Weed Whacker Bike tips

deacon · Aug 7, 2008

I am starting this thread because I am learning a lot of things as I build the Weed Whacker bike. Some of them might help others prevent my problems and if you have a tip please leave it for me.

So let me start with once you are happy with your mount use red locktite on the threads of the screws in the motor case. Be sure you satisfied with the basic mount.

My bolens whacker has two broken spots on the motor case from vibrations of the case. Actually I am considering just drilling out the next one and putting more substantial bolts and nuts on it next time. That pot alum casing isn't really strong enough to hold up under vibrations I am afraid.

deacon · Aug 7, 2008

Of all the motors I have used, I do not have one now but have trashed a few perfectly good ones because I was to stupid to understand how to use them correctly, the Ryobi 31cc and 30cc are almost perfect for this application.

They are full sized and have a 3/8 24 threaded shaft. That is the perfect size for the bmx threaded axle peg. I had an alum set that were about an inch in diameter and they were already textured.

If I have to replace the bolens, I swear I am going with one of those that I know runs. Lots of the used ones don't run for some reason mostly carb problems I expect.

I think it is almost impossible to drag start any 31cc engine that is what I didn't know when I trashed the ones I did.

deacon · Aug 7, 2008

If you are going to buy a junker bike to build to a Weed Whacker bike go with an old mountain bike.

Since you don't need the gears for a motor bike, you can use the controls for your engine and save a bunch of money.

I personally am looking for a rear coaster wheel for the bike so I can have one more brake system. I am pretty sure that I can use the coaster brake chain on the big drive wheel of the mountain bike to stop and start it.

There isn't a lot of pedaling with this setup.

deacon · Aug 7, 2008

If your motor comes with the short throttle cable connected save it. You can marry it to your on bike control cable and if you front mount the engine you probably won't need an additional spring. If you rear mount it you will need a spring to pull length of cable from the front controls. Once you have them pulled back the carb spring can do the rest so still use the cable on the engines carb if you have it

deacon · Aug 7, 2008

I found out something interesting today when working on my chainsaw bike. A turkey baster works great to fill those weed eater tanks that aren't quite level.

Well I know it's not much of a tip but it's the best I could do on short notice.

Ranger · Aug 7, 2008

Is there any solution to fixing the primer bulb? Both of my engines have broken ones. At the moment they are just plugged up but I would like to fix them. I may just breakdown and buy 2.

deacon · Aug 7, 2008

Thats what I had to do but be careful the ryobi bulb comes in two sizes now. You probably have the larger one. I bought a smaller one thinking it was the right one. I'm not sure how you can tell them apart when you order on line. If you have a supplier in your town pull it off and take it with you.

I did try to use a generic but it was a waste of my time. I once used and ear syringe to pressurize the tank. It worked on an old lawn mower but I don't think it would on a weed whacker.

deacon · Aug 9, 2008

Something I learned yesterday when the engine flew apart is that the cast aluminum case of a weekwacker can't be abused too much. Okay so that was then this is now. The cast alum case of a weedwhacker can be twisted not just broken so be careful how you mount it.

Radmanfly · Aug 9, 2008

Hi D,
I recently built two weedwacker bikes. I mounted one engine to a flat piece of 1/8" steel using the four holes around the crank. Had to do some pretty accurate hole drilling and a lot of filing to get it to fit tightly but being mounted tightly to the plastic keeps the vibration from cracking the cast aluminum and the plastic will give before the casting does. This mount plate has a hole in it under the engine used to pivot it for belt slack when I need to clutch. Also a spring to hold the belt tight.

Check out my pics.

The belt drives a pulley welded to the free end of a 10 speed bike hub and the other end ratchets for a freewheel. When engine is running and clutch is out, it drives a bigger gear on the left side of the rear wheel. This gives me the ability to pedal with engine off and clutch out. No belt friction as the 10 speed hub ratchets freely, just the chain goes around but hardly any friction there. A free wheeling gear on the rear wheel would have been better but I didn't know about them until after I built this. I also used turnbuckles to refine the chain tension to eliminate a tensioner there. So far I have logged over 200 trouble free miles and I estimate the top speed on a flat at about 34 or 35 mph. Can't wait to get a speedometer on it to see, but it goes way faster than I can pedal even downhill.

