MFI Throttle Body Help

lwa · 10-14-2008, 11:37 AM

The car is a 71E Targa. I recently received my TBs back from the rebuilder. Prior to sending them out, I worked my way through CMA, up to the point of adjusting correlation. After reinstalling the TBs and carefully setting the linkage, the car idled at about 2200 RPM. I closed up the air bypass screws, all the way in with some variation for correlation and the car now idles at 1350 RPM.

I've confirmed the RPM measurement using the timing light. I've check for leaks using some carb cleaner. I've rechecked the timing. The CO is roughly at 6.5%. Before fix this by adjusting the TB stops, is there anything else that I am missing? Per the C of A, the engine is original to the car, but I don't have the repair history and there is evidence of previous Mickey Mouse repairs. If the engine was rebuilt using T pistons, would this cause this symptom?

Larry
71E Targa

304065 · 10-14-2008, 11:55 AM

Larry, too much advance. Probably your distributor is sticking. What is timing at idle?

Depending on who rebuilt your TBs they should have been flowed on the bench and had the linkages set so they all flow the same. Small variations are taken up with the adjustment screws. Now if whomever rebuilt them left the linkage adjustment up to you, that's a different matter, they should come off the car and be set up first.

lwa · 10-14-2008, 08:07 PM

John, thank you for the reply.

The timing is 0 degrees at 900 RPM (cold engine). I had the distributor rebuilt this past winter.

It appears the TBs were set up on a flow bench when rebuilt. The stop screws are marked with paint, as are the 4 linkages between the 6 throttles. That is why I am cautious about adjusting the stops.

Larry
71 911E Targa

Flieger · Flieger's Garage

You have the 114mm injection pump rod adjusted to spec?

304065 · 10-15-2008, 05:29 AM

Larry,

OK, too much air. Try this. Using blue painters tape and a sharpie, put a little flag of tape around the shaft of each linkage rod. Mark the flag in a way that will allow you to replace the rods in the same orientation, e.g. 1-2, 2-3, etc. Now remove the rods and start the car.

Timing is 0 degrees at 900 RPM, but I thought your idle was 1,350? At 1,350 you should be showing some advance. Try retarding static timing (this can be done with a test light with the engine off)-- you want to eliminate ANY advance because that will make the engine speed up.

This is the classic MFI vicious cycle, by the way. The TBs pass too much air due to wear (not your rebuilt ones) so the air correctors are used to lower the idle. The mixture is so rich at startup (thermostat cold) that the idle is nice and low (10:1 AFR will do that) but as soon as it warms up and nudges closer to Stoch the engine speeds up a little, kicking in the advance and pretty soon you are sitting there on the starting grid with the engine revving 2200 and the marshals looking at you like "I wonder when this guy is going to do a burnout?"

lwa · 10-15-2008, 08:42 PM

The 900 RPM measurement was a quick check made on a cold engine.

I rechecked the injection pump linkage and it is 114 mm center to center.

Is the intention of removing the linkages to ensure there is no pre-load on each throttle? How much should I retard the timing?

Larry
71 911E Targa

Matt Blast · 10-16-2008, 02:21 AM

Larry,

I think John's comment about the linkage is so you can put them back on the same way they were taken off. His point is to make sure the throttle levers are resting on their stops with linkage rods off. Good point. My guess is you will still find that your idle is too high. You have one or two problems with the throttle bodies, or a combination of both. If the throttle bodies are new or properly rebuilt and you turn the air screws in all the way the engine should turn off. Kind of like putting six tennis balls on your intake stacks.

One possibility is your throttle stops are turned in too far which would make the plates open too much allowing too much air into the engine. The other possibility is they were cleaned when they were apart, rebushed or not, and then put back together. The paint on the linkage does not mean they were flow tested it just means they were painted. I'm running out of room.
Matt

lwa · 10-16-2008, 08:33 PM

I allowed the car to warm up and removed the linkages, including the one to the pump. I moved the timing around to see what it did to the idle speed. I made the following measurements:

RPM Timing
1270 0 degrees (this is about where I started the evening)
1000 5-10 degrees ATDC
800 10-15 degrees ATDC

I am trying to contact the rebuilder to confirm he set/tested flow, but so far he has not responded. Is flow the likely issue or do I have a distributor problem?

Larry
71 911E Targa

Matt Blast · 10-18-2008, 02:02 AM

Larry,

Take the throttle bodies off the car, remove the stacks. Go somewhere where the surrounding light is not too bright. A dim room is good. Get a BRIGHT light source (a lamp without a shade works best). Look through each barrel one at a time, from the bottom to the top, with the bright light source directly behind each throttle plate.

With the throttle arms still on their stops it should be difficult for you to see any light. Small slivers of light where the plate is just beginning to open is good, but not that easy to see. Light that is easy to see that is completely around the plate, or on either side where the shaft runs through the housing is a bad sign. I suspect you will find lots of light around the plates. Sometimes the space between the plate and the housing is so big you can slide a small piece of newspaper (about 0.06mm thick) around the plates...a very bad sign. Try this also. This will require some work but its a start. I hope this helps.

Matt

lwa · 10-20-2008, 08:30 PM

To close out the thread...

Thanks to all that responded.

I am confident the TBs were rebuilt. Prior to sending them out, light would readily leak around the butterflys. Using carb cleaner, I could detect some air leakage around the bushings. Even with the stops completely backed out, the lowest idle I could get was about 1300 RPM. The old butterflys were marked Eurometrix, I guess one of the old owners had loved the car.

In this case I sent them to Supertec. The butterflys were replaced. Everything was plated all shiny. Light leakage was not observable around the butterflys. I assume the bushings were replaced. In the course of troubleshooting, I did squirt them with carb cleaner and did not see a change in idle. The stops and the linkages had paint on them, which indicated to me that flow was set/tested. Unfortunately Henry did not respond to e-mails asking for confirmation.

I guess if they were tested, someone goofed and set the wrong flow. I adjusted the stops, linkages, and air bypass screws and obtained an idle of about 900 RPM with good correlation. I ended up backing off the stops about 1 rotation. To amplify another thread, a tall socket and a universal joint allows you to release/tighten the nut on the stops without removing the TB from the engine.

Larry
71 911E Targa