Rejetting Carby

Gidday everyone ,

I have a CB250 that's is only 15 months old and I decided that I would
rejet the carby with bigger jets to give the bike a bit more "zip"

So yesterday I replaced the #35 pilot jet and the #110 main jet with a
#38 pilot and a #115 main jet.

There has been a noticeable improvement in performance , better cold
starts , smoother idling , better and smoother acceleration under load/
up hill and quicker to get up to speed from a standing start.

Generally the performance has improved all round but what I have found
after going for a hack along the freeway ( Perth) is that I have than
increased revs for the same speed.
For example ,
before the rejet 80Kmh @ 5200 rpm
after the rejet 80Kmh @ 6000 rpm

So while I do have an increase in performance after the re-jet , I also
have an increase in RPMs as compared to my normal speeds and I am
wondering if it is really worth it in the long run , so my question to
everyone is :

Do you think that it's worth re-jetting the carby if there is an
increase in RPMs for the same speeds ?
( I am not worried about the fuel consumption at the moment )

Thanks.

 

I seriously hope you're trolling!

It's physically impossible to change the revs @ any given speed without
changing the gearing. You must be reading something different, or the
speedo has taken a sudden turn for the worse.

Cam
'03 954

 

yeah, it's pretty normal for the front sprocket to shrink a little
during rejetting... don't worry about it.

Charlie

 

What's happened is the extra weight of the jets has increased the overall
mass of the bike which is requiring the bike to rev harder to achieve
equivalent speeds, hence the additional RPMs.

I'd recommend offsetting the additional weight by removing weight
elsewhere - perhaps get the front disc shaved to half of it's current
thickness, or replace the rear drum with a piccolo snare or even a small
conga. Replacing the fork oil with something lighter - say sunflower oil
(available from most supermarkets) may also be beneficial (safflower oil
does not deliver as good compression, so it's not recommended).

Manning

 

yeah I think there's a problem there.. you're final drive is a fixed ratio
so it should be directly proportional to engine RPM.... dunno what the
cause would be.. possible slipping clutch?? maybe I'm wrong here...

Later
Andy

 

Its a sad day when the trolls get such a low standard..

 

No , I am not trolling.

I hope you are right because I don't fancy having increased revs at
certain speeds because it will cause more wear and tear on the engine.

I had nothing else done to the bike apart from the rejetting , I read
the speedo right , in fact I had to look at it a few times to make sure
what I was seeing.
The only other explanation that it could be is that the road I was going
along had a rise/hill on it that looked like it was level.

It looks like I will have to go for another run along a road where I
know for a fact what the revs are at 80Kmh.

 

The clutch seems OK and I haven't noticed any problems with it before I
put the bike in to get re-jetted or after , the only other explanation I
can think of is that when I noticed the increase in revs @ 80Kmh , I
could have been travelling along part of the freeway that had a hill on
it that looked reasonably level.

It looks like I will have to go for a ride along a stretch of road where
I know the exact revs @ 80Kmh.

 

Hills don't affect the engine rpm at 80km/h either. Face it, you've fucked
up what you "thought" the bike was doing at a certain speed. Unless there's
been a change in gearing, the bike will do exactly the same revs as before
at the same speed. You need a serious dose of friggin logic.

Cam
'03 954

 

Actually the bike will be lighter due to the bigger holes in the jets. Less
Brass = less weight.

Notihng is lighter than a hole ( old racers terminology )

Maybe the bigger jets cave confused the rider into beleiving 6200 is 5200 -
which would sound about right.

Daron

 

Maybe they told you it had been re-jetted but just changed the front
sprocket to make you think it was quicker.. smaller front sprocket will give
better acceleration making it feel faster and more responsive but higher RPM
at a given speed.. check to see if the sprocket it practically new!!!

Andy

 

They wouldn't do that, it costs time and money to acheive that ....

Cam
'03 954

 

Did you forget to shift to top? Jetting won't change gearing. Have I just
responded to a troll?

 

just trying to offer a possible explanation?? I dunno what the mechs would
do?
ahh well!

Andy

 

More Power = More Fuel + More Air. How have you improved the airflow?

Ahem. That's a different gear, not a different jetting!

It's a bit of a strange question. It isn't an improvement if you
can't use top gear any more!

Go thou and buy the Haynes book of Carbies. It's really good.
Their generic motorcycle book might be a worthwhile investment
too.

-----sharks

 

Hint: count the gearshifts ...

-----sharks

 

If I was in 4th gear , then this goes to show the improvement of the
re-jetting because it certainly felt like I was in top gear ( 5th ).

Next time I go for a run to check the revs , I will make sure that I am
in TOP gear to confirm the revs.

 

Nope , you didn't respond to a troll , I would have made a lot better
post to do that if I was trolling.

Maybe I could have been in a lower gear , but the performance felt like
I was in top gear , I will have to check it out later.

I hope you are right , because the performance I am getting with the
re-jetting is fairly good , I don't want to go back to the original jets
if the revs are somehow higher.

 

Nope , the mechanic would have been too busy to do that and I don't see
any new racing logos stuck to the bike either.

 

So, you richened the mixture right up without any way of checking
how lean/rich it was originally, and now it is as sluggish in 5th
as it used to be in 6th...

Do you want to think that through a little more???

The jetting cannot possibly have changed the road speed / engine
revs ratio for any particular gear - there is some "winning lotto
odds" type possibility that the clutch is now slipping where it
wasn't before, but assuming a working clutch, the rpm per kmh is
set by wheel/tire diameter, final drive ratio, gear ratio, and
primary drive ratio. Note there's no mention of carbs or jets in
that list...

big (I must remember to stop chatting to usenauts under bridges)