Avotec 70cc dirtbike project - Pocket Bike Forum - Mini Bikes
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post #1 of 41 (permalink) Old 02-27-2017 Thread Starter
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Avotec 70cc dirtbike project

Ok i already posted quite allot posts about the avotec dirt bike in the fish's big bloct project thread but id like to seperate them to not confuse people.

So now ill be posting my updates here

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post #2 of 41 (permalink) Old 02-27-2017 Thread Starter
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Re: Avotec 70cc dirtbike project

I did allot work today and im getting closer to finish the bike.

I welded a extention of the frame to lower the footpegs and i also made the new swingarm connection but i still need to put some more welds on it.

I also have the 2 swing arm bars that are 70% finished and i adapted my t8f rear wheel sprocket to the new 12" spole wheel. That was a hell of a job..



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Re: Avotec 70cc dirtbike project



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post #4 of 41 (permalink) Old 02-27-2017
Re: Avotec 70cc dirtbike project

Love that fat rear tire and nice rear arms, things are going good today, lol.

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post #5 of 41 (permalink) Old 02-28-2017 Thread Starter
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Re: Avotec 70cc dirtbike project

Yeah, the fat rear tire makes the 12" only like a little bit smaller then the front wheel

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post #6 of 41 (permalink) Old 03-01-2017 Thread Starter
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Re: Avotec 70cc dirtbike project

Turns out that i have to lower my engine in the frame too if i want to use a lower swing arm.. or i could go with the easy way and lower the swing arm connection with 1.5cm.. its not much of height gain but its something atleast..

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post #7 of 41 (permalink) Old 03-01-2017 Thread Starter
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Re: Avotec 70cc dirtbike project

I also installed the ktm clutch after i downsized the diameter.. unfortunatly my phone battery was empty and i couldnt take a picture of it but i will take one when i have the ocassion to do so.

I also have a question about the ktm clutch.

I read that the ktm and franco morini engines who use this clutch use oil in the clutch chamber.. is this true? And if so. Can the reduction box be sealed of to do the same thing and fill the clutch chamber with oil?

What do you guys think of that?

I personally think that it could be a huge benefit for the life of the clutch shoes when running it wet

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post #8 of 41 (permalink) Old 03-01-2017
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Re: Avotec 70cc dirtbike project

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Originally Posted by Rcfish90 View Post
I also installed the ktm clutch after i downsized the diameter.. unfortunatly my phone battery was empty and i couldnt take a picture of it but i will take one when i have the ocassion to do so.

I also have a question about the ktm clutch.

I read that the ktm and franco morini engines who use this clutch use oil in the clutch chamber.. is this true? And if so. Can the reduction box be sealed of to do the same thing and fill the clutch chamber with oil?

What do you guys think of that?

I personally think that it could be a huge benefit for the life of the clutch shoes when running it wet..



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I wouldnt run it wet as it will most likely cause adverse affects to the crank seal...........

The benefit of running it wet is getting great cooling ......Most high performance setups rely on a dryclutch setup .......

If running it dry I would pre-bake the clutchpads to toughen them up..................

One of the best modifications you can do is get a Horstman axle mounted wet clutch from a racing kart and machining the rim to accept the axle clutch.........Then you can run a straight pinion on the crank ........

Last edited by CAM2; 03-01-2017 at 12:30 PM.
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post #9 of 41 (permalink) Old 03-01-2017 Thread Starter
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Re: Avotec 70cc dirtbike project

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I wouldnt run it wet as it will most likely cause adverse affects to the crank seal...........

The benefit of running it wet is getting great cooling ......Most high performance setups rely on a dryclutch setup .......

If running it dry I would pre-bake the clutchpads to toughen them up..................

