Big End Problems

Hi there,

I need your help / advice.

I have a 1983 CR250, which has less than 15 hours on it since a full rebuild (including new rod kit, main bearings, seals etc) after the big end went after just buying the bike, this I could understand as I have no way of knowing the bikes history, however the big end bearing has just collapsed again with fragments wrecking the piston and cylinder head etc.

I run the bike at 40:1 Putoline MX5 (which I have used for 25 years with no problems), on 98 Octane super un-leaded. The bike shows no sign of running lean, the piston skirt and cylinder bore have no damage, plug is a nice colour.

Has anyone had similar problems, is it a common problem with this model and how do you fix it ?

Cheers for any help,

Rathers

Hi there John,

I'll double check, but 99.9% sure they were correct (my mate actually built the engine up for me, and it is back in his workshop so I can't check).

Weird thing is, he has pulled it all apart today, main bearings all look perfect, no damage to the oil seals ,coated in oil, the small end bearing , piston, cylinder wall etc all coated with a nice film of oil again and no signs of distress, but the con rod bearing has just disintergrated.

So far we think either it was a faulty bearing (it was a genuine Husky rod kit ?), or maybe I have over-revved it ( I don't know if this is likely ?).

Thanks for your help.

Rathers

I've seen detonation in four cycle engines destroy a rod bearing. Some high engine speed detonation will never be heard either. Look at your plug with a lighted magnifier for any signs of aluminum- look like tiny specs on the insulator, also inspect the top of the piston. With this engine having the same failure twice, big end bearing which is rotating, small end reciprocating, sounds like the bearing failure is from impact fracture. Your timing might be ok static but higher rpm it may be advancing. I had the ignition on my 87-430 go bad, it would run fine then act like it was going to die, pulled clutch in and give it some fuel just before it would die and it would run again-backwards! Just a thought?

You may have a point, i did have a problem with the mounting plate for the stator cracking and the timing slipped. I didn't have a replacement plate so we re-set the timing and loctited it down, but i am wondering whether or not it was moving slightly at high rpm and advancing the timing ?

There are tiny specs of ali on the plug but as the top of the piston and cylinder head has been trashed by fragments of the bearing, I don't know whether or not this occured pre or post bearing failure , if you see what i mean. The actual plug colour looks spot on, a nice dark tea colour, so it doesn't look like it was running lean or detonating ?

The thing with plug color is, what rpm/load was the engine running when it was shut off? I have always wondered at the races when guys ride back to the pits then check the plug color, no accurate way to tell what it was doing during running "load" after slobbering back to the pits. I feel that is a very unaccurate way to check fuel mixture. If you were having an engine failure, hearing a rattle, the engine wasn't running right then shut down, it would be hard to tell from the plug. With my drag car I used to shut it down just crossing the line and get towed back to the pits and check plug color. Finally put a EGT probe in each side and never looked at plug color again. Check EGT under load and adjust jetting as needed. I hope you find the problem. I never have liked the inaccurate timing control of magnetic ignition, as magnets age they effect the pre-post magnetics passing the stator which effects ignition timing. I wish someone would come up with a LED/digital ignition for our bikes.

...A plus side of running HEI on mine with just a battery, module, coil and pickup.^_^