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 Running OK but low cylinder pressure? 
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Post Running OK but low cylinder pressure?
Maybe some thought input here would help before I tear into things further...

zuma with Athena BBK...

A little while ago, the scoot started to feel like it had less power and speed. Then it started to die out @ idle when at running temps. Seemed to be tough to get it started again after reaching running temps. All these things led me to suspect cylinder head or piston ring.

So I checked the cyl. pressure when cold, and it was low (~max 60psi). Checked it warm after a quick run and it was much worse (~30psi).

After that, it wouldn't even start.


So to troubleshoot, I pulled the head off, coated the metal gasket with Copper Coat Sealant, and re-torqued (10 ft-lb) the head.

Now it starts and runs much better than before, but cyl press is still low (still around 60 psi).


Anyone have any suggestions to check out before I purchase a new head gasket and piston ring? That's all I know to try at this point.


Tue Oct 31, 2017 9:20 am
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Post Re: Running OK but low cylinder pressure?
You have low compression
How many miles on the kit
What reeds
What carb and what is your jetting
What pipe
What plug
What gaskets are you running base and heat what's your squish
Did you ever seize the engine
Did you chunk a reed
There's a hundred things it could need but not having any details other then low compresion it's hard to say what caused it without seeing anything or having any details
If I could do that then I would be a fornicate millionaire

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2008 zuma dual headlight mod koso mirrors liquid cooled polini corsa kit leo vince zx stage 6 variator rally kit custom red paintFUEL INJECTEDsoon to be 100cc
2008 stella olive custom ported 177 cc kit dechromed ford tear drop tail light alive and breathing and awaiting cosmtic surgery thanks to an donkey hind quater hole cager
1985 elite 150 how long can i leave it stock


Tue Oct 31, 2017 10:37 am
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Post Re: Running OK but low cylinder pressure?
Thanks for the reply! Honestly, I don't know answers to many of your questions :( I don't know much of the history on the bike; I am borrowing from a buddy who upgraded and no longer uses this one.

Not sure on the miles, jetting, reeds.

plug is a br8hs
pipe says leo vince on it and zx-r

base gasket i'm not sure of cause I haven't pulled the cylinder yet

head gasket is aluminum 0.45mm
squish appears to be about 1.45-1.55mm with the head gasket (this would be too high right?)

engine has not been seized


Tue Oct 31, 2017 11:35 am
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Post Re: Running OK but low cylinder pressure?
First issue should be a br9hs for that kit an 8 ous to hot
If it's the stock carb and any where near 1000 miles the kit is just wore out
Yes the squish is high. Less base gasket and no head gasket is about the only way to get to the goal

_________________
2008 zuma dual headlight mod koso mirrors liquid cooled polini corsa kit leo vince zx stage 6 variator rally kit custom red paintFUEL INJECTEDsoon to be 100cc
2008 stella olive custom ported 177 cc kit dechromed ford tear drop tail light alive and breathing and awaiting cosmtic surgery thanks to an donkey hind quater hole cager
1985 elite 150 how long can i leave it stock


Tue Oct 31, 2017 2:12 pm
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Post Re: Running OK but low cylinder pressure?
Ok, I'll try the 9 plug when I get it up and going again.

I took the cylinder off and it appears that there may have been a leak in the base gasket... it was torn in one spot and appears to have residue in that area but it could've just torn when i pulled the cylinder off. Base gasket shouldn't account for the low cylinder pressure though right?

ring and piston looked good to me. there is some residue (no scoring that i could tell) on the cylinder wall in the area of the exhaust port. maybe this is where i am getting a leak past the ring?

photos attached

I'm concerned that the exhaust wasn't sealed well too. it has a strange metal washer-like ring in between the pipe and the cylinder. is this all that is there to seal it up? no gasket?

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Tue Oct 31, 2017 2:16 pm
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Post Re: Running OK but low cylinder pressure?
Take pics of the pistion the sides and top
From what I see that bore kit has been seized and is toast
Those up and down marks by the port are what I'm talking about

_________________
2008 zuma dual headlight mod koso mirrors liquid cooled polini corsa kit leo vince zx stage 6 variator rally kit custom red paintFUEL INJECTEDsoon to be 100cc
2008 stella olive custom ported 177 cc kit dechromed ford tear drop tail light alive and breathing and awaiting cosmtic surgery thanks to an donkey hind quater hole cager
1985 elite 150 how long can i leave it stock


Tue Oct 31, 2017 5:57 pm
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Post Re: Running OK but low cylinder pressure?
Ok well i went ahead and ordered a new cylinder kit. Figured it would be easier anyway. Start fresh with new everything.

Thanks for the help!


Wed Nov 01, 2017 5:32 am
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Post Re: Running OK but low cylinder pressure?
Oh yeah... What about the exhaust? Is that ring the only way it seals?


Wed Nov 01, 2017 5:33 am
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Post Re: Running OK but low cylinder pressure?
It depends upon the pipe
A Leo pipe uses a crush washer style gasket (with a new kit you will need a new one)

_________________
2008 zuma dual headlight mod koso mirrors liquid cooled polini corsa kit leo vince zx stage 6 variator rally kit custom red paintFUEL INJECTEDsoon to be 100cc
2008 stella olive custom ported 177 cc kit dechromed ford tear drop tail light alive and breathing and awaiting cosmtic surgery thanks to an donkey hind quater hole cager
1985 elite 150 how long can i leave it stock


Wed Nov 01, 2017 2:41 pm
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Post Re: Running OK but low cylinder pressure?
Rebuilt and beginning break in now.

I did end up going with the thinnest aluminum gasket at the base and no gasket at the head to get the suggested squish of .9mm

I used Permatex High temp Copper gasket spray on head, base gasket

Let's hope everything goes well in break in!


Tue Nov 07, 2017 10:53 am
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Post Re: Running OK but low cylinder pressure?
Break in went well and she is running strong.

I asked my buddy who own the scoot if he had ever re-jetted the carb and he said no.

so I'm still running with stock carb and jets.

He said the advice he was originally given was to keep the stock carb and jets for reliability and fuel efficiency.

I've read many mixed results on here regarding the jets.


It is running great now, so I'm not sure that I want to mess with it?


Thu Nov 09, 2017 10:30 am
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Post Re: Running OK but low cylinder pressure?
Run at your own risk
No doubt about it who ever told your buddy that they don't know their head from their burro
For a long life the athena needs a larger carb at at bear min larger main jet
Read my thread why use the right jets and you will see the yamaha part number for the oem 82 and 84 main jet

You have to remember that b with an air cooled scooter the incoming air and fuel mix is a majority of the cooling for the engine
You now have a higher compression ratio then stock that mea s more heat
And more agressive port timings also meaning it can create more heat and needs more air and fuel (air is no big issue as the air box will handle up to 90 cc)

_________________
2008 zuma dual headlight mod koso mirrors liquid cooled polini corsa kit leo vince zx stage 6 variator rally kit custom red paintFUEL INJECTEDsoon to be 100cc
2008 stella olive custom ported 177 cc kit dechromed ford tear drop tail light alive and breathing and awaiting cosmtic surgery thanks to an donkey hind quater hole cager
1985 elite 150 how long can i leave it stock


Thu Nov 09, 2017 4:02 pm
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