Thursday, January 26, 2006
The skinny on backfire
I’ve been involved in two separate discussions this week concerning carburation and fuel mixture, specifically with regard to what exactly causes backfires in carburated, high performance motorcycle engines. The popular misconception seems to be that, all free-flowing engines that backfire when the throttle is snapped closed from higher-than-idle revs must be running lean across the rev range. This is not always the case.
Like any good sportbiker, the first thing I do whenever I buy a new motorcycle is to store away its portly, restrictive stock exhaust and install an aftermarket pipe. In the case of my ZX7R, the bike was already lean from the factory, and freeing-up the back pressure made it go even leaner. Crackling backfires on roll-offs were common.
Next, I installed a BMC filter and a big, fat, Factory Pro jet kit. Doing so improved the bike's mid- and high-range performance dramatically, and it's actually slightly rich all the way through the rev range now. But you know what? It backfires just a bad as it ever did. Why? Mikuni has the answer:
It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.If you’ve ever been to a superbike race, you know that backfires are commonly heard as racers slow down for corners. BIG backfires. As the Mikuni guys explained, backfiring is one of the characteristics of a properly tuned, carburated race engine. So be thorough and don’t assume. You know what that does.
The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.
Why this (normally) happens:
1. When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
2. The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
3. Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
4. When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected -- the exhaust then pops or backfires.
5. Completely stock Harleys do not do this until open-end mufflers, such as the popular Screamin' Eagle slip-ons, are installed. The exhaust must be both free-flowing and have an open exit for the popping to occur.
Wednesday, January 25, 2006
2 Wheel Tuesday: The ongoing tragedy
From James Walker on Usenet:
"2 wheel tuesday is a 30 minute show minus 12 minutes for commercials. [That equals an] 18 minute show. So, out of [18 minutes], 10 minutes were devoted to how to make a [motorcycle] commercial. For Pete's sake Speed Channel, can't ya find enough MC news to even fill [an] 18 minute show?"
Amen to that. I desperately want to like this show, but it repeatedly insists on sucking.
First of all, they need to get rid of all the enduro and trials crap once and for all. Nobody gives a flying rat's ass about that stuff. If I never see another trials bike in my life, it'll be too soon. Where I come from, we have a similar device to a trials bike; it's called a Pogo Stick, and it's almost as boring.
Secondly, anything that doesn't have to do with streetriding or roadracing should be kept to a minimum. Dirtbike racing highlights should be shown, but should be collectively no longer than about 90 seconds in length.
Thirdly, I don't want to see another "custom" chopper or S&S-powered piece of butt jewelry on 2WT ever again. Ride On, American Thunder, V-Twin TV, American Chooper, Build or Bust, and a gaggle of other shows on cable have that subject matter covered ad nauseum.
Oh, and scooters suck, too. Every time I see something scooter-related on that show, it's like taking NyQuil.
The old Motorcyclist TV show had it right. They reviewed bikes, gave performance and suspension tips, touched on streetriding issues, and otherwise kept their focus. 2 Wheel Tuesday could learn a lot by digging into those archives and doing something interesting with their 22 weekly minutes.
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Tuesday, January 24, 2006
Remembering a fallen friend
2001 was a rough year for the Odessa sportbike crew. There were lots of tickets written and other headaches from the cops, squids were crashing all over the place, and it seemed like everybody had some kind of story to tell. As if that wasn’t bad enough, we almost lost a group member, Crazy Dave Alders, early that summer when he and his bright orange Slingshot got creamed one morning by a truck, resulting in major injuries. Then, just when we thought things couldn’t get any worse, friend and fellow OSB rider Bobby Miller died.
I got the call from Rodger Gertson, who left the terrible news on my home answering machine. Bobby had been killed the night before on a group ride returning from Lubbock, Rodger explained, and details were still sketchy. I was frightened and shocked beyond belief at the news. I had planned on attending that very ride with Bobby and the rest of the group, but had changed my mind at the last minute for reasons I have since forgotten. But as details of the story began to emerge, my fright turned to simple anger and frustration at what Bobby had done.
