View single post by Mark Rosenbaum
 Posted: 04-05-2005 03:46 am
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Mark Rosenbaum

Joined: 03-12-2005
Location: Kingman, Arizona USA
Posts: 532
The allen wrench used in the adjuster for the mixture needle is 1/8" across flats and about 6" long.  The adjuster tool has a hole down the center for its (included) allen wrench, and one or two little pins sticking out to catch the slots in the top of the vacuum valve so it won't rotate and tear the carb diaphragm during adjustment.  The tool is $9 or so, only two or three times what one pays for a single long allen wrench, so to me it makes sense to buy the tool.

If the carb mixture needles were set to the 'datum' point the mixture should be quite close to correct.  However, this assumes needles of the correct contour.  Hate to tell you but sanding down the needles is a very bad idea, as a change in diameter of a tenth of 0.001" is significant.  New needles are expensive, $25-30 each.  But if you were very careful you may have gotten lucky and won't need to replace them just yet.  As the correct needles for a JH are often back-ordered for months at a time it's wise to order them long before you expect to need them.

The mixture is checked by raising each vacuum valve only 1/32" not 1/8".  If the engine then speeds up the mixture is too rich, and if it slows down the mixture is too lean..  This is quite subtle and it takes a lot of practice to do it right (I'm still learning how).  Alternately you can stick a $150 carbon monoxide tester up the car's exhaust and get the two carbs set just about perfect.  In any case, to adjust mixture, turn the allen wrench part of the adjuster tool CLOCKwise to make more rich, or COUNTERclockwise to make more lean.  Adjust 1/6 turn at a time -- no more, no less.  Recheck carb balance after the mixture seems correct.  Generally the whole process needs to be done 3 or 4 times before it's right.  See section C11 of the shop manual for the factory's way of describing all this.

An engine that's running hot where previously it ran normally most likely has too lean a mixture, or ignition timing that is too retarded, or a combination of these.  Check for vacuum leaks.  Make sure that the distributor's centrifugal advance works.  Make sure that what the timing mark claims is TDC, really is.

The throttle bypass valves don't really affect mixture, they just act as a partly open throttle when activated during deceleration.  The effect of the temp comp valves on mixture really only shows up during an emissions check.  If in doubt, though, these valves can be disabled temporarily for troubleshooting, but I don't think they're part of your problem.

You do have a problem if the timing light flashes triggered by the #1 and #4 spark plugs don't occur at exactly the same relative time (i.e. within one crankshaft degree or less).  There is definitely something screwy there -- radically different compression in the two cylinders, really loose distributor bushings, bad distributor cam, weak coil, bad or different type plug wires, etc.  This needs to be looked at.

Wrong or no damper oil will result in less acceleration but does not have any effect on peak power.  Start with 10W40 or 20W50, which usually works quite well.  Thicker oil means a richer mixture during acceleration.  If you get black smoke out the exhaust but only while accelerating, go to a lighter oil.  Don't use anything that will absorb water, though, or your carbs will eventually freeze up due to rust.

If the power dropped off a lot when you changed the cam timing from 110 IN / 115 EX to 110 IN / 110 EX, something is screwy.  Perhaps you have a non-stock exhaust cam, or the exhaust cam gear is mis-marked, or the cam or gear has the key slot in the wrong place, or something like that.  You can try restoring the cam timing to what you started with (110/115) and see if the power comes back.  If it were my car, I'd want to compare the two gears -- they should be absolutely identical in terms of key cut and timing marks.  I've attached an annotated photo of the most common type of cam gears.

Remember that you may have several problems that act together to confuse the issue.  You'll surely get things solved eventually.

 CORRECTION (7 Apr 2005):  Tom Thomson caught a mistake I made in the above, and advised me of it.  Per section C11 paragraph 6 of the shop manual, the mixture is too rich if engine speed increases noticeably when the vacuum valve is lifted, and too lean if it stumbles.  The text above has been altered to be correct. Sorry for the inconvenience. -- Mark


Attachment: engine cam gears annotated.jpg (Downloaded 30 times)

Last edited on 04-08-2005 02:44 am by Mark Rosenbaum