Tempstar Furnace burners wont light

I have an older (1988) Tempstar natural gas furnace. The furnace will turn on the exhaust fan and I can see and hear the igniter trying to light the burners. I have a Honeywell ignition gas control valve (vr8440p2088) I noticed it has a manual on/off knob that has broken off; it also has push and turn written on the knob. When I replace the broken knob on the control value, I can get the burners to light and normal operation of furnace resumes. The catch is I have to hold on the button down for gas to flow to the burners. Once I release the knob and let it go back up the burners shut off and it the igniter try’s to light it again.
What does that knob do when it’s pushed in?
Any help is appreciated.
see photos
http://img24.imageshack.us/g/dscn1505v.jpg/

 

wow,this is a tough one.....the function of the knob is to allow the valve to be turned to the on position,and yet allow the gas valve to shutdown immediately upon a flame malfunction.the reason the knob has push and turn,is that when you do that,and release the knob,the safety spring and design of the gas valve comes into operation...if the knob is broken,there is NO SAFE WAY TO REPAIR THIS...IN OTHERWORDS,THE FUNCTION OF THE KNOB IS TIED TO THE SAFE OPERATION OF THE GAS VALVE.try contacting honeywell directly,to see if a replacement knob is still available,if not the only other option is to replace the valve.like I said to not alter or operate this valve without it being fixed,or replaced....

 

I suspect the flame sensor. If it is not signaling that there is a flame, the gas will cut off. Look for a metal rod in front of the flame end of one of the burners. It should be held in place by one or two screws and have a wire running from the bottom. Remove it, clean it by sanding lengthwise with fine steel wool or very fine sandpaper. You just want to polish it, not sand it away. Re install and see if that cures the problem

 

Have a furnace repair company replace the valve.

HONEYWELL 24 VOLT GAS VALVE VR8440 - eBay (item 160387434923 end time Jan-13-10 21:00:24 PST)

 

If the burner stays on though, indefinitely, for as long as he holds the (remnant?) of the knob down, that woud not be it. But if the burner fire went out in a few seconds, even if he stayed holding down on the button, then it would be.

 

ECMAN 51, my thought was that he may be over riding the shutdown by manually keeping the valve open. Not being a pro, I will be interested in the final outcome.