Machining a carb jet
Brad Soward Brad_Soward@onaustralia.com.au told us:
G'day all
Progress report, as promised. Today I made a start on fabricating a
replacement main jet for my 10 hp Inter M type. The original is made from
sh#%$ metal or die cast and I wanted to make a replacement from brass. I got
around my problem of a lack of a tailstock drill chuck for drilling the
through holes by using a 'cheat' as explained to me by my dad from his early
years as a mechanic. The main jet is made with two pieces of brass rod with
holes bored right through. One piece is the pickup tube and the other is the
jet tube which has the jet orifice at one end with the other end threaded
for the mixture needle. The purists or genuine machinists should probably
look away about now as this method is not pretty and definitely not out of
the best workshop practices handbook, but it did work. [:-)]
Firstly I chucked my piece of brass rod in the lathe and faced off one end,
then using the tool I gouged a small indent at the centre of the piece. I
then cut the rod slightly over length and faced the other end off to correct
length, also gouging a centring indent. Then using a 3/8" drill I drilled a
small indent into the bench drill table top. This doesn't have to be a
crater, just enough of a depression to locate a ball bearing enough to stop
it rolling around. Then, without moving the table, I placed a 5/16" ball
bearing into the indent and put my piece of rod upright under the drill bit
and on top of the ball bearing, using the centre indent to locate the rod on
the ball. By carefully starting the drill in the corresponding centre indent
on top you can then drill through from either end and know that the hole is
on centre. This was not critical with the pickup tube but was important with
the jet tube as the mixture needle must be centred to the jet orifice. The
pickup tube was drilled 5/32" then the jet tube was first drilled with a
5/64" drill for the jet orifice, then counterbored for most of its length
from the other end with a suitable undersized drill for threading to 5/16"
UNC. It's also neccessary to be careful with the threading operation to keep
it straight, but after I'd performed these operations the mixture needle was
screwed in and seemed to index with the jet orifice pretty well. A bit more
turning in the lathe to reduce the jet end size until it was a good fit in
the carby venturi and it was ready to braze the two pieces together. Once
joined, the pickup tube hole was bored through into the jet tube and voila!
I had a new main jet! [:-)] I still need to get a 3/4" brass plug so I can
make a new retaining nut before I can give it a try, but I'll keep you
posted.
Regards from Oz
Brad_Soward@onaustralia.com.au
Nimbin, New South Wales
Australia 2480
http://albums.photopoint.com/j/AlbumList?u=923731
http://community.webshots.com/user/bradsoward
Reg Ingold randmingold@fastlink.com.au responded only half in jest:
Hmm, Cunning idea but, DRILL INTO THE TABLE? What was wrong with clamping a
bit of scrap steel to the table and drilling that? I am DISGUSTED with your
conduct!! Go and stand in the corner and write out 100 times, "I must NOT
mutilate good machinery!!
Reg & Marg Ingold.
Holmesville. N.S.W.
Australia.
randmingold@fastlink.com.au
http://www.oldengine.org/members/randmingold
This article is part of the Stationary Internal Combustion Engine Frequently
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