MFI air by pass screws -- Effect

[drwhosc]

Hey everyone.

Not sure my throttle bodies are in the best of shape. What is supposed to happen when you turn the by pass screws in (Less air) vs opening the screws, letting more air in.

I am not getting much of an effect if any, and I think my TB are leaky. I have a 71e with a 2.2 and my idle is very low.

[Flieger]

You would change the fuel/air mixture and therefore the idle. IIRC, Check Measure Adjust says to make sure the butterflies are all at the proper angle and thus not sitting on the bores (since this will wear out the magnesium). Then use the air bypass screws to balance the airflow across the 6 stacks. There is a spec for how much air should be flowing. I think then the idle adjustment is via the idle screw while using a CO meter in the exhaust.

[304065]

If you have a whole day to invest, you can remove the aircleaner, stacks, crossbar, then remove the throttle bodies from the engine. Be sure to have one of these handy for when the inevitable metal parts fall in:



Once you have the stacks off, you can use a shop vac with the appropriate sized nozzle, with some duct taped wrapped around it to seal it, on one side of the stack, and your STE synchrometer on the other side, in sort of an improvised flow bench. This should give you a sense of the reading with the airscrew in or out and with the throttle plate all the way to the stop.

If the airscrews don't do anything, it could be that there is too much leakage past the plates as Max indicated, or that the air bypass passages are completely clogged with carbon. The Factory recommended soaking the throttle bodies in B-12 chemtool to remove the carbon-- be careful, it's a toxic substance.

[TEC69E]

You can check the effect of the air screws on flow for each cylinder at idle by using a syncmeter. The process is: 1) get the engine up to operating temp. 2) disconnect all linkage to the throttle plates. 3) pick a stack and position the syncmeter. 4) measure air flow. 5) turn in the air screw until it stops (counting the number of turns required). 6) measure air flow. 7) return air screw back to setting in step 5. 8) repeat for all cylinders.
You should see a diff in air flow between step 4 & 6 and air flow across all cylinders at step 4 & 6 should be identical. If not your mfi is out-of-tune which could be from worn throttle plates or simply an air screw bore that is clogged with carbon.