Gas Gas Txt 250 2000 Running Too Hot...fuel Mix?

[sting32]

Just some "what I have learned and read, and experienced over the past 13 years... Trials bikes are p*** poorly able to survive, for long spurts and rides at over half throttle.

the exhaust is tuned to have power and torque at LOW RPM (idle, or tickover)...

at higher RPMS the exhaust doesn't escape as quickly as the MUCH LOUDER, MOTOCROSS/Enduro tuned engines.

notice the first difference you will see on the 2 types of bike's engine areas, the expansion chamber, and the entire exhaust system. Most trials riders have to flip choke on to richen fuel to hope it keeps engine cooler and lubed,

I just don't ride more than half throttle, and vary the throttle from closed to open all that time I ride higher speeds across a loop or what not.

Worst part is, climbin mountainous areas, as in New mexico and Colorado (ute cups), burns the plastic ends of silencers off the newer bikes, some of that could be jetting for altitudes that we aren't as accustomed to, we flat low landers burn these all the time, unless you go slow and easy, or take breaks (let the bike idle more, or lug the engine more instead of high revs).

Lewissport sells a replacement for the plastic ends, made from aluminum, that I WISH TO GOSH ALMIGHTY, gasgas would go back to using... But the rest of the exhaust gets to friggin hot I melted non trials pants, even with guards in most of the places.

on my 1999 321, I recall it melting the airbox/fender and stuff, after my 1st trip a half mile down a road, racing my buddy who was on our old 79 Bultaco Sherpa-T...

good luck. and your mileage could darn well vary...

Edited September 15, 2014 by sting32

[copemech]

Hi Guys,

My first post on here, I'm hoping somebody can help as there is so little information available on the web for Gas Gases of this age. My old (but reliable) year 2000 250 (249) TXT runs absolutely fine generally but I've very recently road registered it and this overheating problem has only reared its head since I've taken it on relatively long runs (only a few miles though!). I noticed that it gets extremely hot on the road, and only at low speeds really. So much so that it actually smokes from the exhaust, not normal 2-stroke smoke, but hot smoke smoke! It smells like burnt oil...you know, bloody hot oil! The whole exhaust system is ridiculously hot, far too hot to touch, even the silencer. So hot that it's heating up the frame (and petrol tank!!) around it. Strangely, the radiator isn't boiling over but the fan is running of course and it must be close to boiling. I'm sure that the water pump is working correctly too, I've inspected the vanes and the circulation of heat seems fine through the pipe and rad. My thoughts are...

I've been running it at a fuel/oil ratio of approx 50:1. After researching it seems that this might be excessively high, perhaps it should be 80 or 100:1 but again, information online is hard to find. Could this alone be the cause of the overheating (causing a lean running condition)?

I've also been using Silkolene 'off-road' 2-stroke oil, I've read somewhere that apparently Gas Gas suggest that Silkolene oil should be avoided, is this true? I've come from enduro on a 2-stroke KTM where 50:1 and Silkolene oil have been absolutely fine.

Any advice anyone?

Many thanks in advance

I find none of this unusual.

The fact that you have likely been plodding around with that mix which may be a bit much, you see, the excess accumulates in the exhaust, then when you actually take it out on a steady run it really starts to get some heat into the exhaust which will start to burn off the accumulation. This is good! Yes the exhaust may get hot, but that has nothing to do with actual engine coolant temps as the heat leaves the motor, at least under light loads. Get it going like this then let it rest, but do it several times so the crap burns/blows out, then you will be fine and your muff will be better off for it.

More brief periods of harder running will have the same effect, and I allways do this just to keep things from gunking up, and so the stuff in there does not light off badly! It will come to FIRE and melt down if you allow it!(providing there is enough fuel to burn in there)

Smoking will subside after it is clean, although you will allways get more heat into the muff on hard running, as trials bikes are designed for more moderate use and the cooling system will not take long runs at high loading, best kept to half throttle or so. Long runs like this are better suited to the :50 mix, but :70-:80 is fine for normal pottering about and only occasional high revving, some run :100 but seldome rev or run hard and still just fine .

