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 Posted: 09-24-2005 12:25 pm
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Dave
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Hey all,

When I pull off the line 16984 hesitates, runs like a Hugo, until it gets to about 2000 - 2400 rpm then takes off fine. It does this either cold or hot, but actually a worse when hot. It also has a tendancy to want to cut off when I come to a stop when it's warmed up. I have stomburgs and a pertronics. Probably a stromburg adjustment....

Another adjustment question: Jensen's don't have oxygen sensors, but I have an oxygen meter that I can use at the exhaust. How could I use this instrument to adjust the carbs optimum?

Thanks!

Dave

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 Posted: 09-24-2005 03:52 pm
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Mark Rosenbaum
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The torque peak of a stock 907 is at 4800 rpm.  While an engine in decent tune should pull smoothly from 2000 rpm up, it just won't have much torque below 3000 rpm.  In general, one can live with this and keep the revs up, or turn the engine into a 2.2 liter unit, or add a turbocharger, or add a jet or rocket assist, or replace the 907 with some big hulking V8.

That said, possibly things with your engine are not optimal and can be improved a bit.
*  First, set the ignition timing to 12°BTDC rather than the stock 8°BTDC, with the vacuum line to the distributor disconnected and plugged.
*  Second, check your cam timing and verify that the 110 IN mark on the intake cam aligns with the 110 EX mark on the exhaust cam when the engine is at #1 TDC.
*  Third, check to see that you have the correct mixture needles in your Strombergs, and that they're in decent shape -- i.e. no bends, no scratches or grooves, and no flat spots.
*  Fourth, experiment with damper oils -- based on the symptoms reported, most likely whatever you're using now is a bit too light for your particular conditions.

Current oxygen sensors need to be quite hot to work, therefore I'd expect that yours would have to be installed at the outlet of the Y pipe if you have the stock 4-2-1 system, or at the outlet of the manifold if you have an aftermarket 4-1 header.  Normally one welds a suitable fitting to the chosen location.  That done, you can attempt to use the sensor to determine the fuel/air mix under various conditions.

Current oxygen sensors report 'way too rich' and 'way too lean' and have an extremely narrow midband that says 'a bit rich', 'just right', or 'a bit lean'.  For the midband, they  require an accurately calibrated readout device to be usable.  For most purposes, asking for direct traceability to the National Bureau of Standards is going overboard, but I would insist on trustworthy assurances about the calibration.

Without a calibrated readout, about all you can determine is whether your damper oil is way too thick or way too thin, and whether or not if your fuel / air mix is way off.  With decent calibration, you'll get a reasonable picture of the fuel / air mix your engine sees under various conditions -- sufficient for almost all purposes.  And finally, with laboratory-quality accuracy and a lot of data, you can develop a very complete understanding of how the engine is operating -- the sort of thing one does for a brand new engine design but far beyond any realistic hobby-car requirement.

For Strombergs, you would need to correlate the fuel / air mixture information for various conditions with the positions of the carburetors' vacuum valves (sliding piston things) under those same conditions.  Next would be a comparison of the profile of the installed mixture needles with those of other (available) needles, hopefully finding one that might improve overall performance.  You'd then install a pair of the new needle type, and repeat the tests under the same conditions of temperature, humidity, etc.

As an alternative, you can use a carbon monoxide (CO) sensor.  Here, installation is easy -- the sensor probe can generally just be poked into the exhaust pipe from the rear -- but the data received merely indicates how well the engine is burning whatever it's getting, so interpretation is much more difficult.

If you do go to the trouble of installing your oxygen sensor, I'd be quite interested in seeing your data.

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 Posted: 09-24-2005 05:36 pm
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Dave
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Hey Mark,

Thanks as always for details replies. To be more specific about my question: I have a VERY accurate oxygen sensor, it reads accurately from 1 ppm across the scale and it is calibrated against fresh air and a known span gas. That being said, with a well tuned engine what would I expect to see in terms of 02 at the tailpipe across the scale? Would I be hoping to see as little 02 as possible, ie it's all being burned, or should I see something different. Then is it a matter of just experiementing with the oil or do I need to adjust the needle valves, or would I need different needles to get what I'm looking for?

