View single post by Esprit2
 Posted: 11-03-2005 11:17 am
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Joined: 05-01-2005
Location: Minneapolis, Minnesota USA
Posts: 289
Brett Gibson JH5 20497 wrote: for the pump jet filter/spring, I was not talking about the main fuel pump with the large spring over the connecting rod on the outside of the carb, but the much smaller pump jet thats on the top side of the carb under a brass screw cap , 2 per carb quite a small item...  (Snip)...

It was the "filter/spring" thingie bit that threw me.   The part you describe is the Pump Jet.   And it is definitely required !!   There aren't a lot of extra parts in a carb that you can just leave out (like...   none).   They  ALL  need to be there in the proper size combination/ adjustment.   If the carbs aren't fully assembled by the book,  then you may have found the key to your poor running issue.

The accelerator pump is NOT driven by the linkage and your foot.   When you lift your foot/ close the throttle,  the linkage pulls the pump lever up against the small, concentric spring,  compressing it to pre-load the mechanism.   Basically, "cocking" the pump mechanism.

When you open the throttle,  the linkage simply releases the pump lever so the spring can drive it forward with a calibrated spring rate force.   The amount of "injection pressure" is controlled by the stiffness of the spring.   The volume of fuel or "shot size" is controlled by the length of the stroke/ how far the linkage pulls the lever back/ a nut & jam-nut at bottom of linkage pull-rod.   The time period over which the shot is delivered is controlled by the size of the Pump Jet.   All three things must work in concert.

With no Pump Jet installed,  any spring (light or heavy) will immediately dump the full shot size in one big gush as the throttle is opened instead of gradually over a brief time as the shot pees out in a thin stream through the Pump Jet.    Then as the throttle is opened,  the engine will quickly go way too rich and bog down.   And before the Idle Circuit or Main Circuit can establish a fuel flow and take over,  the shot-gush will be flushed out and the engine will go way lean and stumble again.   Sound familiar?


Additionally,  if you purchased used carbs,  assume they are dirty inside.   They are, even if the float bowl and throats appear clean.   Even if you spritz the passages with aerosol carb cleaner and the little red tube.   A modern carb shop will soak a carb body in an ultrasonic solvent tank for 3+ days or so,  then manually clean passages as far as they can.   Sometimes you simply can't get them clean by squirting an aerosol into the open end of those passages you can see,  and Dellortos/ Webers are very sensitive to dirt and varnish build up.   You can't get them "too clean".