re: trouble in paradise

[Jim Adney <jadney@vwtype3.org>]

On 16 Aug 2003 at 11:29, Fiesta Cranberry wrote:

> >When you just widen the point gap, the 
> >spark starts sooner (timing advances) and 
> >lasts longer.  If you just narrow the point 
> >gap, the spark hits later (timing retards) 
> >and has a shorter duration.  

> Can you explain what the practical result of this is?  IOW, when the
> timing advances/retards, what is the effect on the performance?  

After you ignite the mixture, it actually takes some finite time for the 
combustion to occur. The combustion produces pressure in the combustion chamber 
which we need to push down on the piston. This pressure builds and then falls 
off, leaving us with a peak at some point; for best efficiency we need this 
peak to occur at a particular point in the pistons travel downward.

If ignition occurs early or late, then the pressure peak will be early or late, 
this affects the engine output. Too early will simply reduce output; too late 
will too, but can also damage the exhaust valves if the mixture is still 
burning when the valve opens.

This is all made worse by the fact that as the engine speeds up we have less 
and less time for this combustion to happen, so we have to assure that the 
ignition happens earlier. In other words, we have to anticipate the proper 
moment to ignite the mixture, and we have to anticipate it more and more as the 
rpms go up. Thus the dist comes with advance mechanisms, which make the spark 
occur earlier than the idle setting.

Your dist has 2 advance mechanisms: a vacuum advance and a mechanical (also 
called a centrifugal) advance. The latter one is the most important for you. It 
is supposed to make the ignition occur earlier as the rpms go up. You should be 
able to see this with your timing light. Once you've set the timing, watch the 
timing marks as you use the throttle to run the engine speed up. You should see 
the tiiming marks move smoothly to the left (advance) and then smoothly return 
as you let the engine speed drop back to idle. This is an essential feature for 
proper engine performance.

If you don't see this happening, you will experience exactly the symptoms that 
you describe. 

Have you done this test? How did it go?
 
> That is certainly the case with the B.  I have had her tuned and then
> she goes out a week later.  It never lasts long.  One long trip or a big
> hill on a hot day throws her right out.  It's frustrating.
> 
> >The four "lobes" on the distributor shaft 
> >that push the points open as the shaft 
> >rotates. 
> 
> Oh, OK.  How do I know if these are dry or need grease, like Jim said?

The dist cam is just that lumpy thing that makes the points open and close. 
There are 4 lumps (lobes), one for each cylinder. You'll see that there is an 
insulating block on the side of the points that rides against the cam. You just 
need the tinyest smear of lubrication for this so that it doesn't wear.

Start by taking a clean rag or paper towell and wiping the dist cam completely 
clean. Then just smear a very tiny bit of grease all over the cam. Be careful 
not to get any on the actual points contact. Bosch makes a special Cam Grease 
which is the ideal stuff to use, but any grease is better than letting them run 
dry.

I'm gonna write a "Care and Feeding of Distributors" FAQ, but I've not had time 
yet.... Stay tuned.  

-- 
*******************************
Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA
*******************************

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