Dave

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 Posted: 09-24-2005 08:16 pm
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Mark Rosenbaum
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Dave,
Unfortunately we're getting a bit beyond my area of expertise here, but as I understand things, a good-running engine should have relatively little O2 in the exhaust.  The exact figures will depend on whether the caburetors are tuned for economy, power, or some compromise.

Based on old smog test data for my car, I'd expect to see something like 6% O2, 10% CO2, and 1.4% CO at 1000 rpm idle, and 4% O2, 12% CO2, and 0.8% CO at 2500 rpm.  Those figures were taken when the carbs were running a bit rich due to worn mixture needles.  The ignition was a stock points system.  (Aftermarket ignition systems could be expected to give better combustion at idle, and thus lower O2 and higher CO2 readings.)

(A fuel injected car will have much better figures as its computer varies fuel delivery so that exhaust O2 is at whatever near-minimum allows the other regulated combustion products to remain within requirements.)

As far as tuning using an O2 sensor goes, I don't know for sure but would expect that one would adjust each carb's mixture needle so you got the same results when both carbs were drawing equally as when either one was blocked off entirely.  If that method didn't work, I'd want to see the same change in O2 when either vacuum valve was lifted the same small distance by a thin screwdriver or similar tool.

For mixture needle selection, I'd really expect the stock needles to be entirely adequate, and would change them only if the indications were that the carbs were obviously rich, or obviously lean, under some circumstances.  As indicated in my previous post, the new needle choice would depend on where, precisely, things were rich or lean.

For damper oil selection, you'd want the mixture to go quite rich as the engine was accelerating, then taper smoothly to a normal fuel/air mix after a second or two at the new speed.  A very slight single overshoot then single undershoot in the fuel/air mix during this 'tapering' period means you've reached the optimum damping rate for smooth engine response (which is not necessarily what's needed for greatest overall acceleration).

For a better understanding of the internal combustion process than I can provide, you might peruse the article at
http://smogsite.com/5gastheory.html  .

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 Posted: 09-25-2005 05:54 pm
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edward_davis
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Mark,

What is the rationale for disconnecting the vacuum unit on the distributor?

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 Posted: 09-25-2005 11:21 pm
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Mark Rosenbaum
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Unlike most cars of the period, which used a vacuum advance mechanism, the vacuum capsule on the JH retards the timing under high vacuum conditions. Supposedly this reduced emissions at idle.  The facts may well be different.  Here's what I've learned.

1.  The vacuum capsule retards the timing by about 5°, so the static timing of 8°BTDC becomes dynamic timing of 3°BTDC at idle.  My JH ran about 20°F hotter with the retarded timing.

2.  By actual test, emissions were generally worse with the retarded timing.  Two CA smog tests, performed sequentially on the same machine, gave the data below.   The only change was ignition timing.  The tests were close enough together in time, that the coolant temperature remained substantially the same (i.e. within 10°F).

3°BTDC:
HC 498ppm, CO 0.30%, CO2 10.30% at idle (940 rpm)
HC 031ppm, CO 0.10%, CO2 11.80% at cruise (2704 rpm)
8°BTDC:
HC 152ppm, CO 0.45%, CO2 10.80% at idle (985 rpm)
HC 078ppm, CO 0.53%, CO2 11.80% at cruise (2375 rpm)

The difference in idle HC readings is huge, favoring the more advanced timing.  It is possible that the change was due to a radical improvement in gas dynamics, and/or effective compression, between the two engine speeds, but I prefer the explanation that the advanced timing resulted in more complete combustion before exhaust valve opening.

The cruise HC and CO readings are higher at -8° and 2375 rpm, than at -3° and 2704 rpm.  Lacking further evidence, it seems reasonable to presume that this is due to slightly different fuel/air ratios at the two speeds, rather than to the change in ignition timing.

I do realize that my data isn't conclusive, but as I seem to be lacking my own personal emissions tester, it's all I have to work with.  I'd love to see whatever studies Lotus may have performed when the 907 engine was designed....

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 Posted: 10-10-2005 11:32 pm
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Esprit2
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Mark,

I believe the main reason for the vacuum retard had to do with reducing the spike in oxides of nitrogen that occurred on closed-throttle over-run.   Something that's not usually checked during most emissions checks.

The retard also gave a smoother, more benign idle.

Overall,  I think the engine runs better with the vacuum capsule blanked off.   The steady-state power and torque output isn't significantly different either way,  but the throttle response off idle is better without it.

Regards,

Tim Engel

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