pinking rolling off throttle

[1469leach]

Getting a few seconds of pinking when rolling off the throttle after riding on dual carriageway. After running at around 6000rpm for a few minutes I get pinking when closing down at around 1/4 throttle, it stops after a few seconds but that's to much for me! Set up is as follows-

Rapido225 exhaust port widened to 65% bore width and timing at 183 deg, transfer timing as standard(126 deg)
AF Head, squish 1.45mm,champion n3 plug
30mm PHBH carb, jetting 148 MAIN JET,30 DEG SLIDE,AV 268 NEEDLE JET,X7 NEEDLE,2nd CLIP FROM TOP WITH 1 WASHER UNDER,55 pilot jet,70 choke jet, Mixture screw 2.5 turns out, LTH reed valve
NK rally exhaust
M-TEC BOX 23DGRS@1000RPM-17DGRS@ 6000 RPM/BTDC-setting B

Changed atomiser to 270 and its to rich, if I raise the needle also to rich, bigger pilot makes it a bit cloggy low down, strobed the timing and is as I said above, done a leak down test and its fine. Running out of ideas now any help much appreciated.

cheers Jeff

[Darrell Taylor]

Which af head c.r

ps nice to see the transfers measured correctly

did the leakdown include the lth inlet rubber as found they regularly leak on there limited sealing area

[1469leach]

Darell the head is the one they been using for last few years cop ratio 9.5-1,leak down included all bits of manifold.PS thanks for your previous advise about port timings it takes of nice around 6000 rpm just the slowing down is a problem with this pinking!lol

[soullad]

Jeff, is it due to your auto advance set-up? What revs you doing at 1/4 throttle, and whats the timing then?

[1469leach]

its between 5-6k mate, timing should be around 18-19 deg, as I said earlier its 22deg at 1000rpm retarding back to 17 deg at 6000.

cheers Jeff.

[camel]

you could try knocking the timing back a little more to about 15/16 degrees at 6000k if your concerned
ive a very similar set up to you and running an Agusto 6000 to retard to 15/16 degrees
it knocked it back to this as i thought i was getting pinking at full revs,in the end i put it down to the low revving ads3 hitting the end of its rev range
i have to say to hear pinking for a few seconds throttling off is some going....are you sure its actually pinking and not a rattle coming from something else?...stand?panelwork?

[1469leach]

When it's fast cruising can't here anything but its as you roll of,particularly if you are around the point it is on or off pipe.defo pinking I'm used to recognising the marbles in the cylinder sound.

[soullad]

its between 5-6k mate, timing should be around 18-19 deg, as I said earlier its 22deg at 1000rpm retarding back to 17 deg at 6000.

cheers Jeff.

Ok, its just sometimes the ignition curve isn't linear, and I'm not sure of the MTEC ignition curve.
Would be checking that CR, just to be sure.

[1469leach]

Cheers,spoke to Ben at rayspeed and he reckons that head should be safe.He also said that going much lower would give a significant power loss.Acording to the tech info from chisel speed on the mech the retard line is straight and pretty predictable.it does feel as if the piston momentarily overheats which makes me think the air fuel mixture is a bit to lean to keep the piston cool when rolling off following a good run.

[Darrell Taylor]

i did a long reply earlier but didnt load for some reason?
the cr on that head will be fine

i suggested going for a bigger main jet which may sound strange to cure a part throttle issue but would give a better cooling effect on the longer runs where engine overheating will occur where rolling off will create pinking because the motor is past its heat threshold
overheating is the biggest problem to overcome and can leave u with too rich jetting and a spluttery ride when tackled with fuelling and a low power output/ gutless performance when knocking off compression and ignition advance too much ,the choke kits i do offer a cooling solution that allows cleaner jetting round town with improved economy , then laboured and motorway use the choke can be applied to quench the heat.
also run a 9 or 10 plug ,a tight as poss cowling to fan clearance too to get the best cooling
on my 50-80cc liquid cooled race motors no det occurs at all until temps exceed 65 - 70 c ,i didnt fit a bigger jet instead i fitted a bigger radiator and problem solved
so sometimes u got to go lateral thinking ? have i a jetting/compression/ignition curve problem or have i an overheating problem ,if u cant do much with the overheating id pick a bit of retard on the ignition or curve change