Good jet size

[Racingrobby]

I have a 1980 cb750f with a kerker drag exhausts witch way too load and wanted to know what a good size jets to run . I just can't find a sweat spot there it idle right . I adjust the idle screw and it with be fine till it warms up than its idles to fast it goes up and down day to day. I have the fuel screws at 3 and half turns out . If anyone has had this prob and knows that to do let me know thanks

[LukeM]

I think the spec on the base machine with factory exhaust is 1.5 to 1.75 turns out.  With a wide open (no baffles) exhaust, I'd turn the screws out one more complete turn.  So, 2.5 to 2.75 turns total.

You might want to check the following:
carbs balanced/sync'd?
tight connections on the carb boots (input and output sides)?
carbs clean on the inside?

Those are a few of the things that will cause idle to rise and fall.
Luke M

[Vintage]

Racingrobby wrote

I have a 1980 cb750f with a kerker drag exhausts witch way too load and wanted to know what a good size jets to run . I just can't find a sweat spot there it idle right . I adjust the idle screw and it with be fine till it warms up than its idles to fast it goes up and down day to day. I have the fuel screws at 3 and half turns out . If anyone has had this prob and knows that to do let me know thanks

After your carbs are balanced and you make sure you have good plugs, if you continue to have the problem, my suggestion would be to try the next bigger size jet. If you changed the air box or installed aftermarket filters, i'd go with two size bigger jet.

Harry

[Racingrobby]

Ok...  I'll try 2 and a half turns out , The shop said they synced the carbs and plugs are new . I'm running a stock air box  .Oh and new rubber boots. It's just a pain adjusting the idle Screw at red lights when it's fully warmed up.  Thanks everyone Im hoping turning back the screw will do it . And not tearing the carbs apart again..

[FastCletus]

What was your result on this?  I am having same question.

I have Stock box, 4 into 1 exhaust.  The bike has a hard time starting until it gets warm.  Boggs down at 7k rpms