Quote:
Originally Posted by alex.s
there are two jets, a small and a big. they are called pilot and main jet. the needle transitions the big jet on and off depending on how much air is going through the carb. so with two jets and a needle, you have two points and a curve between them. this represents your fuel map... i'll draw it in text:
fuel map:
vacuum gets higher --->
^ ...................................................
^ .........................................******
^ ..............................******...........
F .....................*****.....................
U .............****...............................
E *******.......................................
L ....................................................
... ^-- pilot jet
... needle transition --^
.............................................. ^---- main jet all the way open
adding shims simply adjusts how quickly the needle moves in relation to the vacuum curve. so for example adding shims to the above would make it look like this:
vacuum gets higher --->
^ ........................................................
^ .................................*************
^ ......................******.......................
F ..............*****.................................
U ......****...........................................
E ***..................................................
L ........................................................
you can see the levels for the pilot jet and the main jet are the same, but it transitions much lower in the vacuum range. this is usually NOT what you want. you can see in this graph the fuel stays the same for about the last 1/3rd of the graph. but the air is still climbing at that point, so in this fuel graph, you would probably be super lean up top but nice and rich in the middle. maybe still lean on bottom since the pilot jet hasn't been changed and the needle doesn't effect it at idle.
your fuel curve should match the flow of air as the throttle opens and the rpms climb. if for example at low rpm, there is much less vacuum because the intake is wide open, it pulls less fuel and so the air/fuel ratio becomes lean. that would mean either the airbox needs to change, or if you are set on that air filter setup, you would need to adjust the pilot jet to correct the fuel ratio.
the problem with shims that nobody seems to understand is that it DOES NOT CHANGE your fuel ratio at idle or at the top of the vacuum range. it only effects mid range. so when you change your air flow from exhaust or airbox changes, you need to change the JETS. not the needle. JETS.
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Awesome, that was exactly what I was looking for. Like I said I know fuel injection, not carburetion, so this whole process is a learning experience to me. Im ordering me a jet kit as soon as I get paid and am gonna do some wideband datalogging to get it tuned right.
krolinked- I know my valve adjustment and cam timing is spot on, that is something iv'e been doing for awhile and on much more complicated engines. I like the looser end of the spec because it allows more seat time for the valves, cooling them off more and making them last longer as well as putting less pressure on the seats causing the lash to stay in adjustment longer. Not optimal for top output, but optimal for longevity.
Thanks for the great feedback everyone, glad to be a new member.