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Keihin (harley) carb question..surging

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Keihin (harley) carb question..surging

PostAuthor: JohnnyF » Sat Mar 09, 2019 7:27 pm

'81 XV920. Keihin CV40
I've had the conversion on the bike for a couple years or so now. I put a Vetter Fairing on it recently and just today took it out for a freeway shakedown ride. With the fairing on I'm not being buffeted by the wind (sweet) but I noticed a slight surging cruising at speed, feels like it's starving just a tiny bit. It's so slight and I never noticed it before the fairing. I'm only cruising at maybe 1/8 throttle so I know it's not the main jet size, the pilot is the size everybody recommends and I have 1 washer under the needle.

I've had an ongoing problem with the carb running rich in city (black plugs) but running a touch lean on the freeway. Also, when I decelerate down through the gears to a stop the bike will cough and try to die when I try to leave the stop or roll on through a fresh green light. I can blip the throttle and it will be fine.

It's a little confusing because running in the city (black plugs) I'm using maybe 1/4 +- throttle max so if I add another washer under the needle it'll be even richer but maybe better on the freeway. The only thing I did that was apparently wrong was to open the bleed hole in the slide to 1/8 as recommended by some sites but since then I've read that 7/64 max is best.....hell...I don't know....time for a beer

I'm going to try raising the float slightly but would appreciate any feed-back from you guys.

THX
JohnF
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Re: Keihin (harley) carb question..surging

PostAuthor: bstig60 » Sat Mar 09, 2019 8:59 pm

How many rpm are you running cruising on the freeway? Anything above 2500 is on the main jet as primary source of fuel, for the coughing at the stop light, try opening up the pilot screw 1/4 turn counterclockwise and see if that helps.
Bill
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Re: Keihin (harley) carb question..surging

PostAuthor: Flyingdog » Sun Mar 10, 2019 3:29 am

Johnny...

Never been a fan of that slide mod..what you've done is changed the air characteristics/speed as when entering the venturi in direct relationship with the butterfly/trottle. (Whew..that's a mouthful) Same outcome/results can be done with a weaker spring. But what's done-is-done. Kinda tough making a small hole smaller. Even if you were good a soldering. If you really think it's fat on the bottom end and lean at the top, you could try a smaller diameter needle. And if the needle change didn't work by itself, couple the smaller needle change with smaller main. Tough call not knowing a starting point.
Oh yea..something else notorious bout those things, the diaphragm can get outta whack when putting the top of carb back on. One trick I've found, instead of trying to seat the diaphragm in the groove first, kinda lay it there, lower the top down on it and with a kinda wiggl'n action, you'll feel the diaphragm's ridge fall into the groove. Least it works some of the time. I just know, some Keihins don't seat very easily.
One of the neatest features of those Keihins, you can replace the pilot screw with a thumb screw pilot. Be nice if the Hitachi's were like that. Reminds me of the S&S carbs where damn near any/every adjustment is out there in plain sight.
Does your model have the accelerator pump?
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Re: Keihin (harley) carb question..surging

PostAuthor: JohnnyF » Fri Mar 15, 2019 9:55 pm

bstig60;
I been playin with the pilot screw quite a bit and get no real change on the deceleration thing but completely solved my cough on accelerating from a stop. May go up a jet size on the pilot and see what that does. It is BAD rich city riding tho. Plugs are coal black if I spend the day running around traffic light to traffic light.
The other nasty thing about this deceleration problem is in mountain riding. If I'm doing an old-guy version of corner carving the thing will cough and damned near drop me when I decelerate into a corner then try to gas it out the other side. Took me almost an hour to get the seat cover out of my ass-crack the 1st time it did it.
Also, I carry a screwdriver tip from one of those 6 way screwdrivers, it makes adjusting the pilot screw almost as easy as the thumb screw.

Flyingdog:
A new slide is only about 30 buks, I may go that way. I really need to pull the carb all the way down and see what I actually have in there as far as needle and needle jet size. I bought the carb from a guy that got it in a box of carb parts so something may not be stock.
I'm old-old skool so never thought about the actual slide being out of the needle jet enough to be affected by the main size as I guess it could be with a CV carb.
I'm only at about 1/8 throttle cruising so automatically think the slide is barely up but ....CV....
It's turning about 4500 RPM at cruise. (I hate that)
Yes, accelerator pump, new diaphragm etc.

JohnF
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