Mbk 51 project

I found me a 1990 mbk 51 frame from Dos and plan to make this much deserving bike pretty. I stripped it down and degreased it and went to work on the tank

Dos soaked the tank for me and after shaking the heck out of it with some m8 nuts I was able to get all the chunks of rust out and get clear water to flow through. I dried it out and filled it with premix and called it a day. Will probably clean it again later on down the road

Here it is with a front end mocked up

The forks are 33mm paioli from a ktm65 and the wheel is a kx65. The triples didn't fit but some from an old it or dt175 fit perfectly. I'll need to use mbk cups and bearings though and maybe shorten the forks or put clip ons above the triples.

I have clip ons on my pug and because the seat is so low below the bars they are pretty comfy.

I also found a new Tokico dual caliper for super cheap awhile back that'll work with a bracket fabbed up.

I'm having trouble taking the swingarm off. I have a square swingarm I'm gonna use but the original is frozen solid to the bushings. I tried pounding the bolt out after repeated application of penetrating oil over the last few weeks but it won't budge. The swingarm moves freely but the bolt won't move. Maybe it's bent?

Any suggestions or tips from anyone would be much appreciated. Maybe heating the bushings or I could just cut it off since I won't be using it anyways. Seems like a waste of a perfectly good part if I hacked it off.

I am trying to decide what to do with the frames finish. It's a pretty neat looking champagne-y color or maybe sun faded white. I would leave it alone except for the rust.

There is more rust behind the seat mount area and behind the side rails so I'd like to do something to avoid problems later.

I'm waiting on the engine bits so I would like to make it look nice before I start assembling anything. I have the new Doppler clutch pulley and a giraudo repop variator reserved and coming, along with a bunch of other stuff to build up the engine. Fun times ahead.

http://www.mir35.com/poulie-embrayage-cyclo/491185-poulie-embrayage-mbk-51-doppler-er86-racing-new-2019-491185.html

I am going to be welding some common frame reinforcements soon but need to research gas tank welding safety before I put a torch anywhere near that area.

Tips to avoid an explosion are gladly accepted.

Re: Mbk 51 project

if your brazing the tank, just drain, remove petcock, leave cap off. itll throw flames like 5 time outta cap but wont blow up, earphones help.

Re: Mbk 51 project

> pat splat Wrote:

> -------------------------------------------------------

> if your brazing the tank, just drain, remove petcock, leave cap off.

> itll throw flames like 5 time outta cap but wont blow up, earphones

> help.

Ha. I will be mig welding everything. The frame is in pretty decent shape and the tank is fine but I want to make it a bit stronger with a crossbar and some gussets. It doesn't seem as sturdy as the 103 frame I have is.

Re: Mbk 51 project

You could fill the tank with water . Drain and dry after .

It's the vapors that go boom , not the liquid .

Re: Mbk 51 project

> P D Wrote:

> -------------------------------------------------------

> You could fill the tank with water . Drain and dry after .

>

> It's the vapors that go boom , not the liquid .

I think I once read they can still explode even with water in them. how doe s draining and letting it air out for a long time in the hot Arizona sun then filling with water sound?

Re: Mbk 51 project

You likely read that^ on April 1 .

If the tank is full to the brim with water , where would any fumes be ?

But , take as many precautions as you're comfortable with before you lite the flame .

Re: Mbk 51 project

i've had them harmlessly go 'poof' before, never had a problem. if its drained and sitting open for a few days you should be fine. i'm not a fan of putting water in gas tanks unless you immediately take steps to prevent rust

Re: Mbk 51 project

> Graham Motzing Wrote:

> -------------------------------------------------------

> i've had them harmlessly go 'poof' before, never had a problem. if its

> drained and sitting open for a few days you should be fine. i'm not a

> fan of putting water in gas tanks unless you immediately take steps to

> prevent rust

Definitely drain it and rinse it well before putting any heat to it. Like Graham said, open all the openings and “poof” the tank before anything. I had a heat gun set off an empty and rinsed tank a little over a week ago. Shit is no joke. Still peeling skin off the top of my fingers and also lost the skin on the tip of my nose the next day. Super lucky to have eyebrows still.

5681B28B-1BEE-4459-BA6B-A85AD7926861.jpeg

Re: Mbk 51 project

> Ben Whittle Wrote:

> -------------------------------------------------------

> > Graham Motzing Wrote:

>

> > -------------------------------------------------------

>

> > i've had them harmlessly go 'poof' before, never had a problem. if

> its

>

> > drained and sitting open for a few days you should be fine. i'm not

> a

>

> > fan of putting water in gas tanks unless you immediately take steps to

>

> > prevent rust

>

> Definitely drain it and rinse it well before putting any heat to it.

