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shipdamite

Ty 175 Knock On Decel?

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hello,

well, my rattly old ty is not rattly anymore, having been rebuilt with a relined cylinder, wossner piston, new rod, big end and mains etc.

the timing is spot on.

the jetting is less spot on (work in progress)

not sure if this is a new thing, or if the bike always did it - but didnt notice because of the bad rattles it made before - but on over run, it makes a sort of 'dongk, dongk, dongk' noise .................. I have read about the 'pipe bang' problem (on usa sites mainly).

is this normal for a ty?

pjme did all the machine work and the bike was put together properly - otherwise its all fine.

thoughts appreciated.

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hello,

well, my rattly old ty is not rattly anymore, having been rebuilt with a relined cylinder, wossner piston, new rod, big end and mains etc.

the timing is spot on.

the jetting is less spot on (work in progress)

not sure if this is a new thing, or if the bike always did it - but didnt notice because of the bad rattles it made before - but on over run, it makes a sort of 'dongk, dongk, dongk' noise .................. I have read about the 'pipe bang' problem (on usa sites mainly).

is this normal for a ty?

pjme did all the machine work and the bike was put together properly - otherwise its all fine.

thoughts appreciated.

Might want to raise the needle one notch and go up a little on the pilot jet for a start.

Jon

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if it's only on overrun when the throttle is fully closed, and continues for as long as the overrun is happening, then changing the jetting will not make any difference.

If it is a transitional effect as the throttle is being closed, then a jetting change (pilot jet size usually) will help

The bong, bong, bong on continuous overrun that some 2 stroke bikes do is usually due to the fuel/air mixture going too weak to burn during overrun. Air ingress will usually be worse on overrun because the crankcase vacuum is higher when the motor is being driven by the rear wheel. It can be a bit distracting.

Four stroke motors are susceptible to exhaust bang if they have air ingress into the exhaust system near the head end of the pipe.

On a TY, the bong, bong is caused by air ingress which causes the air/fuel mixture to go too lean to burn for a number of strokes of the motor, until enough petrol accumulates to get the mixture up the point where it can burn in a single violent stroke (the bong stroke)

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thanks for the info.

I braved the cold shed after w.o.r.k today and cleaned out the carby; not bad, but a few gremlins in the bottom of the float bowl. cleaned out petrol tap filter, drained the 'running in premix' and put in some fresh 50:1 fully synth and super petrol.

ran the same, but when choke pulled out, no more dongk dongk dongk on closed throttle over run - so it is weak!

raised need to clip 5 - no difference, so it must be needing a bigger pilot - all the jetting is bog standard by the way.

the next size up pilot is 27.5 (std is 25) - should that be enough?

thanks again.

Edited by shipdamite

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I've had this issue on my old Yam RD400 roadbike - you can't have too many 2-strokes :-) . It was because I was missing a baffle in the air filter box. I know the TY air filter box is a completely different beast, but it looks like the Yamaha engineers realised the issue on the RD was caused by air pressure pulses in the air box affecting the mixture on the overrun, and engineered the air box accordingly.

I'm not sure how this helps you resolve the issue on your TY, but it may avoid you chasing your tail !!

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I have just remembered that the bike has boyeson reeds fitted - not sure if this would have a bearing on jetting or not (std jetting at the moment)

I have just ordered a 27.5 p.j, so will see what that does.

the haynes tells me that the 125 + 175 have an intake duct and a baffle tube in the air filter - dont know if this applies to both models though. my air filter has neither, although it does have a brand new oem cage and filter (originally it had a bit of mesh and some kitchen wash-up type sponge)

my exhaust is standard (although a p.o had crimped the end of the baffle up! [this is now open])

thanks for your replies :icon_salut:

keep them coming - but hopefully the pilot jet is the key

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Hi, the only time I had a real bad knock on low revs was when my pilot jet got a bit blocked. I have the Boyesens fitted on mine and with the fitting instructions it did say that you might have to go up a couple of jet sizes. I'm currently running on a 30. Like I said the knock was caused by a blocked pilot and even with the Boyesens fitted it still ran ok on a 25 a 27.5 and now the 30. Dont forget to adjust the air mixture screw after fitting new piolt jet.

Regards Andy.

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bit of an update.

a new 27.5 p.j arrived in the post today.

fitted it, and the situation is much improved ................ this might sound odd, but the 'dongk dongk dongk' has lessened and has moved to the end can rather than in the middle part of the pipe.

the a/s is on 1 turn out, so maybe a 30 could be tried.

need to give it a good run at rtc central tomorrow and see what the plug looks like - I can't test at home too well as the 'backyard' is a wee bit small.

also did a few other jobs too - finally feel its going to be a good bike.

thanks for all your input.

p.s if it all goes tits up tomorrow, will let you all know!

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the 27.5 appears to have done the job! running well but the plug is dark brown - will try a B6ES.

Dark brown not bad. Reading plug color is only an approximation in a Trials bike due to variations in throttle and load. If you were running an engine under load and full throttle you would generally look for a milk chocolate color, but even that is only a few of the things you look for when "reading" plugs.

Jon

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I would only go for a hotter plug if you experience plug fouling, otherwise you risk a glowing plug causing pre-ignition and then next your piston has a hole in it... :-(

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Thanks for the added input - I will stick with the 7 and see if it fouls at the first trial for the TY this coming sunday. This is the bike at last weeks shakedown at boghopper 350's place. the bike has all new chassis bearings and bush's and the engine has new almost everything aside from cog's and cases. it looks how it looks, but I know that it is mechanically all there. the tank was what was left after removing bad paint and half a tub of filler. there is a fairly good 'new' tank in the shed, but that will be kept in reserve.

fyi, vinegar and coke will clean the inside of a rusty tank really well.

post-17421-0-72010800-1391625042_thumb.jpg

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Glad you have improved the running of your bike. How do you get on with the forks mounted high in the yokes, looks like the tubes are nearly flush with the yokes. I used have problems with the front washing out on a tight turn with them mounted high. Recommend drop in yokes is 20mm. Thought I would just mention it...sorry if you knew :)

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