Retro Content - 1987 Tomos Golden Bullet Moped Mini-Project

[Nomisf]

So...

Something a little like this arrived in the post from the Netherlands,


I got really excited, so the piston and barrel came off with few photos. Here goes the head!



The image below is a drift cobbled out of sockets. The kit I bought is for the A35 engine, which is almost identical in bottom end to the A3 on my Tomos, except for the inner diameter of the wrist/gudgeon pin. Its 10mm on the A3 and 12 on the A35.


This of course, means pressing one out, and replacing it with the wrist pin from the other. Never to be out-done, I decided to buy a needle roller for Ultimate-Street-Ripping-Power. Alas, it turns out they send me the wrong one, or I messed up, or something...


Alas, I thought this might happen - so also ordered a Phosphor Bronze bearing from a Puch Maxi, which is the same size. Although, for some reason it has no oilways in it, so cue the dremel and emery cloth. Old on the right, duh, you can see the diameter difference.


Improvising...


Yes, I cleaned the burrs and sharp edges... Worth noting the two holes in the connecting rod arent directly opposite each other. I really wished for a drill press during this event.


Then I got too excited to take pictures, because I am a giant child. Kit on Block!


This kit converts the engine to reed block, the big square port you can see is the Intake/Reed Block mount.

The carb has stayed as a the Dellorto 14/12 with a stinking great jet in it. I have a manifold for a Mikuni VM20, so I need to find one of those.

Then of course, added a few degrees of timing - and went for a "blast"


IT DOES 42MPH. AND. ITS. TERRIFYING.

Video to come this weekend!

 

[Nomisf]

Bit of a teaser from the other front,

With the now unused jug and head from the black bike, I have the parts I need to complete the blue bikes engine.

Had to cut the piston out of the old seized jug, but otherwise a few new gaskets I had on the shelf and .... A second engine !

 

[Nomisf]

I am just busy breaking youtube, ignore this post !

 

[Nomisf]

The long awaited video of the 65cc power, excuse the odd contrast and sound - new helmet and camera settings to figure out !

 

[Nomisf]

I must have one of these again. So much charm, that 2 speed auto is a gem.

Also, the vid quality is fantastic, what camera are you using?

Its a Drift Ghost S, 1080p 60fps, Normal Colour Mode modded with ND Filter Film behind the front lens.

For these videos it was on +2 Exposure to compensate for the ND Filter, but that seems to have been a little too much - as I am still experimenting. The plan is to get the shutter speed of the camera to slow down, to make things look "faster", but retain the light balance.

 

[Nomisf]

 

[Nomisf]

Lets start with the teardown pictures, everyone likes some carnage!


Oh look, where has the head and barrel gone...


Well thats the jug, and oh look, melted aluminium above the exhaust port!


I guess we know where that came from


Lets just flip this over, and oh, yeah, that got hot!


Now before anyone says it, yes, I was probably running the engine too lean, with too much advance I had pushed it a little far!

In my defense, I usually look for a dark brown coffee colour on the plug when tuning a two stroke, and I thought I had it nailed with this one...



But in hindsight, that's probably more of a milky coffee

What I am trying to understand now, to prevent a repeat - is how the sequence of events looked.

This is the state of the piston,

What I am most interested in is, did it either,

1) Overheat the piston, seize in the bore, and then trash the wrist pin.

2) Wrist pin overheated, failed, piston went off center and seized.

3) Wrist pin has nothing much to do with it, but was failing due to undersized oil holes / bad oil permeation / already lean mixture.




Good news is, the parts to repair total about £30 - so here we go again

Maybe 70cc this time!

 

[Nomisf]

Mine was 70cc and never broke once, although I did always add a little extra oil to the premix. Do it

I was running at 32:1 vs the stock 50:1, might go to 25:1 - blue smoke for days!

At guess the needle bearing with bush combo reduced lubrication and she nippet up.

This was only running the bush, as the needle roller didn't fit through the connecting rod. Although I do wonder if the oil holes could have been even bigger.

I am trying to get hold of a correct size needle roller, fingers crossed!

 

[Nomisf]

Sorry didn't read the thread correctly.

The needle roller for the A3 is £9 let me know if you need a link.

From the states so postage might take the urine.

No worries Appreciate the suggestion,

I might be making an order from Treatlands at some point, but I have a couple of options over here to try first Want their Mikuni VM20 package

The joys of tuning two strokes

No evidence of detonation and it doesn't look like it's been running lean. Are you certain there was enough oil in the fuel (assume it's premix?) as that seize could quite easily be caused by a lubrication failure, and that would explain why the little end bush is trashed as well.

Use can use hydrochloric acid to remove aluminium deposits from cylinder walls if the rest of it is salvageable.

All part of the fun !

I was premixed around 35:1 , although I was quite low on fuel - may have separated in the tank.

Will be changing 2 stroke oil, and bringing it up to 20:1 then working back from there.

Cheers for the tip on HCA, I might rescue to old jug for the second engine - although it is Nikasil lined - not sure if it will play nice with the acid or not,

DPD are out for delivery this morning, so new parts should be with me for the weekend

 

[Nomisf]

Ah Awesome will have a watch later - love RCR!