The Backwards Engine Experiment, 1939 Colson

here's a before and after of my bike. just finished it today, and it rides super friggen awesome.

the build-up process is detailed here: http://motorbicycling.com/f38/rustoration-build-off-13144.html

yeah, the motor's on backwards. that's part of the reason i'm making a new thread here. i'm gonna list the pros and cons, performance, etc... as i go along.

for now, i just got it running today, and besides the "hot foot," it works perfect.

i'll update this later. i need to ride it to the liquor store to get a soda, and that'll probably take me a few hours...

Re: the Resurrection of the 1939 Colson with Backwards Motor

good for you. keep it that way..

i just got back from my first ride, about 20 miles. this thing screams. when i say "it screams," i mean, i blew past a guy on a 50cc scooter, and he couldn't catch up with me. i was riding down Newport Blvd, a 45mph zone, with traffic. keeping up with them no problem. and i still haven't completely opened it up yet.

don't worry, i'll post proof when i can get a buddy to pace me with a video camera.

there's NO four stroking or sputtering at all. none. not even once.

my set-up is:

66cc Grubee Starfire GT4 reversed.

you have to spin the jug and the piston, and match the transfer ports on the case. i also smoothed them all out as far as i could into the jug, and very carefully smoothed the burs at the cylinder ports.

the head and the jug were both lapped on a glass table, probably lost a millimeter on each to increase compression.

the intake manifold was cut down so the carb would clear the frame, which also gives a performance increase by having a smoother, shorter path for the fuel mixture. i also match ported the intake manifold to the jug as much as i could.

the exhaust pipe was straightened and ported, and the baffle and it's internals have been left alone.

the NT carb has been cleaned and polished inside and out, and the burrs and step downs have been smoothed and polished. air cleaner is still stock.

NGK BH6S plug, with filed tip, gapped to .025

solid inner wire plug cable and new boot.

Opti-2 oil at 60-1 ratio

the motor is brand new, and i'm not baby-ing it at all for break in. (if this develops any more power after break in, it's gonna be really scary...)

30 tooth sprocket on 24" wheels.

i'm 135lbs, and can ride this bike off the line without pedaling (but i do anyway.)

the bike itself is a little squirrelly, due to it's smaller size, but i think it has to do a lot with the crappy Duro tires. they came with the bike, i normally hate whitewalls, but they look pretty good on this one. another problem with them, is the whitewalls aren't straight, which gives the illusion of the wheels being bent while they're spinning.

more later...

dmar836 said:

You might be getting a ram air effect from having the cyl reversed. This was experimented with by Yamaha on GP racers back in the late 70s.

Click to expand...

i was thinking that, too. it might be another psyche-out, but it almost sounds like it's sucking in air, like my old mikuni sidedrafts did on my honda racer.

there's really no openings in the air cleaner, though, except for the ones in the bottom.

i think that just the way it's not crammed up into the engine helps, too. it's not affected by the heat, and air can flow into it better.

one problem, which has nothing to do with the orientation of the engine, is the crappy plastic petcock in the carb leaks if it's in a certain position, or if you wiggle it around. it's an easy fix, i'll just take it out and make a block off plate. i don't need 2 petcocks anyway.

just got back from a 30 mile ride. the speed trap radar sign in front of the high school clocked me at 41, 43, and 46mph, in three consectutive runs.

my speedo registered about 3-5 mph faster. i'll need to calibrate it using a crazy method i concocted (it's in the "quarter mile time" thread.)

i'm working on getting a video camera set-up, so this doesn't end up being the usual b.s. about how fast i think i'm going, how fast i claim i'm going, and how fast i'm really going.

still need to work on the "hot foot" problem. checked out some header wrap, but didn't want to pay 30 bucks for a roll, when i only need a little bit. my vintage usa made converse are getting toasted.

otherwise, the heat isn't bad at all, it's just when my shoe rubs on it when i pedal.

that's all for now.

thanks mike, but i think i've got a plan.

i can bend the exhaust closer to the frame almost an inch, and i'm working up some heat shield designs.


fixed the leaky petcock at the carb today, just switched one out (good thing i have another carb,) took the bike out and it wouldn't fire up. sputtered, bogged, and stalled. looked down and saw gas flowing out of my air filter, and out of my exhaust pipe.

