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"Today I thought I would check the timing at higher rpms. With vac advance disconnected and plugged the advance range from idle to max advance is 12 to about 32 deg. With the vac advance connected and no load (racing the engine) the advance ranges from 12 ish to 50 +-2 deg."
Hi Mark,
I think what you're seeing is normal with the vacuum line plugged. The following is from something I found on the net relative to high NOx readings. I've often posted it (with more detail) here on the BB, with several successful results as far as lowering NOx. As I read it, it seems that all the timing control is in the V-C box (with none in the distributor itself).
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Any 240 that has the Chrysler ignition box will pass NOx with flying colors,
provided the cat is at all sound and the ignition secondary is in reasonable
condition, when the vacuum hose to the control unit transducer is plugged.
On the dyno and on the street, unless you're really flogging it, you can tell no difference in performance. What happens is that the "limp home mode" for the Chrysler box closely matches a conventional distributor curve; i.e. advance [timing] retards 5 degrees when the throttle is opened and climbs with engine rpm to approximately 33 degrees instead of swinging immediately to 52 degrees and backing down to below ping levels.
It's this high sustained advance on the verge of ping that drives NOx high. There was a recall on the 83-84 models to install a vacuum delay valve in the distributor hose (to cancel the retard off idle) and a delay valve in the transducer line from before the throttle (instead of manifold vacuum) to delay the advance curve to quell high NOx readings and satisfy the EPA. I rarely see one of those cars that still has that particular modification intact. There were serious performance issues even on cars in a good state of tune.
By disabling the transducer you gain all the dwell extension features of the lean burn system without sacrificing the marginal at best mpg gains that were so highly touted at the system's inception.
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Bruce Young, '93 940-NA (current), 240s (one V8), 140s, 122s, since '63.
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