xr70 nightmare- help me please !

Joined
Sep 13, 2004
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#1
Yes it is a nightmare.
I couldn't get my boys xr70 started, so I went through all the usual, cleaned the carb, air filter, checked for a spark. Still wouldn't start, checked the compression, it was very low so I stripped down the top end and took it to my local Honda dealer to check out. They rebuilt the head, honed the bore and suggested new rings, which I bought.
Now, I've never done a 4 stroke top end but I have done 2 strokes ,and I've got a manuel. So with confidence high I started to rebuild the top end.
As far as I can see I did everything the book said, the T on the flywheel matched the mark on the crank case, the O on the cam chain sprocket lined up with it's mark, the valves where adjusted as per the book. And still the bugger won't start, not only that, it doesn't even try, no backfire, nothing.
There is still a spark, gas still gets through, everythig looks clean and tight, but it's still dead.
What to do ?
I would love some input before I take it apart and put it back together again.

Thanks in advance
 

bro

Member
Joined
Mar 21, 2002
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#2
At this point I usually pull the plug and squirt a little carb cleaner into the cylinder and see if it will fire on that.

They always say all you need is:
- spark
- compression
- fuel

They leave out the 'air' part. You need air for combustion too.
I fought a bike for quite a while once and all it turned out to be was the idle stop was adjusted so low that it wasn't getting any air.

Good Luck
 
Joined
Mar 16, 2007
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#3
Sometimes marks will align just fine...on the exhaust stroke as well as TDC of the compression stroke. Are you sure the timming isn't out 360 degrees? It's an easy mistake to make...not something to even think about with 2 strokes.
 
Joined
Oct 8, 2021
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#6
Just commenting on this even though it's old in case someone is googling for fixes. I'm new to working on bikes, so have just learned by google. But, had issue with my son's 2003 XR70R not starting. First issue was no spark. Don't know how to do the Vmeter tests, so I just started replacing things b/c they're pretty cheap on ebay. First spark plug, then ignition coil, then CDI, then Stator. Of course, it was the stator that was bad. Got it running, and it stopped running shortly thereafter...would not start again. Looked at stator, and wires had burnt. Not sure if it was b/c it was a cheap one off of ebay, or what. But, put wires back together with some electrical connectors, and had spark again. Started it up, drove it for 3 minutes, and it died. But, still had spark. Cleaned carb, tried new carb, nothing...would not start. Sprayed starter fluid, and still wouldn't even kick-over for a second.

I was completely dumbfounded, as I had checked valves, timing, etc. previously while I was fixing the stator wires, and all were correct. Finally, I took the stator cover off, and apparently, even though I had originally ensured the fly-wheel was installed with the key/notch aligned, what I didn't do was tighten the fly-wheel on properly. The fly-wheel was off the notch, so timing was off 180 degrees. Took it off, used a marker to mark the notch so I was sure it was aligned. Tamped the fly-wheel secure with a hammer this time. And, really bolted it on tight this time. Started right up, and has ever since.

So, if you're trying to diagnose, you're sure it's not the carb, your sure it's not anything electrical, and you're sure it's not the flywheel mis-aligned b/c you put it on correctly the first time, give it another look.