> Like Graham said, open all the openings and “poof” the tank before

> anything. I had a heat gun set off an empty and rinsed tank a little

> over a week ago. Shit is no joke. Still peeling skin off the top of my

> fingers and also lost the skin on the tip of my nose the next day. Super

> lucky to have eyebrows still.

Wow. So you were just using a heat gun on the tank?

After draining and drying it out I will definitely try to purge any remaining fuel with a torch. It seems pretty harmless now as long as precautions are taken.

Gonna start drying it out tomorrow.

Thanks

Re: Mbk 51 project

You're making a mountain out of a mole hill .

Just fill the tank with water and get the job done . After you drain it , slosh some 2T oil around in it and get on with the project .

Re: Mbk 51 project

may be wrong but the swingarm bolt might be threaded into one side of the frame. so that the outer nut you removed is more of a locking nut. just loosening the bolt from the other side?

Re: Mbk 51 project

> Yung Spil Wrote:

> -------------------------------------------------------

> may be wrong but the swingarm bolt might be threaded into one side of

> the frame. so that the outer nut you removed is more of a locking nut.

> just loosening the bolt from the other side?

I thought the same thing and tried that early on using a long ratchet and socket on the head of the bolt but was unable to keep the socket on the bolt head.

I always hate coming across those rounded bolt heads with only a couple mm depth on the flats. Used vice grips on the other end too and only mangled the threads. That thing is really stuck.

I couldn't find a free 51 manual to check but the 50/40 manual mentions "driving the bolt out" after taking off the locknut.

Re: Mbk 51 project

I built up the engine awhile ago after getting all my parts in

Stock cases matched to the mvt kit with channel cut from intake to the lateral transfer. Everything sanded smooth with 320. I decked the intake maybe 2mm to get the Reed's in further and opened the stops up to the walls of the intake.

I matched the exhaust port to the ball joint and polished the exhaust passage.

The exhaust passage has a slight venturi and opens up to match the 25mm ball joint so a big carb should work fine.

Still trying to decide what carb I want to use. I have an extra 19mm phbg but want to try one of the cp 21 evolution carbs.

They are supposed to work well over 12000rpm with no fuel foaming issues.

I measured 192/131 for the port timings with a 10" degree wheel and the pipe I have is supposed to rev to 14000+ rpm. Clutch pulley should take care of the low end.

I also started building up a second engine with some Eurocylindro cases

They use am6/kx65 reed blocks. I found some v force reeds for $50 new on eBay and an old ktm 50 radiator. The old ones were single large radiators and should work well.

I found a Bosch pump but it didn't come with a wiring harness.

The pump is # 0392020073

I found a reference number for it's wiring harness but it looks like a universal type connector that may or may not work. Has anyone found something that will work with this pump

Here is a mock up of the h20 engine

Will probably end up stealing parts off the other engine when I get to this one later on.

Going to rectify the AC and run a 4ah battery off a reg/rec to power the pump and some led lights

The heat set in a few months ago so I haven't gotten to the frame yet.

I need to get some more mig practice in and then I can get back to the frame

Re: Mbk 51 project

Johnnie Distefano /

Nice work so far! Loving that pipe

Re: Mbk 51 project

Cristian Luna /

I wonder if those eurocylindro are my old ones I should have never got rid of them haha

Re: Mbk 51 project

> Cristian Luna Wrote:

> -------------------------------------------------------

> I wonder if those eurocylindro are my old ones I should have never got

> rid of them haha

They are actually Turkish copies.

I love them. I got em because the only av10 cases besides stock still available are the polini and if they are anything like the Peugeot Polini cases I have they are garbage

These things are machined really well and the intake is massive, the vforce Reed's fit like a glove.

They came with skf bearings, seals , studs and intake for less than Polini cases cost.

Seller has more . Also has 46mm av10 Gila kits cheap

Re: Mbk 51 project

Anonymous User /
scootertrash

Get a Harbor freight hammer to get the bolt out.

83581D83-AD82-42F1-94EC-4CE21D9C6FB1.jpeg

Re: Mbk 51 project

I just had to deal with this the other day. I used a hydraulic press and a socket extension to get it out. Those fuckers are tight on the bushings!

Re: Mbk 51 project

> Ana L. of the Aurora Roras Wrote:

> -------------------------------------------------------

> Get a Harbor freight hammer to get the bolt out.

> >

I tried a hammer along with every other method. The gap between the swingarm and swingarm mount was wide enough to fit a thin 4 1/2" cutting wheel so I just cut through the bolt on both sides.

I wanna start prepping the frame for welding tomorrow if I get some time.