knew right away the float was either stuck or filling up with fuel.

yup. took the carb apart and the plastic float was full. changed it out (again, good thing i have another carb) and left the petcocks closed, pedaled around, dropping the clutch, letting it sputter and die a few times to blow all the fuel out of the case and the pipe.

fired it up and rode a 40 mile trouble free trip.

there's some really nice straightaways with a bike lane, and one with a gradual downhill slope. with the 30 tooth, i floored it down the hill, never exceeding the RPM limits (but close,) and was paced by a guy in a pickup, who pulled over to talk about my bike, and he claimed he had me paced at 50mph.

i really need a video camera. (i know, "downhill doesn't count," but c'mon, it felt awesome...)

at whatever fast speed i was going, the engine still ran ultra-smooth, not like my other one, where rapping it out caused the whole bike to vibrate like an old electronic football game (kids, ask your parents what that means.)

i gotta do something about these crappy, out of round tires, though. gives the bike this "whump whump whump" feeling. not bad enough to be dangerous, but it's friggen annoying. probably order some monday.

one new problem, is after riding flat out for 30 minutes or so, i stopped at the store, had a smoke, and after the bike cooled for 10 minutes, it took me ten minutes to start it. choked, unchoked, tickled, fast-pedaled, etc... finally, it fired up and was fine the rest of the 2 or 3 miles home.

gotta check the plug later, make sure it's still cocoa coloured.

oh, one more thing. i've been championing the coaster brake only, no front brakes forever, and i just gotta say, i love having a front drum. it's not grabby enough to skid or toss you, and it works great for just slowing down. still love the coaster, and the two combined work really great...

i forgot, and i'm sure some of you are wondering if the carb would suck up any water.

i rode through a mini lake-sized puddle and splashed the whole bike (it's nice having fenders.) the carb got pretty wet, but there was no surging or bogging. don't know what'd happen if it was raining, but hopefully i'll be inside when that happens...

actually, i wasn't thinking performance when i reversed it, i was thinking style, and having a different looking bike than everyone else's.

the performance gains are probably due more to the work i did when i was putting it together, which would probably give you the same benefits no matter which way your motors facing.

i know there's no ram air effect, but having a smoother, less restricted air flow around the carb might be a slight advantage.

the biggest advantage is the shortened, ported intake mainifold. this could also be done with the motor facing the "correct" way.

the transfer ports in the case aren't exactly "matched," since they're cast for the front facing jug, and reversing it partially covers them, but i did have to grind away at the case to match the way the jug fits now. also, just cleaning them up probably helps performance, because they're really lousy from the factory.

honestly, you can probably get the same performance out of the normal orientation of the motor if you do the same stuff i did.

it's more or less just blueprinting it.

or, maybe i just got lucky and got the one in a million motor that goes really fast...

(edit) i'm also not trying to prove that backwards is better. just showing that it can be done, and how it works. if i wanted a bike that doesn't stand out from the crowd, i coulda built a walmart special in one day, and been bored with it the next...)

went for a 60 mile ride today through Santiago Canyon. there's a bunch of hills, some steep, some gradual slopes, and it's got a big bike lane. usually it's full of dayglo pedal-pumpers, but today there were only a few. and most of them waved or thumbs-upped me when i went screaming past them.

i have a new/old gas cap off a pacer moped, so i can fill the tank all the way to the top without leaks, and i've got about an eighth of a tank left, so mileage isn't affected by the motor. it's actually probably better, since i was WOT almost the entire time. the canyon road is about 20+ miles long with no stop lights or signs. went all the way to the biker bar, stopped for a beer and a smoke, then went back, and with barely un-twisting the throttle.

this is the fourth time i've ridden this bike, in case you're counting. aren't you supposed to break these things in before going crazy?

had my speedo pegged at 50 coming down some of the hills, and it sure felt like it. i had my camera set up, but the batteries died.

bike ran perfect. no problems except for starting it. i've got to pedal a bit, drop the clutch, then i jam through a pedal stroke, almost like kickstarting it. i think it's due to raising the compression, and my clutch probably seated itself in and needs to be adjusted a little. might mess with the plug gap, too.

