From the Clinic Article...

"Timing Adjustment Screw

This is the "Green" colored screw just above the Blue duration adjustment screw. The" Mucker" said it best,,,,, This upper screw adjusts the "timing" of the squirt. By altering it's setting, you can advance or delay the onset of the fuel squirt. But probably, it won't have to be touched."

The last sentence discounts what I believe to be the most critical aspect of solving the carb cough....the timing of the accelerator pump squirt.

I have found on both of my stock carbs that the timing cam is adjusted from the factory with a small gap between the cam and the pump plunger. As this allows the throttle plate to begin opening before squirting the gas, a sudden lean condition is allowed to develop which is commonly understood to be the primary precuror of the carb cough. The cam-to-plunger gap also seems to facilitate air bubbles in the discharge tube, thus causing a periodic sputtering of gas instead of a solid, perfectly timed stream of gas hitting the t-plate.

Furthermore, turning in the duration screw masks the true problem...timing...by reducing the amount of available fuel to burn/backfire through the intake tract. So, the apparent diminution of the cough by reducing the duration of the stream foments the logical, though mistaken, belief that you are solving the problem.

In short, if you are experiencing carb coughing on an otherwise properly set PMS and jetted bike, I recommend that the order of resolution to the carb cough should be...

1) Advance the accelerator pump tIming.
2) Increase the idle speed if it is below factory setting.
3) Fill with solder and redrill pilot air jet to .020"-.030"

Please reply back with your experience.

--Chris

Chris,
Thank you for your excellent explenation and information.
Dick

Chris,

Great information!! Can you tell me which way to turn the timing screw to make it less lean? Do you recommend turning it to take the gap out between the cam and the plunger?

Thanks,
Roscoe<br><br>Post edited by: roscoe, at: 2004/07/27 16:55

Hey, Roscoe...

Technically...the timing screw isn't about &quot;lean&quot; or &quot;rich,&quot; that's the function of your PMS and jets. The timing screw's real purpose is to ensure that there is no lag between the opening of the throttle plate and the jet of gas intended to counteract the momentary lean condition caused by the opening of the throttle plate, particularly from an idle or closed throttle condition.

Sooo...the adjustment you will want to make is to back the screw out of the bracket moving it towards the left side of the bike. You have accomplished your goal when there is no slack/slop/delay in the timing of the accelerator pump squirt and the movement of the throttle plate from closed/idle position. I discovered another cough related problem this weekend that I will cross post from the RSR forum in a few minutes.

--Chris

Here's a followup to my earlier post...

Found another problem with one of the two carbs I have that will also cause a timing related cough. I'll do my best to explain.
More specifically, I found that even after adjusting the timing cam (backing out the screw) to the most advanced position on the pump cam, the cam could be rotated slightly back and forth without pushing on the pump plunger. In other words, with the throttle fully closed (idle), there was play in the whole throttle plate-to-accel pump mechanism that could not be adjusted out. I carefully compared carbs to each other and could find no reason for the play in the one carb. Sooo...out came the Vicegrips...tweaked the metal arm the cam is mounted to forcing the whole affair to a zero lash relationship...that is, the cam is under spring pressure against the end of actuator arm at all times...no slop, period.

The bottom line is...the actuator arm that inter-connects the throttle plate with the pump cam can not have ANY slop in it. After a 40 mile ride, purposely doing everything I could to make it cough...I couldn't...not even once!

If I get ambitious, I'll redline some of the carb graphics and post 'em to make it easier to understand what I'm talking about.

Yes, Martha! You can have a cough free bike!

--Chris