Need Help with Low Compression :(

[tzwilly]

Hey Everyone,

I've got a 2007 Yamaha Venture GT with 14K miles on it. I just got the sled from a friend and don't know the history of it but it was cheap enough to take a gamble on so here I am. It runs pretty rough so of course I started at the basics and pulled the carbs and thoroughly cleaned them. That didn't solve it.

On to bigger and worse things..... I did a compression check..... low and behold , 30 On first cylinder (closest to chaincase side) , 175 on the other two. I believe from looking at this sled that it was NOT properly maintained by any stretch. Also when pulling the carbs, from what I could see inside the carb boots, the carbon and general yucky stuff is pretty bad.

So, I'm wondering where to go from here? Do I call this a loss and move on, or do I dig further? What could it be? Thoughts?

Thanks!

 

[theCATman]

Pull the valve cover (which will mean draining coolant) pull the plugs, unbolt oil tank to get at crank timing/inspection plug, have a helper put a 12" piece of mechanics wire, or something similar down #1 to feel top of piston (clutch side) slowly roll the engine over forward direction watching the crank timing marks as #1 comes to TDC & see if the cam timing marks (arrows) on the cam sprockets are pointing right at each other, or really really close anyhow.

If not, the tensioner has failed & there's piston to valve damage.



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[IBABAD1]

I would say piston/rings. Seeing how you have the spark plug out this is what I would do. Pull spark plug, install 1 ounce of automatic transmission fluid in cylinder, install compression tester back up and crank over engine. If compression jumps up a lot it would tell me it is piston/ring. If it is piston then put another ounce of atf in it and let it sit over night, maybe you get lucky and the rings free up and you save a lot of money. Just my 2 cents.

 

[maim]

have the carbs been synched recently? that can also cause it to run rough.

 

[BigDogrx1]

have the carbs been synched recently? that can also cause it to run rough.

carbs won't explain 30# compression on 1 cylinder.

 

[BigDogrx1]

I would say piston/rings. Seeing how you have the spark plug out this is what I would do. Pull spark plug, install 1 ounce of automatic transmission fluid in cylinder, install compression tester back up and crank over engine. If compression jumps up a lot it would tell me it is piston/ring. If it is piston then put another ounce of atf in it and let it sit over night, maybe you get lucky and the rings free up and you save a lot of money. Just my 2 cents.

good advise, might have rings stuck, but odd you have good comp on all other cylinders, hope you have easy fix

 

[engineman]

30# of compression needs to be diagnosed first IMHO. If you take off the valve cover then check the valve clearances. Too tight and it could lower compression value, simple re-shim to proper clearance should remedy. Huge excessive clearance indicates a stuck or bent valve (bent especially probable if the cam timing is off).

Best of luck!!

Steve

 

[tzwilly]

Guys just want to thank you for all the help. Stuck open exhaust valve. All set!

Thanks

 

[IBABAD1]

Guys just want to thank you for all the help. Stuck open exhaust valve. All set!

Thanks

Your a lucky guy. At least it is not a piston and you don't have to tear engine block apart.

 

[engineman]

Guys just want to thank you for all the help. Stuck open exhaust valve. All set!

Thanks

Glad you found to root cause of the low compression!!

What was/is/will be the fix? Inquiring minds want to know.....

Steve

 

[tzwilly]

I got lucky enough that I was able to run power tune through the sled. Basically like a de-carb and she came out of it.

Worked really well.

 

[engineman]

I got lucky enough that I was able to run power tune through the sled. Basically like a de-carb and she came out of it.

Worked really well.

Thanks! And congrats again on the cheap fix.....

Steve

 

[theCATman]

So... you're saying it was hanging open due to carbon buildup?

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[Metallicat]

Guys just want to thank you for all the help. Stuck open exhaust valve. All set!

Thanks

How did you diagnose this? Did you end up taking the valve cover off?