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Johno1970

XR600R 87 xr600 backfires but wont start

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87 xr600 twin carb, recently rebuilt engine: 

new piston +.75, new rings, rebore to suit. New stem seals. Tested head with fuel in the ports (to test valve to head seal on each valve - no leakage. New head gasket Thoroughly cleaned carbs and set them up as per manual. Fresh fuel

I can stand on the kickstart on compression stroke - so hopefully valves etc ok, valve timing marks on either side of camwheel very close to level on the marks against head

Hasn't run since rebuild.

It will pop/bang on demand- ie kick it over and it will backfire/pop/bang not kick, kick, kick, kick then a random pop.

Does anybody recognise whats wrong? PLEEEEAAASE!

thanks in advance people

 

 

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1 hour ago, BPPerformance said:

Sure sounds like timing if it all looks good check the flywheel woodruff key could have sheared from a back fire

 

Thanks I will check. Does it sound like 'a long way out' or close? It turns over cleanly so it cant be miles out-i'd smack the piston on the valves if it was too far out wouldn't i(?)

 

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4 hours ago, baldy110 said:

Did you rebuild the engine or someone else, Have you done any troubleshooting?  

Thanks for your response. I rebuilt it, my 7th Xr and the first totally unsuccessful.. I have removed the kill switch.

I have swapped out the CDI then the carbs then the alternator one at a time with the same results.

Fresh bore and new piston/pin/rings( definitely Xr not Xl piston)

Tested the pickup and it's within spec

Spark is good and I have cut down the lead, changed the plug.

Changed the fuel.

Any suggestions (other than sell the f&+#!Ng thing) welcome please....

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3 hours ago, Johno1970 said:

 

Thanks I will check. Does it sound like 'a long way out' or close? It turns over cleanly so it cant be miles out-i'd smack the piston on the valves if it was too far out wouldn't i(?)

 

Sometimes when they shear they spin slightly and sometimes a quarter turn - either can create a head scratcher...

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On 3/26/2020 at 9:02 PM, BPPerformance said:

Sometimes when they shear they spin slightly and sometimes a quarter turn - either can create a head scratcher...

Just checked the woodruff key/alternator/ crank all ok - as in all inline, mounted correctly, not sheared. Is there anything else that could cause this effect? thanks

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3 minutes ago, Johno1970 said:

Just checked the woodruff key/alternator/ crank all ok - as in all inline, mounted correctly, not sheared. Is there anything else that could cause this effect? thanks

Not sure if you addressed this already but now I would look into the valve clearances if too tight well you know...

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1 minute ago, BPPerformance said:

Not sure if you addressed this already but now I would look into the valve clearances if too tight well you know...

anythings possible-worth a check. I'd be surprised that there was enough compression to even bang once tho..

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1 minute ago, Johno1970 said:

anythings possible-worth a check. I'd be surprised that there was enough compression to even bang once tho..

With fuel in cylinder and spark - bang eventually if valves too tight most fuel will not get there...

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15 minutes ago, cyclewizard said:

I would re-check your cam timing. 

I just have. At the same time i stuck a pencil down the bore and it looks like the marks dont correspond to tdc.  T - which i thought was TDC is not- on the T mark it is between the F and the double strike marks

The magnet hasnt spun ( as BPPerformance suggested it might have).

This makes sense- the cam timing is out but the marks are wrong... EH? So it looks like ive followed the marks and thats why the bike wont start!

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4 minutes ago, Johno1970 said:

I just have. At the same time i stuck a pencil down the bore and it looks like the marks dont correspond to tdc.  T - which i thought was TDC is not- on the T mark it is between the F and the double strike marks

The magnet hasnt spun ( as BPPerformance suggested it might have).

This makes sense- the cam timing is out but the marks are wrong... EH? So it looks like ive followed the marks and thats why the bike wont start!

So it appears as tho my marks arent in the correct place, unless tdc is not the absolute top

Edited by Johno1970
mistake

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6 minutes ago, Johno1970 said:

So it appears as tho my marks arent in the correct place, unless tdc is not the absolute top

Well looks like you probably have found your problem. Let us know Curiosity kills me.

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On 3/28/2020 at 1:29 PM, BPPerformance said:

With fuel in cylinder and spark - bang eventually if valves too tight most fuel will not get there...

 

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FIXED.

BPPerformance gets the virtual beer, it was timing.

Thanks to all who contributed and tried to help

BUT

It was a mistake that i didn't think that i would be able to make. So i can add that to the list of normal mistakes then-

The rotor tag will fit on the crank (maybe just the 87) whereever you like. Whereas before I have been used to basically rotating the tag round gently until the wide gap finds its natural position. NOT RELIABLE!!! I proved three times that the tag would fit wherever i chose as long as it was on whole splines- GRRRRRRRRR

I know and i have read several times that it should only fit on one way/ one position- ie look for the very wide spline and match that to the crank - not true in this case.

It fired easily once: 1.sorted correct engine turning direction. 2.Back up a bit from TDC (mindful of note 1) but not as far as F mark. 3. Tag should fit bang on middle of pickup 4. Fire up.

Hope this helps someone else.

Johno

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