Gas Gas TXT 280 Pro low power and hard to start

[avas.eduard]

Hello everybody!

I bought a 2005 Gas Gas TXT 280 PRO a few months ago t develop better technique for my enduro riding.
I believe I used for less than 30 hours and at my last ride while riding on the street about 10 km (6 miles) to the spot I wanted to train, I felt a loss of power, the smoke color changed and less smoke came out of the exhaust.
After I pulled over I also noticed that the bike is having difficulties keeping the idle, the RPM was lower. It also didn't rev as it should, it felt very lazy, it had no gut.
I turned the engine off, let it cool down and tried to start it, but after way too many kicks it still refused to start so knowing that something is wrong (it usually started very quick), I took it home.
Few days after that I tried to start it again and after a very tiring number of kicks it started and it had the same behaviour - hard to keep the idle, very low power, very little smoke, different smoke color.

I know I have to start working on it but I don't really have an idea where to start from, why is it doing this, what might be damaged, worn, etc.?

Thanks for the input!

[lineaway]

 It got hot from the ride. The orings in the head might have went or the piston did. I would drain the coolant, remove the head and have a look. Posting pics would be good, too.

[d2w]

I suggest checking the easiest and obvious before wrenching the engine:

- Air flow not impeded? Air box, filter, connecting junctions.

- Fuel available? Flowing from tank to carb? Correct mixture ratio?

- Got spark? Clean or fouled plug?

- Do you have compression? Reduced compression?

- Not that it's directly power/performance related, but do you have sufficient coolant? Is your oil clean (and coolant free)? But running with little/no coolant can eventually toast your engine due to excessive heat.

[section swept]

Well avas.eduard, have you been treating your trials bike like an enduro machine? Sounds to me like you have nipped the engine up and messed the rings, probably stuck in the ring grooves causing compression loss and subsequent poor starting. Lineaways suggestion is probably close to whats happened. D2w lists some sensible checks that you could carry out while you strip the head and barrel (jug) off and expose ? the piston. From your description its the classic 2T seizure, I just hope its not too badly scored the bore.? 

[avas.eduard]

Thank you guys for the input!

@d2w checked the airbox, checked the filter, checked the boot to the carburetor and to the motor, everything is nice and clean; yes, I have fuel, I took the fuel tank off the bike and the cock is working, fuel flows; checked the fuel, it has oil in it, i add 200 ml to a 10 liter bottle, that's a 50:1 mix ratio, I use BelRay H1R; I have good spark, tried with the old spark plug and tried with a new one, it starts identically hard and it has the same behaviour; can't check compression, I don't have the tool that I need, I'll borrow one from a friend and write you again; took the coolant off and I had a good quantity (I think around 500 ml, haven't measured it though) and the cooler works

@lineaway & @section swept  took the cap off the engine, pictures attached; before I took the pictures I noticed that the piston top and the cap inside had a black coating, pretty tick, which I took off using a brass brush, but you can see that some of it still remained - is this normal, what is that coating? I took photos of the cylinder from all the angles that I could, I hope they help; please tell me i I have to go further and take the cylinder off to check the piston and rings.

I would like to add that I water flood the engine several times, it was wet outside and I didn't know why it would stop then I read on the forum that this bike has a problem with water in the airbox and I found the air filter wet so I stopped riding it in wet, but I think the engine has been flooded at least 10 times in 2 rides before I realized where the problem is coming from.

And if it helps, the day what I wrote in the first message happened it was very hot outside, I believe around 30 degrees Celsius and I rode it for about 8 km in 6th gear high RPM, then let it go for a while (to avoid seizure), then high RPM again, etc. I don't know how a piston seizure looks like, but the engine did not stop, it just lost almost all its power, dropped idle RPM, made less smoke and different color.

Thank you again for your help, I have no other trial riders in my area. ?

[lineaway]

 I would have liked to see a pic, before you cleaned it up. From the second picture it looks like the cylinder was leaking into the water jackets. I would pull the cylinder off and have a good look at the piston and rings. With using H1R, chances are your rings are just stuck as Bel Ray products leave a real gummy residue. (I used to get all B.R. products cheap and loved H1R until I saw the insides of my motor.) You will need to remove the rings and clean up the grooves.

 So you will need a base gasket, and orings. Rings if you feel like it. Two things to check while going back together would be the water pump and make sure the fan is working properly. Good chance the fan is not coming on all the time. When they start going bad you can spin it to get it started. Good luck, I`m going riding.

[section swept]

Riding in 6th gear at high rpms is NOT recommended that you shut the throttle and coast, then crack it open again, you will rapidly starve the piston of oil with throttle closed. Even on long downhill sections in trials/practice you should be holding the throttle slightly open to allow fuel and oil mix into the cylinder to lube piston and rings.

The black colour is combustion carbon starting to form, on a re-built or new engine after a short run you would expect to see carbon deposits forming as a sooty damp residue....this would just wipe off. However after a long engine run time this residue will fotm into a flakey substance and gain hardness from being continually heated by the combustion flame front. The term de-carbonising didnt come about by luck, it was a d is necessary to allow your engine to run at its best. The carbon does get down into the ring land (grooves) and helps to build up a seal....one of the reasons for running in an engine!

Carry out all of the checks that lineaway has suggested, plus drain the crankcase and also clean the radiator matrix, those fins look like there is mud in between them.

Your spark plug will appear wet if you start the engine, run it and then shut down as no resl hest will have been transfered to the plug tip area...thats a plug chop exercise...Google it saves me writing??

Then there are the crankshaft oil seals to worry about as if these are worn or damaged they will have an effect upon the way the engine runs overall....the list goes on and not to be disrespectful to you I would  hazard a guess that your Enduro mount is a four stroke and your mechanicing skills are still developing. We all learn something new each day so lets hope you get to the bottom of this engine issue ?

Edited December 16, 2018 by section swept

[d2w]

I suggest that you also verify that the piston is installed in the correct orientation; you want it so that the ring retaining pins (and hence the ring ends) do not pass any port openings in the cylinder (where they might catch). Normally, a piston will have a mark (typically a "V") stamped on the crown, and that's to be oriented towards the exhaust port. I can see an "A" on your piston which corresponds to the size (first oversize), but I don't clearly see the "V". Or maybe the "V" is there amongst the other lines/marks. Just have a look.

I will be obvious when you remove the jug as to whether or not the piston is installed correctly; you want the ring ends towards the rear-half of the jug,

[Ian Whittall]

We had a similar problem , turned out to be a broken woodruff key on the flywheel . This caused the timing to be out , hard to start , would not idle and different smoke out of exhaust .Good luck .