[ASK] Ignition Timing VS Carb jetting

[java_scum]

Hello

I have 155cc 2stroke bike (with 8.4CR, avgas fuel). First time use programmable cdi from local product. I have problem with my engine knocking (detonation) start at 6500-rpm (around 17.5deg BTDC) and jetting spec 280 for main-jet.

I try to change main-jet to 340 (60 main-jet increase). knocking gone, but the plug fouling (too rich).

Should I change the ignition-curve to more retard timing-degree or just some jetting problem ???

give my some advice to get best performance.

[flotek]

detonation usually is caused by running low octane fuel .what is your timing advance set at and what fuel and rating is being ran it??? ,what is your squish clearance of the head ? jetting seems fat at 340 though this is a mikuni carb??????????????more info is needed

[java_scum]

I used green avgas-fuel to ran it. 0.9-1mm squish clearance. yes mikuni TMX 38mm carb.

below my advance at rpm - again use green-avgas fuel, main-jet 280 ;

<500 - 2500 rpm at 6 degree
3000 - 6500 rpm at flat 17.5 degree
7000 rpm at 16.5 degree - start slope
7500 rpm at 15.25 degree
8000 rpm at 14 degree
8500 rpm at 12.75 degree
9000 rpm at 11.25 degree
9500 rpm at 10 degree
10000 rpm at 8.5 degree
10500 rpm at 7.5 degree
11000 rpm at 6 degree
11500 - 13000 rpm at 5 degree

I hope this will help

[flotek]

your igition timing curve seems pretty wild for 100LL av gas ...its only rated for 100octane on our scale and your runnig a tight squish too with at some points plus 15 -16 degrees whic h is alot to ask your going to have a nice fat hole in your piston if you dont sto p that ....so either run stright 110 or back those numbers way down why do yo u think yo u need to run such drastic igintion timing advancment?and whos setting this up for you ?

[java_scum]

I get this setup because I don't know what degree should I use.
yesterday I down at 3000-13000rpm decrease more degree 1.5-2degree. seems like light detonation with 280main-jet. and the bike still ran well. and of course the plug fouling too rich.

<500 -2500 = 6deg
4500 = 14deg
5000 = 14.5deg
5500 - 6500 = 15deg
7000 = 14.5deg
7500 = 13.25deg
8000 = 12 deg
8500 = 11deg
9000 = 10deg
9500 = 9deg
10k = 8.75deg
10.5k = 8.5deg
11k = 8.25deg
11.5 - 13k = 8 deg

maybe more retard will give better result.

what peak-advance timing should I use for reference ?? and should I go down for main-jet ?

[flotek]

if your fouling plugs then yes i would drop it dow na size or two ,i would not run more than 10 degrees and run some hiigh test fuel not 100ll try some 110 octane

[java_scum]

not more than 10degree,, at 10 degree BTDC convert to mm BTDC around 0.5mm BTDC. I try to calculate with BELL formula. and 16deg similar to 1.3mm btdc.

Do you mean 10degree peak-advance best for engine with 100-octane fuel??

in this place the 110-octane fuel very hard to find.. the alternative are only green-avgas and methanol with easy to get.

sorry for my english

[java_scum]

Yesterday I back down the timing degree to peak 10deg.
On paddock stand, easier get 11500-12000rpm at WOT.

and back down to 250 for main-jet.

not yet to run,,

thanks to flotek for advice !!

[java_scum]

here is my degree;

<500 - 2500 = 6deg
3000 - 4000 = 10deg
4500 - 5000 = 8.5deg
5500 - 7000 = 9.5deg
7500 = 8.7deg
8000 = 7.9deg
8500 = 7.25deg
9000 = 6.5deg
9500 = 6.1deg
10000 = 5.75deg
10500 = 5.5deg
11000 = 5.25deg
11500 = 5.1deg
12000 = 5.05deg
12500 - 13000 = 5deg