Carbs: Manometer Vs. SK

A couple of years ago when I had the intake manifolds off the 308 and we pried off the (3/16th or so) steel tubular manifold from both intake manifolds and pressed in custom made vacuum nipples. The purpose was two-fold. First, with an appropriate network of silicon hose and connectors, to provide a multi-cylinder vacuum source for an advance sensor on my Electromotive HPX ignition system; second, was to provide the ability to hook up a set of (mercury) manometers to sync the carbs.

First piece works, and has worked, fine. Dials in a bit more advance at partial throttle/overrun/idle, and should help fuel consumption.

Second piece has been more of a challenge, and invariably I have pulled out the SK. Well, I thought I had the carbs sync'd pretty well, but when my Porsche owning neighbor opined he could hear the carbs weren't quite right, I figured I'd have another go. Nothing like a challenge. Having checked/reset the float heights and disconnected the linkages between F, R, I sync'd all barrels to within 1/2 kg or each other. All were now flowing 5 1/2 kg (per minute I assume). Car sounded good. Drove well.

However, the "little man in the back of my head" was saying "try the manometers again". So, I hooked them up and saw a spread of about 1 1/2 inches high to low across the barrels. (This is with P6 cams where there's not a lot of vacuum to start -- typically 8 to 10 inches with a set of restrictors in the lines). Well the first piece is figuring out how you adjust them. Of course, it helps to remember higher flow equates to lower vacuum...

Anyway, an hour of hot work later, the variation in vacuum was down to 1/4 inch. A ten degree turn of the main throttle screws would make a difference, so it is a fine adjustment. After reconnecting the linkage and being careful to get the front and back pulling equally/at the same time (a sixteenth of a turn made a difference here) I took a short spin up the road. Difference is extraordinary. Spins up and accelerates much more quickly. I had thought a slight stumbling low down was an artifact of lumpy cams (the P6s) and big (44 DCNF) carbs running larger (34mm) chokes. Completely gone. Can really feel the lighter weight flywheel now.

Nice when there's a happy ending,
Philip

 

I used 2 "carb sticks", each with 4 columns, available from JC Whitney or similar sources. I don't recall price, but do recall they were cheap. The trick part of the puzzle was the machining of the "nipples" to fit into the intake manifolds. We machined from brass and pressed in.

Peter, I had the SK so I really couldn't see read any difference. Sounded good, ran well. Then I put the mercury columns on. Leads to a new level of precision. Next time you try sync'ing, turn one of the throttle screws 1/16th or 1/32nd of a turn. Can you see any difference in the airflow?

Bottom line: You just can't get the precision with the SK you can with the mercury columns and, yes, I can definitely feel the difference on the road.

 

Less vacuum than if you were drawing lower levels of air, yes. Vacuum and airflow are inverse. Can't opine on absolute level. Also, remember, the port I am referring to is below the throttle plate.

If you spray something flamable like carb cleaner around the base of the carbs and the throttle shafts (not in the venturis) and the RPM increases then yes, you have a leak.

If you are recording high airflow at a normal level of idle (850 or so) that would also suggest low vacuum and a leak.
HTH,
Philip

 

I think it is a commonly(?) understood component of fluid dynamics -- why aeroplanes fly etc.
Philip