Help figuring how to start friction chainsaw bike.

kyle86 · Nov 22, 2012

I built a bike with a 34cc craftsman chainsaw motor which is basically a 34cc poulan. I took the engine out of the case and built a motor mount using the crankcase cover bolts and have it mounted on the rear of my 26" mountain bike.

Since removing the case i have lost the pull start. I figured I would just friction start by petaling the motor and then releasing the clutch however the engine is acting like it has too much compression and the roller just slides on the tire during compression and does not turn.

All rollers have been around 1" and I have tried solid rubber, a smoothe socket, a socket with rough weld all over it, two strong springs, high tire preasure. The roller still slips on the tire and I cannot start the bike. Even the original pull start was tough to crank. Any ideas on how I can start this darn thing? usflg

kyle86 · Nov 22, 2012

70 views and no ideas? I just bought a new bike tire to try...

JonnyR · Nov 22, 2012

do you have a smooth street tire or is it a off road tire off road tires would do this all day

Greg58 · Nov 22, 2012

I don't know how to add a compression release to the 3400 head but that's what you need. Many of the larger two stroke racing motorcycles had them, most were cable operated like the choke on some Mb carbs.

msrfan · Nov 23, 2012

Stihl sells a compression release that screws into the cylinder head. It uses 10m or 12mm threads and you push it in to start the motor, when it fires, it automatically closes. By drilling and tapping near the spark plug to install it, you may also be able to rig up a lever to a cable to push it in from the handlebar. Just a thought.

kyle86 · Nov 23, 2012

Interesting, So with compression being part of the recipe for combustion I'm guessing the compression release only bleeds off a small amount allowing easier starting? Do yall know of anyone who has added one of these to an engine that did not originally come with one? This sounds like a good idea. By the way, my engine does not have a head it just had a solid cast cylinder, and one removable crank cover that clamshells the thing together.

BTW if yall are interested, here is my setup.

kyle86 · Nov 23, 2012

Crap, does not appear to be any easy place to put decompression valve on the cylinder. Angled spark plug on top and fins everywhere else. Hmmm...

RicksRides · Nov 23, 2012

try a knurled foot peg thats what i use

msrfan · Nov 23, 2012

Nice fab job. Not much room on a headless motor. You should be able to attach a starting cup to the flywheel and use a pull rope unless the drive roller is engaged full time. Have to carry it with you.

pocdragon · Nov 23, 2012

nice simple build!

sounds to me like you need more down pressure on the peg. since your setup is one sided it might be awkward to put consistant down pressure on it

knurled peg like pic

the tire looks good, but a little lower presure will increase the area of contact for more oomph

your "gas tank" might do better at a higher elevation then the carb. even tho it has a pumper carb having a little pressure can help it run methinks

Ludwig II · Nov 23, 2012

I was going to ask about a pull rope as well. What about bungee cord wrapped around the fins, not tight enough to start the engine on its own, but when you pedal, the tyre and the bungee gang up on the engine?

kyle86 · Nov 23, 2012

Great ideas! Im at the conclusion that any roller type is going to slip due to the compression. I may try lowering the tire pressure with the new tire. Wrapping around the flywheel is a no-go since the magneto is in the way, however i could attach somethintg onto where the pull start used to engage on the side of the flywheel for a pull rope. Any idea on how I should build it? I have never seen one before.

kyle86 · Nov 23, 2012

I'm honestly thinking about scraping this setup and either building a chain drive with the motor or finding another motor to use that will turn over easier off the roller. I'm getting about fed up with this thing ha ha.

kyle86 · Nov 23, 2012

Ok i just got an idea. For testing purposes, would loosening the spark plug just a tad bleed enough compression off to at least hear the engine run?

kyle86 · Nov 24, 2012

Now just throwing this out there. Like in drag racing, a larger diameter tire gives a larger contact patch on the ground resulting in better traction. Maybe what I need is a larger roller. My current is about 1". Maybe I will look for a larger socket tomorrow at the flea market.

