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karte_m0lim 10-29-2014 04:27 AM

Gokart MZ 250
7 Attachment(s)
as you know, old engine is dead so is time to get new one ..
So I did... I bought motorcycle engine 250 ccm, 20 HP, 5 speed gearbox..

Got it yesterday so only had time to quick check to see will it fit, looks like it will.

Probably gonna cut frame in half and make new back half, make it bit stronger than previous one cause engine is heavier and I saw few mistake in old one (too much flexing beneath @ss ) :p

Must change gear ratio too. Sprocket on engine is under cover so it is size limited. Origninal has 15 tooth, max possible is 19. On other hand , on axle cannot go with too small sprocket because limited by flange.

Also ordered 4pcs Flange Type Ball Bearings for spindles so gonna change that as well.

This engine do not have any rubber shockers so must figure that out to reduce vibrations and shifting and clutch as well..

Engine cost me 200 $ and have to wait to sell some parts before continuing work.

Hope to hit the road again at spring..

Few pics:$T2eC16NHJHoFGlIh3C5VBSP6Ho7V-!~~60_57.JPG

landuse 10-29-2014 04:32 AM

I am glad you found an engine so quickly. You will be on the road soon again :thumbsup:

KartFab 10-29-2014 07:17 AM

Oh wow you are going to be going so fast when it's all said and done

firemanjim 10-29-2014 08:11 AM


Originally Posted by gran_pann (Post 341317)
Oh wow you are going to be going so fast when it's all said and done

Especially if he puts those biggest looking slicks,sitting up on that shelf.
Karte, your shifting and clutch work can all be put on one convenient lever.... Grab lever and squeeze....Then shift...... :thumbsup:. Easy.....
Instead of cutting the back off,I think I'd try to brace it up.......
Nice looking engine, BTW.....:thumbsup:

karte_m0lim 10-29-2014 09:23 AM

I do not like how kart looks and act in back half...

After i bend it, I added L shaped profile to add streingt and it looks really bad...

Second, there would be too many scars of previos engine mounts...

And I think I made mistake by adding right2left instead front2back additional tubes...

For shifting and clutch I gonna to use that solution, but flappy pedal on wheel will be much cooler thing to do...

Also, if there is room, would like to use your idea Jim for starter...

Axx3d 10-29-2014 09:59 AM

Holy Molim.. :)

Thats gonna be too much fun..

Good job on the replacement motor..

karte_m0lim 10-30-2014 01:26 AM

5 Attachment(s)
HI guys. ..

Yesterday did some cutting and removing unnecessary pieces

So need your opinion , should be enough to reinforce chassis with three new tubes (marked red)?

I really do not like this L shape ..

Some more sanding needed to remove old welds and mounts.

Also need to bend exhaust pipe a little bit:

more pictures on LINK

Half-breeder 10-30-2014 02:23 AM

1 Attachment(s)
This is just 'my' opinion... 250cc w/ gears is an awfully large amount of torque/HP for a 'kart' vs a motor bike or a 'buggy'... but from my experiences... parallel bracing (like the design youre thinking) tends to not be as 'ridged' as triangular bracing, using the same amount of metal/length. Not saying there is anything wrong w/ your design (yea Im not too fond of the "L" shape either), but to be on the safe side I personally would do 'something' like the attachment. Bracing strengthens and stabilizes/increases rigidity... but did you also realize it is a transfer conduit of vibrations/shakes. The design I attached will/should strengthen and reinforce the frame for the engines torque, aswell as transfer vibrations towards the outer perimeter framework, vs inner framework... where your butt sits. And trust me... its very uncomfortable to ride a kart that'll numb you nether regions in a matter of afew mins. :thumbsup:

I too am a stickler for appearance... and Ive found that black paint covers ALOTTA :ack2:...:lolgoku:


karte_m0lim 10-30-2014 06:07 AM

1 Attachment(s)
huh, that gonna be stiff :p

I thought to remove this blue lines (tubes) , remove L shaped profile ( green) but left tubes instead and add this red ones.

karte_m0lim 11-03-2014 05:21 AM

So need help , someone to check my calculations ..

On net found few different specs for MZ 250 bike and its tyre size ..

Anyway, there is calculations for gearing:

For tyre size 89/100 r16 , tyre circumference is cca 184 cm
For tyre size 89/100 r18 , circumference is cca 200 cm

My tyres are 25 cm in diameter and circumference is cca 80cm

With bike top speed of 130 km /h or 36,1 m/s and 16'' tyre, wheel on bike is spinning on 1177 revs and for 18'' tyre 1080 revs.

Also found that bike has 19/48 sprocket ratios or 0,39.

Adding that ratio to calculated wheel revs, I have gearbox output spinning on
2864 (16'') or 2769 (18'').

Now, I have on axle 28 tooth sprocket, so I thought to put 20 tooth sprocket on gearbox to have 100 km/h on 2864 rpm or 97 km/h on 2769 rpm .

As sprocket on gearbox is in closed case, and if won't fit inside, will try with 18 tooth sprocket , wich give me top speed of 87-90km/h.

So just need your thoughts and calculations check.

karte_m0lim 11-03-2014 07:39 AM

4 Attachment(s)

karte_m0lim 11-24-2014 02:40 AM

6 Attachment(s)
so, not so much progress lately ..

when try to put sprocket on axle, find out that hole is 0,5mm too big ..

so only do engine mounts with rubber absorbers as I can not use original ones (I do not have it :p)

the big one is from the bus and small ones are from lawn mower

so I thinking to maybe add one more absorber to rear mount..

karte_m0lim 01-12-2015 12:47 AM

6 Attachment(s)
after almost two months , finally some work...

not much, just engine mount...

karte_m0lim 03-09-2015 01:17 AM

finaly some progress ...

I decided to cut frame to the half and gonna build new rear end...

landuse 03-09-2015 01:41 AM

I am glad you got back to this build. I look forward to seeing it finished

karte_m0lim 05-04-2015 12:11 AM

5 Attachment(s)
not much but ..

karte_m0lim 11-10-2015 02:03 PM

8 Attachment(s)
finally restarted this ..

welded frame back together, abandoned idea to use rubber absorbers on engine mount, kept only one ..

anyway, few pics:

karte_m0lim 11-11-2015 10:47 AM

9 Attachment(s)
today's progress:
- exhaust pipe cuted and welded to make turn tighter
- exhaust support
- fuel tank support
- brake cylinder support
- seat support ..

I am trying to use as much is possible old parts so some solutions are not the best...

few pics below...

karte_m0lim 11-12-2015 02:51 PM

9 Attachment(s)
for today:

- finished engine mounts and made them a bit stronger

- holders for brake cables

- started on gear shift linkage ....

please do not write so much, I do not have time to read all :cornut:

karte_m0lim 11-16-2015 01:17 AM

8 Attachment(s)
not much progress over weekend..

finished shifting linkage and clutch...

few ideas , few prototypes and maybe I just buy clutch lever ...

also , there is idea for kick start , as I don't think is safe to start engine by putting leg in middle of frame ...

anyway, few pics below...

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