Hope this is helpful, Scott

gadgetman80 · Aug 9, 2008

Hmm.....

Now where did I put that weedwhacker last time I used it?

come to think of it, I have 3 or 4 of those old homelites sitting in the shed. Might have to dig 'em out.

deacon · Aug 9, 2008

Wow a gold mine of motor bike material..

deacon · Aug 9, 2008

I like that belt drive but I am married to friction drive myself. Just so little can go wrong with it.

I tightened up all the nuts and used loctite and the vibration is gone. I should have done that to begin with. Oh well next time.

handy_Biker · Aug 9, 2008

Radmanfly,
Thats a really nice setup ! Does it start by compression or do you have to use the rope start to get the engine going ??

Also, Looking at the arrangement... it doesn't look too difficult to put a 3 speed hub on that!!

Radmanfly · Aug 9, 2008

Hi Handy Biker,

Thank you for the interest in my bike. This is revision 3. I was going for simplicity and light weight as well as stealth ("invisibility" to the friendly peace officers on the road)

I start it with pull rope, can't compression start it the way I have it set up. If I have to kill it, I can reach back (after the cops are out of sight) and pull start it easily while moving, starts first pull usually. But mostly I just idle down hills and cruise at about 1/4 throttle on the flats about 15-20 mph.

Regarding the 3 speed hub, I actually built it for that reason originally but am having trouble finding one. I'm looking for a good Shimano hub and twist grip shifter. Saw a couple on ebay going for $20-30 plus shipping but I think I'll hold out till I see one on the side of the road for $5 bucks or hit up my local bike shop for a junk 3 speed rim. I wonder if they'll even know what they are anymore....It's perfectly geared for climbing most hills the way it is but the 3 speed hub (1st gear) would help a lot on the steep ones and I can't imagine what top speed would be in 3rd....its already fast enough for me. Probably pretty fast on the flat or slight downhill grade but outside the "powerband" of this engine. If I replace the 6" belt driven pulley with a 7" or 8", It would put it in a lower range and 3rd gear would probably be more within the powerband of the engine giving me awesome low gear, slightly lower 2nd than I'm in now all the time, and 3rd a bit higher, which would be great for flats and down hill. One really cool thing is it never over revs because of the freewheel action of the rear hub/jack shaft! Take care, Scott

deacon · Aug 17, 2008

back to the subject at hand. I think this is going to hold true most of the time. At least it has been in my short experience with the weed whacker engines. If it won't start and the gas lines are intact or almost intact you can probably clean the carb and go.

If they are disintegrated and falling apart, especially if the bulb is as well, don't be surprised if the carb is shot. The things that destroy the lines probably plays **** with the flexible parts inside the carb as well. At least that has been the case with those I have been fooling with. So ask how the gas lines are when you pick up a used one that isn't running. It will make a big difference I think in how much you need to do to them.

Let me know if others have the same thoughts on these please.

aussiedale · Apr 13, 2009

Hi All,

great site !! A big thank you to deacon your info is really helping me on my way, I have been trolling through all the info and trying to get my head around it all I am not very mechanical minded so please excuse my ignorance, I have scored a brand new engine from a weed whacker (we call them whipper snippers in Australia lol), Its a Chinese rip off I am setting about attaching it to my MTB using the friction drive which seems the simplest cost effective way of doing it I have the throttle and kill switch sorted and working on the frame to hold it all together.

I am stumped how to attach the bike peg see picture below, my engine seems to be different to the ones you guys are using, Any tips or points to appropriate threads on how to attach a drive wheel to this engine would be greatly appreciated I don't have a welder but could hire one if it is the only way.

Thank you all

bravofire84 · May 14, 2009

iv certainly been tickering with the idea why not

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