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Decompressor Cam Question

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Bike: 99 XR400

I was riding Sunday and had the bike stall out and have severe knock/clank when trying kick over. I had a slight leakage at gaskets, knock/tick and it smoked while cold so I figured I would pull top end to inspect cam chain, valves, replace gaskets etc. Figured I would find broken ring or something by noise. No huge issues with piston and rings, rocker arms, valves...Some carbon deposits.

I do notice after disassembly that the decompressor cam has quite a bit of slip in different directions on the cam. This doesn't seem normal and the lobe doesn't seem to pick up the pin/spring when laid in place without bearing on shaft but riding in position. Could this explain some of the issues and should I replace cam or???

Edited by SquirlySteve
No bike info

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I was looking at the cam chain tensioner because I noticed that the plunger was all the way in after teardown. I can twist the plunger into the extended position with a micro-screwdriver but I can push it back into the retracted postion with little force pushing with my finger. I also noticed a few fresh scratches on the valve cover. Cam chain is probably stretched but looks in good condition still. Is the tensioner/spring bad and the noise and lack of starting due to cam chain slap and mis-timing?

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I did not double check everything before completely stripping the top end apart. I noticed a slight maybe 1 degree forward mark on the cam sprocket. The upper and lower teeth look good, i'm sure the chain is stretched after 12-13 years of use, the chain doesn't show wear, I am just trying to figure out if the decompressor cam/reverse cam/oneway clutch/plunger and spring should be pulled and tensioner should be replaced. I did not want to start after trying to kick over with tin sound in engine.

I think what may have happened was the tensioner broke and the cam chain slopped around for a second killing the engine and causing a slap possibly valve slap on piston. Valves and everything look fine but I did notice that the decompressor cam has a good degree of slop between the inner surface and camshaft spline area. I figured this may be the valve tick/stick that I have noticed in the past. It appears that the valve cover only has a slight area where the steel cam chain rubbed on the aluminum cover a few times.

All parts seem OK except:

1)Tensioner seems to push back toward the disengaged position (screws out fine with tiny screwdriver but pushes back with finger pressure and I can see the adjustment screw spinning as I push the plunger in. (bad torsion spring?????)

2)The decompressor cam between the reverse decompressor that hits the plunger/spring and the first lobe of the camshaft has significant play back and forth in 90 degree direction from the center of the shaft/spline and a slighter degree of slop side to side (toward first lobe and back toward reverse decomp/one way clutch)

Getting frustrated trying to figure out the route of the problem and not have to order too many new parts.

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Well... the cost of replacing the timing chain and tensioner is not too high to pass on at this point. After this many years, I'd say you got your money's worth! :smirk:

Perc

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All parts seem OK except:

1)Tensioner seems to push back toward the disengaged position (screws out fine with tiny screwdriver but pushes back with finger pressure and I can see the adjustment screw spinning as I push the plunger in. (bad torsion spring?????)

That's not right. You should not be able to push the plunger back in by hand. Being able to retract it with a screwdriver is normal.

When my tensioners torsion spring broke the engine made an awful noise when running and no power.

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Thanks TR42...I figured this was most likely the issue that I was facing.

Does anyway know whether the decompressor cam is supposed to have significant movement? Should I replace the cam or remove the auto decomp assembly from the stock? If I replace, should I go stock with auto decomp and new spring/plunger or run stage 1 hotcam?

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Just going hotcam would be much cheaper. I would go that route for that reason alone, because I'm a cheap ass. :smirk:

You can get a stock cam for less money than a hot cam

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Well...I spoke too soon. It looks like the stock cam assembly with all the auto decomp components is roughly 200 and the stage 1 hotcam is about 180-190. It looks like I'm in the market for Cam chain, tensioner, stage 1 hotcam (guess I'll go to manual decomp), clutch kit, clutch cable, brake pads, & clutch and top end gasket kit.

Wife is gonna kill me at about $450 for the repairs.:smirk::bonk: She might even tell me to kiss her:moon:if I don't sell it to her right.

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If you're replacing the cam the rockers should be replaced too. Search this forum, some CRF450 cam chains can be used as an upgrade and they're supposed to be cheaper.

I found the stock cam for $169

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XR250 stage1 cams are all over ebay for 130-140$. Just as for the XR4, that's why I opted for it, I was on a tight budget then.

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Update...The bike is currently in the process of being rebuilt valves, valve guides, auto decomp removed, cam chain, clutch kit w/ heavy springs, etc., etc. Rebuilding to sell. Couldn't wait so I went to the dark side and bought an 2001 YZ250 2smoker. Couldn't believe the amount of weight savings and extra energy after rides but sure will miss the ability to tractor from a dead stop on ANY hill or take it at 2mph rather than riding on the pipe.

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YZ250! Congrats! they are the best "serious" all around dirt bike ever made.  The steel frames turn better than the newer ones too.

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