I got my radiator and mocked it up to make sure it will be okay. The older ktm50 ones are such a perfect fit for a French bike. Kind of small but will have consistent flow with the Bosch pump.

I am going to put a small overflow bottle just in case.

I picked up a wiring connector said to fit the pump I have and it doesn't go inside the connector housing and it's way too short to fit the recessed pins

I would like to have a proper harness for the pump since the pump and hoses have to be able to flex when the engine variates. Could rig something with two spades if I have to but think a proper connector is best.

Re: Mbk 51 project

> Daniel '' Wrote:

> -------------------------------------------------------

> I just had to deal with this the other day. I used a hydraulic press and

> a socket extension to get it out. Those fuckers are tight on the

> bushings!

Was yours threaded in on the ignition side? After cutting the bolt I pulled out the swingarm and couldn't even tell there was a bolt and separate bushing. Looked like one piece of steel. Fused from rust.

Re: Mbk 51 project

Anonymous User /
scootertrash

SawzaL’d dat bich!

Re: Mbk 51 project

Johnnie Distefano /

I'm curious how those ktm50 radiators do. I'm running a polini radiator on my Peugeot, buy they're getting a bit hard to find. I was also looking into rieju ones. They seem a bit bigger, and is still capped. I dont like the capless ones found on alot of scoots

Re: Mbk 51 project

With a Bosch pump and recovery tank they should work perfectly. The newer ktm50 radiators you would need to use both because each one is about The size of a cell phone

While doing some research a lot of ktm50 riders would overheat when doing a lot of slow speed riding. Makes sense with a crank driven mechanical pump and no recovery tank.

I think you can get higher pressure rated caps for these ones too.

Hard to find small a one oriented horizontally with caps.

There are some brand new piaggio scooter radiators on eBay right now for less than 40$ new. Capless though

Re: Mbk 51 project

The swing arm bushings are always super tight, even as new, to the bolt. Saw through the long bolt and flames are your best friend to get rid of the rubber and get the leftover out. Pull the frame side of the bush out while hot from burning the rubber. The original bolt is M10x140. Silentblocs are 10mm × 22mm 33mm long.

Always use a nyloc nut on the wing arm

> Jesse Simonsen Wrote:

> -------------------------------------------------------

> > Daniel '' Wrote:

>

> > -------------------------------------------------------

>

> > I just had to deal with this the other day. I used a hydraulic press

> and

>

> > a socket extension to get it out. Those fuckers are tight on the

>

> > bushings!

>

> Was yours threaded in on the ignition side? After cutting the bolt I

> pulled out the swingarm and couldn't even tell there was a bolt and

> separate bushing. Looked like one piece of steel. Fused from rust.

Re: Mbk 51 project

I got some welding done today.

Spent most of my time shaping the pieces that were welded to the bottom of the tank.

Fitting the crossbar was easy enough. I used 1/2" tubing and closed one end with my vise so I wouldn't have to cut any of the tank seam to fit it in. Notched it as best as I could with a bench grinder and Dremel

After I ground down the blobby mess I coated everything in epoxy. Going to sand that down to make it look a little nicer.

I should've practiced using thin metal. I had no problem working on 1/4" and 3/16" scrap but the frame and tank are thinner

I beat on the welded joints with a hammer and they seem fairly solid.

Gonna have to find a kids welding helmet for my 4 year old.

He's usually right at my side when I'm working. His favorite tool is the hammer

Re: Mbk 51 project

Anonymous User /
scootertrash

Ha, I hear ya,, and u can blind a dog. rip Rosie

Re: Mbk 51 project

I welded some more reinforcements and decided to strip the frame to bare metal so I could get rid of the rust.

My son wanted to help so I bought the safer gel stripper and that stuff was horrible for stripping paint. Just softened it and still required a lot of work with the wire wheel to get the paint off.

I sanded off the rust I could get to and used phosphoric acid to convert all the other rust forming behind the side rails and behind all the sheet metal areas.

After a few treatments the frame started developing some cool patina but I had already paid for paint and supplies so I started priming.

Going to sand the primer smooth and build it up a bit more before painting it charcoal gray.

I will add some simple mbk decals to the sides of the tank.

Re: Mbk 51 project

DANG - that is looking SEXXXELLENT

Re: Mbk 51 project

> Rebel Moby Wrote:

> -------------------------------------------------------

> DANG - that is looking SEXXXELLENT

I am amazed at how well the primer is filling in all the surface imperfections. This is my first time trying to paint something properly.

Next bike I definitely want a clear coated raw steel frame.

Re: Mbk 51 project

Loving this.

Want to post in this forum? We'd love to have you join the discussion, but first:

Login or Create Account