Hello All,
I just sent my 250R motor off to Forward Motion racing (Eric Gorr). For a new top and, and some mild porting and I really want this to be the last time I have the motor redone for a while (I have burnt through 3 tops and a bottom in the last 2 years). The last time I burnt a piston, I took the top end down and noticed there was some type of oil at the bottom of the crank case. Correct me if I am wrong but there should be no oil at the bottom of the crank case right? Could this have been from a leaking crank seal, or WAY WAY too rich jetting? This last top end failed right at the end of the break in process, went though all the heat cycles and was just now starting to get into 3/4 to full throttle when BAM!!! Lots all compression. The piston was BADLY scored on both the intake, and exhaust side.

The reason I am suspecting something other then just lean jetting is the fact that from idle to about 1/2 throttle the bike ran perfect. This is a 265cc Wiesco with new Hot Rod crank, 39mm Kehin carb with 185main, and 52 pilot in the 3rd clip position, Full FMF all the way back. Rad valve, with new Reeds. To be honest I don't remember what the plug looked like this was about a year ago. Also, the pre-mix I was running at the time was Bel Rays's MC-1 which I just noticed suggests a 50:1 mix? This seems extremely high considing the stock motor calls for 20:1. So the question is, does this sound like lean jetting, some type of engine air leak, or the wrong amount of Premix?

Thank you for your help,
Brad

 

Hi Brad,
I'll try and help...

First of all, a little residual oil in the bottom of the crank case is perfectly normal... (Assuming it's 2 stroke oil, not gear oil)
no big concerns there...

I'd really need to see the piston before I could comment on the cause of failure. IF I had to guess, perhaps 4 corner seizure? this is caused by the bore being set up too tight to allow for piston growth.
Or.... lack of adequate lubrication???
I went thru 2 top ends within 6 months on mine, before I figured out there was something wrong with the carb. (Minute spec of crap plugging the pilot circuit) The best thing I ever did was replace the stock PJ for a PWK (38mm).

I'll get flamed for this one, but I also learned the hard way that belRay isn't the best choice of oils IMHO...
Regardless of whos oil you use, follow THEIR recommendations on ratio's... The old 20:1 ratio stamped on the tank is for the old dino oils of yesteryear. Each oil is specifically formulated to work at the recommended ratio.
Also, remember that when you change oils/ratios, you'll need to re-jet accordingly, because it can make a significant difference.

A leak-down test is ALWAYS a good idea after working on the motor. It will let you know if you have problems before they raise their ugly heads by running lean, then kaboom.

Just my $.02

 

Thank you very much for the reply. I kinda suspected that a little oil down there might be normal and I REALLY wish I could say if that oil was trans oil, or mix oil. Its to the point where if you turn the crank and let it sit for a second and then fip the crank 180 there is some stuck to the bottom, not much but noticable. Another thing I did not mention was when I put the new top end back together this last time, the head gasket (cometic) did not go on very easly at all, and two of the water sleve ports were blocked with I later found out was normal.

I am just going to make sure my builder checks everything this time before it gets sent back to me. I am still leaning toward an Airleak but I have also read that using a lower ratio of oil then recemended will cause a lean condition.

 

There are more than one circuit to check when jetting.
Running lean in any one of them can fry the motor.

 

Here is another question for you 250R quys, when adding a larger aftermarket carb to these things, How are you adapting the new larger carb to the intake boot? I simply took a utility knife and cut some of the rubber away from the inside of the boot and then forced it on followed with a standard hose clamp. Seems to work but I am wondering if there is a possible airleak here?

Has anyone out there used the Cometic top end gasket kit for this bike? It does not seem to fit very well, is there a better alternative other then the super expensive OEM gaskets?

 

You could have very well caused a leak there with your 'custom trim'.
I'm not running the stock boot, so it's a non issue for me. I've machined a boysen rad valve for a cr500 down to fit inside the housing, it utilizes a simple rubber coupler.
On my Suzuki, I've stuffed a really big carb in the factory boot, by dropping the boot into boiling water for a minute or so, thus making it much more pliable and stretch it over the carb... seems to work really well, and it's so stinking tight there's no chance of a leak.

I don't like Cometic gaskets... I run O-rings... If your running the stock head, you can use either a CR250 gasket, or seperate the layers of the stock gasket (There's enough layers there for two gaskets) and use one side (Same component as the CR unit)
{Basically there are two gaskets with a thick spacer separating them all riveted together}
Drill the rivet and only use one side... it also bumps your static compression a little too. I'm thinking the CR gasket is quite a bit cheaper since it's only one of the sides..

 

Good point, would it be a good idea to have Forward motion cut a O-ring grove into the head for better protection if possible? I am not sure how much more i can raise the compression on this thing and still run pump fuel, the last time I had it ported, I also had it decked so its already pretty close to the limit. When I did a compression test on it before it blew on me the PSI was well over 200 but I think my guage might have been faulty.

 

I'm running ~225 psi static compression on mine, but I wouldn't even consider running pump gas. Detonation will kill a motor very quickly, good fuel is cheap (Well, not really $Cheap$) insurance.
it also depends on how your builder set the head up (Squish velocity, etc.)