I don't want to scrap this thing, I've gotta make it work.

kyle86 · Nov 25, 2012

Going to keep talking to myself in case someone finds this thread and maybe has a similar problem.

Update: loose spark plug does not work. I did however get the engine to start 3 times by wrapping a cord around the outside of the flywheel. In the process of trying to ride it around the neighborhood, I chewed the heck out of my new tire. Now I believe the problem is actually not enough friction/tension on the tire. Tomorrow I will go to the flea market and try to find a trampoline spring or some heavy duty bungie straps.

Another problerm arises, my clutch cable is too weak and keeps streching any time I apply decent tension so I will have to either replace it with something more heavy duty, or come up with a different design.

kyle86 · Nov 25, 2012

pocdragon said:
nice simple build!

sounds to me like you need more down pressure on the peg. since your setup is one sided it might be awkward to put consistant down pressure on it

knurled peg like pic

the tire looks good, but a little lower presure will increase the area of contact for more oomph

your "gas tank" might do better at a higher elevation then the carb. even tho it has a pumper carb having a little pressure can help it run methinks

This post is dead on I think!!!
Unfortunately I can't use as peg because my engine uses a reverse thread m8 nut with a 1.0 pitch. I've searched everywhere and can't find one so I cut the clutch up and made my own. I was going to weld a nut to the face, but it is oblonged, and a pair of channel locks puts the tension on quite well.

Anyways, tires are cheap, so I will try to fix the slippage problem first and then possibly replace it if need be.

pocdragon · Nov 25, 2012

to answer you talking to yourself....

1" is a good standard size for a drive peg
smaller then 7/8 and you wont be gaining much more power in the low end
larger then an inch and a half (which is the size of the gnurled peg i posted) and you will have to pedal your buns off to get it started, and will fly in the top end if you can endure the time it takes it to spin up

it doesnt matter if the threads dont fit, drill out the hole to fit on the threads and bolt it tight as long as its centered its ok.

your tiny engine has a good amount of compression but it shouldent be too much to over come to slip on the tire

you need alot of force to hold it on the tire, witch mean that you need even more force to hold the engine off the tire with your clutch system (scissor clutch)
dont use the stock clutch that came with the motor it will burn out and you will have to fix it any way

this pic is of a simple clutch setup you need the mechanical advantage of a lever to overcome the heavy spring that holds it to the tire to pick it up off the tire.
use a turnbuckle and a heavy spring to achieve a good solid hold on the wheel and to dial it in easily. dont use bungee like in my pic they will stretch out and not pull as hard meaning they will need replacement

kyle86 · Nov 25, 2012

Hey thanks!

Well the pedal starting problem is fixed. I bought a trampoline spring at the fleamarket this morning and turnbucked it down. Then I switched to a 7/8 socket which is around 1 and 1/8 diameter. Who would have thought it needed so much pressure. This thing flies!!!! It pedal starts super easy. I had my gf follow me in the truck, she said I was going about 35-40mph on the flat straightaways. Now the only problem is that the engine is surging unless it's at WOT. I tried adjusting the carb but it was no use. I'm suspecting the gas tank is too low causing the engine to starve until the rpms shoot up. I'm going to snatch one off a weed wacker later and raise it. I removed the clutch for now and am running direct drive. The engine stalls when I come to a stop and the easily starts back up when I pedal. I do think I will have to have some sort of disengagement mechanism though eventually.

Thanks again all who helped share their ideas!!!

kyle86 · Nov 25, 2012

Oh yea if anybody was interested, there is roughly $75 give or take a few invested in the bike thus far, though if I hadn't of messed up so much it would have been less. I got the bike for $20 at a swap meet, and the chainsaw was not working when I got it for free. Needed fuel lines, and the chain kept coming off. The rest is metal,misc materials, and hardware.

I'll try to get a video of me riding it soon.

Help figuring how to start friction chainsaw bike.

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