Trying to getting run offroad 50cc (->to 100cc) motorbike

[AndyAndyAndy]

Story behind it is, that previous owner converted this 50cc offroad minibike into 100cc by a bigger head (no idea if it ever run after this upgrade). When he was selling it he told me it starts (no idea if it's true).

Head says "97cm^3 KFM-8" (which is a head from 110cc motorbike). And body of engine says "1P39FMB" (which is a 50cc engine).

Well, it won't start.

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1. Carburetor looks like it's missing a screw? It's a threaded hole.

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2. It has spark.

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3. Trying to start it, carburetor intake actually blows air out. I was trying to put in there a rug soaked in fuel to get it started. And when I'm starting it, it will blow this rug out. Exhaust is blowing air out too though. Exhaust valve is opening/closing when turning engine and by touch it seems that it is set OK. (not too lose, not too tight) I couldn't take off intake valve cover, so I don't how intake valve is moving/doing.

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My first question would be about that carburetor. Is it missing screw, could it be it, why it doesn't start.

My second question would be about carb blowing air out/anybody would know how to check timing? Which mark at what/where/when? Guy said it run. So I don't wan't jump to conclusions to quickly, start tearing apart engine and reset everything. When maybe it's just badly setup carburetor or something.

 

[itsid]

well a lot going on...

first (just to de-confuse) it's not the head it's the CYLINDER that states KFM8 huuuge difference..
so, please do not mess the two up to avoid confusion later on

Now, IDK what bore and stroke the KFM8 should have...
I can work out the stroke for the 1p39FMB knowing it's a 50cc and having a 39mm bore I can say it'll be around 40mm
(close enough for now)

So, if the intended stroke for the bigger top end (cylinder and head)
is bigger than 40mm (the typical 110cc engines I came across are actually 53mm bore and 49.5mm stroke)
the piston cannot reach the top of the cylinder at all, thus not making nearly enough compression to have an strong enough combustion,
i might barely pop.. but surely not turn over.

Ideall remove the head (not the cylinder) and turn the engine over to see how big the gap
between the top of the piston and the sidewall of the cylinder is at top dead center.
if it's almost half an inch (9.5mm to be exact) you wil likely have a cylinder made for a 49.5mm stroke
(i.e. ~52mm bore.. measure that as well to verify!)

And in order to make use of that cylinder and piston you need a new crank which may or may not fit the crankcase

insufficient compression and it's just not running.. no way to work around that I'm afraid.

If the piston reaches the edge of the cylinder nicely however,
compression and setup isn't an issue (well shouldn't be)
the removed head now forces you to readjust the timing chain and the valve lash
(which is the next step oon the list anyways.. this way you cannot forget to )

So reinstall the head ideally with fresh gaskets,
properly align the flywheel mark with the cam marking and install everything as per the engine manual
you will need both manuals I'm afraid, the one for the 1P39 and the one for the KFM8 to be sure the markings are meant to be for TDC on both engines (some use BDC for some markings)

And with that and a verified compression (thumb over the spark plug hole and cranking it)
I think it's fair to assume the engine internals are okay for another try.

add oil and fuel and give it a go.
If it still doesn't want to start, clean the carb
remove als plastic rubber and paper parts and boil in lemon juice [citric acid not freshly squeezed ])
for at least an hour or if oyu have an ultrasonic bath.
reassemble and try again.

If PO hasn't swapped the carb it's likely undersized for the now twice as big engine.
you might need to buy a new carb to accomodate for the bigger trhroughput.
For now it might be enough to increase the mixture for idle and speed to the max..
it should at least allow you to start it up (it might choke on high revolutions though)

But frankly the carb is the least of your worries at the moment I bet.
If the engine cannot make compression because of the new cylinder and piston, you won't need a carb anyways until that's fixed.

'sid

 

[AndyAndyAndy]

Thanks!!!! I will look into that tomorrow and post what I found out.

 

[AndyAndyAndy]

Before taking it apart, I can't make sense of how does it work.

