Subject: Re: Exhaust & Carb Tuning
Author: halc-@comcast.net
Date: Jul 20, 2005, 8:07 AM
Post ID: 1719181185
Joe
After a quick peruse of your email I came up with general approach to your problem.
Do the easiest thing first. Carbs. Is it gas starved at mid-range? Move the needles up a notch.
Though I believe it is a cross over issue. Going from memory, not alway a good thing,
I remember my experiences with and w/o x-overs. Without the x-over there wasn't enough back pressure.
I lost top end. With more restricted cans w/o x-over the mid range was an issue.
So I put on a crossover and ran less restrictive cans (OEM dunstall mufflers) That ended up giving me plenty
of UMPH throughout the RPM range. And a louder exhaust.
I believe Guzziology has a write up about the effects of velocity stacks and exhausts.
I'm at work so I don't have access to my Guzzi knowledge base. I believe Tom Short ,Greg Field or Ken "Lightning" Pitts
may have experimented.
T
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| Your free subscription is supported by today's sponsor: ------------------------------------------------------------------- Get up to $1500.00 Fast and Easy! No Credit Checks! No Paper Hassles! Nothing To Fax! Apply Online and We'll Contact You Within the Hour! http://click.topica.com/caadHk7b1dfltb7hDhIa/911PaydayAdvance ------------------------------------------------------------------- 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" Your free subscription is supported by today's sponsor: ------------------------------------------------------------------- Erase wrinkles without Botox! Nexiderm SP is clinically proven to reduce wrinkles by 68% Click here to get your 30-day free supply. http://click.topica.com/caadHllb1dfltb7hDhIf/Nexiderm ------------------------------------------------------------------- |