installed?

The Bosch OE unit does not put out a decent spark.

I found out the hard way that you need a good CDI to fire the varying mixtures of MFI.
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Old 11-24-2017, 07:44 PM
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spark

Msd on MFI works great.
Old 11-25-2017, 06:46 AM
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I did as tobluforu said. The MFI was clearly out of whack. The pump tuned super lean to make up for the out of adjustment throttle butterflies (not enough air)

After i adjusted the rods and the butterflies (3 were sticking) I richened up the main mixture about 6 clicks and now CO meter reading in spec AND - i reckon i've just added about 20 HP in road testing !!! she's absolutely hammering now.

That said the idle is still super low - around 400RPM. As you say tobluforu is the idle mixture adjustment ONLY effective below 1000rpm ?? if so i think i'm almost there just need get this right to bring up the idle and i should be sweet.
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Old 12-03-2017, 03:07 PM
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Can you increase the idle using the air correlation screws, you will need a synch?
You do not need a new cdi unit unless yours is whacked, geez lets not beat that horse to death yet? I'm still running the original stacks, org cdi, with solex cams, etc. and it runs like a top. Yeah, yeah, yeah, It would run better with new stacks and maybe a tad better with the new and improved cdi, all I'm saying is that you can get these cars to run just fine without upgrading.
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72 911
Although it is done at the moment, it will never be finished.

Last edited by tobluforu; 12-04-2017 at 06:26 AM..
Old 12-04-2017, 06:19 AM
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tobluforu

Yep the CDI is working a treat - i had it fixed - sent it to Florida a while back.

I've got a dirty fat spark and the plugs are brown and furry.

The idle is still a bit too rich (under 1000k RPM) so will try leaning it out first to bring up the idle

My question is though - i assume given i now have the main mixture adjustment spot on (anything above 1000 RPM after warm up is reading spot on) and the thing is pulling like a mother F.......r that idle mixture ONLY has an influence over mixture BELOW 1000RPM ??? I assume it does ??

If it does then leaning this out should increase the RPM and NOT effect my now perfect mixture at RPMs over 1000k ??
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Old 12-04-2017, 01:32 PM
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PS - i have a synchometer - all stacks now at 3 1/2 turns (or close to) and all in perfect synch
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Old 12-04-2017, 01:33 PM
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The idle adjustment at the pump in all honesty doesn't do all that much, but its supposed to affect idle and the transition into the higher rpms. From what I have read its up to about 1600 rpms, but that was years ago I read that and I'm sure the true mfi experts can answer that question. If the car is running great now, leave it be.
Ok, so if the car is idling low, adjust the air correlation screws out. At this point it makes zero difference that they are 3.5 turns out, what matters is that you can turn them counter clockwise and get the idle to 850/950. The 3.5 turns was just a starting point when you adjusted the butterflies. If you can not increase idle this way, then you need to work on the butterflies some more. I went as far as removing all the linkage after the car was warm and adjusting the butterflies so idle was at 900 with the correlation screws all set at 2.5. That was a frickn time consuming hassle.

Good luck
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Although it is done at the moment, it will never be finished.
Old 12-05-2017, 02:44 AM
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Did you clean BOTH by-pass holes ? there is the one that the adjustment screw goes into , and then there is the drilling that comes in from above the throttle plate to join the other one, bringing air in from above the throttle plate.
Old 12-05-2017, 03:47 AM
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The screw on the pump adjusts mixture at idle and low speeds. Think about how it works. More air equals higher idle. Opening the screws at the stacks lets more air in and increases idle. This may make it lean and the idle mixture screw on the pump then has to be turned a few clicks to compensate. If the screws on the stacks do do anything then adjust the linkage to allow more air in to up the idle.

Make sure timing is set correctly with the retard hose attached.

Chris
73 e
Old 12-05-2017, 03:58 AM
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