Xr310 keeps breaking gearbox shaft

[OzzyRZ]

This is the second one now and it's always while trying to kick start it.

I am running a Hot Cams stage one and a 11/1 piston.

Is this a common problem with big bore XR250?

I was thinking of going back to the standard cam with the auto decompressor to try and lesson the load when starting.

Any thoughts would be appreciated

[Oil Baron]

You could shave the top of the piston to stock compression.  

[SteveThe Snakes]

ImageUploadedByThumper

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This is the second one now and it's always while trying to kick start it.

I am running a Hot Cams stage one and a 11/1 piston.

Is this a common problem with big bore XR250?

I was thinking of going back to the standard cam with the auto decompressor to try and lesson the load when starting.

Any thoughts would be appreciated

I have heard of it. However it never happened to mine.

Does the hot cam have a decompression pin.

I run about a stage 2 web cam. It has the stock decompression system. I think.

[Trailryder42]

Have you checked the bearings that shaft is supported by for wear?

You don't have mismatched case halves do you?

What is your starting routine? Do you position the engine to the back side of TDC like you should before every kick?

[OzzyRZ]

Brand new motor and gearbox bearings. OEM bearings too.

I am not running auto decomp because the Hot Cams cam doesn't support it. I never had this issue when it was a stock 250

[OzzyRZ]

Staring-

I push through until I feel the compression. I then squeeze the decomp lever and just push past TDC. I then kick it.

[Trailryder42]

Brand new motor and gearbox bearings. OEM bearings too.

I am not running auto decomp because the Hot Cams cam doesn't support it. I never had this issue when it was a stock 250

I think if the Hotcam with no autodecomp is the only thing I could attribute it to, I would go back with the stock cam too. Reliability over a little extra go is more important, especially in the Outback.

Edited September 1, 2014 by Trailryder42

[OzzyRZ]

The hot cam and 11/1 piston.

So basically I have a little more compression than stock and no auto decomp. But surely other people have the same setup?

[JoeBlow Kokomo]

11.0 is considered high compression on an XR and seems to be where the problems start showing up.  I've had a couple of XR's with 10.5 and no issues, but seems like 11.0 and up are problematic. 

[OzzyRZ]

Yeah when I rebuild it im going to lower the compression.

I wonder how those people get on that choose to run the XRs Only 12/1 280 piston kit?

[Oil Baron]

Yeah when I rebuild it im going to lower the compression.

I wonder how those people get on that choose to run the XRs Only 12/1 280 piston kit?

 

Some mods are known to be reliable in sweltering desert heat and some are good for one quarter of a mile or less.

[OzzyRZ]

Well I stripped it down today and inspected it. No other damage other than broken shaft and bearing.

As this is the second time it has happened I would like some idea why this is happening.

Today I machined off the overhanging cylinder squish that was apparent once the 310cc liner was pushed into the barrel.

This will help direct the gases to the plug and will help lower the 11/1 compression.

The shaft broke when I was kick starting it. It felt like it kicked back while I still had my leg pushing down on the kick starter. I them heard a "snap".

Any suggestions?

I don't want this to happen again

[OzzyRZ]

Well it kicked back while trying to start it today and it broke another gearbox shaft. That's 3 now.

I lowered the compression to 10:1 thinking that would solve the problem but it hasn't.

Do you think it might be because the HotCam doesn't have the decompression system like the stock honda cam?

I have no answers why it is doing this.

[thosforce]

Caveat....I am not familiar with the XR250, have XR400's

 

But my thoughts

I would suggest that you determine why you are getting kickback while starting

Timing?

Is the CDI stock? I have read that the stock CDI on the XR400 has programming to not allow ignition spark until the engine reaches 900 rpm to prevent kick back? (Learned this with estart conversion as the electric starter motor does not spin this high, so will not fire with stock CDI)

 

Member "Baja Trail Rider" runs excursions in Baja and has a fleet of XR's 

He has posted that he and his friends have built up XR250's and XR400's with long service lives

You could PM him about his experience and/or recommendations?

[OzzyRZ]

Thanks mate.

[Oil Baron]

Thanks mate.

 

Seriously, Ozzy, you should talk to BTR on the phone.  He's on baja time at: http://bajatrailrides.com/

[Baja trail Rider]

Ok Here we are.300cc kits all low comp.all have the DC removed all kick back once in a while.Of the over 15 we did.Have not heard that problem,One Guy did tell me he had that problem.I will try to get hold of him to find the answer.Low Comp is what makes the XR live.My 300 was done in 97,bottom end still stock.

[OzzyRZ]

I also forgot to mention that the flywheel was lightened by 500g and the crank webs by 200g.

That's 700grams of rotating mass removed

Other than that is as mentioned before.

Edited September 26, 2014 by OzzyRZ

[Trailryder42]

Wow, that's almost 25 oz.. Seems like a lot. I wonder if the increased compression ratio coupled with the the fact that there's less rotating mass to absorb the torque when it kicks back, that force, instead moves on down the line to be dissipated at the shaft that keeps breaking.

[OzzyRZ]

Yeah I was wondering if that was a plausible theory as well.