Topic: Hi-PO 750 Carb Problems

[litlbrowapc]

Howdy. My Big Brother is modifying a 1978 750. Has two 2-into-1 headers. He is using four K&N cone filters usually used for v-8 valve cover breathers for his air cleaners.  He has stock keihin carbs. The main jet has been drilled out to 564 or .078".
The thing screams on the top end, but will not idle. The plugs verify that it is very lean on the bottom end.
He told me over the phone (550 miles away) that he did not see an idle jet, but has a tube where the idle jet should be. His question of me to research on the net is "Where does the idle circuit pick up fuel?"
 Any thoughts or comments would be greatly appreciated. Thanks

[eurban]

Down load the factory manual here:
http://www.mediafire.com/?wqzmehqvkzl
The stock PD carbs have a pressed in pilot jet which is likely the tube that your brother has seen. These (inspite of what it says in the manual) can be removed with a pair of plyers and a gentle twisting/pulling motion.  Padding the plyers with leather is probably a good idea.  The entire idle circuit must be thoroughly cleaned and the pilots should be confirmed clear by viewing light from end to end.  A single strand of copper wire can be used to help clear the jet.  There are many posts on the subject on this forum and it is very common to have impaired idle circuits that often take 2-3 tries to clean properly.    The PD carbs have an accelerator pump that must be functioning properly for the carbs to perform.  Remove the filters and look for fuel squirting from all four brass nozzles (located next to the choke plate on the bottom of the carb bore, fuel squirts towards the engine) when you rapidly whack open the throttle. . . .  Honestly your brother's .078 drill bit equates to about a 190 main which is ridiculously oversized (stock is 110) for just about any situation based on the 750 engine.  What is the particulars of this setup??  Drilling jets is also a very imprecise science and will give mixed results at best.  In general, it is a complete waste of time trying to jet a set of carbs that aren't fundamentally functioning.  Your brother might want to put down the drill, read the manual and then carefully "baseline" his carbs before making any more attempts at "tuning" I'm pretty sure those .078 drilled mains will end up in the trash!

[litlbrowapc]

Thanks for the quick reply. I'll print off the manual and get it to him, along with your ideas.

[ntintruder]

 What can I expect if I install four into one exhaust and individual air cleaners on stock CB750K2 engine?