Subject: Exhaust & Carb Tuning
Author: joe jump
Date: Jul 20, 2005, 7:39 AM
Post ID: 1719181015
I haven't gotten around to making an appointment with a local
shop that has a dyno, but I've been monitoring the performance of
Junkyard carefully every ride. My seat-of-the-pants dyno has led me to
the conclusion that I have a significant dip in torque from 4000 to 4800
rpm, which manifests itself in 5th gear. Once above that it seems to
pull pretty good, but riding at those speeds draws unwanted attention
from the Boys in Blue.
The drive train is all from an early 850T; 30mm VHBs,
transmission, rear drive. I used the Ambo header pipes (exhaust port
spigot ground off flush), but installed a set of late model HD dresser
mufflers, the big, beautiful, expensive ones those guys all take off &
throw in the dumpster as soon as they sign the paperwork for the $23k
note. I was able to find some thick-wall tubing from a steel supply
house that was 1 3/4" od & that fit onto the Ambo header pretty tight.
But the system does not have a cross-over, as all Guzzi big twins have.
Initially I ran the mufflers as delivered; it was real quiet, but it
seemed a bit corked up. I started to drill out the internal baffles and
each time I opened them up a little more, the better it ran. I
eventually punched the baffles out completely, but then it was a bit
loud. Still not satisfied, I tried using a crusty set of stock Ambo
mufflers I had for a while which yielded perhaps a slight improvement in
mid range power, but they were quite unsightly. I ended up getting a
couple automotive cup-shaped freeze plugs that I drilled 5 or 6 quarter
inch holes through, and drove them back into the HD mufflers; quieted it
down just enough and no noticeable change in power from the stock Ambo
units. Now I'm wondering if putting a cross-over pipe in might help with
my flat spot. It wouldn't be too difficult for me to put one into the
header-to-muffler transition pipes, essentially in the same place where
they are on a Loopframe. Any opinions welcomed.
Another thought I had was perhaps the carb tuning might be off in this
rpm/load range. I initially had the carbs jetted as recommended for an
850T, but I changed the pilot jet, main jet, and (I think...sometimer's
disease) the slide needle to the ones out of the Ambo carbs, which are a
bit fatter than the T-bike's. I'm pretty sure I left the needle jets
alone and that they are the same as those used in Loops, but I did raise
the needles to their highest (richest) position. I once had a
conversation with Ace Mallot (the tuner on MG's race-winning V7 Sport)
about the same lack-luster top end performance I was experiencing on my
Sport. His response was carburation, but he didn't have the time then to
get into the details. On Junkyard I notice that the bike performes
pretty good in town going through the gears with small throttle openings
- I doubt if it's much out of the pilot circuit in those situations.
Even in 5th gear it pulls pretty nicely from 3500 to 4000 rpm. But when
I get onto the interstate and I want to pull 5th above 4000, small
advances in throttle seem to make little difference other than increased
intake honk. More twist on the throttle yields a slow increase in
rpm/speed until the tach is reading around 4800-5000 rpm, which is
getting a little fast for these parts. There is no mis-fire; the
ignition is working well. It just feels like the motor is overloaded,
like it's geared too tall.
What I'm looking to attain is some good roll-on stomp between
4000 & 5000 while in top gear. All my other bikes excell in this area.
There are some pretty good wrenches on this list with Loopframe
experience that is well beyond mine. I'd appreciate any & all comments.
Thanks for your time!
Joe in St Louis
850T-Powered Ambo
"Junkyard"