adjustable carb main jet system?

[steve.emma]

hi all,
had an idea the other day about making adding a external mixture adjuster to the main jet circuit on my bikes carb.
i dont know if this has been tried before or not (probably has..)
but anyway here goes.... my bike has a mikuni TM carb,and at the bottom of the fuel bowl is a 17mm hex nut plug that can be unscrewed to gain access to the main jet. my idea works like this....what if i drilled and tapped the hex plug to allow a fuel mixture screw (like out of the idle circuit etc..) to come up through the plug vertically so that as its screwed in it gradually shuts of fuel flow to the main jet. i would need to use a much larger main than normal, but this screw should give you infinite adjustment to lean the main circuit mixture out as the screw is turned in.
the advantge of this (if it would even work) is that the main system could be adjusted to a much finer extent than with changing jets,and you could do it just by turning a screw instead of having to muck around with removing the drain plug and having to twist the carb around in the intake boot etc..
i know there would be some problems such as preventing fuel leakage past the adjuster screw threads, and how thick the alloy plug is to actually drill and tap it anyway. also the screw would have to be just the right sort of length and taper (maybe out of a car carb?) but i think all of these problems could be sorted out fairly easily.
what do you think? anyone have an opinion on this?

steve.

 

[reelrazor]

http://www.altimizer.com/daj.html

 

[RM_guy]

Here's a good thread on the Dial a Jet
http://www.dirtrider.net/forums3/showthread.php?t=33037

 

[steve.emma]

yeah i have seen the dial a jet system before, but i don't really think its got that much in common with my idea. for a start it adds emulsified fuel that is supposedly load sensed into the throat of the carb and you can adjust how much etc..
my idea is far more simple in that it just gives a degree of adjustability to the main jet circuit,and it doesn't look nearly as ugly or as complicated as the dail a jet system looks.
also my idea could be installed or removed very quickly by simply replacing the standard hex plug with a modified unit and if you wanted to you could swap back to standard again really easily.

 

[KiwiBird]

Should work - make sure you index the hex plug so it goes back to exactly the same place as before whenever you remove it.

 

[marcusgunby]

it will need a fine thread or it may go from a 160 to a 190 in quarter of a turn.

 

[matt-itude]

I think the hardest part is going to be linning the needle up with the jet!

 

[Rich Rohrich]

.
my idea is far more simple in that it just gives a degree of adjustability to the main jet circuit,and it doesn't look nearly as ugly or as complicated as the dail a jet system looks.

You mean something like this from my RD350 dragbike circa 1977? ;)

 

[steve.emma]

Dammit! , thought i was onto an original idea there for a moment. oh well never mind, from the look of those photos that must be a standard fitting for that carb and its exactly what i was thinking of. judging by the abrupt taper on that screw it must give a rough sort of adjustment, i was thinking of a much finer taper because (as Marcus pointed out ) it would change the mixture too much with very little turning of the screw otherwise.
so tell me Rich,what was the idea behind this on a drag bike? and did it work properly? i wonder why this idea was never used more widely on other bikes?
thanks for the feedback everyone.

 

[Rich Rohrich]

and did it work properly? i wonder why this idea was never used more widely on other bikes?
.

It was horrible to put it mildly.

 

[SFO]

adjustajet :blah:
:blah:


lets see pics of the bike :nod:

 

[Ol'89r]

It was horrible to put it mildly.

;) I remember those. Webco sold them in the early 70's. I never tried them but, a lot of guys did and they didn't work very good. One of the problems was they didn't have a detent so you never really knew where you started from.

During the races you could see riders fiddeling with them while coming down the straight-a-ways on the mile and 1/2 mile. IMO, not the time to try to figure out your jetting. :coocoo: