View single post by Mark Rosenbaum
 Posted: 05-09-2005 09:00 pm
PM Quote Reply Full Topic
Mark Rosenbaum

Joined: 03-12-2005
Location: Kingman, Arizona USA
Posts: 532
Gary, it sounds like you've made major progress.

Choke -- You are correct, there is no seal on the choke shaft and it's possible for fuel to leak there.  However, in normal operation, any fuel in the choke is drawn in then out by suction from the two carbs.  I would expect that normally any leakage past the shaft would be quite low.  I suppose it might be possible to fit an o-ring, but that would probably require machining something, and might stiffen up the mechanism so much that a solid rather than stranded cable would be required.  It would probably be far easier just to find a choke mechanism that didn't leak. 

Tuning -- What you describe is normal for a JH.  The slightly rich scent of the exhaust is probably nothing more than an indication that the carbs are set a bit rich at present.  This is normally dealt with when the mixture is being set.

Dwell -- When the points gap gets down to 0.010" or thereabouts the engine becomes hard to start, is prone to missing particularly under load, and tends to have less power, all caused by arcing across the points.  Dwell should be as specified when the gap is in spec.  If not, either the dwell meter is itself inaccurate (this is almost guaranteed if it's an inexpensive unit), or the points cam in the distributor is worn out.  If it were my car, I'd set the points gap to spec, ignore the dwell reading, and wouldn't plan for a Pertronix device unless the car ran poorly.

Tach accuracy -- Guessing here, as AFAIK there are no specs.  Back in the 1970s, I would have expected readings to be within 6% of actual up to 4500 RPM or so, and within 10% of actual above that.  The meter movement itself is probably accurate to 2% or 2.5% and this won't change significantly as long as its bearings remain clean.  The electronics will change as they age, and after 30 years, one might be fortunate if the instrument were to read within 20% of actual RPM.  As long as none of the electronics components have actually failed, the calibration adjustment inside the tach should allow compensation for age-related changes.  Calibration, however, requires knowing the actual RPM.  I would not trust an inexpensive tach/dwell meter for this.

Oil Pressure Rise Time -- What you report sounds quite normal.  A 907 in good condition that last ran within a day or so ought to show oil pressure quite suddenly about 3 to 6 seconds after the engine has started, or within 10 seconds if it's sat for a month.  If you monitor this time (by mentally counting seconds) at every engine start, you can spot potential problems before they get to cause any damage.

For example, if the rise time increases by 3 to 6 seconds after a filter change, the filter has a leaky anti-drainback valve and should be replaced.  This might occur perhaps 1% of the time with any filter brand, and can be more frequent in low-cost filters.  The problem is exacerbated in the 907 engine as the filter is horizontal and will inevitably drain somewhat after engine shutdown.

If the rise time increases to 10-15 seconds, your oil pump is in the process of failing.  The pump typically fails by suddenly losing the ability to prime itself; only in really bad cases will it quit providing pressure while the engine is running.

Oil pressure -- 25 psi when the engine is fully warmed up is pretty good, particularly if the oil cooler is not being used.  You should expect a drop of 5-10 psi (or perhaps far more without the oil cooler) if you should ever have to use 10W40 oil.  As long as you have at least 5 psi pressure at 1000 rpm idle, and at least 30-40 psi at freeway speeds, you should be all right.  OTOH, for racing you probably need at least 10 psi per 1000 rpm.