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Discussion Starter #1
Can anyone tell me how to upload a video file? I have a 91 TRX 300 that I bought a few years ago. With some help from ShadeTree I was able to address some shifting issues. I had both covers off and did not see anything obvious. It starts right up and runs / shifts fine, and the compression was in the 160's. It does not seem to have the power a 300 CC engine should. I am planning on pulling apart the top end to freshen it up. The engine is and always has been a bit noisy. I have a short video clip with the engine running. I am trying to find out if this sounds like the valves/ cylinder or bottom end. Any help would be appreciated.
 

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I can only imagine that since it's making so much noise that something is being damaged as it runs so an inspection of the top end would tell you a great deal including piston slapping or not.
When you pull the top inspect the crank. Check for any play or scaring at the wrist pin and check any up down play at the crank.

Also keep in mind a slipping clutch can cause power loss too.
 

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bad connecting rod, bad wrist pin, piston slap , all these comes to mind after watching your video.
 

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Discussion Starter #7
Thanks. I figured I was going to have to rip it down. I will start pulling the top end apart tomorrow. Shade tree are you leaning toward one of the above or a combination of all?
 

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your in for a top end build by the sound of it, piston, rings & little end. If your pulling it apart to fix up you may as well fit a new timing chain too.
 

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Tensioner does look maxed out, the way to check your chain is push that tensioner back into place, if the tensioner sits flush against the engine case then your chain is knackered. I have 1/8" inch gap between tensioner and engine case and my chain jumped a tooth and put the engine out out of time.
Try it out and post a pic.
 

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Discussion Starter #12
Thanks for the help unfortunately I have everything apart already. Did these come stock with a domed piston? Nothing looks terribly bad but everything is very wet. I will put up more pics.
 

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Ahh ok. As for the piston I'm not 100% sure I asssumed they were flat top pistons, could be differences through the years though I suppose. Was there any play in the little end bearing before you took it apart?
Put up pics of your tear down.
 

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Discussion Starter #14
I'm nervous about the connecting rod. That is about all that is left to be bad.There is not really up and down play but there is side to side play. The cam chain tensioner is maxed but even the plastic guides look good.
 

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Wet looking but notoverly cruddy.
In this post with the photo of the combustion chamber, I can see a gap in the intake valve seat where it fits in the head (look at the valve and seat area nearest the edge of the chamber) and it looks like that intake valve may be in the process of burning too. The intake valve and the valve seat will need to be replaced... I can't tell if the head casting is damaged or not from here. That isn't the source of loud knocking though... just indicates the entire head will need some repair and refreshing.

If you have a good set of micrometers or accurate and consistent digital calipers you should measure the wrist pin diameter all along its length, the wrist pin bore at the top of the rod in at least four places, the cylinder bore in several places throughout the bore, likewise for the piston skirt... check side play and up and down play in the lower rod bearing and be fussy about checking that... its important that you don't miss something or make any measuring mistakes. The service manual has component sizes and tolerances for every one of the parts you'll measure and in many instances shows you where to measure.

You haven't found the source of knocking yet... if you need a machinist to check everything for you you can go that route at this point... but unless you know of an old time craftsman machinist in your area, odds are you'll be wasting time and money on someone's incompetence... and no, good machinists don't work at dealerships or auto parts store machine shops, just the way it is nowadays.

Good luck and have fun with it!
 

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I think I found my piston slap and low compression.. I have quite a gap between the cylinder wall and the piston
The piston is tapered... it is much smaller diameter near the crown than it is at the skirt. Pistons are measured at their largest point on the skirt (which is 90 degrees from the wrist pin ends) not at the crown.

I recommend spending a few days reading and studying the service manual. You aren't prepared to do this yet. And no, you can't just wing it... or get lucky, or rely on a friend, you gotta be knowledgeable and possess excellent work habits and skills to be successful everyday. :) Good help rarely exists nowadays so ya better study up!
 
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