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85' XR100 with a knock.

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Howdy-

 

I just bought an 85 xr100  today ($425), it was very clean (totally stock, even original tires in good shape) and ran fine, it did have a little knock but i went for it anyway, and the owner(s father) suggested maybe loose valves, I figured the same.  I got it home and opened it up-  the valves were very tight; no play at TDC at all, so I opened them up to spec (.002, .5mm) and put it all back together-  same knock, but it runs fine.  thoughts?   also- I would like to get a little more off the line pull out of it- will a larger front sproket help?

 

thanks

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Not sure about the knock. Maybe a worn piston/ wrist pin? Or rod bearings?. Smaller on the counter shaft will make it pull better though. Smaller in the front and larger in the back will gear it down (make it pull harder, but lower the top speed) and the opposite to gear it up.

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How did the oil look when it was drained into a clean,

light-coloured plastic wash basin?

Is the knock possibly due to preignition/detonation?

Due to exhaust leak from where head pipe and gasket

mate with the exhaust port on the cyl head? Checking

nuts at flange might give an indication of loose/tight, cool

weather might reveal vapour leaking.

Edited by EddyCurr

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A smaller front sproket will give you better acceleration, or a bigger rear sproket will do the same. Bigger front, smaller rear = better top speed. Be carefull and dont go too low on the front sproket because your chain will dig into the swingarm if its too small. I would only go up or down 1 tooth on the front and problibly up or down 7 teeth in the rear to be safe. You can make a xr 100 go 75 mph with the right sproket combo, but it will take it a while to get there lol. As for the knocking, i would say either crank bearings, or excesive piston ring wear or cylinder wall wear

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I would also say make sure your running prenium gas in it to prevent preignition, but i dont think you would notice it in a dirt bike like you would notice the knock in a car

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yeah, i was thinking crank bearings- kind of a bummer.  is there any way that i can be more sure without splitting the whole thing open?  thanks, I figure I should switch to a premium non-ethanol anyway.

 

just ordered the honda shop manual and will get on it in the next few days, what else should i tackle while im in there?

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Sorry for the late comment, but i wouldnt be able to tell you for sure what the knocking would be. If i were you, i would replace the topend and make sure everything is in spec. Replace the piston, rings, inspect the jug and replace or bore if its too scored, inspect camshaft and races. Basicly just look for anything that has play in it because that causes knocking. If the knock is still there, then i would be confident that your crank bearings are worn

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Also, take off the flywheel cover and take out the spark plug. Put a socket on the flywheel bolt and listen for any grinding. It should turn nice and smooth. Also try wiggleing the flywheel around with your hand and see if you can feel any play. Thats just a test i would do before taking anything apart to see if its your crank bearings

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