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Old 10-15-2009, 09:50 PM   #1
jak
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Default Valve problem

Purchased a Yamaha Gas cart earlier this week. Not really sure of the model. It’s either a G2 or G9. Someone at one time has removed the model and serial number plate. (At least I can’t find it.) The cart from my minimal knowledge has characteristics of both models. What I’m hoping is that one of you experts can give your opinion on a couple of questions I have. I bought the cart not running from a fellow that bought it not running and couldn’t get it running. One of the first things I did was check the compression with a compression gauge and got no reading at all. That was odd since the engine appeared to have compression when I turned it over by hand using the clutch. I pulled the valve cover and found that the intake valve was not opening, The push rod had no movement. The exhaust valve appeared to be operating normally. I dropped the engine cradle and removed the engine from the cradle. Upon further examination I found the rocker arm support to be badly worn on both ends where the rocker shaft goes through. The push rod does not appear to be bent. Here are my questions.

1. Do you think I should disassemble the engine and check the lifter and cam or just replace the rocker arm support? When I set the push rod on the lifter and turn the engine over by hand with the rocker arm assembly off the engine, the push rod goes up and down normally. I would hate to go through all the work of reinstalling the engine and cradle only to find that I also have a bad lifter or cam. This leads to the next question.

2. Assuming I just replace the rocker arm support. Can I turn the engine over to test compression with the engine out of the cart using the starter by applying 12 volts to the positive terminal on the starter and ground to the ground terminal with out doing any damage to the started? I know from reading several posts on the forum that Yamaha uses a floating ground and that the entire electrical system is finicky and can be easily damaged.

3. Since I have the engine out of the cart and have no way of knowing how many hours of use it has, would this be a good time to consider having the cylinder bored and installing a new piston and rings?
Any input on any of the above would be greatly appreciated.
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Old 10-15-2009, 10:08 PM   #2
sho305
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Default Re: Valve problem

There should be the serial behind the clutch at the bottom of the cylinder fins to ID the motor. On my stock cart it is the same as the cart serial.

I'd say it is a good time to rebuild, but sure would be nice to know it needs it by testing compression. Maybe someone will chime in about the lifter issue. Is the plug full of carbon from oil, that would be a sign.
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Old 10-15-2009, 10:16 PM   #3
Flintrock
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Default Re: Valve problem

Since it sounds like you will need the rocker arm support no matter what, why not start there and then do the compression check. I see no reason a short bench test on just the motor and S/G should adversely affect the rest of the electrical system. ...
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Old 10-16-2009, 08:04 AM   #4
crash12888
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Default Re: Valve problem

Welcome Jak, A visual difference between the G2 and G9 is the front cowl trim that runs up one side, across the top of the cowl and back down the other side
The G2 will have a aluminum trim piece with a black stripe along the center of it. The G9 trim piece will be covered in black rubber.
G2 uses tubular A arms while the G9 uses a stamped, more square looking A arm.
Both very solid carts. This is my wifes 20yr old G2, never misses a beat

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Old 10-16-2009, 02:01 PM   #5
JnB
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Default Re: Valve problem

Hey jak, My 87 G2 serial number was on the back cross member. Had to wipe it down to see it. Here's a picture. Hope this helps...
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Old 10-16-2009, 05:20 PM   #6
smallblock450sl
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Default Re: Valve problem

Did you try to push the intake valve down ? Could be carbon bound or bent. You say the pushrods move up and down when manualy turning the engine, so I don't think a cam or lifter problem. See if you can move that intake valve. Remember, when taking a compression test, you need to have the engine cranking over with a strong battery and the carberator must be at wide open throttle position. Good luck and let us know your findings. Dave
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Old 10-16-2009, 10:26 PM   #7
jak
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Default Re: Valve problem

Thanks to all who have responded to my questions so far.

Sho-- I found the serial number on the block where you said it would be. JG5 09XXX. That would make the cart a 1991 G9 I believe.

With regard to a rebuild, the plug is not carboned up. I pulled the head today to see what the bore and valves looked like. To my surprise the cylinder walls looks great with minimal to no visual wear. There’s no scoring and no ridge at the top of the cylinder. I’ll take the block with the piston in it to a machine shop and have them check it. The valves look good as well. You can still see machine marks on the valve face and seats.

