Help with unisyn and MFI

[pszemia]

Hi, finally i have all the parts i need to do a fresh CMA, i have from an old buy a unisyn instead of the synchrometer.

The problem is that the CMA talks about measuring the air in the throttle body and then adjusting the screws.

If it's possible i don't want to buy the synchrometer because i spent a lot of money in tools to adjust the car.

Is there any tip to measure with unisyn or it's impossible?

Also, in the part-load and idle-speed, i think the CO values may not be the same as the CMA say because there is 30 years in the middle, are there any values i should use, or they are just ok?

thanks!!!!

Ariel

[pszemia]

any help here?

[911st]

That is what I used to do.

Must make sure you get a good seal on the stacks and do not drop anything in them.

Synchronized is synchronized.

If I recall, set it up to test with the throttle on its stops. Use the Unisink to make sure they are all flowing the same by adjusting the air bypass screws.

Then pull the throttle lever up so you are off the stops at say 1500-2000 rpm and check if it is still balanced. I would use the linkage to adjust balance off the throttle stops if I recall.

I used to use a wide band O2 to set A/F at freeway cruse.

Been a while so might want to double check the procedure.

[Grady Clay]

Ariel,

Pelican 911st I ‘right on’.

While ‘synchronizing’ the air flow in the throttle bodies isn’t the first step in CMA, it is useful to review some of the procedures. Remember, one of the hallmarks of CMA is to Check and Measure first, Adjust only later after you think you know what is going on.

At idle there are two paths for the air. First is past the butterfly. Second is through the air bypass port controlled by the idle air bypass screw. Of course there can also be vacuum leaks. With our geriatric MFI parts, the difficulty is determining the contribution of each.

Needless to say it is highly desirable to have clean (no carbon) parts and no vacuum leaks.

The throttles are theoretically controlled by the throttle stops. In practice the wear condition of the throttle plates and bore of the throttle bodies themselves may have a great contribution. Additionally air can leak past worn throttle shafts. These sources can be variable depending on the degree of wear.

The air bypass ports can be contaminated with corrosion carbon or other. The adjusting screw taper must not be damaged. The adjustment is from the screw fully closed lightly ‘finger tight’. Never tighten against the seat with a screwdriver. When tight, check that the spring isn’t in ‘bind’. If so, this is an indication of a damaged seat (usually from being tightened with a screwdriver).

It is reasonable for the highest idle air flow cylinder to have the air bypass completely closed if necessary to keep the idle rpm down to spec.

The three main things you are trying to achieve here are; to have the air flow balanced at idle, idle rpm & mixture appropriate and have all six throttles come off their individual stops at the exact same time. The “Correlation” procedure will determine the throttle position at other throttle settings.

There is a lot of “Chicken or egg” here. I’m going to assume no vacuum leaks, clean components and not absolutely worn out throttle bodies. Make sure all the adjustments aren’t ‘frozen’ from age or corrosion. The engine must have already been through CMA a few times so you know everything is OK and must be in good tune.

As I said above, this is NOT the first step in CMA. If the engine has ANY other issues, you are simply ‘pissing into the wind’ and may get yourself farther from the desired result.


A good first step is to simply measure the air flow of all six cylinders before you start anything. Next would be to pop off ONE end of each link and repeat the comparisons. It is useful to do a ‘spirited drive around the block’ and repeat the measurements.

Assuming there are differences in the air flow, the difficulty is in deciding if adjusting the air bypass or the throttle stop is more appropriate. Always attempt to correct the balance with the air bypass first. You may have to re-clean the air bypass ports and inspect the adjusting screw. It is not uncommon to find that in order to achieve reasonable balance, the idle will become ‘too high’. This is usually because one port is restricted. This is seriously exacerbated by worn throttle plates, throttle bodies and shafts.

Adjusting the throttle stops to a ‘more closed’ position is exceedingly difficult. The problem is that throttle plates may stick in the bore of the throttle body. You must avoid this at all cost. If you can’t get it to have reasonable idle and balance without the throttles sticking ever, you need to have the throttle body assemblies rebuilt.

