It is possible to jump a tooth on the timing chain. I have had that
wonderful experience.
Based on my experience you will also find a few bent valves and a stretched
upper timing chain. The cost to repair then was about thirty bucks for the
upper timing chain, replacement valves, and if I remember correctly an upper
tensioner, and me doing the work. Today it is a little more expensive.
I am very curious about what you find with the harmonic balancer.
I would try putting the engine in TDC as described below and hand rotating
the dizzy with the ignition on until you see the points spark when they
separate. Lock the dizzy down at that point and see how the engine runs
when it is statically timed. In my experience the timing light has always
told me that it was timed right at 0 degrees when I checked with the engine
running. If your engine is running properly you will know the timing is at
about 0 degrees, and you will be able to use a timing light to see how far
the harmonic balancer might have slipped.
Tom
69 2000 - Mr. Hyde
Portland
http://www.datsun2000.com
-----Original Message-----
From: owner-datsun-roadsters@autox.team.net
[mailto:owner-datsun-roadsters@autox.team.net]On Behalf Of Todd Osborn
Sent: Saturday, September 17, 2005 4:42 PM
To: datsun-roadsters@autox.team.net
Subject: Re: high rpm
John,
I suggest you set the engine to TDC as Pat recommends by bring the #1
cylinder to TDC with the sparkplug out. With the engine at TDC, the harmonic
balancer mark should be at TDC, the crank keyway should be straight up (need
to pull the big bolt and washer out of the crank to see the keyway) and the
dowel pin (Phillips screw) on the cam should be straight up (see page 23,
figure EM-83, in the Clymer's manual for the timing relationship). I am a
little surprised you could jump a cam tooth by over revving, how did you
determine this? BTW the harmonic balancer TDC mark is about 25 degrees CCW
from the center of the keyway, not easy to check unless you have a spare
engine or like standing on your head. The harmonic balancer outer ring could
have rotated or shifted fore/aft, I have one that did both (more about that
in a future message). This could certainly cause vibration at a specific
rpm. Another source of imbalance may be the clutch, I've blow the clutch
twice missing shifts as you describe. Most pressure plates have eccentric
weights that increase clutch pressure at high rpm. When you miss a shift the
natural instinct is stomp the clutch and re-make the shift, however shifting
an overrevving engine results in high clutch disk loading and may cause some
friction material to separate (one whole side in my case). Asymmetric loss
of material obviously leads to crank imbalance and new harmonics. Hopefully
you just need to fix the harmonic balancer and don't have to pull the
engine/trans to replace the clutch. BTW the small threaded holes in the
balancer are 6mm 1.0 pitch and greatly aid removal. Drop me a note if you
need more details on degreeing the cam.
Todd Osborn
San Jose, CA
'68 2L DATSFUN
> Sent: Friday, September 16, 2005 1:07 PM
> List, racers,
>
> ...I went to hit third again and hit first gear. OUCH. I sent the RPMS
pretty high. I caught it quickly but obviously not quick enough. After the
incident the car ran fine, but ran slightly rough and had a harmonic
vibration like the balancer was not doing its job. Anyway, after a quick
diagnosis I found the upper timing chain had jumped a tooth. Compression
was at 90 psi on all cylinders so I figured it was a timing issue.
...I guess I can diagnose the timing issue if I "degree" the cam. If any of
you have experience with this, what could be causing my harmonic vibration.
I speculate the balancer could have been damaged. I am thinking of putting
a timing light on it to see if it is still timed where it used to be (outer
ring could have moved radially on its center?) ...
> John Over
> 68 2000
|