Carb cough

[arcenite]

Good evening, All:

A few months ago I rebuilt the carb with the help of you folks (attached link)

It's since gotten cooler out (has been for a bit). I'm not riding it as much as I was when it was warmer but I get it out at least once a week. Once she's warmed up I take her out and she drives great, 99% of the time. Every once in a while when I blip the throttle at a light or while cruising (with clutch pulled in) the carb will cough, and every once in a greater while it will shut the bike off and I restart it and its fine. Most other times I can blip it to my hearts content and there's no issue.

I found this site but have learned in the past to ask you guys first before I muck with anything - http://www.roadstarmagazine.com/2003...elerator-pump/. Especially since it's super intermittent.

I've been chalking it up to her being moody about the cold but I'd like to adjust it if I can because it's starting to get bothersome.

Any thoughts on where I might look would be appreciated.

Thanks!

[MidnightRide]

First thing others will tell you is that these bikes don't like blipping the throttle as a whole. You're probably best to just knock it off. Not like a Honda. I couldn't tell you why.

My less-than-awesome guess would be to make a minor adjustment on your PMS screw, in or out. Remember what you did so you can reverse it. Try 1/8th or 1/4 turn in either direction. I'd start by opening it up a bit myself, which would be counter-clockwise. Quick and easy to do.

In general, if the bike otherwise runs fine, just don't do it and enjoy life while riding...

Let us know if anything changes!

[Brad_G]

As mentioned folks generally advise against blipping the throttle even though we enjoy doing it. While I'm not a carb guru myself (mine's a fuelie) a thought I might have is do you know what your idle speed is set at? If it's set too low I wouldn't be surprised that a quick drop back to idle could cause this.

[Duke]

It sounds like a lean condition causing the cough. Myself, I would turn the PMS screw out about 1/8 turn. You should also check the exceleratior pump settings, making sure fuel is being squirted as soon as you come off idle.

[davej]

Duke read my post below and I think you will agree

[Shores]

Be sure it's turning 900 RPM when warm. Don't let it lope.

[Father Pobasturd]

:win: warn Shores, Really, "win" warm?

[davej]

It's caused from the vacuum slide carb. the slide/needle don't open as quick as the throttle plate causing a lean condition. It is just the nature of a vacuum slide carb. about the only thing you can try is to open the pms screw like mentioned above but I would try about a 1/2 turn This will give a little more fuel thru the primary circuit but will also richen the primary circuit at all time, not just blipping times. The best thing to do is to just quit doing the blip or accept that when you blip this may happen and that is how life will be. don't waste your time trying to completely fix a self inflicted problem that can't be completely resolved with that carb. If you want to blip without any issue spend a bunch of money on a HSR carb that don't depend on vacuum to open the slide/needle

[arcenite]

Thanks Dave

[Duke]

Yes Dave, I agree and can see where the slide movement can cause a lean condition. With the butterfly open and the slide down, there should be a high vacuum signal causing fuel to be pulled up from the main. One way to enrich this problem, would be to move the needle up 1 notch, but only doing this if it's running on the leaner side while cruising.
One thing I've noticed on a lot of carbs I've worked on, is the exceleratior actuator seems to get weak over time, and the squirt of fuel seems sluggish like low pressure. Part of my rebuilding carbs, is to tighten the actuator spring, which has made the exceleratior pumps be more responsive.
Just food for thought. We could go really deep with the emulation tube with the main jet air feed.

[davej]

Duke, Yep it is that split second with the slide down, throttle plate open and continued blipping (3-4 times or so) that cause a disruption of high vacuum when the throttle plate opens. I also agree with you on acc pump sporadic stream of fuel. I have seen that on quite a few carbs. That is not quite as noticeable when you open the throttle and keep accelerating as it is in a blipping situation

[brianmac]

they all do it caused by temporary momentary lean as described above. better to just to learn not do it on RS's . i do it on my bonnie

[Shores]

Blipping the throttle on any bike serves no purpose other than to get other people's attention.

[davej]

hell I even blip my weed whip

[brianmac]

shores, back in the day, it was also a way to make sure your bike did/has not stall out, when u needed throtlle . So its kind of a carryover from that.

[arcenite]

Thanks for the input all. I don't have a great reason for doing it... it's like an involuntary muscle movement I think I do it now to see if the bike is going to cough, which is counter productive. I won't worry about it. I'll leave the pms screw alone too because it's running great otherwise. Has enough power off the line to scare me into not wanting a more powerful bike

[davej]

I think we all do it, I know I do