Assuming you get good balance, proper idle speed and mixture, now you need to address all the throttles coming off the stops simultaneously. All the ball sockets need to be clean and lightly oiled (no grease yet). The throttle link adjustments need to be clean, oiled and free to adjust. Keep them their original length and don’t mix-up their positions.

When testing, use your thumb on the ball socket of the ling going to the bell crank at #3 cylinder and not on the cross-shaft. With very slight movement, you want to have all six throttles come off the stops at exactly the same time. You can get this to happen by slightly adjusting longer one that lags (it is usually the cylinder that pops.)
You can use the hand throttle to get a ‘just off idle’ where all six throttles are off their stops. It is useful to compare the air flow under this condition.

I can’t stress enough that this is way down in the list of the CMA procedure sequence. As you can tell, many other engine condition factors and adjustments have far greater influence and tend to mask (or fool you) in this procedure. Most important is whenever you change the idle air flow you must readjust the idle gas at the MFI pump. The engine will tend to run well when ‘too rich’ so you need an exhaust gas analyzer to tell where you are.

You are correct about the mixture specifications. You will never get proper running that lean. The goal is to have it as lean as reasonable yet still run properly.

You must also be able to intersperse this with some ‘sporting driving’ to keep the engine ‘clean’. MFI does not like to sit and idle for long periods. It wants full throttle and high rpm. Idle is so it still will be civil at an occasional stoplight.

Best,
Grady

[pszemia]

Grady, thanks for all your explanation. After reading it 3 times i understand it

I am still waiting the gastester to arrive so i can truly adjust my MFI engine with the CO values as described in the las part of the CMA.

This question was not to start the CMA i will start with the correct procedure (leaving vaccum leakeage for now) and i was having problem interpreting this point because of my unisyn.

But as you described i will mark some points on the unisyn and try to make all reach the same point adjusting the air pass.

Question about thorottle stop. should i check them with the engine off and try to see from the outside if they "leave" stop at the same time or i will have to take out all the throttle bodies to see it?

Also i was thinking if it's not too dificult i will try to remove the thottle boddies to check if the throttles are carboned up, but i will do this if i can't do "synchronizing throttle valves".

[Grady Clay]

Ariel,

Checking that the throttle stops all come off at the same time is best done with the engine running. Make sure all the link ball sockets are clean. It is best to simply oil them and add grease later. Remember to only push on the ball of the link to the bell-crank above #3 cylinder. Operating the linkage at the engine needs to duplicate the normal operation from the accelerator pedal.

You may find your hand throttle too loose and not be able to hold a throttle position. It is adjustable with the lock-nut on that side of the hand throttle and heater control assembly. There are cork clutches in there and are subject to the tightness of that lock-nut.


You may find that when they come off the stops is somewhat variable depending on the degree of wear in the shafts and throttle bodies. That is a good clue as to the wear. Another clue is how fast and consistently the engine returns to idle. If it ‘hangs up’ at high idle and then ‘settles down’ to a lower speed is another indication of wear but can also be mixture related.

While your (average) tailpipe mixture me be close to correct, if one throttle opens first that cylinder will ‘go lean’ and ‘pop’. Conversely if one cylinder lags all the others coming off the stop, it will ‘go rich’ and if extreme will also ‘pop’ slightly differently. There six cylinders, each doing its own thing.

Almost all of these Checks and Measures benefit from interspersed ‘spirited driving’.


There isn’t an absolute position on the Unisyn, it is a relative comparison among the six cylinders. The Unisyn works best when adjusted so the ball is in the upper-most third of its range. Needless to say you adjust once then compare cylinders to each other. If you readjust you simply re-compare.


Do a Pelican Search on my name using the keywords ‘MFI’ and ‘CMA2’. I wrote an expanded CMA considering the age of our MFI and what has been learned in 30 years.

On these threads I tend to expand a great deal and cover information I’m sure you already know. The purpose is to be relatively complete for someone in the future not (yet) as knowledgeable as you are now.

Best,
Grady

[pszemia]

thanks grady, i will search for that.

the little other problem is that y hand thtrottle connecting piece (the plastic one) disappeared so i have to buy one new and it will take some time to came to Argentina. Si i will work with a partner and not with the hand throttle

thanks!

ariel