MFI Idle Adjustment

[Harry]

Hi All,

Looking for some advice from any MFI gurus out there. When my car is hot it idles too low (400 rpm) and I was wondering if the adjustment is a simple task (that I can perform) or something complex?

Thanks very much.


Cheers,

Harry.

[epilot]

not a simple answer unfortunately

http://www.pelicanparts.com/techarticles/911_MFI/TipMFI.htm

[HughH]

Harry
I think there is a simple adjustment that can be made to the idle level on the mfi pump......... But that may make things worse depending on the real reason for the low idle. It may be the low idle is ignition timing related rather than mfi related and it may be that adjusting the idle by effectively adjusting the throttle travel stop position in the mfi pump will hide another simple to fix problem or make it worse.

Where does it idle when cold or do you use the hand throttle to maintain that. Adjusting the hot idle position without taking that into account, and possibly the thermostat shims into account, may cause a different problem when it is cold.

[Cairns888]

I have never played with these but I am sure there is an air correction on all. Check to see if they are all balanced at idle with a sync meter first. That is usually the first point to go to when you don't have a strong idle. Which is a must.

[HughH]

Jim is correct
There are idle screws, I think, at the base of each of the induction stacks rather than the mfi pump that can be used
Do you have a copy of " check measure adjust" which is the bible on this
If not I think I have a soft copy I can email you if you give me an email address.

[Cairns888]

Any chance you could also copy me in on that email Hugh? Jimcairns2@yahoo.com.au

Many thanks

[HughH]

Jim
I have sent it to you

[HughH]

and on second thoughts, sparked by what Jim said, I am wondering if this thread got off on the wrong foot completely due to Harry saying MFI and people (at least me) automatically thinking he was talking about adjusting the MFI pump or settings when the basic way of adjusting the idle is at the adjustment screws at the bottom of the induction stacks in exactly the same way as with a carb. engine BUT always after all the other settings have been tested to be correct as in the "check, measure, adjust" book

the basic procedure in the manual is attached and the following part from the workshop manual confirms that the adjustment of injected engines is the same as carb ones

[SM911]

Harry,

There is an idle speed adjusting screw, which is adjusted using a tool through the fan assembly. However, when setting the engine up, you need to use the Bosch Check, Measure Adjust method.

Sorry if I'm telling you to suck eggs here, but you need to get the Bosch manual 'Check, Measure, Adjust', and start from step 1. Idle adjustment is toward the end of the process, but everything else needs to be set up properly beforehand.

Believe me, your car will thank you for it. The MFI system can be a pain to get right, but once it's setup properly - using the Check, Measure, Adjust process, it'll be running really well and stay that way for a good while.

PM me if this is all Greek to you. I can also send you the Check, Measure, Adjust PDF if you need it.

[Harry]

Hi All,

Thanks very much for the comprehensive responses.

I have downloaded Check, Measure and Adjust and started with checking the timing.

My distributor makes a loud ticking noise at idle that goes away when I disconnect the vacuum. The idle speed also raises up to 850rpm when the vacuum is disconnected. Any thoughts?


Cheers,

Harry.

[Jeff Eelkema]

Sounds like you are getting retarded timing at idle. Is the vac supply from the right spot? Most carb cars have the vac advance from above the butterfly not below so you get "ported" vac. Below the butterfly will give full manifold vac at idle. Some smog strangled cars did use a vac retard system but don't recall ever coming across this on MFI cars.

Not sure about source of noise but would pull the cap and check advance mechanism is secure etc.

[Harry]

A result

Hi All,

OK, after suffering with a low idle for long enough and putting up with a lot of back firing on the overrun I decided to do something about it.

I did a lot of research on setting up the MFI and read CMA and came across a version of CMA by Leslie (Lee) Rice tat made a lot more sense to me to so I decided to proceed with tackling the procedure myself.

First step was check the operation of the MFI thermostat as I suspected the back firing was caused by being overly rich which a classic symptom of an non-functioning MFI thermostat.

I warmed up the engine and found that the thermostat plunger was still pressing hard against the cam in the MFI pump, indicating that the thermostat was not telling the pump to lean out when warm.

I removed the back of thermostat and found the rod stuck pretty solidly, required a really good tug and twist from a set of multi grips to get it out.



What I found was the the bi-metallic discs and the housing were completely clogged with carbon / oil residue, so I gave the rod, discs and housing a good clean and put it back together.



The result was that the backfiring was gone, and the plugs went from looking like this:



To this:



Next step was to remove the TBs to clean them and setup the throttle stops and air bypass screws.



What I found was that one of the vacuum passages was completely blocked and 2 air bypass passages were completely blocked and one was partially blocked.

[Harry]

They were cleaned of all of their gunk and then the stops and bypass screws setup.



They were then re-installed with the stacks and I then used a syncrometer to correlate the TBs.



I then setup the linkages and checked the air flow with the syncrometer again at idle and 2500 rpm.

Result, an idle that is now at a tick over 1000 rpm (I read that the idle speed would drop by 50-100 rpm when re-installing the air filter housing, this didn't happen), the loud ticking noise from the dizzy is gone (I guess due to the fact the dizzy is now getting vacuum from both banks) and an engine that is running as sweet as a nut.

Follow the link for a video.

https://vimeo.com/153468735

Cheers,

Harry.

[CamBiscuit]

Nicely done Harry

[Merv]

Progress being made Harry. I will avoid the temptation to say 'get some Webers', but do note that the plug threads look very oily, although the electrodes look better of course, in the second photo. The first are too rich of course - not enough air in the mix, as you say. On the syncronometer I would be looking for much more vacuum at 6000, on carbs, but not sure on the EFI. Others will know more about them. Is that set up done at idle or at peak rpm?