I’ve got a 98 WR400F
Somewhat of a newbie here. Quick question – I’ve got a 98 WR400F that I’ve nursed along for a while now. The compression has been fading and an exhaust valve will occasionally stick. Yesterday’s ride presented me with light gray smoke upon start up which cleared up after warm up but didn’t completely disappear. Could the smoke be a symptom of the valves needing shimmed? I’m not too keen on a top end rebuild considering what the bike is probably worth however, i need this thing in running condition when i sell it in about 6 months.
Answers ( No )
I had one of those back in the day. Yeah sounds like rings and valves wearing out. Probably some old oil made it into the chamber.
Change your oil first, reasonable quality synthetic. Motul is ok.
Then pull the spark plug and drop about a coke bottle cap of new oil into the chamber. It's a bandaid job but just helps to seal the upper edge of the rings. Because no oil gets into the chamber usually the top edge of the rings can dry out.
She'll puff a bit but just help her idle along til it clears.
Doesn't hurt to repeat this every couple of rides.
Try not to flog it if you want it to survive. Heat is the enemy.
Ps. Check all the joints on your exhaust and manifold too. Not related but a tiny leak on mine between the block and the header just reaked havoc with the running. Rebuilt the carby twice with no improvement. I blew two carb slides before I worked it out.
There is a leak where the header and muffler join up – seems to be fine where the header joins the motor. I've noticed the rear shock gas resoivoir turning black. No issues with idle, bogging or power delivery once things get warmed up and until yesterday I could usually kick it over (cold) with about 2 or 3 kicks.
Whack a good measure of putty in the joint to be safe. They're pretty tough old bikes but nearly 20 years hard labour would leave anything a bit tired. High compression, water cooled. Not like the old xrs that run forever.
How would leaky exhaust damage the carb slide? Back pressure?
Why not fix it properly? At least do a wet/dry compression test? Otherwise you're just asking 'how do I bodge my bike together so it runs in the shed, and stops in the bush?'
I had to ask the same question. It causes back fires. There's a point in the cycle when both valves are simultaneously open. Milliseconds but it is enough to blast pressure back into the carb. So unless the throttle is wide open the slide cops the force. It's only thin cast ally. Bloody $160 a piece though. Bastards. They changed the design for the 426
Thanks for the feedback guys. I think an oil change, valve adjustment and exhaust leak repair are affordable/logical places to start.