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timing marks....97 xr250

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hey

i just wanted to confirm that the newer XR's (1997) have a "T" mark for TDC. i have not been able to find it although i have found other marking such as the odd "U" that means "micro" (at least it means micro in chemisrty class, don't know what it means as a timing mark.) anyway is someone could clarify how many timing marks there are and wich one means TDC that would be great. just trying to adjust my valves before i put my engine back in my frame.

thanks,

stan

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The flywheel has an "F" and a "T" stamped on it. The F is for checking ignition timing, the T is Top Dead Center (TDC). Both the T and the F are lying on their side, that is they are rotated 90 deg. counterclockwise from vertical. To the left side of each is a vertical index mark that is to be lined up with the index mark at the 12 o'clock position of the timing hole. Look carefully with a strong flashlight, you may be confusing one of these marks for a "U". I have not seen a U on an XR250 flywheel.

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The first time I attempted to adjust valves I swore they never put a mark on my flywheel. You probably just have to crouch down more so you can see higher on the flywheel to find the mark.

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It's definitely there on the flywheel and visible through the inspection hole if you rotate the crankshaft manually.... and, as mentioned above, there is an F followed by a T. Both of the timing marks are turned sideways countercockwise by 90 degrees like this on the flywheel(this is an ASCII text example of the orientation of the "T" on your flywheel):

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ok thanks guys,

i figured it out yesterday, valves are adjusted and the engine is back in the frame. i was confused because the letters were on there side and the "F" looked a hellofalot like a "U" for some reason. also i was turning the engine with the kick starter which wasn't helping.....wrench works a lot better :thumbsup:

thanks

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ok thanks guys,

i figured it out yesterday, valves are adjusted and the engine is back in the frame. i was confused because the letters were on there side and the "F" looked a hellofalot like a "U" for some reason. also i was turning the engine with the kick starter which wasn't helping.....wrench works a lot better :usa:

thanks

Excellent! :thumbsup:

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Resurrecting this post because its got some good info that would remain buried if i started a new one.

I've been struggling with my 1985 XR250. I've just completed a complete rebuild of the motor and it has a bit of backflow through the carb--puffs of air blowing out of the primary carb. You can really hear the air escaping when shutting the bike off or moving the piston up onto compression to start it. The cam sprocket doesn't have a third timing mark on it that I can recall, and I did not replace the timing chain.

I'd read a bit about a "top dead overlap" mark and was thinking that perhaps the air that was coming back through the carb was a result of that. Interstingly, with the head off, it seemed that the top of the stroke lined up better with the "F" mark than the "T" mark referenced in the prior posts. As a result of that, I set up the cam at that point (the "F" mark), thinking that perhaps the other mark (the "T" mark) was the "Top overlap" mark. FOr what it's worth, I couldn't see either mark very well and couldn't distinguish either very well, but taking pictures this evening really made them clear up and I can easily identify the "T" mark that is referenced as the TDC mark in the Clymer manual and everywhere else on the net . Going to use the cell phone more! :devil: .

20120706_2323081.jpg

I'm going to try lining things up on the "T" mark, tomorrow, and if it doesn't work, I'll let everyone know. Perhaps a new timing chain is needed. We shall see.

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Air flowing out of the carb during the compression stroke is most likely one or both intake valves being slightly open due to damaged valve seat/valve face or simply a too tight rocker arm. If the T mark does not line up when the piston is at TDC, check for a partially sheared flywheel key.

Edited by creeky

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Creeky, good thoughts. I hadn't thought about a sheared flyweel key, but I know that is not the problem. I have just replaced the crank and got a good look at it.

I have also thought it could be a bad valve seat, but I've pulled the head no fewer than three times and filled each port with kerosene, searching for a leak. Nothing. All the valve seal well.

The intake valves are new, but they do slide smoothly by hand, without any binding. I'm thinking the problem is related to timing. Look at the problems I'm having alinging the marks on the flyweel and the marks on the camsprocket. Here are some photos.

20120708_150922.jpg

The cam sprocket lines up perfectly when the "F" mark is in line.

20120708_150947.jpg

When the "T" mark is on line, this is what the cam sprocket looks like.

20120708_151250.jpg

When I move the sprocket one link on the chain, this is the "F" mark.

20120708_151310.jpg

When I move the flywheel to the "T" mark, this is what happens.

20120708_160841.jpg

For what it's worth, here is a photo of my cam chain. Does that arc indicate a worn chain? Would a worn chain cause the cam sprocket to be that far off? Maybe a worn chain tensioner? The timing sprocket has light wear marks on it, but the cam sprocket looks unworn.

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The position of the cam sprocket in photo # 3 is not far enough off to prevent the engine from running. However, it does appear that the chain is very likely stretched since the sprocket is rotated slightly to the rear. What is the position of the rearmost wedge in the chain tensioner? If it is located with its' top even with the tensioner body or lower, the chain is indeed stretched and the tensioner is nearing or at the limit of its' travel. If the wedge is far down in the tensioner body and you can you lift the chain off the sprocket any at all, the chain needs to be replaced. A photo of the tensioner would be helpful.

Edited by creeky

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The chain is still tight and won't budge when I pull on it with my fingers, but the wedge is well below the tensioner body.

20120708_201753.jpg

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The tensioner wedge position indicates that the chain is indeed in need of replacement, I would say that the tensioner is very near the limit of travel.

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Thanks, again, Creeky. I'm going to order some parts and I'll report my findings.

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Got my new parts in yesterday and installed them Timing marks on cam sprocket lined up like they are supposed to. New chain, tensioner, and sprockets did the trick. Cost a bunch, but I won't ever have to worry about taht again. It still sounds like it has a bit of an airleak, though, but not nearly as bad as it was, before. I've got some replacement springs coming in, perhaps that will fix it.

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I too am resurrecting this post, because I am having a similar problem. I had a lot of trouble getting the timing set. Seemed that it was off, so I'd move one link and it was still off. Now I am getting blowback out the intake. I had a lot of trouble distinguishing the marks (I did not think they would be at 90*). But will pull everything apart and try again.

Thanks for your post and pictures!

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