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Discussion in 'Technical Q&A' started by petec, Dec 23, 2003.
What is the procedure for setting the idle? I cannot seem to get it to smooth out at 1000 rpms.
The idle speed is controlled with the bypass screws.
You need to mechanically check that the butterfly gaps are both 003" first. Then set the idle while using a vacuum gauge and gas analyser to balance the banks symmetrically.
I'm having similar problems myself...I've just ordered the workshop manual so hopefully that will have the correct proceedure in it. When you say set the thottle butterflies at .003", is this between the butterflies and the throttle body, or between the actuating lever and the idle throttle adjustable stop? Where do you measure the intake vacume?
My idle is also running high at about 1300, I am bringing it to Classic Coach to let them do it, along with repainting my Boxer trim. So much easier, I am so paranoid of F'ing it up.
Two tech. articles here. One on setting the idle, one on tuning a carb'd Boxer.
where you see the stars insert, as one word, ferrari then - then talk.com (no spaces)
The 003" gap is between the butterfly and the housing...so it's very slightly curved and tricky to measure....but as long as they're both identical and as close to 003" as you can get, then you're in the ball park.
The vacuum is simply "t" piece into the ignition/emission gear take offs. Anywhere that's downstream (depression side) of the butterfly. It's been ages since I did one, but about 850-1000 mbar @ 1000 rpm each bank seems to ring a bell. But symmetry is what we're after.
You need to make sure the butterfly housings are clean, and also the bypass passage and screw tip are clean too. Remove them to do this, then screw them both in until they stop, and then out the same number of turns...about 3 full turns to start. Make 1/4 turn adjustments after that.
It's all part of a big service, so check with your history.
Once the butterfly is clean and set you don't touch it again...ever really. It's only people fiddling with what they don't understand that lead to problems in the first place. There (is from new, and) should be plastic covers over the butterfly adjustment screws to stop fiddlers.
Check by pulling the throttle cable that both open at exactly the same time. DO NOT push from the linkage (like so many do). This takes up any slack from the wrong direction and changes the synchronisation. (On carb 308's it also leads to big buzzes as the cable can get stuck on a carb body!)
When you do the tune adjustments, each mixture or speed change will slightly effect the other bank too, so constant back tracking is needed. Follow your exhaust system to be sure you're reading emissions from the correct side......they cross over on a std system. make sure the probe is at least 50 cm up inside the tail pipe.
The mixture screws are very sensitive...make 1/16 turns only, and always go lean (anti clockwise) first, then bring up to be rich.
You're after about 0.8-1.2% CO at 1000rpm with ign set to within +- 2 deg from perfect std settings. HC can be as low as 200ppm on a good car. 500ppm worse case. Use good quality high octane fuel. Unleaded is ok for 75% of the time.
Bring the revs up to about 2500rpm and watch the HC rise initially, then fall to about 50ppm with CO at 0.4- 0.6.
Coincide your tuning adjustments to be between coolong fan cycles, as they effect vacuum and emissions. AC off.
I thought I remembered the book saying .007" ?
Haven't followed your links, but don't confuse the fuel metering plates in the injection unit, with the throttle butterfly.
A .007" gap on the throttles may make it impossible to reduce the idle enough using the bypass screws. Basically, the butterfly gaps should be set (symmetrically) in order to have the idle speed at 1000rpm with the bypass screws in the middle of their range for adjustment. If .003" butterfly gap means the bypass screws are right out to raise the idle speed, then open the butterfly some more and reduce the bypass opening.....
The gap for the metering plate is more like the figure you suggest. But I dont have the specs with me to be sure.