burnt hole in exhaust - any suggestions why?

[geordabroad]

Hello all

as usual i'm on the forum as I have a problem. Got my 06 TXT Pro 300 working fine, after calming it down with spacers below the cylinder and a spacer on top, slow action throttle etc. I was finding it a complete handful, as I've never ridden trials before, but with those changes it calmed down a lot.

So I've been doing a few hours each weekend in the hills near Beijing. All good except for the following:

- spark plug very oily and sometimes means bike stops and needs replacement clean plug.

- side panel started smoking and melting next to exhaust - very hot exhaust, but fan wasn't on all the time so engine seemed to be running at OK temperature.

then I was out last Saturday and engine noise changed and I realised that I'd blown a hole about an inch across in the middle section of the exhaust on the outside right in the middle where it bends. Directly inbetween the 2 mounting points. It had been so hot that the plastic washer on one of the mounting bolts had melted and some of my gasgas stickers were burning and melting, as was part of the side panel.

I've taken the exhaust apart... it's very sooty and where the manifold attaches to the cylinder it's sticky black oily. The frame is coated in soot next to where the hole blew through.

So my question is - what would cause this to happen? Is it lowered compression because of the spacers that firstly causes not enough oil to be burnt (I run 50:1) and Secondly, that this causes temperatures on the exhaust to rise? Is it just that my carb is running too rich?

Finally, is it possible to weld patches on that middle section of exhaust, or do I need to get a complete new middle section?

That's it... any advice gratefully received.

Cheers

Ian

[b40rt]

What's the ambient temperature, and are you riding it on the road fast ?

[cvgmmartin]

Is putting a spacer below the cylinder a recommended way to soften the engine and how thick a spacer did you install. I can understand the spacer on top to reduce the compression, but a spacer below would change the port timing, eg the exhaust port would open earlier and close later! Is this a proven method for this engine?

[chewy]

50 to 1 would actually make the air to petrol ratio weak (nfinitesmally), two strokes have two compessions , one under the piston and one abve it, if it was running too hot the plug wouldn't be sooty nor the front exhaust pipe, weld repair is feasable but essential no oil to get into the weld puddle, it seems to me that all trials bikes made today are desighned to be light ( and made to a price!) the "cost" of this is going to be more faliures of components sooner than we would expect/hope for. Does it ever kick back ..it may be a bit retarded which would dum it down but make it run hotter and burn fuel less efficiently. Also our opionions from here are based on our fuel "quality" experience it is quite probably very different in China. I'm sure this will stimulate some discussion of your problem ...good luck

[laser1]

IMO you are running too much oil - and that oil is gumming up the works. It sounds like the oil residue in the midbox got hot enough to catch fire and melt it. Does it smoke after a hard run down the road? As far as the mid box, Have a welder put a patch on it - weld may look poor as above mention of contamination, if not cleaned well. Should be OK if u take precautions. FWIW: 80 to 100:1 is typical. (w/a good synthetic oil)

[arnoux]

Big chance you have blown a crank seal and you have transmission oil leaking into the crankcase, hence the very oily conditions and the big temps created in the exhaust system.

[geordabroad]

Thanks everyone for your replies.

I'm not feeling that the engine is running hot, as the fan was kicking on and then off and the plug was oily, pipes sooty etc.

I agree with the point about the spacers on the bottom of the cylinder and the retarded timing... I have about 0.24 inch of gaskets and tried a retarded ignition to calm the bike down, but it was running so badly I moved the ignition back to more or less the original position. It still only really pulls smoothly at full open throttle and very low revs, the rest is quite stuttery... but it'as ok for me and means it's pretty calm while i learn to ride it properly.

What I'm going to do:

1. I'm contemplating taking 1 or 2 of those spacers out, but I think first I will screw out the mixture screw on the Keihin carb and try and get the bike running without so much oil in the mix...

2. so I'm going to try 1:70 as I have a nice synthetic oil to use. I would like a nice colour on the spark plug

3. I'm looking to see if I can get some aluminium welding done, but as yet, not too much luck. Lots of 'chop shops' for tatty bikes, but I need a specialist welding place. I'm also contemplating buying the mid section new, as it will have new wadding (or whatever you call it) inside, and I'll be in the UK in a couple of weeks, so able to pick it up. I didn't see oil in the mid-section, much more sooty than oily, so I'm not sure whether it was on fire.

One theory could be that the mid exhaust section was just slowly failing, and the heat on the side panel etc. was evidence of the aluminium slowly disintegrating...

What I'd hate to do is spend money on a new section for it to happen again because there is a cause I have not addressed.

Please feel free to comment on my suggestions or offer other advice.

By the way, what's the easiest way to take the cylinder head off without spraying coolant everywhere? I have never managed to do this in a controlled way.

Cheers

Ian

[laser1]

Did you say .24 INCHES of Gaskets? Did you mean MM? I hope so. If its really inches - I would pull them and limit it to 1 extra base gasket. +1mm.

How did you change the ignition timing? Not easy to do on a Pro.

Most of the time, holes in a AL mid box are caused by fires. Just because your fan is kicking off, doesnt mean the exhaust temp in the mid box is OK. Usually, the tell tale sign of this is that the bike continues to smoke even after its shut off.

[geordabroad]

Hi Laser1

you're right to query the .24 inch... it was actually 2.4mm of base gasket.

re: the timing... I slotted the holes for the flywheel pickup and moved it anticlockwise.

