Jerky gas gas

[Steveyboy]

Hi.  I'm new to trials and have got a gas gas txt racing 225. I'm finding that at low revs and in 1st gear the bike sometimes feels jerky, almost as if I was in too high a gear. Just wondering if anyone knows what could be causing this. I notice the chain could do with tensioning, could that cause it or is the gearing all wrong? It's got 9/48 sprockets.  I've fitted a slow throttle too but find it very responsive despite the slow throttle. Any ideas/help appreciated. Cheers

[lemur]

The bike will be more prone to jerky throttle response in 1st gear, same reason you shift up a range on wet slippery muddy terrain, higher gear range with more clutch control will apply power to the rear wheel more smoothly and with greater range.  and I suspect it is only a 125 and not 225 so there is not much power range to begin with so good clutch and throttle control is imperative. 

[RonH]

Hey! I agree with lemur 100%. I had the same issue with mine, when I first got it, and soon found out that trials bikes are extremely responsive with just lightest touch on the throttle in first gear. It’s intentional to go from 0 to quick very quickly to hop from rock to rock on the back wheel for example. That’s not the style I ride though so I put a flywheel weight, a 1mm head spacer, a slow throttle and changed the front sprocket to an 11 from 9 on it. It tamed it down considerably so I, a novice, can handle the bike easier with better throttle and clutch control. The flywheel weight and front sprocket change had the most effect on mine. Aside from all that, starting out in second gear may help you also to relieve the “jerkiness” out of it. Also, like lemur mentioned, use your clutch to smooth it out.

Also, try reading this thread.

https://www.trialscentral.com/forums/topic/78583-s3-low-compression-head-replacement/

Edited yesterday at 01:39 AM by RonH

[faussy]

The 225s are rather tame. The jerkiness could be a whole number of reasons. Poor carburation, a bad ignition or you could just not be accustomed to the type of throttle response trials bikes provide. Are you coming from a road bike or a mx bike?

Has it got a flywheel weight added? You can tell if there's a 10mm spacer fitted between the casing and the flywheel cover. A flywheel weight is usually added when converting from 125 to 225 and helps to smooth the power out. I remember Kev from GGUK used to run 2 flywheel weights

There should be a few posts on the forum here for 225s. 9/48 is more 125 gearing but is rather low for a 225 or 250. It could be the person put a 225 kit on a 125 and didn't change the gearing. Gearing up may help

Edited 14 hours ago by faussy

[Steveyboy]

45 minutes ago, faussy said:

The 225s are rather tame. The jerkiness could be a whole number of reasons. Poor carburation, a bad ignition or you could just not be accustomed to the type of throttle response trials bikes provide. Are you coming from a road bike or a mx bike?

Has it got a flywheel weight added? You can tell if there's a 10mm spacer fitted between the casing and the flywheel cover. A flywheel weight is usually added when converting from 125 to 225 and helps to smooth the power out. I remember Kev from GGUK used to run 2 flywheel weights

There should be a few posts on the forum here for 225s. 9/48 is more 125 gearing but is rather low for a 225 or 250. It could be the person put a 225 kit on a 125 and didn't change the gearing. Gearing up may help

Had road bikes in the past but new to trials apart from riding my boys sherco 125 around the field. I'll check for the casing spacer. His seems to ride really smooth in 1st gear and low revs. I've not looked at his gearing to see how that's set up. I'm just working may way through this bike. Took the back wheel off last night to sort the rear wheel bearings and shook the swing arm and the dog bone bushes are shot. The front sprocket is well worn too. So I'll get it back together and have a play. At the moment it just feels like when you are in the wrong gear in your car and you get that juddering. Did my first trial at the weekend and felt like I was just on the clutch the whole time I was trying to go slow. I mean that could be normal, I don't know as I'm a complete beginner🤷😂

 

[Tr1AL]

@Steveyboy This is what the specs were when the 225 kit came to market as used by a man referred to by most as Factory Kev who was the head mechanic at what used to be GasGas UK when John Shirt was the importer , he  is still around working on bikes as far as I know you may find him on Facebook??  His name is Kevin Hipwell. he helped design the 225 conversion and used to ride one in competitions himself . 

Anyway here is the basics from my notes at the time:   Gearbox sprocket 10 teeth  Rear sprocket 42 Teeth  Fly wheel weight 1020grams    Jetting on a Dellorto PHBL BS26  is same as a 125cc bike pilot being a 36 but change the main jet from the standard 125 jet which is a 105 to a 110 for the 225cc , If you want to run the bike with a Keihin PWK28 pilot is 45 or 48 and a main jet of 118. That is it really The 225cc must have the heavy 1020g flywheel weight fitted or it will be too responsive perhaps you dont have one fitted as they have been un obtainable brand new for years now , they were interference fit and had to be heated before fitting to the 125cc flywheel and when it cooled it gripped although for better security it was recommended that some spot welds were placed on the join seated area ,although that could cause the  unit to be un balanced which is bad for the motor. As you cannot get the flywheel weight you will have to make or get one made by modifying a 250cc one that are available and produced by a company called S3 , you can find these at the importer JSM Motorcycles @ their web shop www.trialendurodirect.com.  When you have done all the right mods the 225 works great not jerky but very smooth on take up . Best wishes Alan.

Edited 12 hours ago by Tr1AL

[Tr1AL]

@Steveyboy Oh this may be of use to you , if you need a new piston you want to buy the 250cc TXT EDITION version made by S3 as that piston has a 16mm gudgeon pin to fit the shorter length 125cc Con Rod and that EDITION piston is the one that is in the kit when bought new , The 225cc cylinder is just a 250cc PRO cylinder with 5mm cut off the top of it but you cannot use the 250cc piston for the PRO model as it has a 18mm gudgeon pin. 

Edited 12 hours ago by Tr1AL