[harryharls]

Thanks Sting and Copemech, really good advice. It certainly makes sense actually that it's just burning off accumulations of oil. I think the way forward is to have a look and possibly clean the exhaust system anyway just for peace of mind and then make sure the tuning is correct and stick the choke on when on the road. Brilliant help everyone, thank you so much

[harryharls]

Oh just another thing, has anyone got any thoughts on what I think I've read about Gas Gas suggesting that Silkolene oil should be avoided?

[copemech]

Oh just another thing, has anyone got any thoughts on what I think I've read about Gas Gas suggesting that Silkolene oil should be avoided?

Don't recall hearing that, but you may want to ck with GG UK on it!

[thats_a_five]

I fully agree with Sting32 and Copemech. Your exhaust is plugged with excessive oil and on long pulls that oil heats up and burns off, creating lots of smoke and hot exhaust parts. Remove and clean your front and center sections as described, also disassemble (drill out the rivets) of your silencer, clean it and repack.

I did want to discuss gas and oil as there is often confusion about too rich, too lean, etc.

Oil is mixed with the gas in 2 stokes for lubrication of bearings, pistons and rings. For engines running at high rpm, load for extended times, there needs to be more oil. 35-50:1 is usually recommended. Trials bikes are only run like that on road or trail between sections, and normally only for short periods of time. As such they can run much less oil (80:1 is pretty common) and still be well lubricated. Running more oil will clog the exhaust because there is not enough prolonged heat to burn it off well.

Gas (or petrol for my non-USA friends) is mixed with air to make power. The carburetor provides a certain ratio of gas to air for a good burn and good power. The leaner this mixture, the more power but more heat is generated. Extreme lean running will melt holes in pistons. Extreme rich running will give less power with cooler running, and even fouling of plugs.

Here is where it gets interesting and where many people are confused. When oil and gas are premixed, this changes the lean/rich condition but in the opposite way might be expected. The carburetor actually mixes the liquid to air mixture, not just the gas.

As more oil is added to the mix (50:1), there is less gas in the same volume of liquid. This might be called oil rich and gas lean. This creates more power with hotter combustion temps but lower exhaust temps with resultant exhaust clogging.

With less oil added to the mix (80:1) there is more gas in the same volume of liquid. This might be called gas rich and oil lean.

Changing jets, or adjusting fuel or air screws, only affects the quantity of liquid per volume of air. Adjusting the oil:gas mixture changes what is in the liquid. Sometimes I hear people say "my bike is running too lean so I will increase the oil mixture." Result is the bike is even more lean!

While I am ranting, choke and fuel enrichment are indeed different. The choke restricts the air flow creating more vacuum which sucks more fuel (richer mixture for cold running) but the restricted air flow prevents the motor from revving high. Fuel enrichment open an additional passage for fuel to flow through (richer mixture for cold running) but does not restrict the air flow so the engine can rev high. More fuel lowers power but also lowers combustion chamber temps. For high speed, continuous running of a trials bike (like on road sections), an enrichment circuit is better than putting on a choke. Which one you have depends on the carb that is on your bike.

[huski]

Should be ok at 45 mph on the road in 6th gear check the sprocket sizes a previous owner may have lowered the gearing All 2t trials bikes smoke for a while on the road it's just oil accumulated in the pipe burning off with the pipe getting hotter It may have never been used on the road so there's years worth of oil in there to clear out I don't feel there's anything to worry about

[dadof2]

For extended higher speed running a 5 plug can be a bit hot and could lead to a holed piston, 7 or 8 may be advisable.

Check the plug base and electrodes colour it should be pale / milk chocolate brown, not white or just off white which indicates too weak / too hot. Richen mixture if necessary, raise needle a notch (move the clip down) and go up maybe 5 or so on main jet.

50:1 mix is OK (I use 32:1). I had not heard Gasgas comments about Silkoline but I agree with them and would not use it.

Oil build up in exhaust and subsequent burning out during higher speed running is normal. Use a decent 2T oil and you will not get build up. Shell scooter oil and Exol fully synthetic racing 2T are virtually deposit free.