Compression – GT125R

[Sam]

Hello everyone I have a 2009 gt125r and just did a compression test my results were front cylinder 131psi rear cylinder 135psi are these results OK thanks for any replys

[♠️ MARCEL]

How was the compression done ?
– Was the bike hot first ?
– Both spark plugs out ?
– Full throttle while cranking for compression?

I will have advise after your next answer 🙂

// Meditation doesn't mean you have to sit still....

[Sam]

• Both spark plugs out bike was cold and throttle fully open

[♠️ MARCEL]

Sam wrote:

• Both spark plugs out bike was cold and throttle fully open

Quote

Try to have the bike running hot for 10mins , i mean hot then wait a few mins before taking both plugs out. Twist full throttle and let us know fella, as compression does change when bike is warmest .  Korea hence forward tells us all to do it when engine is warm/hot if possible

// Meditation doesn't mean you have to sit still....

[Sam]

Got bike running over ten mins nice and warm took bother plugs out  testesed rear cylinder reading of 144 psi increase of 9 from cold reading however when testing front cylinder  psi was 131 the same as the cold test I’d did

[♠️ MARCEL]

Sam wrote:

Got bike running over ten mins nice and warm took bother plugs out testesed rear cylinder reading of 144 psi increase of 9 from cold reading however when testing front cylinder psi was 131 the same as the cold test I’d did

Quote

Okay, thank you for this, according your PSI i’ve converted them to Korea’s language of using KG/CM2
– Rear is about 10 KG/CM2
– Front is 9 KG CM2

OEM limit is 8 or 9KG CM2  (115-130 PSI ish)

My advise for you would be to try and

• Check the valves for tight clearances , as that can affect compression and adjust them.
  Korea uses the “Cam Under Bucket” style tappets with shims (that you replace) hiding under the buckets which the cams hit.
  Ie. If REAR Valve “A” is  exhaust….
  It uses 175 shim but clearance on ice cold  (overnight ice cold) is 0.35mm    (too loose)
  I would replace 175 shim with 160 or 165 shim  , that should just about bring it to the 0.20-0.30 range Korea says for “all exhaust” valves.
  All intake valves are 0.10 – o.20mm max.Optimal for hyosung GT125  (all 125 carb vtwins for that matter) in my experience fixing them is  0.13 intakes and 0.25 exhaust.  They puur like kittens after that and throttle is easy, bike starts easier , the bike seems to have maybe 1 or 3 mph extra each gear before redline.
• Push comes to shove, be prepared to buy new parts for the piston & rings    (as long as the valves are okay and cleaned up, with no signs of a slight bend)
• Replace tensioner (better to go new anyways) and probably a fresh cam chain, it will love you for overhauling it and will give you more good years of running well.
• Head Gaskets (ofcourse)

dont panic at what i said, you’re not exactly at the limit but your engine is getting close to the 8KG CM2 limit on one of the heads.

Hope this helps!

// Meditation doesn't mean you have to sit still....

[Sam]

Thank you so much for your reply and I will definitely take on board what you said very informative and helpful.I will do what you have advised and let you no how it goes thanks again

[Author]

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