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I had a problem a week ago or so where the tachometer in my TR4A was =
making a lot of screaching noises and figured it was a dry bushing from =
50+ years of life. I figured I would try fixing it myself. What could =
happen??
As I was taking the tach apart, I noticed there was a black paint mark =
on the drag cup that was connected to the needle so I just noted where =
the mark was with the needle "at rest" against a point on the frame and =
continued taking it apart. From reading information on the topic, I knew =
that that little spring attached to the drag cup was very delicate and =
was very careful to hold the tach upside down and lift the "drive" end =
off the drag cup, not the other way around. Once I had separated the =
frame, I just set the drag cup half on the bench in that same =
orientation.=20
The other half of the "frame" contains the magnet, shaft and bushing =
which connects to the tach cable. Sure enough the little shaft that the =
magnet was attached to was dry as a bone. The bushing and the shaft =
looked OK so I figured it just needed cleaning and lubrication. After =
careful cleaning of the shaft and it's bushing with some carburetor =
cleaner, I lubricated it with a few drops of 3-1 oil and put the magnet =
and shaft back into the bushing. MUCH BETTER.. The shaft would spin very =
smoothly by hand.=20
The assembly went well. Keeping the frame half with the drag cup upside =
down, I dropped the "drive" half back on the drag cup half and =
reinstalled the 5 little screws. It took a little trial and error to get =
the two pieces together where the drag cup would move smoothly around =
the magnet. The two pieces can only go together one way so all the screw =
holes line up and there are two keying pins to help hold the thing in =
alignment but there still is a slight amount of movement so it's not a =
quick assemlby. Next, I aligned the mark on the drag wheel to the spot =
on the frame I found before taking it apart at pushed the needle back on =
the shaft. By the way, I didn't put any lubrication on the drag cup =
bearings.
The tach is now running smoothly and the best of all, no noise.
The only problem is that somehow the tach is running about 500 RPM below =
what I think it should be. At idle, the needle just floats off the pin =
from "0". I think I can get a pretty accurate reading of the actual =
engine RPM using a Harbor Freight digital tachometer ($39.00). My =
problem is that I am wondering which way I should turn the needle on the =
drag cup? I have to take the tach out of it's case to move the needle as =
I have to hold the drag cup while I twist the needle on it's shaft and =
don't want to end up taking it apart 10 times..
One way I thought about doing the "calibration" was to pick a RPM, say =
2,500. With the tach out of it's case and the cable out of the firewall, =
have someone hold the throttle at 2,500 RPM and paint a new mark on the =
drag cup. Stop the engine and move the drag cup to the new mark and =
install the needle pointing at 2,500 RPM. At least it would be accurate =
at mid-range.=20
What do you guys think??
Dave Connitt
Thanks,
Dave Connitt
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<DIV><FONT face=3DArial>I had a problem a week ago or so where the =
tachometer in=20
my TR4A was making a lot of screaching noises and figured it was a dry =
bushing=20
from 50+ years of life. I figured I would try fixing it myself. What =
could=20
happen??</FONT></DIV>
<DIV><FONT face=3DArial>As I was taking the tach apart, I noticed there =
was a=20
black paint mark on the drag cup that was connected to the needle so I =
just=20
noted where the mark was with the needle "at rest" against a point on =
the frame=20
and continued taking it apart. From reading information on the topic, I =
knew=20
that that little spring attached to the drag cup was very delicate and =
was very=20
careful to hold the tach upside down and lift the "drive" end off the =
drag cup,=20
not the other way around. Once I had separated the frame, I just set the =
drag=20
cup half on the bench in that same orientation. </FONT></DIV>
<DIV><FONT face=3DArial>The other half of the "frame" contains the =
magnet, shaft=20
and bushing which connects to the tach cable. Sure enough the =
little shaft=20
that the magnet was attached to was dry as a bone. The bushing and =
the=20
shaft looked OK so I figured it just needed cleaning and lubrication. =
After=20
careful cleaning of the shaft and it's bushing with some carburetor =
cleaner, I=20
lubricated it with a few drops of 3-1 oil and put the magnet and shaft =
back into=20
the bushing. MUCH BETTER.. The shaft would spin very smoothly by hand.=20
</FONT></DIV>
<DIV><FONT face=3DArial>The assembly went well. Keeping the frame half =
with the=20
drag cup upside down, I dropped the "drive" half back on the drag cup =
half and=20
reinstalled the 5 little screws. It took a little trial and error to get =
the two=20
pieces together where the drag cup would move smoothly around the =
magnet. The=20
two pieces can only go together one way so all the screw holes line up =
and there=20
are two keying pins to help hold the thing in alignment but=20
there still is a slight amount of movement so it's not a quick =
assemlby.=20
Next, I aligned the mark on the drag wheel to the spot on the =
frame I=20
found before taking it apart at pushed the needle back on the shaft. By =
the way,=20
I didn't put any lubrication on the drag cup bearings.</FONT></DIV>
<DIV><FONT face=3DArial>The tach is now running smoothly and the =
best of all,=20
no noise.</FONT></DIV>
<DIV><FONT face=3DArial>The only problem is that somehow the tach is =
running about=20
500 RPM below what I think it should be. At idle, the needle just floats =
off the=20
pin from "0". I think I can get a pretty accurate reading of the actual =
engine=20
RPM using a Harbor Freight digital tachometer ($39.00). My problem is =
that I am=20
wondering which way I should turn the needle on the drag cup? I have to =
take the=20
tach out of it's case to move the needle as I have to hold the drag cup =
while I=20
twist the needle on it's shaft and don't want to end up taking it apart =
10=20
times..</FONT></DIV>
<DIV><FONT face=3DArial>One way I thought about doing the "calibration" =
was to=20
pick a RPM, say 2,500. With the tach out of it's case and the cable out =
of the=20
firewall, have someone hold the throttle at 2,500 RPM and paint a new =
mark on=20
the drag cup. Stop the engine and move the drag cup to the new mark and =
install=20
the needle pointing at 2,500 RPM. At least it would be accurate at =
mid-range.=20
</FONT></DIV>
<DIV><FONT face=3DArial>What do you guys think??</FONT></DIV>
<DIV><FONT face=3DArial>Dave Connitt</FONT></DIV>
<DIV><FONT face=3DArial>Thanks,</FONT></DIV>
<DIV><FONT face=3DArial>Dave Connitt</FONT></DIV></BODY></HTML>
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