mainnozzle m5

for my 37 knuckle(61) i have the M5 carb,also have a chamfered mainnozzle,seems to be an original early knucklenozzle, but: the shoulder(4.1mm) on that nozzle is shorter than those on other (flathead)nozzles i have(6.2mm).I heard its meant to put a gasket on to fill out,if so for what reason and with what material, would it harm to let it out?

Damn good question! I have a pair of M-5s on the shelf (I run others on my two '37s) but I cannot be sure if their nozzles were original. In Palmer's "How to.." it mentions the gasket, I think for M74B's, but I have never found any such gasket in the factory literature. I could cut them out of nylon in minutes, but since my M-74B's run so well without them, I wonder: Why?

ok,found out my knucklenozzle is:1258-40 for 1-1/2carbs with 1-1/8 venturi (thanks rene) B.Palmer tells in his book, that nozzle 1258-40(not shown) used on 1941 -74OHV and 42-48 models with special?? carbs is identical to the earlier model, but:with shorter shoulder ,also he says:"be careful not to put a nozzle requiring a gasket in any carb without the gasket,or it will fit to loose!" This i don't understand,because the springtension is always there! Maybe H-D just lowered that shoulder to bring the nozzletip to the relatively same position in the later(larger) venturi!This would mean that if you use(d) the later(shorter shoulder) nozzle in an earlier carb,ONLY THEN you need that gasket!! This could be the reason why you don't find it in the partbooks!I hope i am on the right track here

very good picture rene.As you say,the position of the tip is important.Since i have the third one from the left,i was convinced to put on a gasket(or spacer) on that short shoulder to fill out the difference and have the same dimensions as 1258-36.Maybe H-D delivered both a(new)short 1258-40 plus a gasket for your 36-39 knuckle when you needed one lets say in the late forties? Anyway,today I did put in the kh gasket+"short" nozzle,started...excellent idle,have to bring it on the road though ,thanks!

BTW: using an un-necessary gasket (or otherwise lowering the nozzle) will make emulsion pull out of the nozzle at an earlier point in rpm/flow, sometimes helps off-idle response, but it also changes the fuel curve by raising all air correction holes in the fuel level - effect not known.

by putting a short shoulder(1940)nozzle in a 1937OHV(where a long shoulder nozzle is required) to my opinion you must need to lengthen that shoulder(by using thick gasket or spacer) in order to have the same dimension as the genuine long shoulder one.then nothing has changed! You are right about the fact that H-D in fact brought up the elevation of the 4 holes by shortening that shoulder by going from M5 to (M35?) carbs.
Like you say this probably had some(wanted?) effect on fuel curves, i feel you know more about that than me!