Shouldn't one turn of camshaft should be equal to exactly X turns of flywheel/rotor? It isn't.

My camshaft is at position "just closed exhaust; about to open intake". I turn camshaft 360° to the same point. But flywheel is now positioned differently. For example off by 40°. I turn camshaft again 360° and now flywheel is off for example 120°. If I turn it for example 5 times or 6 times they will eventually get into "starting sync position". This is making it impossible to record original settings before taking it apart.

Also air intake is behaving every opening differently. I will start rotating camshaft and intake will start opening and closing:

First opening: it will blow air out at the start of opening
Second opening: it will blow air out at the end of opening
Third opening: it does nothing
Fourth opening: it sucks air in
...


So maybe somebody could explain this behavior to me. Or point me to some material, that would explain why does it work like that?

 

[itsid]

Well the camshaft should rotate exactly once per every two crankshaft rotations

You can quickly check if the ratio is off by verifying the marking on the rotor to be correct.
the "T" marking should be inline with the mark on the crank case every single time the piston is at top dead center.
(remove the spark plug and get a dowel in to verify! or do that tomorrow when you remove the head )
if the position is off, you need to align the internal gears correctly,
if the ratio is off you ned a new crank or flywheel rotor or intermediate gear... or all of those

Once that's verified however to be correct, the ratio of the cam in regards to the crank
can be verified as easily...
first pass of the T mark exhaust just closed, intake's about to open,
second pass: both are fully shut,
half a turn later exhaust opens and closes approaching the third pass of the T marking.

And yes.. it is possible the cam gear isn't correct and causes absurd valve timing on your engine..
everything is possible with a stupid enough PreOwner I'm afraid

'sid

 

[AndyAndyAndy]

I lined up T mark on rotor with | mark on crank case. Through spark plug hole I can feel that piston is top dead center. If I rotate rotor 360° piston will again come to top dead center, as they come to marks.

No issues here.

But I can't make sense of valves.

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(T mark = when rotor and crank case marks are lined up; inbetw - during the 360° rotation)

T mark: exhaust closing
inbetw: intake open/close
T mark: -
inbetw: -
T mark: exhaust fully open
inbetw: intake opening
T mark: intake closing
inbetw: -
T mark: -
inbetw: exhaust open/close
T mark: intake fully open
inbetw: -
T mark: -
inbetw: exhaust open/close
T mark: -
inbetw: intake open/close
T mark: exhaust fully open
inbetw: intake open/closing...
T mark: ...intake still closing
inbetw: -
T mark: exhaust opening
inbetw: exhaust close
T mark: intake fully open
inbetw: -
T mark: -
inbetw: exhaust open/close
T mark: intake about to open

 

[itsid]

I can feel your pain..
but that table is pointless I'm afraid .

AS long as we cannot verify the cam timing is correct (and I have the fear it'll be off by a few teeth)
TDC might not be the 'control point' for the cam ATM...
So instead of observing the valves now and trying to figure out what valve timing might be ground into the cam
and by how many degrees this cam might be off...

See that dot on the cam gear sprocket?
Align that with the triangle cut on the head's gear cut-out (opposite side of the head bolt)
you can clearly see both in pic 6 in your first post above.

have that? great.. now get a sharpie and make your own marking on the flywheel. to align with the case mark.
rotate the flywheel exactly twice (do NOT look at the cam at all!) back to your home made marking.
NOW look back at the cam, the dot must be a whole number of rollers off of that triangular timing mark
(ideally ZERO of course)
if not.. we need to know the number and if it's ahead or retarded in the direction you turned (clockwise or CCW)

making sense of the valve action only helps if one is sure everything else is properly aligned already

'sid

 

[AndyAndyAndy]

OK, my try on following your instructions:

 

[itsid]

Sooo, you rotated it CCw if I'm assuming correctly by your circular arrwow ..

