Topic: drilling a jet

[KRONUS0100]

Howdy folks.  Dumb ass question here....I have done the requisite search and come up empty.  I have a 76 CB750F.....it has a mildly built ( I suspect a cam change)...K2 motor now.  I am running a full Dyna Ignition System complete with coils and wires, power for coils coming straight from battery, not the harness.  The carbs are running 40 pilots, and 120 mains withclip in 4th from top slot.  Plenty of power, runs 115 with me on it alone at about 7 grand.  Idles like #$%*.  Pilots are clean, air screws 2 turns out.  I have an extra set of 40 pilots I want to drill out to 45 or 50.  What size drill do I need?  I know this is a lost art, but trying to find the right pilots is a pain in the ass, so I am considering the drill bit.  Any I deas?  I know the 120 mains are supposed to be like 12 thousanths, so would the 40 pilots be like 4 thousandths?  Help please.

[Soos]

For keihin carbs, the 135 designation denotes 1.35mm (or 0.053" for the americans).
You would need # drills.
Or oddball metric ones.


mm X 0.03937 = inch equivalent. (for ex, 1mm=0.03937 inches)

With the diameter you need, look up # drill size diameters.
Any (big)hobby shop should have them, as well as any industrial tool supplier.
Even harbor freight has a # drill set... although I wouldn't trust them far... they are from harbor freight after all.



l8r

[TwoTired]

Application of choke will draw more fuel from the existing idle jets, whatever their size.
To simulate larger jets you can apply choke at idle.  If this cures your idle issues, then perhaps larger jets may help.

If the engine will still not idle, you have a problem other than jet size to contend with.

Why did you pick two tuns out on the idle air bleed screws?  This has the effect of making the idle jets smaller.

It is a traditional problem that cam changes that drastically improve the higher RPM power, often do so at the expense of smooth air flow/ loss of vacuum at idle speeds.  The vacuum is what draws fuel from the jets.
>If< the cam is causing your idle issues, it is not just the fuel jets that are the problem, but also lack of oxygen (air) at idle.  Then throwing more fuel at it with bigger idle jets, won't improve idle matters.

When you vacuum synced your carbs what was the average vacuum level monitored in the intake runners at idle?

Cheers,

[heffay]

my only argument... wait, i have two:

1)  JETS ARE CHEAP
2)  just make sure you take those damned drilled jets out of the bike before you pass it on... jets that were once something and are now something else don't make it easy for the present owner to map out all the mistakes the previous owner made.

[KRONUS0100]

I don't know what cam is in it as of yet...have not measured.  The 2 turns out seemed like a good number to start with.........I did not check what the pressure (vacuum)  was when I synched the carbs....but it was pretty low on the dials.  If I disremember wrongly....the old F motor I blew up did not want to idle either.

[low-side]

Kronus, have you tried advancing the ignition?  Helps in certain situations, especially with big bumpsticks.  You saw all the nonsense and f***ery I went through before I found my solution.  I drilled mains while experimenting but the problem is that you have to account for half a thousandth or so because of the tolerances involved; it's a rare drill press that has no runout, an even rarer hand drill.  Not so bad on mains because 140 and 142 or 144 are in the same ball park as far as a tuning tool, but for an idle jet if you try for 50 you could hit 52 or 55.  A .020" bit would get you around a 50, and if it slopped around .001" you would get a 53.  I'm sorry, I just reread your post and saw your pilots are 2 turns out - try turning them in if you want it richer.  If it idles better but starts doing weird stuff to the rest of the band, advance the ignition.  Anyway, good luck with it.

[KRONUS0100]

OK.... what ARE the correct settings for the idle air screws...seem to have misplaced my service book......about 1/2 to 3/4 turn out?  Thanks guys

[TwoTired]

OK.... what ARE the correct settings for the idle air screws...seem to have misplaced my service book......about 1/2 to 3/4 turn out?  Thanks guys

For the stock bike, with stock displacement, stock cam, stock air filter, and stock exhaust, the idle screws are set for 1 +/- 3/8 turn.