Thanks for the comments. The Hawk
they tested was stone stock except for the Dynojet. Dynojet did offer a
Stage 7 kit for the Hawk.
Slide springs were not changed. MCN had good
results at midrange. Top end was not improved significantly. The
point of my comments was that drilling slides is not for everyone. It
may cause a loss of low end reponse. To undrill the slides is very
easy. Another point that can be taken from the MCN article is that there
may be little variance in stock slide hole diameters between different Mikuni
carbs. That to me is really interesting because it may say that Mikuni
developed what they consider to be an optimum slide hole size.
I am not trying to discount the unfortunate experience that MCN had with
their 650cc Hawk ... and I certainly believe them when they state that from
their article ....
"Remember that the all-new GSX-R 750 has computer-controlled slide
venting. Our test indicates why. Smaller vents will give better
off-idle response, and larger vents give better response at high rpm.
Now we know why Suzuki went to all the bother."
.... as I think this might be true with everything else remaining
stock ..... and I am not attempting to challenge their expertise or that
of their dyno people with this response or to attack you in any way ... I am
only pointing out that I think it is unfair to make such a statement without a
lot of qualification. Additionally, if Suzuki "went to the bother" to
computerize their slide opening the computer must be monitoring something
beside screams or foul language from the operator like ... airflow ... vacuum
... etc .... The slide simply reflects the standing pressure difference
between the area in front of the slide and a hole located below the throttle
plate ... it's retraction is an indication of increased
airflow.
I wonder however if the 650cc Hawk kit came with lighter springs as the
DSP044 springs that come with the Vmax Dynojet kits are 2 1/2 inches shorter
than stock and only offer 1.5 grams compression
resistance as opposed to 5 grams for the stockers, (more than three
times less effort to compress the slides and the needle it is pulling on
.... Thanks to Patrick Ahern for measuring the tension on these guys ... )
.... IMHO ... I think that everyone knows that none of the above would have
any affect for top end horse power .... as once the slides are open you are
running on the main jet and how much air you can get into the top of the carb,
(while still being able to create enough vacuum to pull some fuel along
with it). The size of the slide vent hole works against the #2 purge jet,
(which is an air vent located below the throttle plate), to overcome the
spring tension and open the slide.
I have mixed feelings about the lighter
springs. They do offer better high end throttle reponse. No one
will argue with that. But down low, the lighter springs may
flutter. I have tried it both ways with my Venture (with Max intake),
and find I get a little smoother running down low with stock springs and not
that much better reponse up on top with the lighter Dynojet
springs.
As far as the DynoJet arrangement improving performance ... I don't
think I would bother if I wasn't opening up the air intake on the bike in the
bargain .... as airflow potential is maxed out in stock condition
with the "Y" in place ... despite there being a K&N, UNI or no filter
in the air box, (thanks to Terry Hayden for measuring the "suck" resistance of
these airbox combinations). If no additional air comes in the front door ...
you can add all the exhaust systems in the world and you would not likely see
more than about 4 RWHP ... because despite what gets exhausted .... you can't
make that much more HP without getting more air into the engine and the gains
in efficiency from a superior exhaust would be greatly suppressed.
On the other hand ... take the "Y" off the Vmax and don't do anything
else and see where your low end response goes ... check the mid-range on the
dyno .... As you well know it is not uncommon for some of it to disappear but
often give you an additional horse on the top. Sooooooo ... we didn't change
any slide vent holes ... where did it go ?? Should I make the slide
hole "smaller" to counteract this modification ... will I get it back ?? ... I
think not ....... Was I flowing more air ?? Of course I was !! That was
reflected in the gain at the top end ........ Soooooooo ... the easy way to
fix that is to put a washer under the needle and make sure it is retracted a
little more than it used to be for the same position of the slide ... and wala
!!! my bottom end comes back .... annnnnnnnddddd I get to keep the gain on the
top .... Clearly, (and I am not trying to be funny here), we did not lose our
low-mid range in this scenario because of changing the the size of the slide
hole ... we lost it because we did not adjust our needle properly to provide
an optimal amount of fuel within this RPM range ... and guess what .... on the
top end we gained because it didn't care about the needles ... at that point
they were fully retracted, (of course a main jet adjustment there might have
given us another couple of horseys).
Consider something in addition to raising
needles. The original Venture came with a 42.5 pilot jet. Most
Maxes and all 1300 Venture went to a 37.5 pilot jet. A friend of mine
was running a stage 7 with his Max (stock exhaust) and had a mid-range hole as
big as a house. He tried the 42.5 pilot jet and claims it eliminated the
hole. I am installing Max heads on my 1300 Venture and will also test
the 42.5s on it this spring.
