Warm up first?

Originally Posted by ogden

If Nikasil counts as chrome plating for the purpose of the discussion, the RGV has/had it.

2 stroke see ... even more of a need for good quality oil.

[SVGrandad]

Originally Posted by Spokey

General consensus sugests that chrome plated bores tend to be in two strokes ( not any I've come across though ) and as these have the lubricating oil as part of the fuel mix so we assume that this is enough to lubricate the Parts required - and pistons tend to be turned so the grooves in the skirts will carry a certain amount of oil in them during operation ?
Apparently Nikasil is slightly porous so acts like sintering where the oil would stick in the porous parts ...

Spokey

Thanks Spokey - I can imagine two strokes having chromed bores because with all the ports in them there is not a lot of metal left to hold it together and mid bore wear rates can be high. I seem to remember some four stroke bores being "hard chromed" for half their stroke, to avoid the wear step at the top of the bore. Your research much appreciated.

SVGrandad

[Kalessin]

Okay, so I wasn't too far off the mark then. Thanks for all the replies; I wish I'd posted a poll now!

[embee]

On the cold start enrichment theme, as an addition to the good descriptions already given, it's worth appreciating that quite a lot of heat is absorbed from the intake tract/inlet valve to evaporate fuel at normal running temperatures.

With cold starts and carbs, the whole of the intake port between carb and valve is wetted so you tend to need more enrichment than with port fuel injection where the fuel is targetted directly onto the back of the inlet valve and picks up heat sooner after the start. This is just one of the reasons why carbs are so bad for HC emissions.

In the interests of emissions of regulated pollutants (specifically HC and CO), and for combustion stability (cycle to cycle variability), port injection is usually timed to start just after the inlet valve closes to achieve maximum heat transfer and best combustibility of the mixture. Sometimes for acceleration response there might be an extra injection when the inlet is open to match air flow "gulp" when the throttle is snapped open.

Most modern car engines now use sequential injection, where each cylinder is timed in the same way. Early ECUs didn't have the processing capability and used "grouped" injection where two or more cylinders injectors used a common driver. This compromises the injection timing for at least one cylinder and makes the engine rougher running and worsens emissions.

An unavoidable consequence of this evaporation is that fuel vapour (as opposed to atomised liquid) occupies a big volume which would otherwise be air and thus reduces the potential power output of the engine (volumetric efficiency in air terms). Direct injection gasoline engines don't suffer this effect, with the fuel being injected directly into the cylinder rather than the port, and typically achieve several percentage points better operating volumetric efficiency. However they are not without other problems arising from not cooling and washing the backs of inlet valves, and problems with exposing injectors to combustion conditions.

Racing engines with port injection tended to have them way back at the bellmouth so fuel was held in the whole of the port and didn't pick up as much heat so didn't evaporate to the same extent in order to get better volumetric efficiency. Emissions weren't considered.

Note also that pump fuel volatility varies throughout the year, with typically 3 specs for summer/transition/winter. Using summer fuel in winter gives difficult starting due to lower volatility, likewise winter fuel in hot weather gives excessive enrichment and particularly hot fuel handling problems, vapour locking of fuel pumps etc. Volatility is measured as "Reid vapour pressure" or RVP, and there are strict limits in certain markets.

Originally Posted by Lissa

Warm it up a bit. Start it, then put helmet and gloves on. Zip up any fleece pockets that may contain wallets

+1 - Also allows you to not look like a numpty if you put all the kit on and then try and workout:
a). Why the things not starting or
b). Realising you've left the disc lock on and have to try and fanny around removing it while fumbling without being able to see the lock and constantly headbutting the bars or fork stantions...

Not done either of those, of course!