Topic: 1978 CB750F Carb Question

[jshea]

Pretty general question with a longer explanation - if my air/fuel mixture screw is turned all the way in (fully seated) and the bike seems to want a richer mix, does that mean I should increase my main jet?

My bike has 4 into 1 pipes with a straight and no baffle. It also has K&N pod filters (RC-2314, I believe). I can't remember what size the main jets currently installed are, but I can check. 

The reason I've been adjusting the air/fuel screw is because I was getting some serious popping on deceleration when the screw was 1-1/2 turns out. It sounds better (not perfect, but better) with the screw all the way in but I feel like acceleration has suffered. Its also harder to start and the fuel smell is stronger.

I am happy to install baffles if this would help eliminate the popping and regain some power. Or change the jets. Open to any suggestions. Thanks

[Bankerdanny]

The air fuel screw doesn't impact much but idle and limited throttle, so 100% closed or 100% open shouldn't have a big impact on acceleration because with the throttle far open you are running almost entirely on the main jet.

An open muffler and pods likely will cause a need for bigger jets.

[jshea]

Thanks bankerdanny. Maybe the acceleration loss was in my head. I'll check on the jets currently installed and plan to step up in size.     

[TwoTired]

The way the pilot adjust works on the PD carbs is a tapered needle into the final delivery port to the carb throat.  Turning it all the way in effectively shuts off the entire pilot circuit.  This only leaves the slide needle exit to provide any fuel to the motor, which should be pretty near minimum for properly working carbs. The idle mix should be dominated by the pilot circuit.

It shouldn't even idle with the IMS screws turned all the way in.  This is a problem to be corrected.  Are the needles broken off at the tip? Are the exit holes now blocked or elongated?

Popping on decel is either leaky exhaust gaskets, or a too lean idle mix.

The main jet sizing is done with the throttle in the 3/4 to WOT range.  The slide needle further restricts fuel flow regulating between 1/4 and 3/4 throttle position.  While there is some bleed over from main, Slide needle and pilot circuits, relying on a main change to fix and idle or mid-range issue, is simply the wrong approach.  The main is selected to provide for the maximum demand at WOT.  The slide needles are set to provide optimum fuel at 1/2 throttle position.  The needle taper angle then sets the mixture for 3/4 and 1/4 throttle position.  And the pilot/IMS is set for leanest idle mix.  The accelerator pump provides for throttle twist reaction.

Cheers,

[jshea]

Thanks TwoTired. Sounds like I need to disassemble my carbs and investigate. Would you suggest I get a colortune to see whats happening in each cylinder? 

[TwoTired]

Never used a color tune.  I read the deposits on the spark plugs to determine combustion conditions.

Cheers,