Flintrock__ I agree with replacing the rocker arm support and doing a compression test based on the condition of the cylinder as mentioned above.

Crash___ Based on your explanation of the difference between a G2 and G9 and the engine serial number I found on the block per sho’s input I’m pretty sure I have a G9. The cart has the black rubber trim across the top of the front cowl and the stamped A arms.

Good looking G2 your wife has there by the way. The silver looks great on it.

JnB___ I looked on the back cross member per your recommendation but no serial number. I think we may have cracked it with the info from crash and sho. Thanks for the help.

Dave____ Let me clarify. When I first removed the valve cover and manually turned the engine the exhaust valve operated normally but the intake valve would not move. It was in the closed not open position and the push rod was not moving. It was only after I removed the rocker arm support that the intake push rod would travel up and down when I turned the engine. The intake valve moves freely in the guide. That’s what concerned me that either the lifter or the cam may be damaged. I attached a picture on the extent of wear on the rocker arm support. Do you know if this is common for this engine?

I have another question. As mentioned above, I pulled the head to examine the cylinder wall and valves. The cylinder visually looks in great condition with no wear ridge at the top. I attached a picture of the top of the piston. Can someone tell me if this is a standard size Yamaha piston on not larger rebore piston? I can not see any marking indicating a rebore.

Thanks, John
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Old 10-16-2009, 11:28 PM   #8
crash12888
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Default Re: Valve problem

Jak, I have read or heard Yamaha uses a higher than standard percent of nickel in their cylinders so unless it has seen alot of low oil abuse i'm not surprised to hear it still has the cross hone marks in it.
Unless there is an oversize stamped in the top the only way to tell is mic it.
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Old 10-17-2009, 03:47 PM   #9
jak
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Default Re: Valve problem

Assuming the attached picture works this time, as can be see, there's considerable wear (about 1/8 inch) on the rocker arm support. Has anyone seen this before? Is it normal? Anything else I should be concerned about with this amount of wear? The rocker shaft has no abnormal wear and the rocker arms show no excessive wear on the shaft. Nonetheless, I'll replace the shaft along with the rocker arm support.
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Old 10-17-2009, 04:26 PM   #10
Monsterezgokid
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Default Re: Valve problem

Quote:
Originally Posted by jak View Post
Assuming the attached picture works this time, as can be see, there's considerable wear (about 1/8 inch) on the rocker arm support. Has anyone seen this before? Is it normal? Anything else I should be concerned about with this amount of wear? The rocker shaft has no abnormal wear and the rocker arms show no excessive wear on the shaft. Nonetheless, I'll replace the shaft along with the rocker arm support.
WOW
Quote:
Originally Posted by jak View Post
Has anyone seen this before?.
NO

Quote:
Originally Posted by jak View Post
Is it normal?
NO

Seeing that leads me to believe that there is a major issue with the top end oil feed tube that goes to the head. There could be something pluged. either way, ill post a pic of a good rocker support

In my opinon, the top end oil dlivery system on the early yamaha engines were very poorly designed, and didnt feed enough oil to the top due to the way it worked with the splash system. If you ever take one of those engines apart, youll see what i mean. it was just a small hole in the top of the crank case where the hose hooks up, and the oil thats splashed on the inside of the case was supposed to travel slowly up the tube. but it was iffy.

But they are a good engine.

Also you asked about doing rings and such.. If it has never been done, then yes, i would say that you should get it done soon, give it about another 2-3 years of moderate use and she will slowly start to smoke. if its alreay been done, then you should be good to go for a while The reason that i say this is because for some reason ive seen alot of g9's recently needing a rebuild because of shot oil ring, older g2's on the other hand not as much. Now i know many will say whats the difference?.. well the g2 was made in japan, and the g9 was the first to be made in the US. Im assuming that they also build the engines over here as well for the g9, and probably got a different supplier to make the rings, then those for the g2 in japan.. these g9's just wore out faster.

Its just an assumption that ive made from seeing many like this.


Whats your compression like now that you have the intake valve free?

Duh.. never mind that i asked that. lol.. if the intake dont work, you ant gonna get a proper reading.. i was in 2 stroke mode for some reason.. Been working on skiddos today. lol
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