Topic: cyl #4 not firing on cb750f

[Andrew Foss]

hey guys
my #4 cyl. was not firing and i didnt realize it until it kicked in one day while riding. i thought that the problem was my coil but i am now leaning towards carburation. here is the story:
 at first i had no #4 at all then it kicked in one day and i decided to figure it out. i cleaned the plug and got it to fire when i really nailed it. if i let the bike idle it would then stop working until i took the plug out and cleaned it again. this made me think i was running rich, so i backed out the mixture screw and had nothing at all. then i went in one turn from where i started and got the cyl to go at about 3/4 throttle. then i went in another turn and it went from 1/2 throttle...the richer i go, the better the response is. the air/fuel screw is now about 1/2 turn out from being seated. the cylinder fires as soon as i give the bike some throttle, but not at idle. i cleaned out my float bowls yesterday and they were filled with rust peices so i put an in line filter on and then started doing the adjustments above. if a peice of rust is in my idle jet, would the mixture screw have much effect on the bike at idle? why does it help the cylinder once i give it some throttle? i have a general understanding of how the carbs work but i guess not how the mixture screw works, i thought it was only to adjust mixture at idle. anyone who can give advise, i would appreciate it. if i have to take the jet out, since it is an outside cyl. i should be able to do it while it is on the bike correct?
-andrew

[Tim2005]

Is it the 750F with the clip on float bowl or the one that's held by screws? In any event, sounds like a jet (or two) is blocked so I'd pull them all out and clean them. As there's rust in that one, give your tank a good clean out and consider cleaning the other carbs out, could make quite a difference and prevent other problems too.

[Andrew Foss]

clip on bowls, which jet is the idle jet? and how do i remove it. the cyl. having problems is an outside one so i can probably get it out without removing carbs...

[scout18]

ok so you have a  blocked slow speed jet. It is the small one if you are looking at the bottom of the carbs. You need to take it out and with a very small piece of wire I mean like a strand of wire from your speaker cables get it to poke through the jet. Put the jet back in an it should be fine.

You should probably get your carbs out and clean them all and clean the passage that feeds gas to all the carbs then clean out your tank and put in an inline fuel filter to keep the crud from getting to your carbs like I saw on another post "If a job is worth doing it is worth doing twice."javascript:void(0);
Wink

[TwoTired]

The 750F came with two styles of carbs.  The 76 had removeable slow jets, the 77-78 had pressed in jets.  I'll assume you have a 76 since turning the idle screw in, closes off air supplied by the slow circuits and would make the throttle valve act a bit richer.

The slow jet is up higher (shorter) in the float bowl.  Unscrew it, clean 'till you can see thourough it, and put it back in.

If your fuel supply has bits of crud in it larger than 0.016 inch, they or the ones in other carbs can plug up again, though.

[Dunstall_74]

I'm having the same exact problem w/ my 1974 CB750 K.  Let us know what you find.  I was thinking that it might just be my timing, but the throttle response you are describing sounds almost exactly what's happening to my bike. Although I just Had my carbs off, completely cleaned them, set the floats and stepped the main jets up to 125's.  I guess after I set my timing in the morning, if that doesn't do the trick, I'll clean out the slow jet on the #4.   

[Dunstall_74]

I just went and cleaned out my slow jet w/ some very small guage wire, and that was exactly what it was.  PLugged slow jet.  Give it a try.  Also,  thanks scout 18!  I'm back in business!

[Andrew Foss]

cleaned out my slow jet (it had some crap in there) the fouled a few plugs because of the richness....i think im in buissness now though, thanks!