cam timing chain jumping a tooth ???

[Langbolt]

I couldn't believe my eyes.....after spending days trying to re-jet my bike from a rich situation....I got down to a 140 main 38 pilot and noticed a slight backfire through the carb off idle.

I removed the Air box & looked in the carb throat and could see a flash!

I thought maybe I sheared the Timing Key....popped off the flywheel and it's fine.....So off comes the rocker cover.

Cranked the motor over by hand to line up the 'T' mark and the lobes down.....I look at the Cam Sprocket and the lines were Advanced!!!!!

I had to Advance the chain clockwise one tooth to get the lines on the sprocket to line up.

I put it all together and cranked the motor over a couple times.....reset it to 'T' and checked the sprocket.....the lines are aligned.

What the Heck!!!

How is that possible ????

I installed new tensioner blocks and the automatic tensioner has lots of tension when released....

I'm stumped.....It's too late to take it for a spin......I'll update on the results tomorrow.

Any one have any ideas of what might be wrong ?

I sure hope the crank sprocket is still good!!! I don't wanna have to tear it all down again to replace the crankshaft.

:confused:

[dxcody]

i dont have an answer for you, sorry about that, but this has me stumped to and sparked my iinterest, so i am curious to find out what the problem is. ??

I guess the only way to know if you have a stripped crank sprocket is to check but i kinda think that would be odd to go bad.

hope its nothing too serious! :confused:

[hybridracer686]

might be able to check the timeing chain gear with a flash light from the head by looking down in? :confused:

[Stickman400]

Or just tear off the clutch cover and have a look. That's about the only thing that gunna be a guranteed way of finding out. It ain't that bad to do anyway. Atleast you don't have to tear your basket and inner hub off also.

[01boneless]

your cam chain is probley streched mine did that befor

[Langbolt]

I checked the chain for stretch.....bu grasping it on the top of the sprocket and lifting....it only move up about 1/16"

It was a brand new chain when I put it in last year and the bike hasn't had many miles.....it's the girlfriends ride.....maybe 50 mile total since the rebuild.

I sure hop I don't have to put a new CRANK in it....Is the bottom cam timing sprocket a seperate unit ???

I see I'm not the only one having this issue.....some XR400 guys too!

:confused:

[CJM]

You do realize the timing key advances the timing, as such why your cam didnt line up right..

Remove the key and see what happens imho. Its a waste of money imho anyways.

[Stickman400]

To answer your other question, yes the sprocket is a seperate piece. It slides right off the end of the crank.

[Langbolt]

CJM: I have the stock key....not the advanced key....so the "T" mark is timed to the crank correctly

Stickman400: glad to hear that....so if it is the bottom sprocket....I don't have to buy a complete crank,

I had it out for 2 days this past weekend.....it's running great with no issues.....I've heard if you're accelerating and the chain is flexing outwards (centrifical force) with the tensioner compressed and you jamm the breaks or slow the engine quickly the chain can skip a tooth....that's why APE makes manual chain adjustors.

Knock on wood...she's runnin' great!!!!

:D

[CJM]

Sorry musta misread it.

Its probably just a stretched chain, they do wear out eventually.

Id also check the tensioner, and the guides to be sure everything is a-ok.

[400exrider707]

The stock chains notoriously stretch like crazy, this is why everyone recommends the CRF chain. Your tensioner could also just be bad too, remove it and try to collapse it with your hands, if you can do it, replace it.

Pulling on the top of the chain wont tell you much, since it's still attached to the sides of the cam sprocket...