out of the canyon, when i hit the streets again, and traffic, the bike got waaay louder, and there was this weird rattly ping-ing noise, and i thought, "that's it. i killed it."

it was only the exhaust baffle, hanging on by a thread. the bolt had almost fallen out, despite my best efforts to keep it in there, so i pulled into a parking lot, let it cool down till i could stick it in my pocket, and rode it home 10 miles, baffle-less.

despite the awesome, head turning, old man annoying, cop noticing, leaf-blowing sound of a nitro-circus back-flipper, it ran really good.

i mean, i hate to admit it, because i still think it's a hillbilly hop-up, but with no baffle, it accelerated faster, had a higher top speed, better throttle response, and except for the sound, it ran friggen awesome!dunno what no back pressure might've done to my motor, but i doubt it had an effect.

so now, i'm thinking up ways to make a better baffle. i still want some sound reduction, but i want to keep the power. a little louder would be ok, but not open header funny car loud. i've got plans for an adjustable baffle system, akin to the "supertrapp" mufflers, only different.

i know, i could just get an expansion chamber, but i'm trying to keep the motor stock, at least parts-wise, so anyone with some tools could do what i'm doing, without spending a lot of cash on bolt-ons.

it's easy to buy horsepower, it's harder to make it from nothing.

(here's a good time to add a disclaimer. i'm not looking for longevity here. i'm not trying to make a bulletproof, 50,000 mile motor. i'm trying to beat the living daylights out of it, break stuff off of it, make it spontaneously combust, and see what she'll take. anyone who uses my experiences as a how-to, do it at your own risk. i've already voided my warranty, and i look at it like "it's a cheap motor. i can get another," not like "holy crap how am i gonna get to work...")

actually, just got out of the garage. the plug is perfect. looks better than any other spark plug i've had on any vehicle in my entire life. seriously.

also pulled the head off, just because it was easy enough, and it's got barely any carbon build up, and same with the piston. cylinder walls are still shiny smooth (i was a little worried i might have got a few shavings left over after porting.)

there is one shiny spot on the piston edge, might be from detonation. i'm running 91 octane and opti-2 at 60 to 1, i'm gonna mess with it, maybe go to a colder plug, and check it again next time.

been doing some exhaust research, i think i might've found a way to quiet it down, without sacrificing power. won't tell ya how yet, i'll let ya know when i've proved it...

(disclaimer #2: bairdco industries is not responsible for melted shoes or burnt pants...)

some pictures...

did some exhaust work today. i was reading Graham Bell's Two Stroke Tuning (don't have a link, just gogle it and get the PDF) and got some cool tips.

i encased my stock exhaust with a piece of aluminum pipe, which quiets down the "tinny" sound and keeps it a little cooler.

i got a thin aluminum pipe, sliced it lengthwise, slid it on and clamped it tight, then alumiwelded the seam. hopefully it holds up to the heat. it should, but if not, oh well.

i also found some cool mini diamond plate thin aluminum sheeting, so when it's raining all week i'm gonna make a cool heat shield.

the exhaust is a little heavier now, so i'm gonna make a mount that comes off the back of the clutch cover. i don't want to mount it to the frame, because if the motor moves and the frame doesn't... well, you can figure it out...

also put in a brass 90 at the tank, so the petcock came out at a better angle. before, i had to ziptie the line to the frame so it wouldn't rest on the head.

i made a braided stainless fuel line, too. i had a braided wash machine water line laying around, so i cut the ends and pulled the stainless sheathing off, slipped it over some 1/4" fuel line, and heat shrinked the ends. also added a piece of heat shrink where it rubs on the frame.

i hate the way the carb inlet is 1/4", and the petcock is like 5/16" tapered. when i try to screw a hoseclamp on, it just works it's way off. so i took a sharp stanley knife and gouged some barbs in it.

i know, there's no inline filter. i was gonna put one in, but i still have the one in the tank, and when i took it all apart it was nice and clean, so i figure if nothing's getting past that one, then shine it. who cares. i clean my carb out on a regular basis anyway, so i ain't worried.

that's about all i did today. i'm gonna make an exhaust manifold this week so i can match the exhaust port perfectly.

stay tuned...