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Accucraft GS 4 Chimney&Petticoat

2205 Views 8 Replies 4 Participants Last post by  Charles

After carrying out some of the modifications recommended for the alcohol-fired version of Accucraft's GS4 (incl. Gordon Watson's combination levers and nozzles and new wick material) I am still not REALLY happy. The performance is quite OK but could be better.
So I referred to some textbooks and articles on the subject, e.g. Martins Evans' books and J.v. Riemsdijk's articles in the G1MRA Newsletter.
As regards the front end, i.e. arrangement of nozzle, petticoat (chimney extension) and chimney, both authors recommend the same geometrical dimensions. I built two templates to check these dimensions on my Accucraft GS4, see first picture

The template tapered 1:6 governs the height of the chimney top above the nozzle, which appears to be OK, see second picture

However, after applying the second template tapered 1:3 which determines the extension of the petticoat, it appears that the petticoat is at least 1/4" too long!

How's this? This template has 8 stripes green/yellow each 7/64" wide. The template shall sit in the blast nozzle with the tapered part still inside the chimney extension or petticoat by about 3/16". So at least six of the eight stripes should show underneath the petticoat, however, on my GS 4 only three and a half are not covered by the petticoat. That means it is too long and most likely impedes draft!
Has anyone else observed this?
Has anyone unscrewed the petticoat? How is it fixed? I would like to shorten it to make another test.

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RE: Accucraft GS 4 Chimney&Petticoat

Hi Michael,

My GS4 has the same problem as yours. Could not hold pressure above 50 psi while running.
Two fixes seem very important. 1) reducing the diameter of the blast pipe; It is not the exit volume that is important
but the exit velocity. 2)The fire box arch

It sound like you have done the first and should try the second.

After a year of fiddling these two changes made the difference.

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