this means the cam is retarded by 4 teeth
and since we talk two revolutions on the Flywheel side,
it's either the cam needs a four teeth smaller sprocket or the flywheel a two teeth bigger sprocket

So yeah clearly PO lied to you.. that engine NEVER started up.. not at all.. it cannot and will not ever with those internals

I would poop on his doormat let's say

Anyways....
Are you really wanting to fix that abomination?
I mean it will take more than just two minor parts and a set of wrenches to fix that.
you could try to get the 50cc parts from PO and convert that back perhaps (that might be a viable solution)
Or buy them based on the engine model of the crankcase
finding the parts and buying them will likely cost you upwards of $75 I'd say...
Worse if you try to keep the bigger combustion cylinder in place,
I'm not sure that's a good idea; since the very first task would be to find the original parts manuals for BOTH donor engines,
and that alone can prove to be difficult.

I'm sorry.. that's not good news, is it?
And we haven't even checked if that piston is even reaching the top of the cylinder (it will most likely not)
and then you surely cannot make it work without a new crank anyways... and what about the carb at all

Since you can get a fresh 110cc engine for ~99USD off alibaba.com I think it might not be worth the hassle.
example:

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Cqjb 110cc Electric Kick Start Assembly Motorcycle Engine - Buy Motorcycle Engine Assembly,Motores 110 300cc Del Motor Kayo 250cc Motorcycle Engine Complete 4 Stroke Bike Engine Kits For Sale,110cc Motor 300cc Used Motorcycle Engines 150cc Loncin 250cc Del Motor De La Motocicleta Mini Gas 50cc...

www.alibaba.com

just saying

'sid

 

[AndyAndyAndy]

I was looking around and if I wanted a new 110cc engine it would cost me at least 250$. That's too much. Alibaba and shipping is problematic.

Checking Facebook Marketplace I see some used 125cc engines for 20$ - 40$. And even a whole 125cc motorcycles for 100$ (i need carburetor, throttle, break lever etc. anyway...this might be actually cheapest solution).

Tomorrow I will open current engine, look inside. Post some pics. Which will be plan A. Plan B will be Facebook Marketplace.

 

[AndyAndyAndy]

Piston actually comes to the top of the cylinder.

But first sprocket has 14 teeth, while second one has 32 teeth.

Are we going to buy 28 teeth? It's a 3$ part. Let's go for it? If so, what about chain, won't it be too long? Or will chain tensioner take care of that?

 

[itsid]

that fist pic is unfortunately a smidge out of focus..
I can't tell if the piston reaches the end of the cylinder or not

I'd rather increase the crank sprocket if you have the choice
and no.. you must install the correct length chain... the tensioner can only take up the schain stretch not compensate for a wrong size

'sid

 

[AndyAndyAndy]

 

[itsid]

uhm.. that's not too insignificant..
(5 or so additional cc that turns out to be 40% or such bigger combustion chamber.. meaning similarly less compression)
you happen to have access to a compression tester (maybe a friend has one perhaps?)

Oh wait.. maybe that is NOT a 110cc cylinder then.. it's the 90cc cylinder perhaps (I think that might have a similar stroke to the 50cc...)
could you do me a favour and measure the bore of the cylinder,, maybe that sheds some more light on this frankensteined engine.

'sid

 

[AndyAndyAndy]

51mm

 

[AndyAndyAndy]

I don't know if there is a point in messing with this engine anymore. Not only I need internal parts like crank sprocket, timing chain, possibly cylinder and ?head?, but also I need things like shift lever, throttle, possibly carburetor...all included it's like 150$ - 200$. And even after that running motorcycle is not guaranteed.

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Looking at the marketplace I see motorcycles for 80 - 100$. For example this one has manual 4 stroke 110cc engine. Also comes with all extra parts I need. And asking price is 90$.

So maybe I could change the theme of this project from 'saving an old engine' to 'butchering a running motorcycle for parts'? I have MIG welder and angle grinder for misaligned engine mounts....don't know what else could be an issue here.