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Info on 155z auto decomp cam

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DHZ_XR8cing

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Good Evening all. I have a quick question. The 155z has a auto de-comp cam.
Is this correct? If so is it a possibility that this play's up and stays on when the bike is running?
 

sean01

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well the newer advertised z160HO (still z155 tho) engine specs list -New camshaft design with anti-kick back
but the older advertised z155 don't..

even the 175's listed on dhz say they have a decomp but i have a revmx 175 and it doesn't..so!!!

im guessing your talking about the newer one..

anyways, no doubt the decomps can act up a bit... just verify your rocker clearances are correct and if they are then inspect the decomp mech, maybe even remove the bits of it if you can and try it for comp..
 

DHZ_XR8cing

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This PSTO I have is a weird one. Some times when it starts it runs rough, back fires when I rev it and is a dog. I hit the kill switch and re start it and it runs smooth. Sometimes it does it 4 or 5 times before it clears. I have tried changing all electrics and it still does it so I was thinking the decomp plays up.
 

themountain

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Anti-kick back mechanism on the cam will have a tab on it to hook up and stop it rotating backwards (in theory)
In reality the anti-kickback mech is made of soft/thin metal and just bends
and stops functioning after a few hits.
Auto decompression cams ,as the name says ,reduce the compression in the starting process for easier starting big cc engines to save them kickstart breakage
 

crazzy #1

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The cam above is anti-kick back, auto decomp holds one or more valves open when a engine is spun over slow, when the engine comes to live centrifugal forces hold this particular lug in place (which is the same od as the cam) and allows to fully close the exhaust valve(s)

Simpleset way I can explain it

Well that's how it works on my Honda anyway
 
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themountain

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The cam above is anti-kick back, auto decomp holds one or more valves open when a engine is spun over slow, when the engine comes to live centrifugal forces hold this particular lug in place (which is the same od as the cam) and allows to fully close the exhaust valve(s)

Simpleset way I can explain it

Well that's how it works on my Honda anyway
+2..exactly as you said..and a 155ho does not have a decomp or you got one of these fancy +Rheads with the cam in the picture ;)
 

sean01

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This PSTO I have is a weird one. Some times when it starts it runs rough, back fires when I rev it and is a dog. I hit the kill switch and re start it and it runs smooth. Sometimes it does it 4 or 5 times before it clears. I have tried changing all electrics and it still does it so I was thinking the decomp plays up.
decomp works off the exhaust valve not the inlet, so it wont really make it back fire or anything like that. would be down on power and maybe slightly pop out the pipe..

running a molkt carb too i presume.. throw it in the bin..

The 155z does have decomp. Im sure as poo about this.
pull the cam cover off and shoot us a pic...
but if its a decomp cam it will be evident, if its not it will just be a cam gear with 2 bolts in it...
 

DHZ_XR8cing

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Its just a cam gear and 2 bolts in it. It went well today at mx central and didnt miss a beat.Got me stuffed what it was that was playing up.
The molkt went in the bin the 1st day the bike was in my shed. What a waste of space them fuel leaking sods are.
 

sean01

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cool, maybe you have a internally troubled carb. strip the carb completely including the emulsion/main jet housing and blow the hell out of i with air..

just be careful not to kill the rubber gasket in the housing..

a dodgy choke or choke circuit can act like this too.
 

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