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Lost/found tappet locking nut. Now only firing once every few revolutions

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TLDR: valve locking nut fell off. found the nut and replaced it. Bike still won't start.

I turned up to a trail ride a few weeks ago, started the bike 2nd kick as usual, rode across the paddock to another car in the car park and stalled. I assumed it was because the engine was cold and I've been meaning to check the valve clearances. I tried to start it for quite a while but it would only sometimes fire or backfire, then stall straight away. Was looking all over the bike to figure out what's wrong. After opening the valve inspection caps I noticed the locking nut for the right rear intake valve had disappeared. So I took the valve cover off in the car park to take a look around. Couldn't find the nut so I called it a day and took the bike home.

Now I've been working on the bike every night since last weekend and It's still a problem. The first step was to find the missing locking nut. Checked under the cam, removed the head (not that I needed to, just wanted to torque up the cylinder bolts). Checked under the clutch cover. Managed to use a magnet to shift the nut across the bottom of the sump, then suck it out of a small opening using the magnet.

The nut was oval so I put it in a vice to make it circle again. Turns out thread need to be quite accurate so it's not going to work. Thankfully I have a spare xr250 to borrow a locking nut. After Installing the locking nut and checking all valve clearances, the bike wouldn't start again. It's still doing the same thing it was with no locking nut. I double checked the cam sprocket is aligned properly. It's about half a tooth retarted, which is fine according to other people on here.

Yesterday I tried towing the bike behind a car to start it. I noticed very consistent and rhythmic firing from the bike. Roughly (but consistently) every 8 or 16 revolutions, the bike will fire once. After it warmed up slightly (from dragging it around behind a car), It would fire twice instead of once.

Today I swapped the carb with my spare XR to make sure it's not a carb problem. I haven't tried towing it around the paddock, but it feels the same kicking it over. Sometimes fires and stalls immediately. So I guess the locking nut has damaged something in the engine to cause this but I have no idea what. the only damage I saw was denting through the head where the cam chain goes. There is a small, what looks to be spring loaded oiling line that touches the cam. Because of the denting, it's no longer spring loaded and stays in the compressed position. There's a tag on the cam that pushes on the oil line in. No idea why but just though I'd mention it.

I suspect this could be a very difficult problem to isolate so I'm thinking of selling the XRs as is and upgrading.. After having just spent $600 on a rebuild :(. Any ideas on what I can do? The bike with the problem is a 2001 XR250R, My spare is a 1994 XR250R. Are there any parts you suggest I swap between the two for testing?

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I bought a 94 XR250L, and it was previously sold, as a bike that was hard to start, needed carb cleaning. the next guy, found that it would start cold, but not restart if hot. he sold it to me thinking the head was cracked. I found not only was the head cracked, but the seat was falling out. This was why it was hard to start when hot. in the process of replacing the head.

 

IMG_5957.JPG

IMG_5960.JPG

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It sounds to me like you're describing the plunger as spring shown in the picture. The engine will not start and run without the plunger and spring.

XR250 plunger.JPG

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12 hours ago, Trailryder42 said:

It sounds to me like you're describing the plunger as spring shown in the picture. The engine will not start and run without the plunger and spring.

XR250 plunger.JPG

 

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I pmed sharft6 instructions and fiche of head last night. He thought pin was cam oiler as it was jammed in hole,spring couldnt move it. Guess tappet banged head next to plunger hole and peened hole a little tighter and jammed it.Hopefully he will be all set,was gonna remove pin and clean up hole.

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Your tappet nut fell off and got so deformed that you couldn't reinstall it? You think it got deformed from an 13 inch fall from the top of the engine to the bottom of the engine? 

I suggest looking for whatever part that ran into that nut with enough force to squish it into an oval. 

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2 hours ago, firstplacephoto said:

I pmed sharft6 instructions and fiche of head last night. He thought pin was cam oiler as it was jammed in hole,spring couldnt move it. Guess tappet banged head next to plunger hole and peened hole a little tighter and jammed it.Hopefully he will be all set,was gonna remove pin and clean up hole.

Sounds like a credible scenario.

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17 hours ago, cjjeepercreeper said:

One little point here.  You said you removed the head.  I hope you didn't reuse the head gasket.

Yeah. I only bought the head gasket late last year so I just re-used it. Will replace it if it leaks.

 

Yes I thought the plunger was some kind of small oil line and didn't see it's importance. I'll sort it out tomorrow and let you know how it goes. Thanks for the advice!

Edited by Sharft 6

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3 hours ago, Sharft 6 said:

Yeah. I only bought the head gasket late last year so I just re-used it. Will replace it if it leaks.

 

Yes I thought the plunger was some kind of small oil line and didn't see it's importance. I'll sort it out tomorrow and let you know how it goes. Thanks for the advice!

Head gaskets, like most gaskets, are one time use.  Bad idea to reuse it once the motor has been run.  Its like replacing a clutch pilot bearing and not replacing the clutch in a car, you went through all that work to get there you should do it right the first time, not have to go back in.  A head gasket is only $25.

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yep the plunger was all it was. Thanks so much!

in case anyone else runs into this, it can be a bit tricky getting the plunger out when it's almost flush with the head. I was going to use a drill bit to bite into the plunger and then pull it out. But a 1.5mm bit is too small and a 2mm bit i think may have been too big. Besides, I would need to remove an intake valve or use a drill bit extender in order to drill straight anyway. So drilling isn't the answer.

I ended up getting a piece of lockwire. I think it's about 0.5mm in diameter. Folded and crushed the end so that it's 2x thicker at the end. But it's not perfectly crushed so it's quite scratchy and barbed like. Anyway, I stuffed that into the middle of the plunger and pulled it out quite easily. Then removed the spring the same way. Then I used a piece of paper towl to absorb the pre-existing liquid down the hole. put the spring back in, coated the plunger in fresh engine oil, Put the plunger back in upside down because the exposed end is slightly bigger so it can help boar out the hole slightly. used pliers to move it up and down. Then flipped it up the correct way and it was moving nice and freely.

Bolted it all back together and it starts easy as with the 94 carb. I'll put the stock carb back in and I'll be away :)

Edited by Sharft 6

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