One of the best modifications you can do is get a Horstman axle mounted wet clutch from a racing kart and machining the rim to accept the axle clutch.........Then you can run a straight pinion on the crank ........
Looks great that horstman clutch, is it fitable on the engine? And are all crank seals affected by the oil? Heres a picture of the ktm clutch mounted

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post #10 of 41 (permalink) Old 03-01-2017 Thread Starter
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Re: Avotec 70cc dirtbike project

I also mounted the 14 inch wheel again today. I just need to do some finishing stuff on the swing arm like a chain guide and tensioner and closing the front of the swing arm tubes And mount the rearwheel. Here is what it looks like now



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Re: Avotec 70cc dirtbike project

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Looks great that horstman clutch, is it fitable on the engine? And are all crank seals affected by the oil? Heres a picture of the ktm clutch mounted

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Your best bet would be to soak a spare seal in the lube you intend to use..

The Horstman clutch Im talking about could be integrated into the rearwheel hub and eliminate the need to have the clutch on the engine.....as youre well aware theres nothing from stopping one side of the engine to heat up more than the other when the clutch starts heating up.......

This clutch is fluid filled and is fully adjustable aswell as rebuildable....It grabs instantly at its preset rpm and doesnt gradually slip in and slip out which will make the bike pretty intense to ride and compete against......





On this engine it looks like it has an engine mounted clutch....Its where the MotoTec Ignition generator generates its power from aswell as powers n triggers the coil........

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Re: Avotec 70cc dirtbike project

Right now Im in the middle of hopping up a Techumseh HSK600 3hp 2 stroker engine that has a 53mm bore and a 36mm stroke with a 5/8" shaft...

Its 98cc's in size and should be more than adequate when modded to around 12hp pumped up on methanol/alky mix hooked up to a TAV or manual clutch setup...,

I have a 4hp version on my drag minibike that was pushing close to 16hp and propelled the minibike well over 70mph on just a Comet centrifugal clutch without grenading but its getting old n tired and in need of a rebuild aswell....LOL

This newest build is going on another homemade stand-up scooter project......
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Last edited by CAM2; 03-01-2017 at 06:33 PM.
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post #13 of 41 (permalink) Old 03-02-2017 Thread Starter
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Re: Avotec 70cc dirtbike project

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Your best bet would be to soak a spare seal in the lube you intend to use..

The Horstman clutch Im talking about could be integrated into the rearwheel hub and eliminate the need to have the clutch on the engine.....as youre well aware theres nothing from stopping one side of the engine to heat up more than the other when the clutch starts heating up.......

This clutch is fluid filled and is fully adjustable aswell as rebuildable....It grabs instantly at its preset rpm and doesnt gradually slip in and slip out which will make the bike pretty intense to ride and compete against......





On this engine it looks like it has an engine mounted clutch....Its where the MotoTec Ignition generator generates its power from aswell as powers n triggers the coil........

I get it, + u have the advance to not have a still engaged clutch when u stop giving throttle, i had this issue on the bigblock cag engine when it wasbat high rpm and its not so healthy for a 2 stroke engine. I will definitly look for more info about getting a horstman clutch instaĺled on the rear wheel hub

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post #14 of 41 (permalink) Old 03-02-2017 Thread Starter
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Re: Avotec 70cc dirtbike project

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Right now Im in the middle of hopping up a Techumseh HSK600 3hp 2 stroker engine that has a 53mm bore and a 36mm stroke with a 5/8" shaft...

Its 98cc's in size and should be more than adequate when modded to around 12hp pumped up on methanol/alky mix hooked up to a TAV or manual clutch setup...,

I have a 4hp version on my drag minibike that was pushing close to 16hp and propelled the minibike well over 70mph on just a Comet centrifugal clutch without grenading but its getting old n tired and in need of a rebuild aswell....LOL

This newest build is going on another homemade stand-up scooter project......
Wow that minibike sure is a beast! So is the kart when you get the mods done to the engine! Whats the powerband on those engines when you modded it to 10hp+ ?

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Re: Avotec 70cc dirtbike project

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Wow that minibike sure is a beast! So is the kart when you get the mods done to the engine! Whats the powerband on those engines when you modded it to 10hp+ ?

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The minibike was a crazy handful and smoked them race briggs bigblock builds pretty good when it was tuned on alky......I had to modify the clutch and clutch spring to grab almost double of stock and yet it wasnt enough....On the next go around Im gonna put a TAV on that bike too............

