Crf70 won't run right

Ok so I bought this 07 crf70 for my little brother last Winter and it would always be hard to start, and when it did it would have some bluish tint to the exhaust. so I thought oil was getting into the cylinder and I replaced the rings. Now it really doesn't like to start? I have checked the timing it's spot on done a leak down on the valves there sealed, and have checked the piston rings and cylinder they all apper to be good. It just doesn't like to start and when it does start you have to have the throttle wide open to keep the bike running it bogs like hell and you put it into gear to go it hardly has any power. I have cleaned the carb several times to I'm at a loss I don't know what it could be any suggestions?

Check your ignition, timing 180 degrees off, valves out of spec. It's gotta be one of those three. Try a new spark plug and check your timing once more, and spec your valves.

Alright will do

 have checked the piston rings and cylinder they all apper to be good

How was this accomplished?  You can't do a visual and know that the bore isn't out of round, or doesn't have excessive taper or too much clearance between the piston and cylinder.  This all needs to be measured.  Also, if you put new rings on, it's generally considered standard practice to at least hone the cylinder if it measures within spec.  Seeing that's there's still cross-hatch isn't good enough.  I know some people can get away with just slapping new rings on the piston, but it's not the best thing to do.  Cylinders glaze up during use and the glaze needs to be broken to allow the new rings to seat properly.

 

How is the compression?

I will be checking compression again once I have time to grab my buddy's compression tester. I doubt the bore is out of round but I will check to see if it is with in spec. As for the glazing on the cylinder it's a xr/crf there bullet proof I highly doubt it would be the cause but I will pull the head again and take it to power seal to get honed.

Before you go ripping things apart and spending money, make absolutely sure all the basics are good - compression, valves, clean carb (it can be tough to clean), cam timing, spark plug, rings and piston installed correctly, etc.  I would hope you're using the Honda shop manual.  You should be consulting the manual for troubleshooting.  It contains the information you'll need.  It's easy to overlook something and assume the worst.

 

If you determine you need to have any cylinder work done, you should have the piston and the cylinder measured by someone with a good set of micrometers and inside mics.  You can't do it with a set of calipers.  Most people don't have a good set of mics but just about any machine shop should have them (some automotive guys don't do small engine stuff and they might not have the right size).  The measurements will tell you what you need to do. 

 

Just because something is bulletproof, it doesn't mean it can't be worn out or that it wasn't abused or neglected prior to you buying it.  I discovered this the hard way when I bought a "bulletproof" Z50.  Hopefully your CRF70 is better off than my Z50.

Edited by Punkinhead

You can use calipers to get an idea of what size the piston is currently.  meaning, is it stock, first over bore, second, etc.  But Pumpkin is right.  Check the basic first, and if you feel a top end is needed, get the next size up and take the piston and cylinder to the machine shop.  Do not have them bore out the cylinder first and then buy a piston to fit.  If they have the piston, they can get an exact measurement and bore out the cylinder to match perfectly.  and yes, they are bullet proof, but a cylinder is a cylinder.  It should be honed no matter what it looks like in there.  Even a few seconds on the drill will help clean it up and get it ready for the new rings.  

 

but in all honesty, it really sounds like a valve issue.  If they are out, it can be hard to start, lack power, and it will smoke.  .002 for intake and exhaust.

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