"Jittery" just off-idle

[jdosher]

I'm fiddling with jetting on a brand new 2009 EC300 and would like a bit of a sanity check. The bike came with a 178MJ, 45PJ and N1EF needle. Based on how far out I've been turning the air screw I've been working down PJ size and currently have a 38 in it. I also now have a CCK needle in the bike (and a CCL on the bench).

Here's what I'm observing:
If I roll along in 1st gear just barely off-idle the engine runs smoothly, if I then give it just a tad more throttle I'll get rough and "jittery"; I alternately feel power, then engine breaking. This is slow, like 8-10mph. My assumption is that it's four-stroking? If I then give it just a tad more throttle (we're at about "three tads" of throttle now, heh) it cleans up and runs smoothly again. I've tried the trick of running it on the choke and it's blubbery, so I don't think I'm dealing with a lean condition.

With the 38PJ if I turn the air screw out a ways - like 3-1/2 it reduces that jittery stumble. So, I'm thinking that the PJ is still too large, but that really surprises me, hence I'm hoping for some advice so I'm not running this bike too lean on the bottom. I've moved the needle up and down, but it hasn't made any difference (it also did this with the stock needle). The next size smaller PJ I currently have is 35.

Specs:
178MJ
38PJ
CCK-2
stock slide (#7)
FMF Gnarly/TC2
50:1 premix
50F
400ft
RH 80%
29.80 in Hg

Riding is a mix of tight 2nd gear single track and twisty 3rd gear open trails, so I'm hunting for clean jetting on the bottom. I've looked over other people's jetting and I see (at similar conditions to me) a wide range of PJ's, up to 48! I don't see many 35s.

I haven't really addressed MJ and needle clip yet since this is all under 1/8th throttle.

Thanks

-Jesse

 

[Krieg]

Try a 35 pilot at 1.5 turns. My 11' 300 currently has the 35 in it and I'm on the verge of going to a 32.5.

 

[stainlesscycle]

i believe this is a rich slide cutaway issue. i tried going leaner on the cutaway, which removed the issue, but caused other problems. i am just gonna live with it. leaning the pilot removes the issue also, but causes other problems. i'm assuming we have the same issue - at 1/16 or 1/8th throttle, it feels almost like a pv debating on whether to open. it's 'juttery' or a 'stutter' for lack of a better term - kinda just when you're idling along. - right?

 

[jdosher]

i believe this is a rich slide cutaway issue. i tried going leaner on the cutaway, which removed the issue, but caused other problems. i am just gonna live with it. leaning the pilot removes the issue also, but causes other problems. i'm assuming we have the same issue - at 1/16 or 1/8th throttle, it feels almost like a pv debating on whether to open. it's 'juttery' or a 'stutter' for lack of a better term - kinda just when you're idling along. - right?

Yes, exactly. For me, I can roll along slowly (1st is more pronounced, but it's present in 2nd as well) with a tiny amount of throttle and it's okay, if I then give it a tad more throttle - like 1/16th to 1/8th throttle it'll start doing it. A bit more and it goes away. "juttery" and "stutter" describes it well. I'm going to try the 35PJ and if that lessens it, then at least I'll know I'm on the right track. I haven't even gotten this bike out on the trails yet, but in the really tight stuff that idle-1/8th range needs to be buttery smooth'.

The bike is new, so I want a few hours on the engine before I start doing 1/2 to WOT plug chops to dial in the main and needle position. Maybe it'll all clean up when everything is more dialed in. I know the main has nothing to do with 1/16 throttle, but they do all interact ever so slightly across the range.

I wonder how the RB head mod would address this?

 

[fabulous furry]

i have the same problem with my racing 300 09 as a result it fools plugs all the time even with very low jetting :35 Pj / 168 main / cck needle/2% mix / 30deg C / sea level.
the throttle feels lean and when you slow down after running it will rev high until you give it some choke but still the original plug last 2-3 hrs no more .how is your plug situation ?

 

[Bailey28]

In hot weather I don't like the K diameter. I am in the process of installing the CCL I also have. The smaller diameter will clean up the bottom end in the 0-1/4 throttle range. I am at sea level, 80* F temps.