The HSK600 will have a powerband thats unlike common 2 strokers and will be dictated mainly by the TAV..........Im looking to build a scooter thats quick n nimble and fast as heck to smash the twin GP460 speed record............

On the build seeings it has a 2pc rod I can get rid of the industrial steel unit and fit-in a much lighter billet aluminum one....Im still researching the best piston for the task as I may overbore the cylinder by 1mm..

Im also debating on whether to go dual carbs on it with a hybrid reed induction on the crankcase melded with modified piston ported induction............

My old Mini Trike of Death build-up utilized a hopped-up and modified Techumseh TC300 from the early 80's which is very very similar to the HSK600...............

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Last edited by CAM2; 03-02-2017 at 01:03 PM.
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post #16 of 41 (permalink) Old 03-03-2017 Thread Starter
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Re: Avotec 70cc dirtbike project

I mounted the rearwheel yesterday evening and it turns out that i cant get it in center with the chain running straight.. so i mounted it out center just to get a feeling what is like to drive the dirt bike. Then i tryed to start it and then my piston was hitting the head... lame.. i will see for a headgasket and a cylinder base gasket today. I hope to find this in a shop today, else i wont be able to ride it this weekend..

I will also move my engine a bit to the left so that my chain runs straight with the rearwheel centered and look for a rear brake disk that fits and weld the footpegs on the new position + adding the auto chain tensioner and the chain guide once i centered the rearwheel.

After all that i just need to paint some things and then the bike is finished!

When all that is done then i might try out to run the clutch wet

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Re: Avotec 70cc dirtbike project

Getting close now, I had many of the same problems when building a custom modified motorcycle. The piston is hitting the head, you have run this engine before right?? How thick is your base and head gasket now. You have a head gasket more like a 110cc 4 stroke Honda clone engine right?? I would leave the head on the cyld & gasket alone and pick up the base of the cyld, get four .010" to .015"shim strips, take something like JB Weld, a thin coat on the cases, then the base gasket and another thin layer, then lower the base of the cyld down, put I shim 4mm in on 4 sides and lower it down, install your head/cyld mounting bolts, very light finger tight. The next day pull the shims out from the edges, then torque your bolts. That will raise the cyld .010" to .015" leaving more room for the piston and combustion chamber, with out a Big Hassle. How is your piston pin bearing, lol.

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Re: Avotec 70cc dirtbike project

Didnt you do a squish calculation of the engine during its build phase?


I usuallu use a laser to help find my placement for the engine or to check to see if theyre in-line accurate.........

first thing is mounting the rearwheel evenly across the rear swingarm then you attach the laser to the rear sprocket and then align the rear sprocket to pinion then mark n mount the engine with trans........


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post #19 of 41 (permalink) Old 03-04-2017 Thread Starter
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Re: Avotec 70cc dirtbike project

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Didnt you do a squish calculation of the engine during its build phase?


I usuallu use a laser to help find my placement for the engine or to check to see if theyre in-line accurate.........

first thing is mounting the rearwheel evenly across the rear swingarm then you attach the laser to the rear sprocket and then align the rear sprocket to pinion then mark n mount the engine with trans........

I did attempt but due the big diameter bowl in the cyl head it is impossible, i did notice that the outer side of the bowl has a very small space of like 1mm that does get close to the piston. I added a 0.6mm base gasket and added a tiny layer of jb weld on the cylinderhead to be sure that it is sealed. The engine did not come with a cylinderhead gasket.. but the squish should be around 0.6mm now instead of 0.04? + -

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post #20 of 41 (permalink) Old 03-04-2017 Thread Starter
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Re: Avotec 70cc dirtbike project

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Originally Posted by CAM2 View Post
Didnt you do a squish calculation of the engine during its build phase?


I usuallu use a laser to help find my placement for the engine or to check to see if theyre in-line accurate.........

first thing is mounting the rearwheel evenly across the rear swingarm then you attach the laser to the rear sprocket and then align the rear sprocket to pinion then mark n mount the engine with trans........

Thats a great way to do this. Though i do not have a laser kit like that unfortunatly

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