Pinking at high speed with barely open throttle

[J1MS]

Ive read that cover to cover many times, I still have a few notes I copied out of this it was a library book, befor computers and printers/scanners so I hand written a few chapters in my best scrawl...

Its a decent book still today, more so for the scooter tuners today, but its starting to get dated now, nearly all the modern bike engines are all electric with engine management systems starting to be the normal...

[jason frost]

It sounds like the slide to me

[GP Kevo]

What do you mean by "barely open throttle"? Less than 1/8 throttle? 1/8 to 1/4 throttle? Above 1/4 throttle? Do you know what throttle opening your carburettor is at at any given position of your throttle twist grip? Maybe make a throttle index on the twist grip as Sudco suggests, marking out 0, 1/8, 1/4, 1/2, 3/4, and WOT. Then do plug chops to get the jetting right at each throttle opening.

Jason Frost said, "It sounds like the slide to me"

Yes, I agree here. The slide cutaway also has an effect especially between 1/8 and 1/4. Your slide cutaway may be too lean.

You said you lowered the Jet Needle. The Jet Needle doesn't affect much until 1/4 throttle opening with its largest effect above 1/2 throttle with a diminishing effect just past 3/4 throttle. So, by lowering your jet needle, you have leaned the jetting between 1/4 to just above 3/4 throttle. Then you increased the size of the pilot jet which has it's largest affect at 0 throttle with diminishing effect up to 1/4 throttle.

Which Needle Jet are you using? I believe the TM24 has a Needle Jet. The Needle Jet begins to affect running conditions at 1/8 throttle with maximum effect to just above 1/4 throttle then with slowly diminishing effect to nearly 3/4 throttle. In my experience the pilot system has the largest effect on idle and the closing down of the throttle and the results (does it quickly come down to idle or does it want to rev on).

But, Sudco states that

Sudco says in their Mikuni Tuning Manual, "motorcycle carburetor fuel circuits are additive. This means the main system delivers fuel at all throttle settings, therefore the pilot circuit will have some influence on the decisions to be made regarding the main circuit."

They suggest that you first find optimal pilot circuit settings, then proceed to throttle slide, needle jet, jet needle, and main jet.

Sudco: "Work in progression from 0 throttle through full throttle."

I would highly recommend Sudco's Mikuni Tuning Manual.

Also, as has been stated, riding at speed down hill or drafting other cars or trucks then closing down the throttle can have disasterous effects but especially if your jetting is already lean at lower throttle openings. I learned this the hard way (using a Delorto PHBH 30 mm) when slipstreaming a lorry at 70 mph on my Honda 205 and instead of using the brakes to bring down my speed I closed the throttle to 1/4, then below, with the result that the piston siezed and sent me going sideways down a very wet M6 for a couple of seconds then highsiding and sliding next to my scooter for I don't know how far but it seemed like a looong time. Since then I've been very careful about closing down the throttle from high speed and prefer to use my rear brake to bring down the engine speed before lessening the throttle and while doing that I'll pull in the clutch and blip the throttle to get more fuel into the cylinder for some extra cooling.

[ArmandTanzarian]

I wrote to Allens Performance regarding the problem and they said....


Hi there,
That sounds like a tricky one. Possible solutions would be an octane
booster, a larger bore needle jet, a different ignition curve or a slightly
altered needle.
Unfortunately the needle would be a tricky one because Mikuni do not release
dimensional data and they don't have a logical progression of needles. This
kind of trouble is normally sorted by simply changing the diameter of the
needle jet or changing the clip position as you have already tried.
What needle jet do you have in there?

[oldbiker]

Going down long, steep hill stretches at speed with the throttle barely open will result in the motor overheating, as there will be little cooling fuel and therefore not enough lubrication from the two stroke oil to cope with the piston speed.

This sounds right to me, but the thing about being rich when doing normal city riding is a bit iffy ????????????????


edit > as Jason frost says (slide)

[jason frost]

try taking 0.5mm off the bottom of the slide to make it richer.

[oldbiker]

Sorry what carb are you using..

Is there a richer slide??

[sean brady scooters]

try taking 0.5mm off the bottom of the slide to make it richer.

that will make it weaker.......as it lets more air through.......

sorry ,ive just realised what your trying to say here. .5mm off the bottom of slide (as opposed to .5mm off the cut away)........
yes that would i suppose allow the slide to seat lower down and reduce the relative height of the cutaway by .5mm hence riching it up....
but why not just choose a richer slide to start with ....one with a lower cutaway....

[jason frost]

try taking 0.5mm off the bottom of the slide to make it richer.

that will make it weaker.......as it lets more air through.......

I know it friday and I have had afew red wines but a smaller cut away less air.

[sean brady scooters]

yes ,see above post jase .........
i did not understand what you meant at first..............