XBR tuning problem

[guest526]

Well I've been out test riding my XBR cafe racer ( http://www.thumperclub.com/smf/index.php?topic=2859.msg20084#msg20084 )today,and it's badly out of tune.To be fair I was expecting problems,I know that removing the airbox on a CV carb can lead to all sorts of issues ,but its integral to the new look of my XBR and I'd love to get to run ok with it's current K&N filter.

Set up as it stands:
Motad exhaust ( 2 into 1 standard not performance)

main jet#165
pilot jet #48 (standard)
mixture out 3 turns
hose out of airbox utilized as a velocity stack fitted with a large K&N filter off a MX bike (not shown in the pics,I only fitted it today)

Problem:
Starts fine,and idled fine after I slightly enriched the mixture,the real issue starts  under load between  2000 and 4000 rpm the bike hesitates and stutters after 4000 rpm full clean power up to the red line.

First off ,anyone removed the airbox on their XBR  and got it to work ok?,what jet sizes etc?
Any ideas ? ,luckily it now only takes 10 mins to take off and refit the carb ( no airbox remember )
So messing round jets is less time consuming,probably be a week before I get another chance to look at the bike so I'm hoping some of you will chime in ideas and a possible solution,thanks.

[KungFooBob]

I was always told that when tuning a carb it's not the Rev's it's the throttle position, certain parts of the carb only work at certain throttle positions...

Full throttle is mostly main jet, mid throttle is more to do with needle....etc...

I'd have thought a 165 was a bit big (standard is 152 iirc), but it's more than likely going to be the needle if it's playing up mid throttle, you could try shimming it up and down (I can't remember if the standard carb has an adjustable needle?)

Mine's running with out the airbox, but I've got a 41mm Kiehin Flatslide on it, originally it had a 165 in, but it had no top end pull at all, I went down to 160 and got an extra 250-500rpm in top, I'll probably go even leaner when I put it all back together.

[guest27]

Guess it is tricker with a CV as the needle is moved by vacuume not the cable, but it is throttle position under load that is the issue.

R

[Steve H]

Guess it is tricker with a CV as the needle is moved by vacuume not the cable, but it is throttle position under load that is the issue.

R

With a CV carb its the slide position, rather than the throttle position thats important.   

[guest7]

I'd have thought a 165 was a bit big (standard is 152 iirc),

Honda put the size up to 158 within a year of the XBR's launch. I have used a 165 when running with a standard airbox and (very) open pipe and it ran great. Can't help with the no airbox thing though 

GC

[guest526]

Well I have had a kind of successful day of pragmatic carb R&D and tuning.

First off KungfooBob was right about the #165 main jet being too big,my Motad silencer must be more restrictive than I thought,anyway I refitted the original #152 main jet,took it out for a quick test ride and sure enough the mid range hesitation had all but gone ,however the bike still wasn't accelerating as strong as it should and the throttle response was still pathetic.

So I did a bit of research on the net,this became a somewhat confusing and frustrating exercise mainly because there seems to be so many contradictory ideas on how to tune CV carbs.To me the most logical issue, was the slide was not opening quick enough, because the air pressure required to lift the slide wasn't high enough after the air box was removed.I have a couple of spare slides, so I figured I could afford to do a little experimenting.So I enlarged the 2 holes at the bottom of the slides to 2mm this is supposed to reduce the amount of air pressure needed to lift the slide.Finally I raised the needle with a small washer in a bid to improve the acceleration .

In went the carb for probably the tenth time,and out onto the road and ,wow, what an improvement ,throttle response is instant ,noticeably better than stock and acceleration through the gears is back to where it should be.

Well maybe I got lucky,and sure, I still need fine tune the jet sizes (50/158 I recon),but I'm well pleased with this afternoons efforts.