Topic: tuning/carb oddity

[bucky katt]

i have a 74 cb750k with a k6 motor. it has .75mm over pistons and a cam from a 75 750f. i'm running points and stock coils with D8EA plugs, stock airbox with a k+n filter in it, 122.5 jets with the needle in the center position, idle screws 1 turn out, new carb and airbox rubbers as well, a kerker header and megaphone with the stock baffle (fiberglass packing removed because it was too quiet). at idle and just off idle with part throttle it's sounding like it's really lean, plugs are showing the same thing. what i'm wondering is if anyone here that has a similar setup might could tell me where your idle screws and needle clips are set at? should i maybe try a different slow jet maybe? i pulled the carbs off and re-cleaned them to be sure there were no clogged oassages or jets, made sure the petcock screen was clean and i have fresh shell 87 octane fuel as well. i know i can go and start tuning without the information but if someone else has a similar combo maybe i can cut a few steps out of the process.

[Hush]

"BUMP"  cos I can't help you any other way Bucky.

[scottly]

You might try backing out your idle screws a bit and see if it helps.

[bucky katt]

that will be the next thing i try. tomorrow as long as we dont get the snowy weather thats supposed to hit Atlanta

[wdhewson]

As long as you don't have a lean "stumble" your carburation sounds spot on and ideal.  Keep things lean and hot at lower throttle, but richer and powerful when you un-cork her and work the engine harder.

[mlinder]

Which carbs?

Also, not to be contrary, but idle should be rich on mechanical slide piston carbed motors with no pumps, for increased low end throttle response.

[TwoTired]

Also, not to be contrary, but idle should be rich on mechanical slide piston carbed motors with no pumps, for increased low end throttle response.

Agreed.  If you suspect it is idling lean or going lean during transition, you want to turn the air screws IN.  Do a quarter turn and drive for effect.

You could also try plugging a hole or two in the mains emulsion tubes (ideally you would engineer the existing holes a tad smaller).

Also, you could try raising the fuel level in the carbs to make it easier for fuel to be lifted into carb bore.  (this will also effect mid-range and WOT mixtures, too.

Usually High RPM cams lose vacuum at idle speeds compared to stock street cams. That vacuum loss is what you need to compensate for, as well as a low restriction air filter and a non-restrictive exhaust.  Lower carb throat vacuum draws less fuel through fuel metering devices in the carbs.

[bucky katt]

Which carbs?

Also, not to be contrary, but idle should be rich on mechanical slide piston carbed motors with no pumps, for increased low end throttle response.

theyre the stock carbs from a k6 750 motor. once it's rolling the thing pulls like a freight train, alot stronger than i expected a mostly stock motor to be.

[mlinder]

I can't remember which carbs those are. Where's the idle mixture screw at? If at the 'head' side of the carbs, and sticking out to the side instead of straight down, turn them in (as TT said) 1/4 turn, and give it a try. (actually, turn them all the way in, then turn them 3/4 out.)

[bucky katt]

on the airbox side i think. ill check once it stops raining/sleeting

[mlinder]

Ah, crap, I hate those carbs.

Dunno what stock turnout is on those, but try 1 1/2 out. Work from there. You'll have to readjust idle, prolly.
I'd prolly just go with next size up idle jets, but I can't remember if those are pressed in or not on those carbs.

[bucky katt]

theyre screw in, 40's my local honda shop has all the jets for old carbs in stock, priced pretty competitively when youre looking at shipping costs from an online vendor, nice to give a local shop business when i can too.

[scottly]

Hey bucky, did you get the chance yet to play with idle screw settings?

[bucky katt]

no, it's been cold wet and nasty outside today. arthritis was kicking my ass so i stayed in and had a few hot cocoa with generous amount of baileys in it.

[scottly]

Uuumm,,Baileys....