Hi, I've recently purchased a Sherco st 300. Bike went fine first couple rides out on it but then started smoking badly. It was a grey kind of smoke and smelt really bad so I suspected it was burning gear oil. The crank oil seals have been replaced and I've also replaced the head o rings and water pump. The smoke has mostly cleared after a good run out but there's still a puff and the bike feels underpowered. The carbs been cleaned out well and I've adjusted the air screw to suit. For a 300 it feels gutless compared to other makes I've had. I've done a compression test and it's 160psi. The gearing in the bike is still standard with the 44t rear sprocket. Do people keep this standard for clubman trials? Below is a picture of the spark plug after a few hours out it's maybe on the lean side?any help much appreciated

Edited October 22, 20214 yr by Chris87trs

Also the bikes just been serviced spark plug ect and I've also replaced the reeds, replaced the fuel filter and checked the tank breather

 Did you pull the cylinder and check the piston and rings? You could also just pull the header pipe.

I've had the cylinder head off and there was a fair bit of carbon on top of the piston but the cylinder looked good no scores or unusual wear ect. Yea il remove the header pipe tomorrow and get a look at the rings

Is the timing standard ?

Yea timing is standard as far as I know everything looked good in there when I was replacing the crank seals on both sides

59 minutes ago, lineaway said:

 Did you pull the cylinder and check the piston and rings? You could also just pull the header pipe.

Surely the compression test would flag up any problems with rings ect?

This spark plug look too white (learn) for me. Brown ir the gold spot.

Had this spark plug for how long? If you rode few hours with this one, you need bigger jets. 

4 hours ago, cascao said:

This spark plug look too white (learn) for me. Brown ir the gold spot.

Had this spark plug for how long? If you rode few hours with this one, you need bigger jets. 

Yea I thought it looked a little lean but instead of factory 125 main jet I'm running 128 and in the needle circuit I've added a washer under the needle as advised by splatshop and it still seems lean so It must have been crazy lean as standard?

For reference, my ‘16 ST300 plug looks more like this. I need to pull it and check it again soon, this photo was a while ago.

 Of course, mine is on the ‘too rich’ side, or else I have massive carbon deposits in the cylinder somewhere. Once warm I don’t get any noticeable smoke out the exhaust. 

On 10/22/2021 at 6:53 PM, Chris87trs said:

Hi, I've recently purchased a Sherco st 300. Bike went fine first couple rides out on it but then started smoking badly. It was a grey kind of smoke and smelt really bad so I suspected it was burning gear oil. The crank oil seals have been replaced and I've also replaced the head o rings and water pump. The smoke has mostly cleared after a good run out but there's still a puff and the bike feels underpowered. The carbs been cleaned out well and I've adjusted the air screw to suit. For a 300 it feels gutless compared to other makes I've had. I've done a compression test and it's 160psi. The gearing in the bike is still standard with the 44t rear sprocket. Do people keep this standard for clubman trials? Below is a picture of the spark plug after a few hours out it's maybe on the lean side?any help much appreciated

I'd be concerned if my plug was like that, is your air filter well oiled ?

4 hours ago, b40rt said:

I'd be concerned if my plug was like that, is your air filter well oiled ?air filter is brand new and well oiled...

that plug color - if that color was at lower rpms, its too lean. pull the carb, remove the pilot jet and blow it out - do not run wire tru it to clean it.  I have no background about your bike.-  so- this is a wild guess from me. Trials bikes are most times are running low rpms. bigger hand fulls of throttle every now and then.  The muffler- the bigger part of it close to the header connector could have been loaded up with wet black exhaust goop. as lean as that plug looked, if you were hill climbing or just running the bike hard, the smoke perhaps is the mentioned exhaust goop burning off.  -- this happend to me on low hour 2020 scorpa factory 300.  I cleaned carb- air filter- checked plug color on the ally behind house- paper bag color,-- took it for its 1st trail ride - rode 1st- usually 2nd on ocassion.  for approx 3 hours- taking breaks - cutting trail with saw & loppers.- came back to truck- ate food- gassed bike up- hit the good stuff- at least 45 degree hill climb on a old trials bike hill climb. it was epic ! about 2,800 feet constant- no break-  the scorpa was smoking badly - real badly- i have tempeture stickers on raditor & engine jug. 159 degrees. perfect--- I simply cooked all the wet goop out of the lower exhaust area-- the packing was brand new- cleaned -replaced before the ride--  That bike sucks the gas compaired to the beta techno 250.  the scorpa hit reserve about 1/2 mile from the truck on the way back--  i was not reving the bike either-- about 1/4 throttle in 2nd & 3rd. on the hill climb.--  It does not smoke any more after its 1st good pull for about 3/4 hour.---  i bet your bike is the same way- needs the same treatment that my bike needed.  pardon my spelling--- db.   my spelling-- god o mighty-- lol,

Yea it's too lean down at low rpm but I just don't understand why. I've cleaned out the carb pilot circuit many times now 

3 hours ago, Chris87trs said:

Yea it's too lean down at low rpm but I just don't understand why. I've cleaned out the carb pilot circuit many times now 

Eggs sucking here… on the outside of the carb, are you definitely turning the air/fuel screw in the right direction?

Sometimes people forget which side of the carb the screw is on and what it means. You might be adding air/reducing fuel when you think you are adding fuel/reducing air.

 

Mine was running so rich it would foul the plug and bog out. Even leaned out a bit, it has that black plug. I’d turn your air-fuel mix all the way in, counting how many turns you had it at, then report back.