New bike in preparation for colder climes and WTF is that mod all about...

Carb heater wivva PWM controller, though I can't read what the IC is
easily. The 'Temperature' marking on the PCB kind of gives it away.

 

Low compression would imply a large combustion chamber, so plenty of
room for valves to do their thing without poking the piston. I think.

 

That's the ambient temp. that the sensor uses as the switching point. I've
only had one bike that seemed to carb ice and that occurred at well above
ambient freezing, so a switching point of over 5C ambient temperature would
seem right to me.

It's a long time ago since I got rid of that bike, but IIRC, it was
humidity that caused the problems, so carb icing happened more on cold,
foggy mornings.

 

Really? I've only built car engines, but I've never met one yet where
a valve stuck fully open wouldn't get clobbered by a piston. On the
DOHC Fiat the piston tops had to be relieved so as not to get
clobbered by valves when running normally.

 

1600cc Ford OHC engines didn't, the 2.0l did.

 

PWM?

 

Looks like a makeshift carb heater to me.

Could be useful.

 

Pulse width modulator.

http://en.wikipedia.org/wiki/Pulse-width_modulation

 

Ta. Every day's a school day. And I see chains are becoming popular
again.

 

IIRC, SAAB H type engines (the ones in classic shape 900s) are chain
driven and had a spate of chain breaks but were non-interference engines
so it wasn't too bad.

 

Indeed. My 08 Mazda 6 with MZR petrol engine whizzes the knocking sticks
round by chain drive.
--
+-------------------------------------------------------------------+
| Pete Fisher at Home: |
| Voxan Roadster Yamaha WR250Z/Supermoto "Old Gimmer's Hillclimber" |
| Gilera GFR * 2 Moto Morini 2C/375 Morini 350 "Forgotten Error" |
+-------------------------------------------------------------------+

 

The 2.0 didn't either. I broke the cam belt on an old MK4 2 litre Ghia
and the engine survived the experience.

 

Oh ok, I broke a belt on a 1.6 banger racing - I was told the 2.0l
wouldn't have survived. Not that I can see me ever having another 2.0l
Ford OHC. Ever. Again.

 

This was many years ago when I was more into bangernomics.

The only good thing about those old engines was that they were easy to
work on but apart from that they'd seen their best days long before
the MK4 came out.

 

Um. Guesswork here but gluing a resistor on the outside is the easy
option; heat will conduct through the metal to a certain extent and
the fuel will carry heat to the venturi (or rather, the warmed fuel
won't take so much heat from the venturi). Also, the sides of the
float bowl are fairly flat and easy to stick a power resistor onto;
the top of the carb is a poor shape for easy heat transfer.

Trying to heat the incoming air stream would be physically harder and
might introduce a restriction and the risk that if it broke then bits
of heater wire might get ingested.

 

Umm. Surely that would require a potential divider circuit? If so then
I can't see a heat sink to dissipate the heat from the other half of
the potential divider.

Could that blue box be a relay? There's a diode there (actually there
are two on the PCB). If so then maybe it's some sort of saw-tooth
oscillator that relies on the relay not activating until the current
is high enough. The transistor and one of the spare amps could provide
suitable buffering.

 

http://www.velleman.eu/downloads/0/minikits/manuals/manual_mk138.pdf [1]
has a scematic. It is a relay but my memory of analogue circuits seems to
be shot because I can't give it a name...

[1] from the link posted by Burnt

 

I would've recognised it as a comparator without the feedback loop from the
hysterical resistor. I can't remember any other op amp circuits, though, so
wondered if that configuration had a different name. It's 8 years since I
had anything to do with electronics and most of that was digital stuff.

 

Aah.