Bogging on takeoff  By  lain (Read 6,845 times)

JOEYBEE had a great photo of the needle, which shows pretty clearly what causes the fuel/air mixture to be more lean or more rich (shown here again):




I happen to enjoy knowing how and why things work as they do, and these carbs are simple to understand.  So, for those who are not yet familiar:

Here is an overview of a typical carb.  The top part has a cap; under it a diaphragm/piston with the needle sticking down.  The bottom part (body of the carb) has a fuel bowl, the main jet screwed into it (and the idle or pilot jet that I'm ignoring):




Here is a nice little diagram showing how the thing works:




The very top (vacuum chamber) IS at a vacuum since it's connected to engine vacuum.  The diaphragm/piston assembly seals the top part from the airflow, and as the vacuum increases the piston is pulled UPWARD.  Since the needle is connected to the bottom of the piston, it rises as the piston rises.  So, the whole purpose is to either lift the needle upward, or let it go back down under spring pressure, depending on the demands of the engine.

So what does the rising or falling of the needle do?  Well, the needle sticks down right into the main jet, which is a sort of tube.  Now take another look at the first picture (the one Joeybee posted) and notice the needle is TAPERED ~~ the thing is fat at the top and becomes progressively thinned at the bottom.

BELOW the main jet is the pool of fuel in the fuel bowl.




So if the needle is dropped all the way down the THICKEST part of it slides right into the jet.  In this case the needle pretty much fills the entire tube of the jet, so there is very little room for fuel to get past the needle (and into the airstream).  But if the needle is pulled all the way UPWARD, only the thinnest part remains inside the jet tube.  In THIS case there is plenty of room between the needle and the jet, and PLENTY of fuel can get past the needle into the airstream.  FYI:  the carb is designed to create a low pressure spot in the airstream exactly where the needly and the main jet meet - creating a sort of vacuum there.  It is this low pressure or vacuum that SUCKS fuel up past the needle.  The fuel is under ZERO pressure in the fuel bowl, so ONLY this vacuum gets fuel up into the air (one reason vacuum leaks, even is small, really mess up the engine's performance).

For those who have never taken their carm top cap off, there is what it looks like with the cap off:




and here is a photo of the needle/diaphragm/piston assemble that is inside the top of the carb (along with a spring):




So, anyway - this might help explain why putting the circlip in a higher or lower slot of the needle causes a leaner or richer mixture ~~~ because it makes the rest position (engine off) of the needle in the jet lower or higher.  And, with the needle starting out at a different position (and thus thickness where it meets the jet tube) it will ALWAYS be either higher or lower as the engine vacuum causes it to rise and fall according to throttle and demand.

Cool posting of the mechanics of carbs, rcq.
Tried to like but having some king of compatibility issue with browser. I'll get it in with a dif one.
Thanks

You mean 391 , what can I say , lol

That's good to hear. How is starting with the "racing CDI"?

And this is why many of us here recommend the advancing (not advanced) angle CDI that tvnacman sells, it prevents kickback at low RPM, and advances as the RPMs increase. The stock CDI has a similar curve, but it is much steeper in the lower RPMs and flattens out, the advancing angle CDI has a smoother curve.

I've had the same blue racing cdi for the past 3 yrs or more , as long as my battery is strong it works great , but if my battery is weak it starts the kickback . I'm sure its because of the advanced timing at startup .

Can you shoot that with a timing light and let us know what you find ?

John