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-   -   Jetting (https://ecomodder.com/forum/showthread.php/jetting-21976.html)

BHarvey 05-19-2012 05:50 PM

Jetting
 
Had the bike jetted for the 14-1 range on the bottom end but going up to 12.8 on the top end. Decided the bike might pull better down low with a little richer (smaller) needle diameter and indeed it does. But the trade off was 10 mpg!

Thought that by having a little better bottom end, I could use lower revs, but it wasn't enough. The problem was when I ordered the new needle, (same length and taper), the only one that was identical except for diameter was 3 sizes smaller.

2 tanks now verify this so time to put the summer needle back in....

2000mc 05-19-2012 06:32 PM

Non abjustable needle? Or just still too small with the clip all the way up? Then you could try other needles, I was always surprised how different the performance was even tring to get very similar tapers / diameters

BHarvey 05-19-2012 10:12 PM

The carb is an FCR 41 and the possible needle selection is pretty large. You can vary clip position, length, taper and diameter to get the jetting you need.

renault_megane_dci 05-20-2012 04:57 PM

Nice list of mods on your bike !
And good MPG too ! Standart sprockets ?
It looks much more like a fun toy than a MPG champion : what is your ride ?

If it is on you big single, I got from advrider that fitting a pumper carb on (some DR 650), you can go some size down as the higher flow of an even smaller carb will not hurt top end while higher velocity through smaller diameter might help low end.
Maybe a FCR 39 rather than the 41 could have helped more ?

Anyway, running lean is not an issue if you have no pinging problem.

How about less gas in the pump circuit (or whatever it is called) ?

BHarvey 05-20-2012 08:38 PM

Rear sprocket is standard, front is up one tooth.

Thought about the 39mm FCR but had a big cam in it at one time and re-installed the factory cam for more low end. Helped a little in the mpg department but I do miss the BANG in the power curve! It's more linear now.

Built as a fun toy, but getting older means I can still have fun without turning the throttle so much, so why not enjoy saving some fuel.

renault_megane_dci 05-21-2012 05:13 AM

I had a XF650 Freewind before the GN 250, it was converted supermoto, +1 front sprocket and free flowing exhaust.
They come standart with some kind of fairing wich can help.
Best MPG was 4 liters for 100 kms (corrected for the smaller front wheel) on my commute to work wich involved 60% sat on the freeway around 70 mph.
=> hence my question about your ride (highway / town traffic)

Later on I fitted a 10.5 CR piston from JE but never saw good MPG again, the added "Oomph" probably dragged me toward the dark side of the Force ...

Anyway.

A big single with 12 to 1 CR and pumper carb, I wonder how it can be called linear ;-)

The "saving fue"l side of driving is what allows me to keep my driving license so I know what you mean.
But still, impressive and consistant numbers for a bike wich is probably able to spank some ass in the twisties.
Big single rules !

BHarvey 05-21-2012 04:55 PM

Maybe 2% of my miles covered are highway.

renault_megane_dci 05-22-2012 02:55 AM

Even better then !


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