HELP! ! Carb experts!

HELP! ! Carb experts!

Post by Leslie Voro » Tue, 24 Nov 1998 04:00:00


The VM38's on my stock '90 Indy 500 with PSI pipe are soon to be
replaced with Keihin flatslides, like the ones on the new XC's.  With
the main objective being improved throttle response, would it make sense
to install 40mm flatslides, or go with smaller carbs for higher air
speed?  how 'bout combining the 40's with power jets ?

                                                Thank 'yall,
                                                            E-GR-2XLR8

 
 
 

HELP! ! Carb experts!

Post by light » Tue, 24 Nov 1998 04:00:00

If throttle response is your main goal you will want smaller carbs ..40mm
carbs MIGHT give you slightly more topend power if you can spin it high
enough to get to peak(clutching) but you will lose that midrange punch that
you are looking for.If you look at Yamahas across the board the carbs are
slightly smaller  than machines of simmilar size from the other
manufacturers . They strive for good velocity rather than absolute volume at
top end .This sacrifices a bit of power on top for better throttle response,
lower (engagement) ,wider power band. So unless you modify the motor to pump
more air smaller carbs would be the answer.You would have a much easier time
tuning with the power jet carbs to get your mid range to wot transition just
right.Why not install power jets in your present carbs .Alot cheaper .
Steve
97 600SX
Quote:

>The VM38's on my stock '90 Indy 500 with PSI pipe are soon to be
>replaced with Keihin flatslides, like the ones on the new XC's.  With
>the main objective being improved throttle response, would it make sense
>to install 40mm flatslides, or go with smaller carbs for higher air
>speed?  how 'bout combining the 40's with power jets ?

>     Thank 'yall,
>     E-GR-2XLR8


 
 
 

HELP! ! Carb experts!

Post by Leslie Voro » Thu, 26 Nov 1998 04:00:00

Quote:

> If throttle response is your main goal you will want smaller carbs ..40mm
> carbs MIGHT give you slightly more topend power if you can spin it high
> enough to get to peak(clutching) but you will lose that midrange punch that
> you are looking for.If you look at Yamahas across the board the carbs are
> slightly smaller  than machines of simmilar size from the other
> manufacturers . They strive for good velocity rather than absolute volume at
> top end .This sacrifices a bit of power on top for better throttle response,
> lower (engagement) ,wider power band. So unless you modify the motor to pump
> more air smaller carbs would be the answer.You would have a much easier time
> tuning with the power jet carbs to get your mid range to wot transition just
> right.Why not install power jets in your present carbs .Alot cheaper .
> Steve
> 97 600SX

> >The VM38's on my stock '90 Indy 500 with PSI pipe are soon to be
> >replaced with Keihin flatslides, like the ones on the new XC's.  With
> >the main objective being improved throttle response, would it make sense
> >to install 40mm flatslides, or go with smaller carbs for higher air
> >speed?  how 'bout combining the 40's with power jets ?

> >     Thank 'yall,
> >     E-GR-2XLR8

I appreciate your thoughts on my carburation matter.
A local performance shop also suggested that I replace the stock vm38's
with simple vm34's, perhaps bore out the 34's to 35.2's in order to
maintain a decent top-end.  Apparently the 500 engine is conservatively
built, airflow mostly being limited by the ports, not the stock 38mm
carbs.  Thus the carbs are already somewhat oversized..  This sounds
quite logical, as he was explaining that a decent porting job will boost
the output by ~15 HP.  Like you said, the bottleneck to airflow is in
the porting on this particular engine.  Since I'd like to keep the motor
stock, smaller carbs are the the way to go.  

LED SLEDHED