Ty250 Engine Knock

[mike55]

Hello, I'm new to this and I posted this in the wrong place, so i'll try again.

I have just bought an'85 TY250 mono and when the engine is warm either on overrun or light load, you know how they only fire

or pop erratically well each time it "pops" there is a loud metallic knock. Is this normal because I thought something was going to let go!

Any help would be much appreciated,

thanks

Mike.

[tltel]

Hi I dont know much about TY'S , but it sounds a bit like the problem I had with my gasser when I got it, it sounded serious,quite a heavy knock, I changed small end bearing and it was still the same, turned out to be the squish clearance was too tight (practically none existant). Fitted a thicker base gasket and it cured it.

TLTEL

[azzy]

Hi Mike. Before you start any engine work try adjusting the carb as it sounds like its running lean to me. Cheers Azzy

[2stroke4stroke]

Worth trying super unleaded.

[jon v8]

Hi Mike. Before you start any engine work try adjusting the carb as it sounds like its running lean to me. Cheers Azzy

I had the same problem with my TY250R back in 1988 - it too was running too lean,sorting the carb out got rid of the knock,and made it rev/pull like a train and the plug lasted for ages.(Which it didnt before)

[dombush]

Mike

This sounds very much like pre-ignition, where the "clang" is caused by the fuel / air mixture being ignited by something other than the spark plug. This happens too early (similar to massively over advanced ignition)

This "something other" can be a few things:

• something over-hot in the combustion chamber (carbon build-up, wrong grade spark plug, too high compression for fuel, too large a squish clearance etc.)
  
• ignition too advanced, causing over heating of the combustion chamber leading to similar issues to above
  
• Incorrect exhaust / silencer causing back-pressure, leading to similar issues to above. This is very rare, so unlikely.
  
• Running too weak = running too hot.
  

The over-run pre-ignition is not that dangerous, this is becuase the engine isnt under load and because its infrequent. Pre-ignition under load is much worse and can lead to melted / holed pistons.

Be aware that squish is confused with compression, they are inter-related but completely different facets. Some of the suggestions made already are potential solutions. Do the easy things first, suggestions are:

1. Using super unleaded could solve the compression issue.
   
2. Check the ignition is set correctly, check the plug type.
   
3. Clean the carb thoroughly, check jetting vs std spec.
   
4. Check the squish clearance by crushing some soft solder inserted through the spark plug hole.
   
5. Remove the cyl head and barrel. De-coke the cyl head and piston crown.
   
6. De-coke the exhaust port and exhaust header pipe.
   

Dom

[mike55]

Thanks to everyone taking an interest in my problem.

Can I just confirm that 50:1 is the correct fuel/oil ratio? Is there a particular make of oil that suits this engine, I think some are synthetic?

Did I mention that I am a duffer when it comes to 2 strokes, especially when one opinion is that the squish may be too tight and another is that

the squish may be too big, whatever squish is?

I have remembered that the chap I bought it from said that he had recently had the middle exhaust box cut, re-packed and welded back up again.

Could this have a bearing on things?

thanks for your help,

Mike.

[andy.t]

Hi, if it is the pilot jet/air screw mixture is causing the problems then try giving it a bit of choke when the engine knocks and see if it improves the situation. My ty175 knocks like mad when the pilot jet gets a bit of snot in it.

[tltel]

Hi Mike55

just to clarify what my problem was with squish clearance, I too was not used to 2 strokes and had always had 4 stroke bikes, so when I bought my gas gas and took it out for a run I was horrified by the knock when the engine got hot.

After some research and firstly changing the small end bearing as the noise did sound like the top end, I discovered squish clearnce, very basicly its the distance between the top of the piston and the cylinder head. If you check the forums on here its explained how to measure it with a piece of solder wire.

My bike had a very small clearance (cant remember size) so I took it up to max recomended gap think it was 1.5mm by fitting two cylinder base gaskets.

Being a 350cc it also made the bike easier to kick over and start.Two years later it has'nt missed a beat.

Not saying this is your problem but it made me understand the bike a bit more, presumeably you would get a similar symptom if you needed to decoke the top end.

Good luck

TLTEL

[twinshocktosser]

Hi mike55

If you listen to virtually any other Ty mono you will find it will make a similar noise to yours. Some worse some better.

It is difficult to cure but the general advice is to ensure the engine is bored to a tight clearance by a Ty specialist.

The cylinder head then needs some work in a lathe.

The engine is based on a Yamaha yz490. They are renowned for pinking, if you do a Google search you will find details how to alter the cylinder head and stop the pinking.

With regard to squish, a tight clearance (1mm) is beneficial in preventing pinking. However care must be taken not to increase the compression when reducing the squish.

[mike55]

Thanks everyone, I'm learning so much!

Just goes to show that an old fart can still learn. I am quite looking forward to solving this little mystery now (negative into a positive and all that).

As I don't need all the power of this bike would it be an idea to put on an additional base gasket anyway, especially if it makes it easier to kick over. If there is a head gasket, could the same result be had by adding another head gasket?

Please keep your opinions coming,

thanks again,

MIke.

