Testing Vacuum Advance

[Griz]

Getting near to firing up the BJ7 for the first time, and prior to setting the static timing I decided to check that the vacuum advance was working. I've read the procedure to do this and that there are supposed to be three numbers on the vacuum unit that correspond to point were advance begins, point where maximum advance is achieved, and total degrees of advance. Problem is, there were no markings on my unit. Here are the readings I got:
Vacuum advance begins at 7" hg
Max vacuum advance at 17" hg
Does this sound about right to you all?

Griz

 

[vette]

Sounds about right to me. You'll note that max is at 17" hg. And at idle most engines are around 15 to 18 or 19. But the thing about vacuum advance is to study what your engine is doing while you are driving. An interesting exercise is to hook up a vacuum gauge with a line long enough to bring the gauge into the cockpit with you. then go for a ride.

 

[Griz]

Thanks for the confirmation Vette. Good idea about having the vac gauge along for a ride, but I'm probably 6 months away from driving the BJ7 at my current rate of progress and with Montana winter coming on. But I do hope to have it fired up in the next few weeks.

Griz

 

[John Turney]

I'm not sure about other Healeys, but the vacuum port on my rear HD8 doesn't show measurable vacuum at idle. It increases with RPM.

 

[vette]

Hello John, idle vacuum can be affected a lot by how the carbs are adjusted. On carburated V8 engines it is significantly affected by the position of the throttle plates. On hot rodded engines people tend to open up the throttles more because the cam won't let it idle. They'll start to open the secondary throttle to keep it idleing. This kills idle vacuum because all the suction of the engine is then being pulled thru the open throttle plates instead of a smaller area of just a slightly open primary throttle.
About HD8 carbs, I have them on my BJ7, people have argued with me about the correct way to tune them (adjust them). If you read the Carb adjusting section in the Bentley, Austin Healey workshop manual you will see that HD8 carbs are to be adjusted for idle with the throttle plates "COMPLETELY" closed. They are to run on the large idle screw on the right side of the carb. The jet is adjusted with the small jet adjuster screw, ( it becomes apparent that the HD8 carb is different than most SUs). With the mixture set right with the jet adjuster screw it is intended that at idle all the fuel/air mix is drawn thru the large idle screw. In reality the HD8s don't work as good as the book would make you to believe and most people will open the throttle plates just a little to get a good idle. This then also necessitates going thru the carb balancing route to get the carbs balanced. This also will reduce the vacuum available at the vacuum take off port because the throttle plates are open some. I ran my BJ7 with the throttle plates closed at idle for years (this also eliminates the need to balance the carbs because the throttle plates are completely closed at idle, how much more balanced could you get), But back to reality , the large idle screws do not flow enough volume to let the engine idle sufficiently and my idle was always low, like around 400 or 500 rpm. At first i thought this was neat but as time wore on I wanted it to idle more near 800 so I too opened the throttle plates just a teak.

 

[RestoreThemAll]

Griz! We're still at about the same place in our BJ7 restores. I tried to fire mine up last weekend. I had no spark and a flooding carb late Sunday night. Those issues are fixed. My friends are coming on Saturday to give it another try. I did the pre prime routine and ended up with over 55 lbs of oil pressure spinning it with no spark plugs. That's a good start.

I also won't get to drive the car until spring. I'm in NYS where we may not get Montana weather but can get hammered with snow and ice. They use too much SALT here. I need to take care of loose ends and put the body back together.