View single post by Mark Rosenbaum
 Posted: 05-07-2005 04:52 pm
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Mark Rosenbaum



Joined: 03-12-2005
Location: Kingman, Arizona USA
Posts: 532
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Congratulations on waking up your car after its long sleep.  Sounds like you have the usual sort of teething problems, all of which can be fixed (eventually).

The fuel leak from the choke implies that either the front carb's fuel inlet valve is not sealing properly, or the float level is set too high, or perhaps both.  Over-tightening the fuel inlet valve may distort it to the point that it won't seal properly.  Probably you'll have to pull the carb to find out what's wrong.  Whatever it is, is also likely to be the cause of your black smoke.

The rebuild kit should have included the choke-to-carb and choke-internal gaskets.  See the attached photo.  If your kits were missing these, and if the old gaskets are not reusable, contact me directly as I've doubtless got some left-overs I could send you.

Unless the temp compensators on both carbs are disabled, carb balance needs to be set after the engine is fully warmed up.  From your post it isn't clear if you set the balance before that.

As far as mixture goes, it's hard or impossible to set the mixture properly until everything else is taken care of, specifically including the elimination of all fuel leaks and incorrect float level settings, so I advise returning the mixture needles to their 'datum' positions for now.

Don't know if you've read the Stromberg rebuild article that's currently appearing in the JHPS magazine but I do comment there on rebuilding the vacuum switch, which may be possible in your case.  This switch, and the throttle bypass valves, allow the engine to return to idle relatively slowly when the throttle is closed.  The fact that these are currently disabled is quite possibly contributing to your near-stall upon returning to idle, though there may be other factors involved as well.

Normal idle speed drift on a stock 907 engine in good condition, with stock cams, seems to be about 25-30 rpm on either side of the nominal 975 rpm setpoint.  This is actually pretty stable for an engine with cams as radical as the stock grind.  The 200-300 rpm you observe is a bit too much.  Unbalanced carburetion, too lean a mixture, and low manifold vacuum can often cause such drift, but as Kurt points out, ignition is often the true culprit for an alleged carburetion problem.  Things to check include:

(a)  Is your vacuum retard mechanism hooked up?  If so, disable the thing while you troubleshoot.  If the internal diaphragm is damaged and has a leak that varies with diaphragm position, it can cause the hunting idle you describe.  (For that matter, so can excess space between the carb piston [vacuum valve] and the carb top, or a lack of damper oil.)

(b) Have you checked the distributor and made sure that its advance springs are okay and that the advance weights move freely?  If these are intermittently sticky, the ignition advance will vary erratically and can cause all sorts of odd problems at idle and elsewhere.

(c) Are the wires connecting the points to the blade connector, and the rotating plate to the body of the distributor, properly limp?  Stiff wires here can do really odd things to the ignition advance/retard.  Things can get quite misleading if the wires' flexibility varies with temperature.

(d) What sort of intake manifold vacuum are you seeing?  With a somewhat retarded spark such as the 7^ BTDC that you mention, you probably have only 12-13" Hg of vacuum.  Try advancing the timing to about 12^ BTDC while watching the vacuum gauge.  With stock cams, the manifold vacuum should climb to 15" Hg or slightly more, and you should see upwards of 18" Hg momentarily when closing the throttle.

And finally, if you're getting lots of oil in the combustion chambers, then the car won't ever run perfectly.  As Kurt notes, going one step hotter on the plugs may help, if the oiling isn't too bad, but going too far on the plug heat range can result in major engine damage.

Attachment: Stromberg Choke Assy Gaskets.jpg (Downloaded 360 times)