Edited February 7, 2013 by mike55

[old trials fanatic]

IMHO better off adding an extra head gasket as if you add extra base gaskets you also alter the port timing. If you are thinking of following that route though as per my original comments why not double check the piston clearence, rings and little end ? Worth making sure while you are there and it's easy enough to do on a 2 stroke especially.

[plodder]

Hi, i've been working on a TY250 Mono that knocks like yours does. The problems i've found are; The bike was designed with a very wide squish clearance , depending on which piston is fitted it can be as much as 2.4mm which is excessive. There are TY 250 pistons with conventional crowns (Piston top ) & Later pistons with two crown heights which have a lower crown height on either side to bleed the fuel/oil mixture into the cylinder earlier through the transfer ports. The upshot of all this "Jargon " to the layman is that the Squish clearance is to high . If you measure the suish clearance as mentioned in the previous posts & say it comes out at say 2.4mm then the fuel/oil mixture will not totally be burnt causing the "Knock " you mentioned ( if you remove your cylinder head & turn the engine over to TDC look at how near the piston is to the top of the barrel, there will be a soot line left by the carbon deposits ). Some people remove the head gasket ( measures 1mm thick ) & lap the head to the cylinder to close up the squish, but i would advise turning 1mm from the base of the barrel then raising the exhaust port 1mm to get you back to where it should be, The transfer & inlet ports will be o, the optimum squish clearance is 1.1mm to 1.4mm. Also the TY250S road model has a 28mm wide flywheel weight & the TY250R competition model has a 17.5mm wide flywheel so owners of the TY250S can get a more responsive bike by having the flwheel weight machined to the TY250R width. Other things to note are ; always use fresh fuel at 50:1 pre-mix( 100ML to 5 litres of Petrol ) with a good quality synthetic oil , Ignition timing static is 2.8mm BTDC but you can experiment slightly 0.2mm either way won't do any harm. Keep the exhaust clear & a good silencer fitted. The carburettors on these bikes are usually worn out and one of the biggest problems is leaving the bike with fuel in the tank for a while , then the float valve lets the fuel flow into the crankcase & this can lead to all sorts of running issues the next time you use the bike ( unburnt fuel sloshing around in the crankcases ) .

So the number one thing on a TY250 mono is to remove the bash plate & you'll see a 6mm bolt with a fibre or copper washer under it , remove this bolt & see how much fuel pours out !! if it doesn't you can leave some fuel in your tank but personally i'd at least remove the fuel pipe from the tank. I hope this is of help . I'll do another post on the jetting when i've finshed this bike. Regards Plodder.

[mike55]

Hello Plodder, thanks for all your advice, i will try to get my head around it.

In the meantime I have stripped and cleaned the carb and made a note of the settings. The jet sizes seem to be wrong from what seems to be standard.

The carb is a TK26

The main jet is a 135

The other jet is a 38

The bike is a 250mono S, 59n

Are these correct or can any one tell me what they should be and could this be a source of my "knocking" problem?

Also between the main jet and the bit it screws into (marked S10) there is a sort of washer, should it be there, I've never seen one before!

thanks everyone,

MIke.

[plodder]

Hello again, The TY250 59N model you have originally had a restrictive airbox lid & would of had the following jetting as standard :

Tekei Y26P/1 carb

Main Jet 110 , Needle Jet 2610,Jet Needle& clip position 5C95-3, Cutaway Slide 2.5,

Pilot jet 48, Air jet 2.5mm ,Starter Jet 0,7, Airscrew 1 1/2 turns, Float Height 22mm , Spark plug NGK B7ES. Ign. Timing 2.8mm BTDC. Compression 7.6:1

TheTY250R Model number 38V/44J/44H differs in having a free flowing airbox lid & lighter flywheel. Carb specs for this model are: Tekei Y26/P1

Main Jet 140, Needle Jet S-00 , Jet Needle & clip position 5C95-3, Cutaway Slide 2.5,

Pilot Jet 40, Air jet 2.5mm , Starter jet 0,7, Airscrew 2 1/4 turns,Float Height 22mm,

Spark Plug Champion N-5G , Ign. Timing 2.8mm BTDC. Compression 7.8:1

The TY250R(last model ) 2AF 38V-010101 Different Piston to earlier models.

Carb specs for this model Tekei Y26/P1

Main Jet 135, Needle Jet S-10,Jet Needle & clip position 5C99-3, Cutaway Slide 2.5,

Pilot Jet 38, Air Jet 2.5mm , Starter jet 0,7 , Airscrew 1 1/2, Float Height 22mm ,

Spark Plug NGK BR6ES , Ign, Timing 2.8 BTDC , Compression 7.0:1

A few points to take into consideration: It's very easy to chase a jetting problem when the carb is worn, especially the slide, as this could be letting air past it.I put some new jets in & the bike was a lot easier to start.Also it's possible to have the carb settings correct , but be spoofed by a too rich pre-mixture (wrong oil ratio )or old fuel which will both give the symptom of weak carb settings.

Hope this helps your situation, Regards Plodder.