DIY 11 HP flathead minibike

[nico_v]

Hi all, I figured I'd start a thread here about the minibike that I built last summer.
It all started out after accidently seeing some minibike videos on Youtube, that looked like a lot of fun so I decided I needed one too. One problem, over here in Sweden we don't really have minibikes of the classical type. Closest thing we have would be a Honda Monkey I guess.
But, problems exist to be solved, so I decided to build my own. I seriously considered buying a frame on ebay but shipping costs were just too high unfortunately.

So, I got myself an old snow blower to donate the engine, an 11 HP flathead. A VW roof rack (for a T4, don't worry, I didn't cut up some priceless vintage roof rack), and a bunch of moped parts that I had laying around.

Just stacked some things together for inspiration


Most ingredients were here now.


I switched to Photoshop instead and came up with this


Took measurements in Photoshop and converted them into a simple frame jig. This is where the roof rack came in, it had nice and rounded corners so I didn't have to buy a tube bender.


Here the jig became 3 dimensional, the right part is angled inward to meet up with the steering tube.


Quick test fit

 

[nico_v]

I'd post more but there seems to be a limitation on the number of pictures I can post in a new thread So I'll continue whenever it gets approved or something.

If you can't wait, you can see the finished product and more on my Youtube channel. The latest 2 videos are about this summers project which is doing a bunch of performance mods to the engine
https://www.youtube.com/hollaendaren

 

[RobertC]

Welcome Nico...!

Your bike looks bada$$...!

Keep up the good work.

 

[Li'l Popeye]

Nice to see there are other Europeans active with cool minibikes.
Your bike has a nice sound and twice as much power as mine. Link to my Dutch minibike build.
I like the homemade stuff. You did a nice job on your billet flywheel.
Greetings

 

[CarPlayLB]

Awesome bike! Looks like a great project! Please keep us updated

 

[nico_v]

Thanks!

Let's see if I can get some more pics in..

More jig stuff, the planks had grooves milled in them to keep everything parallel.


Test fitting the engine again.




That looks about right..


The frame made it to the floor, fork in place and setting the height for the rear.


Some new parts arrived. The handlebars and everything on it I had to buy new, because I didn't have any that were high enough. The tank I got off ebay, it's off a Hercules moped.

 

[nico_v]

Starting to look like a bike now


I got cheap and went for a 30 series, just wanted to try it instead of a 40 to save some space as well. So far it seems to be doing fine.


Time for a quick rattlecan paint job.


All back together


I used a VM28 carb off a Suzuki 125, this turned out to be a bit of a disaster.


Originally I wanted to have the exhaust go straight back, but I ran into clearance issues because of the carb, so I used the header pipe of that same Suzuki 125 for an exhaust for now.

 

[nico_v]

And here are 2 pics of the finished product.


It was in desperate need of some new rubber, I went with the fattest that would fit. Clearance is probably around a mm in the front

 

[nico_v]

So that was all last summer.
This summer it's time to start tuning the engine a bit.
We still have some snow but snowmobile season is over, so I got started about 2 weeks ago with some baseline runs. I did a 95 km/h (59 mph) run with it last year, which was way faster than I want to go, so I decided to gear it down a bit for everyone's safety, and gain a bit of acceleration at the same time. It now tops at around 80 km/h / 50 mph which is plenty on this tiny frame.

So gearing was kind of the first mod, I did have to make a sprocket adapter though because I didn't have any sprockets that would fit this hub and couldn't find any that were large enough. You can see the process and some test rides in this video:
https://youtu.be/SF-QNnARWlM
[YOUTUBE]SF-QNnARWlM[/YOUTUBE]

Next up was to lighten the flywheel. The stock flywheel weighed about 6.5-7 kg / 14-15 lbs and the engine felt pretty tired because of it. I kept the stock center and pressed/bolted on an aluminium flywheel and plastic fan. The plastic fan didn't work (too tall fins) so I replaced it with a cast aluminium one that was a bit heavier but shorter. The flywheel ended up at around 1760 grams with the alu fan. Around 1820 now that I've balanced it


You can see the flywheel fabrication video here:
https://youtu.be/WawmJwiCcLM
[YOUTUBE]WawmJwiCcLM[/YOUTUBE]

