Hi, I would really appreciate some help with a problem I've got.

A few months ago I bought a worn out YFM 200 89 and I've been gradually restoring it back to its former glory. Unfortunatly the cylinder head is damaged and the camshaft and rockers are completely knackered. Looking on bike bandit I noticed that the XT200 82-83 has the same cylinder head so when I came accross one for sale cheap on ebay complete with camshaft and rockers all in very good conditon I couldn't believe my luck. The problem is that now that I've bought it I've realized that the camshaft and rockers on the XT have different part numbers then those on the YFM.

Does anyone know if these parts will still work in my engine?? I've noticed the bore, piston stroke and compression ratio of the two engines are the same. Also the cam chain, sprocket and valves have the same part number.. Does anyone know what the difference is between these two cams?? If I install it in my engine is it likely to greatly affect valve timing and damage my new piston or just change the characteristcs??
Any help would be greatly appreciated as my knowledge on cam shafts is very limited. Thanks for your time and sorry for the long post.

 

My guess, and it's only a guess is the cam is ground a little bit different to push the torque curve around a bit. No biggy if I'm correct.

If it has the same piston # the new cam is not going to open the valves enough to wack the piston so your fine there.

I can't say about the rockers. I would look VERY hard at those and see what's up. Maybe a different ratio? I just can't say.

All in all if the bore and stroke and all of that is the same I would not worry too much.

Let us know what you find out.

 

Hi, thanks alot for your reply doctorturbo.

I've just realized that the YFM and XT pistons aren't the same! The piston rings, pin and con rod are, but the piston is a different part number.

I guess now I can't be sure that the piston is not going to hit the valves?

I suppose I could buy a piston kit for the xt200 and then there would be no need to worry but having already bought the yfm piston this just seems like another expense I could do with out.

Anyway, thanks again for your reply and I'll keep you updated.

 

Another guess, but it's probably just a slight compression difference.

I personally don't know of any stock cams that have enough lift to wack the piston when in time. Now aftermarket with crazy compression, that is a different story.

Something else to think about; if the con rod is the same and the crank is the same the pin highth of the piston is going to be the same.

 

Hi Doctorturbo, thanks for all your input it’s much appreciated.

Just measured both of the cams and they are almost identical, I think the slight difference between them is simply due to the YFM cam being more worn. I guess there could also be a very slight difference in the angle like you said, but certainly nothing to make me think the valves will hit the piston. As for the rockers, they look different in design so I'm not too sure..

Unfortunately I've now got another worry.. With both cylinder heads being the same part number you would expect them to be identical but for some reason they take different valve sizes (Even though Bikebandit states these valves have the same part numbers). The YFM having larger valves than the XT.

Do you think this could just be some aftermarket work that has been carried out on the engine? And also is this something I should be worried about or not?

This would've all been so much easier if I had just been patient and waited for a YFM cylinder head

Thanks again and srry if some of my questions seem a little silly, this will be the first 4-stroke engine I've rebuilt so I'm still trying to get my head around a few things. Cheers.

 

Well, I've just solved the valve size mystery.

Turns out the guy sent me an XT125 cylinder head instead of the 200cc They look identical apart from the valve sizes and the slightly different bore size.

On the flip side the 125cc cam shaft is the same part number as that of the YFM200. So no more worries about that but I've got to admit this build is turning into a real headache!

 

Wow, well that sucks.

Sounds like your on it though. Good catch!!! It's very cool that your doing this on your own even though this is your first 4 stroke.

Sounds like you are taking your time and looking at every little detail which is what you should be doing.

Let me know how it goes.

Woooo Hoooo, race day, working on the car today and racing tomorrow.......got to go!!!

 

Hey, sorry for the delay but I've only just got the engine back together and in the frame.

Overall I think the build went pretty well, the only problem I had was when after installing the cylinder I let the timing chain drop down into the crankcase.As you can probably immagine I then had a lot of fun trying to get it back out.

Unfortunately now it seems I've got more problems because I just can't get the engine to run right. Its probably just jetting problems but I would very much appreciate some input on this.

The engine starts up but gets very hot and starts spluttering and popping. I assumed it was running lean which would make sense seen as I've now got a .50 oversize piston in there but when I pulled the spark plug it was pretty black. Also the engine starts without the choke, but if I pull the choke when the engine is running the engine dies. All suggesting a rich mixture?? The problem is I've played around with the pilot screw and it makes no difference same as the jet needle position.

I'm 90% sure there are no electrical problems (I've tested everything with a multimeter) its got a new ignition coil, wire harness and spark plug. Plus theres a very strong spark.

Everything inside the carb is standard so having installed an oversize piston surely you'd expect the mixture to be to lean right??

Any help, ideas or opinions more than welcome. Thanks.

 

Could be a lot of things.

Most of the time you don't need to go up in jet size because the bigger piston will draw more fuel through the jet.

From what you have said I would double check the "valve" timing and lash.
Both of those would do what you say the engine is doing.

 

Hi, thanks for your advice Doctorturbo you was right!. I double checked the valve timing and clearances and found that the exhaust valve clearance was way out. I clearly didn't torque the locknut enough when I set it during the build.

I haven't yet had a chance to find out how the engine is now running because I've taken the exhaust of and prepped it for spraying but I'll keep you updated.

Thanks again, the valve clearance would have been the last thing I'd have checked given I was so sure I had done it right the first time. You've probably just saved me alot of wasted time and money.