Thread: dirt bike rebuild, are there any tell tail signs when its needed?

Originally Posted by Savage1

They're pretty bullet proof those engines, especially in comparison to other brands. I wouldn't say they're particularly rattly. Re shim valves and check the cam chain isn't at the end of its adjustment. Try running on 95 or av gas if you're not already and see if that makes a difference.

I put over 200hrs on one of my YZ250Fs and never touched the head on it.

If it's still bad or rattly under power I'd be pulling it down and checking the main bearings, big and little ends. I'm OCD with engine rattles in MX bikes.

cool thats reassuring, its not bad rattly just a bit noisier than other bikes i ride with, i also run 98 or force10 in it, smells nice that stuff.

The 2005 model is water colled f I recall correctly. Another way of keeping the temp down when idling thru the thick bush is to replace the engine oil with a very thin one - say 10-30 which is what I use.
YZ350 1999 been going strong since a rebuild x3 years ago.
If you're only using it occasionally then when finished, fill the tank right up. This prevents condensation of any moisture in the air in the tank, therefore stopping water in the fuel.
The plastic on these bikes gets stiff over time so I use Armour all on them, it's like giving the missus some hand cream. It keeps the plastic from "sweating" thus remaining soft and supple. Amazing stuff.
I hope you get as much fun from it as we do!

hey guys , I am amazed that savage got such great milage out of his , one of my hobbies is working on bikes , and the recommended cam chain replacement for the yz250f , kx250f , rz250f (same motor) is 10 hours ,,,, the one I just stripped and told the guy who owns it that it is fukked had 20 hours since last rebuild ... the cam cover caps were over tightend and stripped whixh allowed the cam to vibrate and tore it to bits , ... it would start and run but was rattly as hell , I know we are talking about the bigger motor , but be aware some of these bikes are on the very fine line for servicing , I know they are not as cool or fast , but xr250 400 , 600 etc etc are almost bullet proof .... same with Yamaha xt 500 etc etc .... big over engineered lumps .... I know some on here will cry "bull shit " etc ... but this is my experience yes modern bikes are very well made , ... but they are also very reliant on servicing . YMMV
cheers

Yea bro Ill cry 'bull shit' haha, those service intervals are due to manufactures covering themselves and to push parts. In saying that remember that these bikes (yz,kx,rm etc) are designed as race bikes where servicing has to be done at regular intervals. Without skimping on oil changes/filters/fresh petrol and other basic servicing most of these 250/450 4stroke race bikes can go for a very long time without major servicing if they are not raced all the time.
I used to see it all the time when I was fixing bikes for a living and riding for fun. From memory we had a customer (used as a farm bike) with 190hrs on his WR450 without having the engine apart. Most of the massive hemorrhages I've seen in them have been race bikes(gear boxes,cranks, valves bedded in pistons).

Savage never said if he's a racer, weekend warrior or a trail rider. It's possible to get good hours out of them, but it's certainly not the norm. The wr450 is a very understressed engine compared to the yz250f, you're not comparing apples with apples.
In case you're wondering, I've been in the motorcycle parts trade for nearly 20 years.
I'd have to agree with stumpy on almost every point he's raised.
to the OP, what's your intentions with the bike? That would help with giving you better info on rebuild intervals.

I wouldn't go ignoring the manufactures recommendations, 4 stroke motor x bikes are not known for their reliability in the dirt bike world.
I would be doing a top end on a 250 around 40-50 hours and 80 odd on a 450.

The thing that kills them is that to get them to make power they rev high, and end up with a very short conrod and short skirt piston.
A piston can go a lot long than the recommended 50 hours but if it wears excessively it will start to wear the bore out, then when you do replace the piston at 100 hours the new piston is still crap in the bore and the piston skirts can crack, or the pistons fly to bits.

Checking the valves is also a good thing, but you need to replace them when they say so, as it's not uncommon for valves the snap off and drop into the cylinder head.

They are not as reliable as your car petrol engine that you never do any work on.
It wont take long reading through dirt bike forums to see 2 strokes are regaining popularity due to less tendency to blow up for little or no reason.

Also don't assume that riding slow causes less wear as the engines are designed to rev a lot all the time and but not running them as they were designed can cause extra wear.
There are some 450s that are the exception, like the wrf450s that are known to last a lot longer, but thats typically with very regular oil changes and good servicing.

