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Yamaha majesty 125 valve timing, low compression (SOLVED)

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liladams1
Two Stroke Sniffer



Joined: 06 Jun 2015
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PostPosted: 22:37 - 20 Mar 2017    Post subject: Yamaha majesty 125 valve timing, low compression (SOLVED) Reply with quote

Hello, wondering if anyone can help?

My scooter has 35K miles bought it with 31K been fine for last 4 months with minor issues
Anyway it had a new carb fitted off ebay when i got it and was working fine with it
Now from one day to another the bike was harder to start, would need to ask someone with my jumper leads and couple of days ago it finally gave up, still sounds like its about to fire up but no

Anyway i never taken apart 4 stroke bike but my flatmate knows bit more than me
We decided to pull the cylinder head but didnt mark timing
Anyway so cleanee valve tops it was dirty had thick carbon deposits and exhaust valve had carbon piece stuck between it eo it didnt close fully cleaned it gave it new head gasket put it together but its the same nearly firing but nope
When i was turning the flywheel before dissasembling it was easy to turn so we thought it has low conpression

But cylinder was spotless and no scrapes bought a compression tester it gave 2.5bars top so what could be wrong?? Did we put timing wrong or i cant think of anything else that would cause this
Spark is good plug is wet

Any help would be great this is my work horse
Its a t reg
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Last edited by liladams1 on 22:57 - 22 Mar 2017; edited 2 times in total
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Paulf
Borekit Bruiser



Joined: 24 Jan 2017
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PostPosted: 22:52 - 20 Mar 2017    Post subject: Reply with quote

Compression seems low to me 6 -10 bar is about right. 100 to 150 lbs per square inch.
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liladams1
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PostPosted: 00:09 - 21 Mar 2017    Post subject: Reply with quote

but what could cause low compression, got new head gasket, valve seats and valves itself looked ok.
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jaffa90
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PostPosted: 00:34 - 21 Mar 2017    Post subject: Reply with quote

T reg 1999, try a wet compression test (after squirting some engine oil in the plug hole).
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liladams1
Two Stroke Sniffer



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PostPosted: 00:36 - 21 Mar 2017    Post subject: Reply with quote

will try tomorrow morning
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liladams1
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PostPosted: 10:08 - 21 Mar 2017    Post subject: Reply with quote

These are the correct marks to se timing?
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Paulf
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PostPosted: 19:50 - 21 Mar 2017    Post subject: Reply with quote

Low compression worn piston ring, valve not closing, valve seats, head gasket, cracked head, worn bore, etc

Compression test warm engine if possible, insert compression tester, open throttle all the way and crank engine. The maximum is reached in 4-7 secs. Compression pressure 14 bar or 199 psi ( figure from honda workshop manual honda fes 125).

Repeat this time with a spoon of oil 55mls if the figures are greater suggest the rings or bore are worn. No change head problems gasket, valves or crack in head.
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jaffa90
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PostPosted: 02:11 - 22 Mar 2017    Post subject: Reply with quote

Paulf wrote:
Low compression worn piston ring, valve not closing, valve seats, head gasket, cracked head, worn bore, etc

Compression test warm engine if possible, insert compression tester, open throttle all the way and crank engine. The maximum is reached in 4-7 secs. Compression pressure 14 bar or 199 psi ( figure from honda workshop manual honda fes 125).

Repeat this time with a spoon of oil 55mls if the figures are greater suggest the rings or bore are worn. No change head problems gasket, valves or crack in head.



Well stated, better than i did.
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liladams1
Two Stroke Sniffer



Joined: 06 Jun 2015
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PostPosted: 22:56 - 22 Mar 2017    Post subject: Reply with quote

Valve was cleaned with dremel it had no big marks and was ok i out a new gasket anyway tried the wet compressiom.test and it shoot to 8 bars but it was cold put it together and its running been using for 2 days now
Think the rings got stuck as i removed the cylinder head and maybe moved yhe barrel slightly

If it goes bad again it will get a new oiston ring set or a piston kit or a whole cylinder kit as aftermarket cylinder kit 80

Thanks for help guys
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