2-stroke piston window, rear boost port.

[Lozza]

Need the next leanest needle I don't know the Keihin numbering.

[evo550]

The pilot jet is down from a 68 to a 50.

  What increments are those jets in? If 2's that a massive jump.
How does a cooler piston crown stimulate a jetting change ? My understanding is a optimum fuel to air ratio is around the 14:1(correct me if I'm wrong) this ratio is altered by the amount of fuel being delivered through changing jets. So unless you've had a serious change in atmospheric conditions, why the dramatic jetting change?

[John Orchard]

Yep I agree it is a massive jet change! That's why I've mentioned it.  And the plug is still black !!!!  And the bike starts cold without the choke.  I just think Honda got it so wrong in the first place?

But I have compensated with a richer needle, as mentioned earlier in my ramblings, the big engine, combined with the odd PE38 carb, draws fuel from both the pilot and the needle jet with the twist-grip closed, and up to 1/8th throttle (straight section of the needle).

[Lozza]

More likely  would be increased air speed past the slide. AFR 12:1 makes the best power on a 2T not that AFR is a reliable guide on a two stroke

[John Orchard]

With regards the cooler running crown, my thoughts are to use a hotter plug, hence my drop from the stock 8 to a 7 to keep the plug burned clean.  :-)

[evo550]

More likely  would be increased air speed past the slide. AFR 12:1 makes the best power on a 2T not that AFR is a reliable guide on a two stroke

If its increased air speed wouldn't that require richer brass not leaner?

[sleepy]

John just curious as to the fuel oil ratio you are using. Perhaps a bit more oil in the fuel may be enough of a change to clean up the mixture with the added benifit of cooling the piston even further. Some years ago as an experiment I got a guy with WR490 (I think thats what it was) with a very rich bottom end to go from 50:1 to 20:1. He was very reluctant to try it cause he had been told to much oil would destroy his engine or some such BS.
It made a dramatic improvment to the light throttle running and the transition onto the main. He went back to the 50:1 on the advise of the local bike shop sales idiot.

[John Orchard]

Yes more oil would lean it out more, I am using 28:1 at the moment but the bike's not dribbling splooge from the pipe joins.  I'll try running it as it is for a couple of rides and see what thoughts enter my head  :-)

[Lozza]

More likely  would be increased air speed past the slide. AFR 12:1 makes the best power on a 2T not that AFR is a reliable guide on a two stroke

If its increased air speed wouldn't that require richer brass not leaner?

No higher air speed drags more fuel out of a circuit.

[Brian Watson]

The way I recall it Sleepy ..is yes..the presence of more oil will lean off the mixture.. but the fuel does the cooling .. the oil does the lubricating.. so more oil  will not make the piston any cooler..

[sleepy]

The way I recall it Sleepy ..is yes..the presence of more oil will lean off the mixture.. but the fuel does the cooling .. the oil does the lubricating.. so more oil  will not make the piston any cooler..

Everything that goes in cool will take away some heat but the extra oil means less frictional losses which in turn means less heat. I recall seeing dyno tests years ago on oil ratio/head temp and from it was quite clear that less oil meant higher head temp which would equate to piston crown temps as well.

Although in this case with a 28:1 mix there would only be a small gain as it is already more oil than most run.

[Tim754]

Cutting the iddy biddy little bit of glass to go in the windows a prick... still that's easier than makin the catch to slide it open and closed.

[John Orchard]

Cutting the iddy biddy little bit of glass to go in the windows a prick... still that's easier than makin the catch to slide it open and closed.

Lexan  ;-)

[Brian Watson]

I know it has been done to death a million times on here.. but I can't for the life of me, with modern synth oils , work out why anyone would want to run 28:1.. even on an old banger aircooled engine..40:1 is fine..

[Daryl Jones]

A hotter than standard plug might keep the plug cleaner & firing,
but may not have too much effect on the total combustion process.
Maybe Honda didn’t get that bit of the engine design wrong.
i.e. perhaps they didn’t put in the piston window boost ports for a reason.

The performance aim is to achieve maximum possible cylinder pressure after TDC.
(We are burning fuel and oxygen to heat and expand the Nitrogen in the air.)
If the piston crown is now running much cooler, then the produced pressure will be lowered.

Increasing the compression ratio will produce quicker & hotter combustion, but if this is not desirable,
for mechanical or physical starting issues, you might need to look at the head and upper cylinder finning. 
Perhaps there is now too much, for the boost port cooled piston?

I wouldn’t start chopping fins off (yet) but you could conduct some tests with airflow ‘reduced’ to the head fins
and see what effect that has on combustion chamber temp (use the std. plug) and the resulting power output.

Have fun, DJ