2 Strokes Down Under

Hi all. Had this 74 T500 for a while. Done points, timing, plugs, carbs, exhausts, coils, etc. Convinced everything should be right. When it gets into a run, could be 5 mins could be 10, it suddenly starts to miss, pop and bog down. Gets worse and can just nurse home. The only thing I can put it down to that I have not checked so many times is the crank seals. Just now did it and upon returning took the plugs out, looked lean. Always look lean no matter that I have replaced the 97.5 main with 100. Most importantly right plug shows oily around the outside and some vapour or smoke came only out of the right plug hole. Top end rebuilt. Took off fuel lines and petrol pored out so it`s not starvation. Not blowing oil from exhausts except just after starting, and tends to be both. Just can`t diagnose it beyond the crank seal which apparently is common to blow the right one. Any ideas?

I should start by saying that I know next-to-nothing about the T500 , but...

Even though you have proved your fuel tap and lines clear, there could possibly be a blockage in the carb(s). I have checked this in the past by removing the float bowl, placing a container under the carb to catch the fuel and turning on the tap. The fuel should flood through and you should be able to stop it by gently lifting the float.
This method checks the entire fuel supply circuit in situ (as well as the sealing capability of the needle valve), just in case there is a kink in the fuel line(s) which disappears if you pull the line(s) off to check the flow.
Some admittedly much later model bikes have small "sock" filters in the fuel inlet passages where the lines plug on. I had similar trouble to yours with my RG500 around 1991, eventually finding that rust particles from the fuel tank were clogging the small sock filters, allowing fuel to just seep through instead of flowing properly (luckily they were doing their job!).

There are of course many other possibilities for the cause of your problem. If you are sure the engine is running lean, there is a good chance it is related to lack of fuel. Does the problem go away (even temporarily) if you let the bike sit idling (or with the engine off) for a while?

Regards,
James

Hi seals dont usually play up 5 mins 10 mins into a ride unless plug is fouling. Id be looking at battery not charging correctly even overcharging, condensors, or coils etc getting hot and breaking down. Is it one or both cylinders.It shouldnt look lean though anyway so play it safe and run premix for a while and jet up as well as pumps do play up on occasion. T500s also love to crack heads and it opens up when hot so check real careful from plug hole out I have a box full off road bikes with cracks then ended up casting my own for racing. Link

Seal on the rhs fouling plugs I should say. lh one never fouls plugs just sucks nice gulps of air,mmm.Also if you chuck tranny fluid in the box you can smell it in the smoke if the centre seals are gone. Link

Ignition problem and why run points anyway