Air pulse system on airheads

[rban]

Photo of ruined exhaust valve from my "84 R100RS ... compared to intake valve on left. That knife edge would soon fail, causing valve to retract far enough for keepers to separate and then valve to hit piston.

What was the tip-off for you, was it the lack of adjustment remaining on the rocker arms? Or something else?

 

[stevenrankin]

My Valves on my 84 R80RT went at about 56K, the tip off was poor idle, and more frequent valve adjustments. Instead of the 15k adjust interval, I was adjusting at first at 5K then the mileage got less between adjustments. I recall at the time the interval reached 5K, I pulled the heads and had them rebuilt. St.

 

[mneblett]

Burnt valves?
Really?
Anyone want to explain exactly how the Pulse Air System works?
I’ve asked this question for 40 years and have never heard it correctly explained.
Even the Rubber Chicken Racing dude couldn’t answer the question correctly.

What's to explain? It's well known that pressure in the headpipe momentarily drops between the high-pressure pulses of exhaust gas passing down the exhaust. This phenomena is the basis for the art of header tuning, which attempts to vary header length, diameter, bend shapes, adjacent cylinder exhaust pipe cross-connection location, arrangement of pipes to take advantage of multi-cylinder firing order, etc., etc. to cause the timing of the peak of the lower pressure to arrive at the exhaust port at the same time the exhaust valve opens in the next combustion cycle to assist in cylinder evacuation (the increased exhaust evacuation from the cylinder leaves less residual exhaust gas in the combustion chamber, thereby allowing the descending piston to generate a greater intake vacuum to suck in more intake air/fuel into a cylinder to increase power output).*

In the pulse air system, the moments of reduced pressure allow air from the airbox to be sucked into the exhaust, right next to the exhaust valve (note where the pulse air connection enters the front of the head).** The excess oxygen causes unburnt fuel in the exhaust to ignite (unburnt fuel being common in the days of sorta-accurate mixture-generating carbs, unlike today's hyper-controlled fuel injection and closed loop oxygen sensing systems). The burning of the excess fuel gave both the benefit of a reduction in unburnt hydrocarbon emissions, and the detriment of generating a large amount of additional heat right at the exhaust port, which in turn increased temperature at the immediately adjacent exhaust valve. Higher heat = compromised long-term component integrity. A pretty straight-forward cause-effect arrangement.

* Ever wonder why BMW added cross pipes between the boxer headers? The cross-connection allows one cylinder's exhaust to help with improving the timing of the pressure waves in the other exhaust. Similarly, control of the same phenomena is behind the inclusion on modern BMWs of a variable-opening valve near the muffler, which can alter back pressure (i.e., alter pressure wave behavior back at the exhaust port) to suit the current throttle position. rpm, etc., etc.

** The "pulse"of the pulse air system comes from the air flow through the pulse air tube not being continuous, instead passing into the exhaust in pulses. When the pressure in the headpipe is decreased, air is sucked down the tube; conversely, when a high-pressure exhaust pulse is passing down the head pipe, there is no airflow because the backflow-prevention in the system blocks exhaust gas flow back to the air box.

 

[kbasa]

What was the tip-off for you, was it the lack of adjustment remaining on the rocker arms? Or something else?

For both my '83 RS and my '84 CS, the tip off was frequent valve adjustments when it would stop idling well and running out of adjustment at the rocker.

It's gonna happen, so on the CS, my experience with the RS's valve recession issues helped me see it sooner and deal with it.