Spacers under carburettors

gt alex

Been Around the Block
I have converted my bike to a single 41mm Fish carburettor in place of 2x 39mm Mikuni carbs. and first dyno run was pretty good except for rick idle and part throttle, WOT wasn't bad for first go. Great torque better than stock and only 2hp down at peak and no big drop off even at the rev limiter.
I have got the Idle and part throttle behaving and have cleaned up the small plenum area blending from the flange to the runners.
Now before dyno session 2 Should I fit a spacer under the carb to increase plenum volume or fit the cams I have from a later version that has more valve lift or both?
The goal is making more power than stock at all revs with the single Carb.
Am I missing something I should try?
Feed back welcome
IMG_20210211_093830.jpg
 

gt alex

Been Around the Block
A bit out of date now but this is from my only dyno session to date. The bottom end mixtures were rubbish but I think they are much better now. this compared the toque of my 02 650 with fish carb to a 05 FI model with k&n filter and slip on. My goal is to match the top end power of the FI model once the better cams are fitted. My thoughts are that adding plenum volume will help the top end?
IMG_20210211_141724.jpg
 

cxman

Over 1,000 Posts
DTT SUPPORTER
give it a try i guess but i doubt it will help but its not a big deal to try it
what is you afr showing the curve almost looks like its starving at high end by the way the power drops off is it going rich?
or are the sharp edges in the intake causing reversion?
 

gt alex

Been Around the Block
give it a try i guess but i doubt it will help but its not a big deal to try it
what is you afr showing the curve almost looks like its starving at high end by the way the power drops off is it going rich?
or are the sharp edges in the intake causing reversion?
Mixture once on full throttle was constant 12.5:1 the throttle plate was only opening to about 11 o'clock(sorted) and there was a unintentional venturi shape in the plenum(sorted but unproven). It was all a bit rough first time out.
Not being a normal carb but rather a mechanical Map sensor, fuel flow at WOT is set first then the mid range which is calibrated buy moving the throttle plate in relation to the shaft. Since this was designed for a 3-4litre car engine it wouldn't idle at all, but after the dyno I had a good idea how to re-calibrate the hole unit which I have done and all runs well but I need dyno again to check.
As I have implied with all that was wrong I am hoping they is a lot more to give.
 

ex119x

Been Around the Block
I agree that a spacer would probably not add enough volume to make a difference. But, I wonder if a spacer would add enough length between the carb flange and the first intake tract turn, that it would smooth the flow out and add power in the whole range by not having the intake charge make as sharp of a turn.
Like was said, it wouldn't be hard to do and the dyno will tell the truth. BTW, I would do the cams before the next dyno trip as they might require some more carb tuning, too.
 

gt alex

Been Around the Block
I agree that a spacer would probably not add enough volume to make a difference. But, I wonder if a spacer would add enough length between the carb flange and the first intake tract turn, that it would smooth the flow out and add power in the whole range by not having the intake charge make as sharp of a turn.
Like was said, it wouldn't be hard to do and the dyno will tell the truth. BTW, I would do the cams before the next dyno trip as they might require some more carb tuning, too.
I was going to leave to cams till after the fine tune but then it will change things so I know you are correct in saying I should change the cams first.
 

DTT Bike Of The Month Gallery

DTT Light or Dark

www.jadusmotorcycleparts.com
shop.themotoworks.com
www.cognitomoto.com
https://www.townmoto.com
www.speedmotoco.com
www.lostapostle.ca/
www.sparckmoto.com
Top Bottom