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

[harryharls]

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

[lbhbul]

Harry, The mixture is a bit rich but should not be the overheating cause.

I have experienced the hot exhaust only at low speeds and at that time it was a bad plug.

2001 txt 280, these things act totally different than a Bultaco when they foul a plug.

I don't know what speeds you are running but if this continues it will not live. You may well need to

go up on main jet & or raise the needle. Finally put in colder plug. PS the hot oil smell is from the accumulation

of oil in your silencers. Good luck.

Regards Larry

Edited September 14, 2014 by lbhbul

[harryharls]

Thanks for your reply Larry. I had no idea that you got hot/cold variants of plugs! The manual states an NGK BPR5 ES, mine has an NGK BP5 ES, basically the same I think (R is for internal resistor or something I think). Could you suggest a plug that runs cooler? Shouldn't the stock plug be correct though? I'll certainly be trying it with less oil in the fuel and I'll tune up the carb to make sure it's not running lean in the meantime. I'll also order the BPR5 ES plug and try that too unless anyone's got a better suggestion for the plug type.

Ah, just Googled NGK spark plug codes and I see that the number (5) relates to the heat rating (low being hot). Perhaps a 6 then??

Nice tip on the burning oil smell, I must admit I was wondering about the source of the smoking oil.

Many thanks

[lbhbul]

Harry, The best plug for me is the Iridium NGK of equal heat range to which ever you choose.

I seem to remember some recommendation from the SSDT pertaining to road use, running @ speed,

applying the inrichining device. One would tend to think that trials jetting is a little lean.

Best of luck.

Larry

[ben888]

Hi Harry,

It could be that your exhaust needs de-coking, something that should be done as part of a major service, particularly on a trial bike that usually chuggs about, building up carbon.

A partially blocked exhaust will cause excessive pressure, not only restricting gas flow, but also the flow of heat away from the engine.

When used for relatively long periods at higher revs, such as on the road, the carbon in the header can begin to burn inside the exhaust. This would explain the burning oil smell and very hot pipe.

Before changing to a different plug temperature, I would definitely pull the pipe and ensure its nice and clean.

Good luck and let us know how you get on

Regards, Ben.

[harryharls]

Good thinking Ben, that certainly makes sense to me and well worth a try. Looking at the plug, it looks lovely, no untoward signs at all, no fouling, no signs of excessive heat so it could actually be literally just the exhaust that's getting too hot and, in fact, the engine running at an acceptable temp...hence the rad not boiling over! I'll whip the exhaust system off and have a good look.

Larry: Interesting thought on the enriching device. The choke is basically an enriching device, could be interesting to see if running with the choke on helps. But I'll try the exhaust tip first.

Brilliant advice chaps

[harryharls]

Oh btw, any advice on how to clean the system if it is coked up excessively?

[ben888]

Either burn it out if you have a gas torch and air line (see you tube) or soak in caustic soda. Either way is messy but not particularly difficult. Worth re packing the silencer at the same time.

[lbhbul]

Harry, The choke & the enrichining devise do the same. There is not enough room to elaborate on the terminology,

it is simply a richer mixture. Cleaning out the rear muffler & repacking is a good idea, however you might remove it

and give the bike a go and see if this helps, might be to noisy at this point though.

Let us know how it goes.

Larry

[harryharls]

Thanks Ben, I'll have a look for the burning out method. I've found the previously mentioned link to the Scottish Six Day Trial prepping advice and it does indeed suggest using the choke:

14. YOU MUST USE THE CHOKE ON THE ROAD. Keep easing off to the ‘rich’ spot is the best technique. Tip; vary

your speed and throttle when you are on the road. Holding the throttle in a constant place on any 2 stroke is fatal.

The engine will run very weak, detonate and easily seize up.

Always ride a long with a finger hovering the clutch lever. If you feel the engine tighten or if the piston ‘nips’, pull

that clutch lever fast ! If you seize the piston, don’t panic. Let the engine cool down a little and try and push the

kickstart down. If the kickstart is absolutely solid, you are in trouble ! The cylinder needs to be removed and

renewed with a new piston. If the kickstart will push down, you’ve been lucky. Start the bike and carry on, steadily

for a while. The engine will rattle but usually quietens down after a few miles.

http://www.motomerlin.co.uk/blog/wp-content/uploads/2011/03/GG-Pro-SSDT-2011.pdf

I think a combination of all the above (except perhaps the spark plug) should help/solve the problem. I'll keep you posted on the results, could be a week or two though!

[harryharls]

Harry, The choke & the enrichining devise do the same. There is not enough room to elaborate on the terminology,

it is simply a richer mixture. Cleaning out the rear muffler & repacking is a good idea, however you might remove it

and give the bike a go and see if this helps, might be to noisy at this point though.

Let us know how it goes.

Larry

Thanks Larry, I didn't realise a choke was also called an enriching device. I will try it with the packing removed before repacking too

[ben888]

I've read that ssdt exert before but I don't agree with all of it personally. I've been a marine engineer for over 20 years, and worked on many 2 stroke outboard engines (as well as many of my own bikes).

Many 2 stroke engines will happily run at a constant throttle, such as outboard engines, lawn mowers, chain saws, bush cutters, scooters, etc. As long as the carburettor is jetted correctly, with the needle set correctly, holding a constant throttle will not cause a lean mixture situation. Outboard engines spend most of their lives at extended constant throttle, and with the drag of the water, it's the equivalent of constantly going up hill under load. This doesn't cause them to overheat and seize.

In my opinion, trials bikes would be more susceptible to overheat at extended high revs due to their small (light) coolant systems designed to cool a bike that revs occasionally. Also it may be in part due to an exhaust that is tuned to make power at low revs, might be restrictive at higher revs.

However, the advise isn't bad, and I certainly don't want to put down the author, Pulling the choke would assist in cooling a 2stroke engine because the additional fuel itself would help cool the engine. A rich running engine is a cool running engine. But, The engine isn't running weak, it's overheating due to its design characteristics.

All that said, it's still worth making sure your exhaust and carb are in top condition to maximise your on road experience

Good luck, Ben.

Edited September 14, 2014 by ben888

[harryharls]

That's a very good point Ben, some things are so obvious once pointed out!! With the exception of motorcycles, the majority of 2-stroke engine applications require running at constant speed. You're probably right about the restrictive exhaust design but one might at least expect to be able to do a few miles on one of these things, especially considering they provide the high 5th gear which is obviously only intended for such use. Therefore a thorough check and clean of the exhaust system, correct tuning and use of the choke on the road should hopefully be the answer. I just need to get the science technician at school (I'm a teacher) to make up some NaOH solution for me to clear the exhaust sections

[ben888]

You should expect to be able to do a few miles without issue. Although not on the road, my old 97 Beta Techno has been fine around the woods for a few miles at 5th gear speeds

I don't know where in the world you are, but here in the UK, NaOH (caustic soda) is readily available from amazon, very cheap. I prefer this method, but be very careful with skin, eyes, and aluminium bits!!

Edited September 14, 2014 by ben888

[lbhbul]

Harry, I am pretty sure the Gasser has 6 gears.

Larry