By all reliable accounts, Bobby and several other riders had been radared by an oncoming DPS trooper, and Bobby bolted. This was his fatal mistake. On an unfamiliar highway, in the middle of the night, and at speeds in excess of 180 miles per hour, Bobby went off the road and into a field. His bike disintegrated in the process, and by the time his poor, mangled body finally came to rest, there was very little life left in him. A Helivac was called, but the paramedics could not stabilize Bobby for transport, and he died at the scene.
In an instant, one very bad decision to run from the cops had not only robbed Bobby of his life, but it had also robbed a daughter of her father, and a wonderful wife of her husband. It also robbed the rest of us, too -- of an unforgettable friend and riding partner that we’ll never be able to replace.
As motorcyclists, we owe Bobby the honor of not only remembering him, but also of learning from his death. All you young guys out there who’ve maybe only been riding for a year or two should keep the story of Bobby Miller in mind any time you start thinking that running from the cops is the thing to do. No ticket or trip to the pokey is worth dying over. Sure, most highway speed law is crooked, along with the officers who enforce it, but don’t let them take your life on top of your liberty. Running is never worth the risk, and by accepting that fact, we can gleen something meaningful from the otherwise senseless loss of Bobby Miller.
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Sunday, January 08, 2006
Your tax dollars at work #546
I had an interesting (albeit infuriating) conversation today with a sometimes riding buddy of mine who owns an aviation maintenance company at Midland (TX) International Airport. It seems he has contracts to work on several law enforcement aircraft, including a helicopter owned by one of the local departments, and being a motorcyclists, has had multiple conversations with the LEO pilots concerning local sportbikers.
As the story goes, these helicopters and their hi-tech gear are being utilized for the surveillance of sportriders -- often from miles away -- with infrared cameras. My friend said the officers had shown him videotape of 15 or 16 bikes parked on the side of the road out in the middle of nowhere as two other motorcycles drag raced.
“You could mainly see the infrared hot spots on the videotape,” he remarked. “But it was definitely sportbikes.”
The tale caused me a silent anger far beyond my ability to fully articulate it. These bored LEOs, with apparently no real matters of law enforcement to contend with, are spending taxpayer moneys to the tune of around $2,000 per hour of aircraft operation to spy on a bunch of harmless motorcyclists doing a little late-night drag racing out in the desert, miles away from civilization. Behold, your tax dollars at work.
As for street racing being illegal, that’s another issue for another day. My main beef is that there has been a rash of unsolved burglaries and other crime in both Midland and Odessa since the beginning of the 2005 holiday season, and rather than doing their jobs, these guys are apparently out in their whirlybirds playing ‘I Spy’ all night while fiddling with expensive, unnecessary toys that were purchased by the likes of you and me.
At this point in American history, tales of government waste often come as no surprise to the complacent, desensitized ears of the American public. I suppose that’s the way of things these days, but it shouldn’t stop us from stopping it. Demand fiscal and civil responsibility by government, before we allow ourselves to be whipped into Orwellian submission at their hands.
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Friday, January 06, 2006
New year, new ideas
After a holiday season that seemed to last forever, I’m slowly getting myself back on track for 2006. For me, that mostly means getting back into racing trim (down to about 145 pounds) and going back to a regular work schedule. I’ve also gotten the ball rolling in accomplishing some tasks that I’ve been telling myself for a year or more that I was going to get done.
Firstly, I finally lowered the suspension on my ZX6R after six years of ownership. Using a set of Vortex lowering links and a come-along, my friend Rodger and I completed the job last night. The bike is now 1.5 inches lower and looks great. Finally, I can sit completely flat-footed, and the reduction in ground clearance will hopefully keep me off the edge of the tires all the time.
Projects to come include lightening the ZX7R and researching FI conversions for both bikes. Details as the develop. Happy New Year.