After getting my 2012 crf250r motor reassembled and back in the bike after rebuilding it, I’ve been trying to start it with little luck. The bike spins freely, and after kicking it a few times it will pop and backfire out of the exhaust, but not much more. It did actually start up at one point and was responsive to throttle, but I was only able to keep it running with throttle and it would not idle. Once off it does not want to start again. I rechecked valve timing and clearance twice, and the cam looks in time and the valve clearances are in spec. I’m really stumped as to why this won’t start as it’s efi. Have strong spark, fresh fuel, clean air filter etc. it looks like I have everything I need for it to run and yet it will not. The time I did get it to run it was smoking out of the exhaust a decent amount.
if anyone has any ideas I’d appreciate it, this isn’t my first rebuild or anything, and I’ve gone through everything spark, fuel, air, valve timing and clearance.
My guess would be base engine timing is off. Other possibility is the flywheel is off causing your ignition timing to be off. Was the woodruff key ok and not bent or damaged?
Yeah it was ok, I pulled both the clutch side and flywheel side timing covers off. With the dot aligned, the flywheel mark was also aligned and the cam aligned as well. I suspected the dot on the gear might be off, but the flywheel mark was aligned as well. Once I got it to fire up after about 50 kicks it was responsive to throttle and all that, but was smoking. Once off though it didnt want to start up again. I’m kind of leaning towards compression whether that be head gasket or rings due to the smoke.
I’m fairly certain it won’t matter but would rotating the engine a full rotation without the cam connected and resetting the timing after that 360 degree rotation make a difference? I don’t think it would because I believe the spark fires every stroke whether power or exhaust
Could the marks on the cam somehow be inaccurate? I have no idea how to check this other than based on memory of how the cam sits from a 2010 I used to have.
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Don’t rotate the crank with the cam installed and the timing chain off or you will bend valves, and it wouldn’t make a difference anyways. The dot on the gear could possibly not be in the correct position on the crank. If I remember correctly don’t those have two marks on the flywheel? I would make sure it’s lined up with the correct one. Also, when installing the cam chain.m, are you making sure all your slack is on the tensioner side?
I think there is two marks on the flywheel, I didn’t check to see which one was lined up but I think the second mark is for valve timing and the first being spark. I’ll have to check that tomorrow, when I check all the timing marks it’s with the tensioner already set by screwing it all the way in and letting it spring back out. Im not sure about the slack in the chain being on one side or the other because usually there isn’t actually any slack, I also always replace timing chains since I had a yz250f blow up due to one, but that’s getting off topic. I don’t have a compression tester for this motor, since the plug threads and side aren’t the same as the standard 5/8 spark plug. I’m really suspecting compression, but the backfiring really seems like valve clearance or timing yet the valve clearances are in spec.
I guess what I’m trying to say is that the symptoms seem like valves being out of spec, but everything there checks out which is what has me stumped. I’ll triple check timing again tomorrow and let you know. I think we can rule out fuel issues because when it was running it had good throttle response.
What was the reason for the rebuild? what color is the smoke? Sounds timing related.. I’d run a leak down test to make sure everything’s ok with valves/ rings... what was replaced during rebuild?
I bought it locked up, ended up being main bearing failure which also destroyed the left side crankcase. I put a new left case and all that in it and now I’ve been trying to get it to run, I don’t know how it behaved before I’ve never seen it run. I put new rings in it, but not a piston as it looked almost brand new. I’m suspecting that to be a problem and I have a piston kit coming tomorrow I’ll be installing.
Most main bearing failures I’ve seen on 4 strokes ;especially ones that cause enough damage to need a case need a piston... with the amount of metal shavings being pushed though the engine it normally causes some pretty heavy scuffing to the skirts and bore... I’d still recommend a leakdown test itll pin point a valve/ring seal problem instantly
How did you clock the piston rings? Poss blow by Did you change valve guide seals? may be leaking cause of smoke Do the leak down to make sure valves sealing Popping is timing or electrical clean injector
If it’s smoking that bad you could have a bad valve seal or ring issue considering you reused piston . Oem piston and Oem rings ?
Backfiring sounds timing related .
Possibility the gear spun on the cam causing it to be off enough resulting in what you describe . Hard to tell by the naked eye . I’ve had one with similar symptoms and found this out the hard way..A quick swap with a known good cam is best way to eliminate this as a possible issue.
Sorry I haven’t updated, I’ve been busy today and fedex didn’t bring my new piston kit anyway. I used wiseco rings, as I could tell it wasn’t an oem Honda piston based on how there is an arrow pointing towards the exhaust and not a dot to mark intake, since Honda oem does that opposite of how wiseco does it. The rings looked identical to the old ones but like you guys said, maybe there’s a problem with the piston itself, tomorrow it’ll come and I’ll be sure to hone the cylinder real good again, measure ring gap etc.
If the engine locked up, there is a very high chance that the cam gear spun on the cam. When this happens, the timing marks will still line up but the cam itself won't actually be in the correct spot. Only real way to verify is to degree the cam or you could visually compare it to another cam. Also, when checking TDC, I always use a screwdriver or dial gauge on the piston down the spark plug hole instead of using marks on the flywheel. Most bikes have multiple marks on the flywheel and it is easy to use the wrong one, the screwdriver down the spark plug hole seems like a crude way of doing it but it is the most accurate in my opinion. If you are unsure if you have a compression/ring/valve issue then you need to do a leakdown test. Just guessing at stuff just ends up costing you more money and time.
this top photo is the cam that currently came with the bike, and the second will be the worn cam which is pretty obvious.
Looking at the center of this, you can see where it points to on the cam gear itself.
So I ordered a cam off eBay, it doesn’t come with an Auto decompression, I have two here I can use, but the problem is I’m not sure how to remove it. I have the manual for this bike but I was just wondering if anyone could help clarify how to remove the hot start on these cams, I don’t see a nut or bolt to take out.
Did you ever find the solution dealing with the same problem now
Yeah ended up being the cam out of time, the gear is only pressed on so when the motor locked up it somehow spun a few degrees on the cam causing it to be out of time. Threw in a new cam and the thing fired right up
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