WOT stumble

D

DaveJ

Gold Level Site Supporter
So I don't know if this has just started or has always been there. I must admit that I wasn't the best rider when I bought my bike but I've improved enough to actually need full throttle. The problem is when I crack the throttle wide open the engine stumbles, I have to back off and let it build rpm's. Even when going 50mph twisting the throttle wide open produces a bad stumble, if you slowly open it up, the bike will speed up. It also seems to have lost its 'hit'. Any suggestions on where to start, I've checked the power valve operation with the engine running, it does open up. I have a 178 main, lowered the needle one clip when I got it and just raised the float a little bit after I had lowered it earlier in the season due to gas leaking out all the time. I always use fresh gas but I've been using regular pump gas. Should I try some higher octane premium fuel?
Throw some stuff out that I should check, this is my first real 2 stroke bike and therefore my first foray into jetting.
Thanks
 
Check the reeds. Make sure they aren't chipped/frayed. Make sure they lay flat and seal against the reed block.

Check for air leaks between the cylinder and the carb and air box. Most do this by spraying carb cleaner or starter fluid around the joints while the engine is running. If the idle speed changes due to sucking in the chemical, you have a leak.
Do so at your own risk and at least wear safety glasses. I've never had a problem but...

There's 2 cheap, easy things to check that you'd want to do anyway.
 
Partially plugged silencer

Might be worth a shot to pull the silencer apart and make sure its not plugged and repack it. Had a couple of bikes sputter on the top end related to my spark arrester getting clogged with packing.
Ryno
 
Dave,
I've been using Shell super only. Runs great, no detonation, no bogging anywhere.
You sure you aren't riding a lean running 4T ?
It does sound like you've got the float a little too low, but many other things could be happening too.
Mike
 
Hi!

I have exactly the same problem with my 07 250. You can read the other thread about "not 100% syn oil in winter..".. Main jet 180, clip in 2nd position (it was in 3rd.. now it's better), air screw 2,5 turns out. With -10 C I tried a 185 but it moved this stumble to almost all the rev range. Right now I get much less spouge out of the silencer and the bike works great except this full trhottle thing. Should I try a 178 jet? Where is this air restrictor if there is somewhere?

Thank you!

Leon
 
I haven't had a chance to try anything yet, apparently nice bike riding weather is also nice 'do crap around the house' weather too.
I don't want to do too many things at once. I think I'll put some fresh gas and a new plug in it first and try that. I do have an NGK plug in it so maybe I'll try a different brand. I also tried going up to a 180 main, that made a big improvement in the midrange power but i still had the WOT stumble. I may try the 178 main and raise the needle after I figure this stumble out. Next step may be to take the carb off, clean it well and check the reeds. I'll keep this updated as I try stuff just incase someone else runs into the same problems.
Oh, and no intake restrictor and no spark arrestor so those are ruled out.
 
gasgasman said:
Change your spark plug.
I've had a rash of bad NGK plugs.
Click to expand...

This was my initial thought or gut feeling also.

I have had that happen, but only with the standard NGK BR8EG. My bike would still run but felt like the jetting on top was way fat. Put a new plug in and it was like a new machine!

I have been using the NGK IRIDIUM IX BR8EIX for a season and these seem to last. I have never had one foul. You are however gonna pay more for them.
 
I think that's the plug that is in it now, the NGK BR8EG. As for paying more, it's only one plug and I'd rather pay an extra $10-15 more than chase engine problems around. First thing I'm going to do is get a better plug, some fresh premium fuel and take it for a spin, that is right after all this new snow melts. It was 15C on Tuesday and all the snow was melted, today it's -7C and woke up to 4" of fresh snow...argh.
 
Also try a non-resistor plug, the bikes run fine with them. Check the HV coil and cap connections. I never fouled a plug on a GG, and only change it once or twice a season.
 
Hi!

Checked the OEM Denso plug and it was quite black with some deposits on the top of the "anchor" (I don't remember the word..). Changed the plug to an NGK B8EG (has much smaller electrode than the Denso), put a 178 jet (one size smaller..) and adjusted the air screw to 2 turns out (it was at 2,5). Needle has been at the 2nd position. Bike runs even better and the WOT stumble is almost gone. Not 100% gone but close. Maybe it needs a bit more time to clear out. The filter needs a bit cleaning too... it's the next thing to fix.

Very soon I will change the silencer packing because the bike has been running rich for 2 months (for three rides I had the needle at 3rd pos and 185 jet... it was awful. The stumble was even at mid range rpms)... Maybe this helps too? What I like now is that even after minutes in tight woods the bike stays responsive and ready to rock whenever I open the throttle.

Thank you all for the help!!

Leon
 
leon: it sounds like your in a cold area. make sure that the fuel you pick,is not a winter fuel,as they oxyegenate them and also add methanol to the fuels up here in canada EH,to help with cold start/freeze ups etc. this could give you some unusual results as far as jetting goes,so keep it in mind. I've been using a brp 8es plug that has a protuded tip, and no it doesn't hit the piston,but it burns cleaner than a lot of the other plugs.
 
So I put in some new premium fuel and an Iridium plug. I pay about $6 for the stock plug and this one cost $14, since it's only one I think I can handle it. The bike does run a bit better but still 'misses' at WOT. The winter gas has got me thinking that I should wait until we start getting summer gas before going any further.
 
Hi!

Yesterday I replaced my filter for the upcoming race and I saw a big restrictor plastic plate in the face of the filter! This explains everything..! There were only two small holes on this plate allowing some air to pass to the carb. The bikes come ready here for street use and even though I got an unrestricted bike, probably the dealer forgot this plate there... At least even at -20 C the bike was rich... There isn't much time to really check the jetting but with the 178 main the bike should be ok for around +10 C. And I should feel a big increase in overall power.

Thank for mentioning something about restriction..!

Leon
 
Throw all your previous jetting notes in the trash. Make sure your not too lean on the needle now, when in doubt err on the rich side.
 
Hi!

The bike worked perfect for 3 days, 950 kms, 7 hrs, 59 minutes, 59 seconds total time of special tests (better than expected..). I just adjusted the air screw at 1,5 turns out (from 2,5). Needle at 2nd pos, main 178.

Crazy race for crazy people..!! Extreme is very tiny word to describe what I experienced there....

Thank you for all the help!!

Leon
 
Hello,
I'm new here today...looking for jetting info to help my Bro with his new '07 EC300. I have a KTM 300XC.

Reading this post, my first thought re this wide open stumble is that the 178 main is too rich to allow the bike to run. I think it is drowning in fuel. The clue is that the larger main makes it worse, and brings the problem down lower in the rev range. One must recall that the main jet affects the entire range, as the needle taper works within the main jet.

I would suggest trying a much smaller main, maybe a 170 or somewhere in that range. I think it might just transform the bike...however it may require a needle position change as well.

My KTM, which uses the 36mm carb, uses a 160 main jet at sea level.

Just food for thought. I may be waaay off, but these are my thoughts.

good luck with it.

Mark
 
Ok. Tested my theory.

Brothers 07 300 EC. Stock 180 mj, needle 3rd from top. 38 pilot(not positive of this)
Would not rev out...blubbered at WOT.

installed 172 main, clip 2nd down from top of needle. Same stock pilot.

Big improvement. 65`, 2500 feet elevation.

cheers
 
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