Jump to content

Recommended Posts

Is anybody an expert at adjusting the metering units on the PI, I would like to lean mine off a bit for the RBRR, to improve fuel consumption, my pi is getting between 20 to 24 mpg at present. I am led to believe that 25 to 30 mpg can be achieved with a bit of fettling?

It would be worth doing over 2300 miles, certainly worth a few beer tokens for anyone wishing to help at the drivers meeting?

Link to comment
Share on other sites

I wouldn't suggest making adjustments blind. You can end up too lean without knowing until its too late. I'm thinking of trying to lean off mine on the TR6 but I've just ordered an AFR meter. It should arrive tomorrow so I'll see how I get on over the weekend.

If you must do it blind, then maybe the way to do it is to slip a shim in rather than turning any nuts.

Link to comment
Share on other sites

Simples, just either loosen the bottom big collar off, and take the others with it,
and then tighten back down
or, just slacken the middle one, taking top one with it.

BUT, check to see its not sucking air from some where eles fust.
easiest way, put finger over idle hole, to stop all air going to idle,
if it dies, then its ok, if it still runs, that the cause of higher fuel use, as the vac is lower, so links will be lower.

adjustment is easy really, you can leave the top off, BUTT, you must put the wee screws back in, as it seals the chamber .
at a riugh estimate a 1/4 turn at a time.
trial and error, as all are different to a certain extent.

Note, leaning off too far with out turning top one doon, can cause it to stall, as it cuts of most of the fuel on owa run, so may not come back on fast enough to keep engine running.
but done right, it can be an advantage, as ye use very little fuel on over run.


Link to comment
Share on other sites

I've got my air fuel ratio guage fitted and considering I've got a sound engine pulling vacuum in spec and a recently rebuilt and calibrated metering unit, I've got some way to go.

At cruising speed, I'm seeing around 11.1 and on overrun just over 13.

At about 30 mph, I'm seeing about 12.5.

Yes, the fuel enrichment lever is fully home.

Link to comment
Share on other sites

Before you do anything, you need to make sure the rest of the engine is in tip top condition.

Injection system is the last thing to look at.

Ignition system, valve clearances etc all critical.

Once you have this sorted, remove the black plastic cap from the side of the MU, and check the roller clearances. Engine off, rollers at the bottom, (ie full fuel position) the clearance should be 55 thou.
Remove the top, take out the springs, replace the top. Lift rollers up, clearance should be 2 thou.

Bigger gap = rich.

Mess with the lock rings at the top at your peril.

Link to comment
Share on other sites

OK so I've double double checked the ignition system.
Compression check shows 14 bar on all cylinders
Valve clearances checked, all OK
Checked balance pipes for leaks - none
Took servo pipe off and blocked hole in manifold
Double checked vacuum at servo inlet on inlet manifold, found to be within spec

None of this has made any improvement.

The car runs with the air fuel ratio above.

I'm itching to get at the metering unit but there isn't much clearance to get the side plate off with the unit on he car.

Any other thoughts before I start playing with my nuts?

Link to comment
Share on other sites

If you need to chech the clearance, then the side panel can be took off.
needs to be about 58 - 60 thou
more likely its ok, but the culprits are the springs.

middle spring will make the  cruise / mid range adjust ments.
slackening the bottom spring too much will / could reduce throttle sharpness, as it act like a
sort of fuel dump,when foot is floored, due to spring pressure.

I never could see how some one in a shed, even with the calibration gear, could get
the fueling right for so many different engine quirks.

I can remember Dads last PI, never got more than 20 ish to gallon, no matter what
plugs always black at moderate speeds.
got at it, later on when he had finnished with it.  and it went up to 28/30 ish, not bad for an auto PI

so it can be done easily.

ohhh, whats the air filter like,!!!  if it is clogged up a little  it will run rich, but really clogged up it will run  weak,!!!

Link to comment
Share on other sites

I'm really hoping that the a/f meter is going to help with this but I'm trying to put off messing with the MU as it has been recently rebuilt.  

I was getting about 28mpg with the old metering unit and about 24-25 now but even at 28 it was running rich.

Putting the same a/f meter on my modern BMW I get about 16 when the TR is at 11.

I'd be happy to see 14 on the TR.

Link to comment
Share on other sites

on the PI, you should be able to run about same at cruise as the BM

Lambda one equates to aboot 30 on my ox sensor
im running 0-2 on  ox  raw feed back signal. and it runs OK which is well below L1
as when ox feed back  is on, the ECU is adding 15-18 % extra fuel.
but this is just around the cruise / low throttle spots, up to throttle opening of 10 TPS
which is about   35 thou  open throttle discs.

have a fiddle with the springs, they can easily be put back .
ohh leave the cap off, but PUT the screws back in to seal chamber.


Link to comment
Share on other sites

I've made some progress.

There is a vacuum leak but I'm not sure where it is.

When I suck on the vacuum pipe to the metering unit the arm moves up but if I leave my tongue on the pipe, the arm and roller slowly slip down.

It doesn't appear to be the connection at the pipe, the screws for the adjuster cap are in and sealed the diaphragm is recent.

Link to comment
Share on other sites

Now we're making progress. It turns out that the original vacuum pipe that ha been on the car since 74 has gone porous. I've never seen that before.

So the engine is now as it should be but the a/f ratio is still showing little improvement so I've made adjustments to the metering unit.

It's looking like this has improved matters massively.

Many thanks for all the advice.

RBRR - here I come :) :)

Link to comment
Share on other sites

what is the foot to floor pick up like now, hesitant,!!!

my self, I would turn big one down a wee bit, losen middle off, as this is the cruise / mid range set up.
then when thats ok, loosen top nut off, and  re set till it just touches the stop inside at idle, then move 2 turns further up..  this only affects the over run fuelling,  too far down it uses alott of fuel on over run,  too farr up, it may stall when it comes to an idle quickly, but will soon sus it oot.

28 not bad, could get more though. keep fiddleing, and you will get it way up.


Link to comment
Share on other sites

Eagerbeaver wrote:
So is the big nut for low speeds, the middle nut for motorway cruising and the small one for overrun?

NO, the bigg one is for mainly the pree load and accel setting, too far up and you get hesitation on initial foot to floor BUTT will effect fuel all over range.  this why I said turn it down a bit, and re set middle one.

middle one is the main cruise one. as the spring is alott finer,

top is for over run fuel. no spring at all.

NOTE, some set ups had 3 springs not 2, so there will be an additional setting.
BUTTTT, all depends on what you want as well. this why I said make a note of where they were.
as you can have the bottom one well up, and the others well doon, but it will run, and run ok,ish but not pick up right.
slight mods to each will pay bigg benefits. as I used to get 30-35 out of my engine. on normal  every day type drives. and steady speeds 35-40 depending on speeds.
happy tinkering

Link to comment
Share on other sites

  • 3 weeks later...

After much tinkering I have lost where I started from and have a couple of issues.

At idle, the car smells like it is running too rich but on the air/fuel meter, it's showing about 12.5 which I thought would be about right. On acceleration, it pulls well but the meter oes down to about 10.5 which sounds rich to me. On overun, the meter goes to about 13.5 which again sounds OK. On cruise, the meter is about 12.

My biggest concern though is that after a run, No 1 plug is as white as a a white thing with added whiteness and No 6 is as sooted as a child extra from Dickens!!

Before I started, all plugs were equally sooted.

Any suggesteions as to where to go from here?

Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...