Clearly ... the instructions for both the DynoJet kits have everyone
setting the needles too rich for default but once they are set up properly
they do infact provide whomping throttle response compared to a stock setup,
(IMHO). I don't know if the carbs on the Hawk have bypass holes either, (pilot
mixture holes on top of the throttle plate), as the Vmax does and these along
with what is coming out of the mixture screw holes, (underneath the throttle
plate), and what ever escapes around the needles is what the bike is running
on when we go off-idle. IMHO if someone is tuning a bike and depends too
much on slide movement to save their butt on an off-idle condition then
they are asking for trouble and should be making additional adjustments in
their pilot circuit. If they are looking to for more fuel when they womp
on the throttle they need to give the thing a little more needle or remove
some of the resistance from the slide operation. Also ... IMHO .. if
someone is looking to launch themselves over a parked car when they peg the
throttle ... these are the kits that will give you the ability to do it
.... because the slides will open ... quickly ... Of course this presents
another opportunity to mis-tune by using as much needle with this slide
arrangement as you are used to with stock vent holes and stock
springs .... there is just going to be too much fuel dumping in there
and responsiveness will suffer as a result .... also on 4500 RPM
cruising you will foul your plugs in short order because you will be running
entirely too rich ........
Sorry for the length here ... you guys can attack now ... sigh ..... NO
.. I don't work for Dynojet or Redline oil ...
campbell
Finally, we are all in the same boat. We
are all trying to figure out which works best and what Yamaha was thinking
about when they engineered this motor. At the end of the day, it is
still just a motor.
Regards,
Rey
----- Original Message -----
Sent: Tuesday, March 07, 2000 12:41
PM
Subject: Re: Drilling Slides and
Reversing the Process.
I reversed my Dynojet slide holes by filling the holes
with J.B. Weld. I redrilled holes adjacent to the filled holes.
There was an article in Motorcycle Consumer News by Dave Searle, dated
May 1996 a while back which addressed drilling of
slides. The bike they tested was Honda 650cc Hawk. After
drilling the slides and installing the rest of the components, throttle
response was off. Following are some interesting quotes from the
article:
"Resigned that we must have done
something wrong, we scratched the head of Joe Youkhan, the dyno operator,
who'd had considerable experience with these kits. Regarding the
sluggish throttle response, he said that he doesn't ordinarily drill slides
as suggested unless the bike is really intended for the
racetrack, because which throttle response at high rpm is enhanced, it is
worsened at low rpm...Hmmm? Back at the shop, we called Two
Brothers Racing to see if they had any tips for us, assuming their extensive
Hawk knowledge would provide the answer. We were told, 'We don't sell
the Dynojet kits, because nobody can get them to work.' "
"We called back the folks at K&N,
which had provided the kit. Acknowledging the company's reputation for
quality and expressing satisfaction with other Dynojet installations we'd
done, we gave them our results. We also relayed the remarks we'd
received from the dyno operator and Two Brothers. They said their dyno
comparison showed a gain of nearly five horsepower and offered to send
another kit although our description of the needle dimensions seemed to
indicate we had gotten the correct parts. We were told however, as Joe
suggested, that drilling the slides would hurt off-idle response
but that the carbs would reach full throttle soon and therefore gave an
acceleration advantage in spite of the derivability problem. We were
told the drilling does not actually influence horsepower, so we resolved to
leave the slide holes alone on the next attempt. Remember that the
all-new GSX-R 750 has computer-controlled slide venting. Our test
indicates why. Smaller vents will give better off-idle response, and
larger vents give better response at high rpm. Now we know why Suzuki
went to all the bother.
MCN used a 0.109" drill to redrill the holes to stock
size.
In November '96 in response to a query of a Yamaha FJ 1200
owner who wanted to undrill his slides, MCN called Yamaha. There is no
Yamaha spec for size of slide holes. Yamaha suggested calling Mikuni,
and Mikuni said the sizing had been worked out in Japan in concert with
Yamaha engineers. Dynojet's tech rep said they have information back
to '91 models, but suggested most stock holes are in the neighborhood of
0.110". A #35 drill is 0.110", and a 7/64 drill measures
0.1094"
Rey Kirkman
----- Original Message -----
Sent: Tuesday, March 07, 2000 8:08
AM
Subject: RE: Going back to Stock
Jets.
If the slides are drilled out, it CAN be
reversed.
Take some good 2-component glue. Close the hole
with the glue and let it dry thoroughly. Drill in the original size holes
again. However, at the moment I do not know the size.
Anyone??
Patrick Kuyper
Vmax Club Holland
The Netherlands
