250mmHg idle vacuum debunked.

Hi gang. There're several very well written TB synch procedures, but almost everybody got the vacuum figures wrong, including me. Most said we need to synch at 250mmHg, including me. Others that we needed to adjust for altlitude, which is not correct either (it happens automatically). And a few others, including Radman, that it didn't matter. Well, IT DOESN'T MATTER indeed. Here's why.

First, the common sense answer: you CANNOT alter the vacuum level at the same rpms when TBs are synched. Sure, if you move one air screw, vacuum goes up or down on that cylinder, but when you synch the others to it and adjust idle to the same 1K rpms, vacuum level will always be the same.

Second, the visual inspection answer. If you check your air screws folks, and follow all vacuum hoses, you'll see that all are fed by a vacuum valve connected to the air box, and regulated by...... drum rolls please........ the idle valve screw. So it doesn't matter if you turn the air screws out and the idle screw in, or viceversa; as long as you can set your idle speed to 1K rpm, you're fine, and vacuum will always be the same, whatever it is (depends on altitude). In other words, the idle screw is the MASTER air screw, feeding individual air screws.

Third, for the ones who don't understand, the physical experiment. Tried the #3 air screw 1/2-turn out, then 1-turn out, and when the other TBs were synched and idle speed adjusted to 1K rpm, TA DA!!, same vacuum level, which at my 4K' of altitude is around 230mmHg.

The best setting IMO was with #3 1-full-turn out, then the other TBs synched to it to 1K rpms. Idle speed dropped slightly less on hot restarts; guess air flows quicker that way when starting the engine.

Oh, and when you use your throttle lock to adjust the middle screw (unauthorized TB synch), REMEMBER TO DISENGAGE IT AFTERWARDS. Had my 411-mile-old engine bouncing against the rev limiter for the second it took me to figure what the f*ck was going on. Geez. The good thing was engine was hot (4 bars), and oil fully hot; hope I didn't take a few thousand miles of life out of my engine :angry: . By the way, just changed oil to Mobil1 15/50 full synthetic, which supposedly keeps cylinder walls lubricated even when engine is off. Words of encouragement welcome folks.

What matters is that the TBs are synched to each other, that is all. The 250 mm Hg is a relative, baseline setting, but is not critical.

-BD

I don't think the air flows for each of the TB air screws passes through the idle air adjustment valve. The idle air adjustment valve was added on the '06 and '07. On the earlier models turning the idle adjustment knob moved the closed, resting position of the butterfly valves. But on the '06 and '07 the butterfly valves' closed position is fixed and not adjustable. The idle adjustment now introduces an adjustable flow of air via a hose to each of the TBs.

So the total air flow through each TB is the sum of the air screw flow, the idle air valve flow, and the butterfly valve flow. I believe the air screw flow and idle air valve flow are distinct and separate.

Also when the coolant is below normal operating temperature an additional air flow occurs in each TB. The cold high idle rpm is a function of air flow via the melting wax control assembly below the TB assembly.

So there are four separate air flow mechanisms on the '06 and '07. The cold high idle air flow which is at its maximum at a cold engine startup. This flow gradually tapers off to zero or some small value as the engine coolant warms. The air screw flow which is adjusted for equal TB vacuums. The idle air adjustment flow which is adjusted for idle rpm. And finally the butterfly valve flow which is controlled by the throttle grip opening.

Constant Mesh said:

The air screw flow which is adjusted for equal TB vacuums. The idle air adjustment flow which is adjusted for idle rpm.

Click to expand...

Without any of the PAIR crap, you can clearly see the idle screw feeds the air screws, so they're redundant controls in a way. Both change vacuum levels, which in turn alter rpms. Open all your air screws the same amount and your idle goes up (and viceversa), which is exactly what the idle screw does. This balancing act works with the air screw on #3 open from 1/2 to 1 turn out from seated. Any more or less and you might not be able to achieve a 1K rpm idle.

But you're absolutely right about the cold idle enrichment and butterfly valves also supplying air. And yes, the days of adjusting idle via the idle stop screw are gone. Later.

JC