Flat spots on very low throttle.

[goose202]

Hi all,

Just hoping for confirmation that I haven't missed anything here before I start chasing my tail.

My stock TRX has been running fine, but I decided to freshen the Mikunis with the Keyster kit since I had around, before balancing the carbs.
I didn't use the emulsion tubes or needles because there are aftermarket items in of some description and they look fine. The pilot and main jets are also not stock.
Float levels are correct.
It is running one of cobba's ignitions with the out of the box TDM/TRX curve, apart from an extra degree of timing in the low throttle, <5k RPM areas.

However now there's a bit of a flat spot at about 2500-3000rpm at very low throttle openings (say 5-15%) as you get back on the throttle after engine braking. Clears instantly with more throttle. Kicking it down a gear and bringing the revs up to about 4000 makes it unnoticeable.
It feels like lean to me, but pulling the choke on slightly doesn't fix it, it makes it worse, suggesting to me it's actually rich.

Since it didn't do it before and I didn't change any jets or needle positions, I'm thinking maybe it just needs the pilot screw wound in a bit? I did set the new pilot screw from the kit at the same turns as the old (2 or 2.5 turns out, I forget now).
It idles fine and pulls well throughout the whole range otherwise.
Anything I've missed/am misguided on?

Thanks,
Bruce.

[BigAl-TC]

Emulsion tubes. Classic symptoms.
Your welcome

[goose202]

I'd agree, but the symptoms weren't there before I had the carbs apart. I can't have worn them out just by re-assembling them, right?

[bagmaker]

Ya too rich down low.
Will be worse in the hot weather, not as noticable when motor cold?

[cobbadiggabuddyblooo]

The only thing you changed is the pilot screw so start there.
Just remember when turning the screw it's not a linear adjustment as such.
1/2 turn from 2.5 at present will has a lesser effect than if you where at 1.5 turns out as the taper on the needle gets closer to the seat..

[dandywarhol]

did you turn on the TPS mode in the Ignitech software?

[goose202]

Ok, well it doesn't sound like I've missed anything obvious, so I'll start with the pilot screw and see how it goes.

Dandy, nothing's changed there. I've been running the Ignitech for sometime, TPS enabled.

Thanks guys.

[goose202]

There isn't a trick to getting to the pilot screw without lifting the carbies up out of the boots, is there?

[Rod.s]

There isn't a trick to getting to the pilot screw without lifting the carbies up out of the boots, is there?

http://www.ebay.com/itm/Motorcycle-Tool ... 1085453329

[goose202]

Now that is a cool thing!

[dandywarhol]

Did you check the float levels?

[goose202]

Yep (I did mention that at the start...)

I've wound in the pilot screws about half a turn. Haven't ridden it again yet to see what that did.

[goose202]

So the pilot screw at two turns in made things worse, it felt just the same as when the needle was too low in the past.
The choke did clear it up a bit.
Resetting the pilot screw to 3 turns out seems to have fixed it, although 3 1/4 might be spot on in this colder weather right now.
I wouldn't have believed from previous experience on other bikes that a single turn could make such a difference.

It hadn't occurred to me until thinking about what had fixed it that the pilot screw on these bikes works opposite to most carbies. Usually more turns out means a leaner mixture since it then bleeds more air into the metering circuits, but on these it's actually a fuel flow control screw.

[dandywarhol]

Generally, if a mixture screw is below the fuel level then it's a fuel control and turning it out richens the mixture. A screw higher than the fuel level is the opposite.
I'm aware you checked the fuel level but it is often overlooked as to how critical this can be and also how it can be used to fine tune the fuelling. Even s few mm make a difference, the lower the level, the harder the engine has to work to draw the fuel in, making it run week - and vice versa

[goose202]

I do agree, dandy. I did double check the float level before adjusting the screws for that reason. No point trying to fine tune if you don't have the basics right.