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Measuring piston rise

To: "'mgs@autox.team.net'" <mgs@autox.team.net>
Subject: Measuring piston rise
From: Larry Colen <lrcar@red4est.com>
Date: Sun, 30 Mar 2003 18:18:26 -0800
It took me all weekend to do it, but I finally made a widget that will
tell me where the carb piston is, while I'm driving. I don't know if
it is peculiar to my setup with the blower, but if I take the damper
cap off the dashpot, the car won't even start, so I need a way to
measure the piston position, with the dashpot cap on.

I'll spare you the details of all the things I tried that didn't work
and just cut to the chase. 

You'll need:
   some 1/16" brass rod (several inches)
   some brass tubing with a 1/16" ID (less than an inch)
   a 1/16" drill
   a 3/32" (or whatever the OD of the tubing is) drill.
   a propane torch, preferably with a fine flame
   various hammers, punches, pliers etc.


The goal is to attach the brass rod to the sheet metal hat thingy that
fits inside the top of the damper tube on the piston. The problem that
I had was that I kept trying to fasten it in ways that didn't let the
"hat" move in relation to the rod. 

The purpose of the tubing is just to act as a sleeve for the rod going
through the cap. You may be able to get by without it.

Pull the metal shaft with the brass valve out of the plastic cap.

Remove the "hat" from the shaft.

Drill a hole in one of the ears of the hat, just big enough for the
brass rod. If the rod won't fit through the hole you can file it down
a bit so that it does.

Drill a matching hole in the cap. Make sure that the hat is centered
on the stub that the shaft mounts into. You can either make this hole
the diameter of the rod, or if you use a sleeve, the diameter of the
tubing.

If you use tubing, cut a short section, make sure that it slides
easily on the rod and insert it into the hole in the cap. It should
not extend past the plastic on the inside of the cap.

At this point, you may want to mark the rod every 1/8". I used a
triangular file, then used sandpaper to smooth everything down so that
the rod would slide well.  The other option is to mark something that
the rod would move in relation to. In retrospect, I would have taken a
flat piece of metal and drilled 1/8" dia holes, 1/8" apart then cut
down the center to get nice arrows that would show up in sillohuette.

Another option is to take a hotmelt glue stick, apply some flame to
one end to soften of the glue and attach it to the top of the
cap. Then use a sharpie to mark it every 1/8"

Heat up the brass rod to soften it near one tip. Use a chisel, punch,
pliers or whatever to flatten out a small section of the rod near the
tip. You want the deformed section to be as short as possible, and
wide enough to prevent the "hat" from sliding up the rod.

Put the rod through the hole in the "ear" of the hat. Make sure that
the dome of the hat faces away from the rod.  Heat of the tip
of the rod and flatten it out so that the hat won't fall off. This
way, when the piston drops, it'll pull the rod down with it. You must
make the metal extending through the "ear" as short as possible as
there is not a lot of clearance.

Put the hat back on the damper shaft, and reinsert the shaft into the
cap.

I finally realized that it is easiest to remove the dome. Screw the
cap into the dome, then put the whole assembly on the piston and carb
body and screw the dome down to the carb body. You will want to remove
the air filter and make sure that the piston rises and falls freely,
and that the rod tracks the movement of the piston.

I removed the bonnet from my car, attached my camcorder to the
rollcage so that it could see the carb (manual focus, manual exposre)
and went for a drive narrating what was happening during the
process. Ideally, I would have been able to mount my DMM attached to
my O2 sensor in view of the camera as well, but at least I now have a
feel for where on the needle I am under what conditions.

I've got some pictures of the whole business at:
http://red4est.red4est.com/jasmine/carbpist/

With larger pictures at:
http://red4est.red4est.com/jasmine/carbpist/fullsize

My digital camera takes crappy pictures so most of the detail shots
are fuzzy. The first 11 shots are of pevious attempts at this.

-- 
I've found something worse than oldies station that play the music I used to
listen to. Oldies stations that play the "new" music I used to complain about.
lrc@red4est.com                                    http://www.red4est.com/lrc

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