If you've watched the video, you've seen that it no longer wanted to idle. I've always had low speed issues with the VM28 carb, probably because it was setup for a 2-stroke. I decided to try a TM28 instead, and lo and behold, it idled just fine with that. I had to weld a new intake, because the TM was a bit taller than the VM and I ran out of room for the throttle cable. That's an upcoming video



With a working carb now, it doesn't like traction very much It's either pulling wheelies, or throwing mud, or both. I have a lot more coming so in order to prepare for that I'm probably going to stretch the frame a little bit, that should help a lot with the takeoffs.
I'm also going to be experimenting with grinding my own cam, porting, and last but not least a smog pump supercharger.
I'll keep you updated with pics and videos!

 

[TheDullCarbide]

I'd post more but there seems to be a limitation on the number of pictures I can post in a new thread So I'll continue whenever it gets approved or something.

If you can't wait, you can see the finished product and more on my Youtube channel. The latest 2 videos are about this summers project which is doing a bunch of performance mods to the engine
https://www.youtube.com/hollaendaren

Wow! Hollaendaren on OldMiniBikes. Your Bombardier builds are amazing!:bowdown: I've followed your channel for awhile.

 

[nico_v]

Wow! Hollaendaren on OldMiniBikes. Your Bombardier builds are amazing!:bowdown: I've followed your channel for awhile.

Thanks! Yeah I love my Doos!
It feels good to be on an actual active forum, seems like most forums (of all kinds of topics) have kind of died out over the past couple of years, that's why I started doing Youtube build logs.

 

[TheDullCarbide]

Thanks! Yeah I love my Doos!
It feels good to be on an actual active forum, seems like most forums (of all kinds of topics) have kind of died out over the past couple of years, that's why I started doing Youtube build logs.

Are you part of DooTalk? Somebody posted a 4 cylinder 440-ish type engine from ebay. It looked much like the one on your channel. Or...was it the one on your channel?:laugh:

 

[brown boonie]

Fantastic build. Looking forward to more of your builds. Looks like you have a shop full of machines to make what ever you need.

 

[nico_v]

Are you part of DooTalk? Somebody posted a 4 cylinder 440-ish type engine from ebay. It looked much like the one on your channel. Or...was it the one on your channel?:laugh:

Yeah I'm on DooTalk every now and then, have to check if its the same video, everyone is free to share them of course That 4 cylinder is my friend's sled but he wanted me to do the video. We shot some more material a couple of weeks ago but I haven't edited it yet, too busy with minibike stuff
Edit: I found the thread on Dootalk, different engine but the same principle. Someone did link to a video of my friends engine though. For those wondering what we're talking about, this 900cc 4cyl 2-stroke:
https://youtu.be/954fEMsTiig

Fantastic build. Looking forward to more of your builds. Looks like you have a shop full of machines to make what ever you need.

Thanks! Yeah, I live out in the woods so shipping times are a problem for me, and you know how it goes, when you need a part you need it yesterday.. On top of that I can be a real cheapass. So my solution has been to get the machines I need to create whatever parts I need unfortunately they do take up a lot of space so I'm going to have to expand my garage a bit. I have another larger lathe and a horizontal mill waiting to get inside that just won't fit atm.

 

[delray]

hey great job making it work. was that in grams you weigh the flywheel. if so that would of been around 3.692 pounds. I was surprise you didn't have a unwanted balance in the flywheel. most aluminum flywheels have a small counterweight on the other side?
I think if you wanted to improve the performance more I would check and see where the timing is. not hard to do if you have a automotive timing light and a 12v battery. more then likely your factory timing will be somewhere around 20+or- degrees
you could of also made that hub with slots . this would allow you to adjust the advancement in the timing. lets say 25 to 30 degrees.
I think if you go that route you might want to make that new hub out of steel. I don't think I would trust that cast hub for longevity. it just might crack on you? one other way you can advance it is by making a slotted bracket to off set the coil to the left.
also if you can pull the factory valve springs out and find a replacement spring with a 16+ spring rate. this will allow you to rev the motor higher without floating the valves. just don't go over 18pound springs or you might start to chew up that stock cam. porting the block/bowls would help. just clean up any bad casting and so. don't go overboard if you don't know what your doing?
I didn't really see what you had for exhaust? if you make a true one piece header pipe that will make a difference with all the other modifications. you almost need to do all of them at the same time to really notice a difference in power.
the torque converter engagement from 2200(stock) to around 2500 would help too. the next big problem would be is the rod strong enough to hold these modifications?.........