Anyway, you say the engine might be running OK but the mid box could be too hot. What would cause that? all the messing about I've done to calm the bike down with the gaskets below and above the cylinder? If so I might remove a few and see how it behaves.

Any advice on how to remove coolant without getting it all over the place?

Cheers

Ian

[sting32]

Hi Laser1

you're right to query the .24 inch... it was actually 2.4mm of base gasket.

re: the timing... I slotted the holes for the flywheel pickup and moved it anticlockwise.

Anyway, you say the engine might be running OK but the mid box could be too hot. What would cause that? all the messing about I've done to calm the bike down with the gaskets below and above the cylinder? If so I might remove a few and see how it behaves.

Any advice on how to remove coolant without getting it all over the place?

Cheers

Ian

Base gasket cant be 2. something mm. we're talking tenths .x and or hundredths .0x of mm in thicknesses of available gasktes.

AND, JUST FYI! the ENGINE was assembled in factory with a "standard thickness base gasket" then the clearances on piston to head is measured, then gasket thickness is determined to make sure minimum clearances exist, sometimes less thick gasket is used or thicker gaskets is used. FWIW, mine had 2 gaskets from factory I think total .08 thickness. I doubled that for my 06 300 pro, loved it. I didnt do that stator adjustment nor head spacer.

Coolant drain, remove radiator cap, remove brass part showing at waterpump housing, (uses not an allen wrench, but a Star wrench, BTW this part is also the bearing for the impeller shaft, it has a tiny thin thrust washer on it, watch to see where it ends up. refer to gasgas.com website with vids on youtube about the waterpump so you might now what you have lost or not. you never get all the water out, but I have removed the hoses then blew air in to head holes (make sure radiator cannot be connected via hoses at all!!!!) to help clear more of it.

I think your muffler could been cracked at mounting points, I know I had to have mine welded up and built up a bit on the 300, especially as much as we fall down practicing. that silencer can have a lot of leverage on muffler, you know.

hole in muffler, then fast rides down trails with excessive oil getting spewn then lit with hot-high rpm exhaust temps, plus outside air, it would be like a chiminea http://en.wikipedia.org/wiki/Chimenea

if you race down a trail at high RPM, best to let bike cool itself (especially exhaust) by idleing (or riding like trials) for a minute, IMHO.

Edited March 16, 2011 by Sting32

[laser1]

The std base gaskets are available in a number of thicknesses - 1.0mm and less. So it sounds like you have 3 in there. The OEM ones I have pulled have been in the 0.8mm to 1mm range. Maybe Jon (JSE) will explain what this is probably doing in regards to your issues. My own experience has been that novice riders using 50:1 tend to need to clean out there exhausts on a regular basis to retain proper performance. Maybe the altered port timing is making this even worse. I do know several guys who added a base gasket to there bikes to smooth it down, but my dad has always preferred the head spacer route with customers. Personally, I would clean out the exhaust port, head pipe and mid box when you have it all apart to patch it (Check the tabs for cracks as mentioned above) and service the silencer with fresh packing material. Run it at 80:1 to 100:1 for trials riding. More oil ONLY if you do alot of road work. Check the jetting and take it from there. Sting's draining method is spot on. (I always forget to tell folks about the thrust washer and its easy to miss +1!)

Do you know how many degrees you changed the timing? I wouldnt think it can be more than a couple degrees max, so shouldnt be a problem. You do need to keep an eye on the GAP of the sensor and FW. Too close, will rub when hot. To far, weak signal. Dont know the target number off the top of my head. Do a search on here if you can. (Sting - do you know what the nominal is?)

[richt]

Have you considered fitting a flywheel weight?

I used one when I was learning and found it quite effective.

[geordabroad]

Thanks everyone for the support. Have found alu welder and exhaust is in for patching. Mounting tabs look fine.

Will clean exhaust, repack end can, turn out air screw to lean the mix a bit, as well as move to a 70:1 80:1 mix and see how the spark plug reads after a good run.

If plug is looking better then I may think about removing both or one of the bottom top gaskets and seeing how the bike carburates. But want to do things in stages so I can tell how each thing affects the bike.

Cheers... no doubt you'll hear more from me if I run into further difficulties!

Ian

[arnoux]

keep a very close eye on your transmission oil levels, as I still feel you could have a crank oil seal that is faulty.

[baldilocks]

This is just theory but

in my 280 and 300 pros I fitted a 1mm base gasket as this more reliable and calms down the power delivery, even going 0.8 to 1mm you can tell the difference. 1mm is more reliable as the bikes often come with a 0.5mm base gasket which the bigger engines can blow at the edge of the cutaway for the transfer port.

Now I suspect that 2.4mm of base gasket is way to much and i'm wondering if this also has the effect of making the engine run hotter as you should have more unburnt fuel. As the combustion area at TDC is now compartively much bigger the fuel wont be as compressed when it ignites perhaps leaving more unburnt?

retarding the timing also makes an engine run hotter. The combination of these two may well be your issue.

A 300 is quick but I found with mine that it was actually harder to ride with a low compression head insert.So you may find a 1mm gasket, slow throttle and a flywheel weight gets you there. Lots of VERY good riders run a flywheel weight and my theory was if they need one I'm only kidding myself if I think I dont.

Let us know how you get on.