Ohhhh here we are the old parts man aye, just chuck em in so I can meet sales targets hahaha.
Ive got plenty of examples of " moto x" bikes with no engine work an many many hrs on them.
Haha we just might ride like old ladies here though.
For what they are they are fukin reliable.


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Nope, not a parts man at all. Wholesale importer mate, so I'm not at the frontline trying to sell what's not needed.
I've seen a number of blowups from lack of maintenance, but have to agree that there's a good number of bikes out there with huge hours and little upkeep.
I'd still be on a smoker by choice, but that's my preference.

Oh yea we would have stocked some of your parts if you're working for one of the major ones. Yea two strokes last ages too.
Im just used to seeing a lot of paranoia from owners who only know enough to be dangerous.... and believe every write up they see in the monthly magazines. Both shops I was in always had extended service intervals, otherwise they spend more time in than out. Even our new tractor has the service interval pushed out as suggested by the franchise dealer.


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I'd like to say the biggest, but that'd be bragging.
Misinformed owners can be a good thing, easy money!! My personal protocol on that situation is to provide correct info, as that's a better way to retain customers, especially if they find out you tried to rip them off (or actually did).

I was defiantly a racer running A class and not a trail riding warrior. I did have a riding style where I didn't spend all my time on the limiter though.

By not touching the head I mean the valves. I'd replaced the piston a few times.

I miss my old 2004 CR250, still blew that up a couple of times though.

At the very least, check the clearances. If you have no idea on how many hours the valves have done, it may pay to replace them. OEM ones have dropped dramatically in cost to what they were in the early days of hipo 4 strokes, and have improved in reliability, almost to the point where aftermarket valves aren't worth it. Part of the reason we stopped bringing them in.

Take the exhaust off go T.D. torch in the port and look at the piston skirt ,then kick star lever lower the piston and you'll get a lookthru to the back of the barrel you wont see everything but you will get an idea of or if theirs any bad scoring ,if it looks bad it will give you an indication things mite not be so good else where internally ,other then general wear the biggest contributor to fucked engines are lack of clean air filters and oil i dont like running thinner oil thery be cause of the heat there 4 strokers produce viscosity go's out the window quick enough with out doing that, not knowing how to fit the air cleaner correctly is another , because these bikes run hot i have a rule of thumb ( one ride change oil regardless of hrs 2 ,4 or or 6 hrs and use full syn ) when i sell a bike iv owned it will come with at lest three used air filters ,even at some trail rides i change a filter don't matter if its hot dry wet cold muddy ,a trail ride still getting 200 plus k in a day unless your there to eat your lunch

Originally Posted by NZHTR

Take the exhaust off go T.D. torch in the port and look at the piston skirt ,then kick star lever lower the piston and you'll get a lookthru to the back of the barrel you wont see everything but you will get an idea of or if theirs any bad scoring ,if it looks bad it will give you an indication things mite not be so good else where internally ,other then general wear the biggest contributor to fucked engines are lack of clean air filters and oil i dont like running thinner oil thery be cause of the heat there 4 strokers produce viscosity go's out the window quick enough with out doing that, not knowing how to fit the air cleaner correctly is another , because these bikes run hot i have a rule of thumb ( one ride change oil regardless of hrs 2 ,4 or or 6 hrs and use full syn ) when i sell a bike iv owned it will come with at lest three used air filters ,even at some trail rides i change a filter don't matter if its hot dry wet cold muddy ,a trail ride still getting 200 plus k in a day unless your there to eat your lunch

Holly shit ! sorry guys i have a young guy boarder likes to read this site uses our PC , does a lot of dirt riding wrote this shit buggered if i know what it means ,or if it is fucking dribble ?

Originally Posted by NZHTR

Holly shit ! sorry guys i have a young guy boarder likes to read this site uses our PC , does a lot of dirt riding wrote this shit buggered if i know what it means ,or if it is fucking dribble ?

Yeah he is thinking of a two stoke because you sure as hell can't look up the exhaust on a four stoke and see the piston or barrel. It is a good trick on a two stroke though.

Changing air filters often is good, however they are designed to choke the engine out before they let anything through, I have seen idiots running dry filters before.