Gasgas Txt Engine Knock

[howie314]

Hi all,

i have a 2001 txt 250 that has a really annoyed engine knock.

Only seems to do it from idle and quick throttle response, but occasionally at consistent high revs too. sound is a very mettalic clunk not consistent just every now and then.

Sounds like detonation or knock but i have tried the following to no avail yet....

I have tried idle jet 30-35 with all needle clip positions.

I have tried 118 and 122 main jets.

new plug, both this and old plug are almost perfect in terms of colour and no deposits.

advancing and retarding timing to limits of travel (seems worse when advanced from stock though)

decoke head and new piston kit, big end checked and ok as are crank mains. If anything noise occurs slightly more often with new piston kit and decoked head.

Flywheel and clutch all ok.

waterpump seems tight andfree of play.

The fault is present in neutral as well as gear so assuming nothing to do with gearbox.

Have exhausted my ideas, can anybody think of anything else i can try????

thanks in advance for your help

[benbeta23]

Possibly Pinking?, mine usually does it a lot, try 125 main jet, 45 pilot, JJH needle with clip in middle position.

see if it stops then, if it doesn't try faffing about with the air screw otherwise.

I get it happening on mine also when you blip the throttle & going up a hillclimb.

From the description you have given us, i think it is very likely to be pinking, but could possibly be something else?

Edited September 15, 2009 by GasGasben

[howie314]

have you tried this arrangement?

thanks for advice by the way.

[benbeta23]

have you tried this arrangement?

thanks for advice by the way.

Hi,

I am currently using this Jetting & it has more a less stopped, my engine is set up completly different though by the factory which was done for Sam Haslam (Team GG UK), & before he sold it to me he messed about with it & made it worse by taking the original carb off, which has made it start pinking.

Edited September 16, 2009 by GasGasben

[howie314]

hi,

took your comments along side answer recieved from gasgas usa guru.

I now have 122 main with stock needle on clip 4 (lowest) still with 35 pilot.

fuel screw 3 1/2 turns out and bike runs with no knock what so ever.

I intend to goto 124/125 main & 40 pilot, on order. I have a 126 main but this is little rich on clip 3.

Seems to be very much a balancing act for this bike, but now i am nearly there and armed with right info i can check settings to hopefully find a happy medium at all throttle positions.

bike is far happier now though and pullsa like it should. i think i was litterally too lean at bottom and full throttle positions, but spend most of time at mid throttle which was about right, hence nice plug.

if i find that happy place i will let you know so you can try yours.

thanks for your help.

[benbeta23]

Glad to help mate, i did try running my 300 of a 34 pilot jet which was in for a while because for some reason with the 45 in, it would eat fuel like no tommorow, anyway i put the 45 back in & it's not eating fuel now, i still aint got rid of the pinking though.

[peterb]

Hi Howie,

I don't think the original problem has really gone away, on the older pre PRO motor's, we have always had to lean off the original main jet on the DellOrto carbs ever since the early 90's. The knocking description is very likely due to a weak mixture and may be due to the inlet manifold area leaking in air, or the inlet rubber may have perished, or the mag side crank case seal has worn. For a 26 PHBL carb on the older 250's, these carb settings work well:

Main 115

Needle D36 2nd notch off bottom

270K or 268K needle jet

33-36 pilot

60 slide

Pilot screw (fuel) out 3.5 to 4 turns

Completely strip the carb and blow out all air passages, check the slide has not worn. Check the float height, float arms should be parallel to the carb body when upside down, there is a measurement for this but I find eyesight is fine to line this up.

Bye, PeterB.

[howie314]

hi peter,

thanks for your advice.

I have previously checked all the areas you speak of. but i will check again.

I am yet to find out what the effect of running 40:1 oil will have on jetting, most likely a small effect but will have some, and i assume means i must richen the air/fuel ratio due to that fuel being replaced by oil.

I have always ran 2t engines on 40:1 and really dont like the idea of 100:1

I also have yet to check consistent running etc, any failure here would indicate air leak as you suggest.

Will keep all informed.

thanks in advance for your help.

[howie314]

Glad to help mate, i did try running my 300 of a 34 pilot jet which was in for a while because for some reason with the 45 in, it would eat fuel like no tommorow, anyway i put the 45 back in & it's not eating fuel now, i still aint got rid of the pinking though.

may sound daft but have tried a real good mess with airscrew, in my case air screw setting is like a switch as screw in between normal running and consistent "pink" on fast throttle response.

cheers