Hi guys can I get some advise please my yz 250 08 seems to be running rich no...

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Hi guys can I get some advise please my yz 250 08 seems to be running rich no matter wot I do iv rebuilt the engine top to bottom with a few mods ect iv tried different jets pilot jets new reed block the lot am I missing something ? your help would be greatly appreciated....

Mods are.....

Barrel has a polished exhaust port along with debured transfer ports Its also been port matched to the crank case...

Carburettor has been polished on the intake only before the throttle slide...

Dep front pipe with pro circuit rear can that has been shortened 3 inches...

V force 3 reed block brand new...

New vertex piston....

Crank bearings engine seals ect all new no air leaks ect

Ignition timing spot on at 0.18mm from tdc used a dial gauge...

New plug air filter ect

Using super unleaded at 32-1

Great compression....

Iv used all different size main jets from 175 to 185 and pilot jet 50 to 52

I'm so stuck and about to give up the bike has loads of torque but falls flat really quick I have no problems with starting it either... can't seem to get it to idle right either....

%d comments
  • I live in the UK and when I bought the bike it had a 175 main jet and 50 pilot jet with the need clip position at the top ( lean ) and the bike is still running rich....

    Would upping the compression ratio make the bike run rich ?

  • Have you examined thoroughly how your reed petals are seated well and your float bowl height? Check also your slide if you run the stock slide.

  • Reeds are brand new float hight is spot on and I'll check the slide tonight....

  • There could be more Dancer.... Better yet Dancer, you might wanna tighten your squish clearance. UK-spec YZ's could have a wider squish clearance than US-spec'd ones (up to 2010 US model YZ's).... Can't believe you still run rich at 175 main mate granting you even live in a colder country, where the weather is almost always "gloomy" skies and cold.

  • Is your air very humid there Dancer Michael (especially those near the sea?)

  • Hi Geoff

    Global-spec YZ's compression ratios:

    8.9 : 1.0 (powervalve open)

    10.6 : 1.0 (closed)

    0.5cc larger head volume than US-spec YZ's

    US-issued YZ's compression ratio (up to 2010 model)

    9.2 : 1.0 (PV open)

    10.9 : 1.0 (PV closed)

    0.5cc lesser head volume

  • my 99 with a stock size wiseco, stock porting was 230 cold..lol

  • Just done a compression test and iv got 200 psi cold engine wide open throttle power valve closed....

  • Right had my bike put on the dyno to get the fueling setup.... we started of with a 175 main jet witch is wot the bike came with and it was way to rich so we went down to 170 then to 165 160 and ended up with a 155 but as we went down a jet size each time the bike started to develop a miss fire but it got worse the lower we went ? I'm so confused with this bike...

    could it be that when the bike had the 175 in and it wasn't reving as high... and the lower we went the bike revved out more but had more of a miss fire... if the bike had to much compression could it blow out the spark ???

  • Yes, you can suffer from spark blow out and also after reading this again have you checked that your powervalve is all assembled correctly and working properly. The powervalve not opening properly just kills the top end.