 

[Phil1958]

nice job on that bike!

i would tread carefully with engine mods and a stock rod. i think you made the right
decision on the flywheel., i put an hs-50 on a bonanza with the stock flywheel
and the engine had a poor throttle response both on acceleration and deceleration.
too much inertia for a minibike but fine for a snowblower.

:thumbsup:

 

[Havasu Dave]

I just spent a very enjoyable time watching your videos. Great job.

 

[Dr. Shop Teacher]

I just spent a very enjoyable time watching your videos. Great job.

Make that another person who enjoyed watching (and drooling over the machines). Very cool. Since you probably can do it--machining a billet rod for that motor would be a good idea as Phil said (and now that I say that--maybe an ARC rod for a vertical shaft Briggs might fit).

 

[nico_v]

hey great job making it work. was that in grams you weigh the flywheel. if so that would of been around 3.692 pounds. I was surprise you didn't have a unwanted balance in the flywheel. most aluminum flywheels have a small counterweight on the other side?
I think if you wanted to improve the performance more I would check and see where the timing is. not hard to do if you have a automotive timing light and a 12v battery. more then likely your factory timing will be somewhere around 20+or- degrees
you could of also made that hub with slots . this would allow you to adjust the advancement in the timing. lets say 25 to 30 degrees.
I think if you go that route you might want to make that new hub out of steel. I don't think I would trust that cast hub for longevity. it just might crack on you? one other way you can advance it is by making a slotted bracket to off set the coil to the left.
also if you can pull the factory valve springs out and find a replacement spring with a 16+ spring rate. this will allow you to rev the motor higher without floating the valves. just don't go over 18pound springs or you might start to chew up that stock cam. porting the block/bowls would help. just clean up any bad casting and so. don't go overboard if you don't know what your doing?
I didn't really see what you had for exhaust? if you make a true one piece header pipe that will make a difference with all the other modifications. you almost need to do all of them at the same time to really notice a difference in power.
the torque converter engagement from 2200(stock) to around 2500 would help too. the next big problem would be is the rod strong enough to hold these modifications?.........

Thanks! Yep, that was grams. The flywheel was out of balance quite a bit (60-something grams) but I was too eager to test and couldn't wait until I fixed my balancing rig. I figured the incomplete aluminium fan would almost counterbalance it. I have balanced it now, have only tested idle so far though because of a combination of late nights and bad weather, so have yet to see if I feel any difference while riding. It does seem a bit less shaky at idle at least. The balancing process is on Youtube now, link coming in a bit.
I plan on making the timing adjustable on the fly, I have some ideas but haven't worked it out completely yet.
As for the hub, I'm not too worried. What's left of it is a lot thicker than the stock flywheel at its thinnest part and has a lot less weight pulling on it at high rpm. I'm also not planning to turn it 9000 rpm
I bought some valve springs (a dual spring setup with an outer/inner spring) that might work but I have yet to check.
Porting is coming of course. I know my way around 2-strokes, but I have never really ported a 4-stroke other than a bit of match porting. We'll see how it goes!

 

[nico_v]

nice job on that bike!

i would tread carefully with engine mods and a stock rod. i think you made the right
decision on the flywheel., i put an hs-50 on a bonanza with the stock flywheel
and the engine had a poor throttle response both on acceleration and deceleration.
too much inertia for a minibike but fine for a snowblower.

:thumbsup:

Thanks! Yeah, the rod is my main worry atm. I'm trying to find out if a horizontal 28 cui rod would work, those seem to be popular in lawnmower racing. I think they are the same engine, but the 28 is bored a little larger than my 25 cui. But I want to be sure before I order anything of course.
The flywheel completely transformed it, that was a mod well worth doing