98 YZ400 pair of strange problems

[darrenposton]

So I've had this bike for about a year now and ridden it a couple dozen times and had really low hours on it when I got it. It's always started first kick even hot or after a crash and it still starts just fine and up until the last ride this weekend has always run really strong and smooth.

This weekend however, 2 problems popped up practically at the same time.
1) At midrange rpm and up at any throttle opening it developed a miss....it felt just like the bike was running out of gas, but it had plenty. At low upto midrange rpm at any throttle opening it ran as always - no miss what so ever.
2) On the way back to the truck after the first problem popped up it became apparent that sometimes after letting completely off the the throttle the engine wouldn't return to normal idle speed as if the throttle were stuck open at between an 1/8 and 1/4 throttle. However, the throttle was not mechanically stuck at all and moved just as freely as always. Furthermore, a quick blip of the throttle would bring the idle immediately back down to normal.

Last night I pulled the subframe off to gain easier access to the carb and found absolutely nothing wrong in it and everything looked very clean. I started it up with out the subframe on it to see if I could duplicate either of the problems and maybe see what could be causing it, but neither problem showed up.

The airfilter was pretty dirty but not to any extreme. However, the last time I serviced it I over-oiled it and the excess dripped out the bottom of the airbox. These problems showed up on the third ride after servicing. I live in west Texas so it's really dusty and 3 rides is just about the limit for how long you can go on a air filter service.

At this point my only conclusion is that the over-oiled filter is actually a lot dirtier than it looks and that is what's causing the problem.

Does anyone have any thoughts of what else I might need to look for? I just find it really hard to believe this bike could be so touchy about the air filter and be so obviously effected when it gets too dirty.

Thanx
D

 

[BSWIFT]

For a start, check the valve clearances, insure that all vacuum lines are in good condition and properly seated on the carb. If your bike has been starting the first kick(time) every time then your bike was well tuned. The high idle makes me think of an air leak but I it is only a guess.

 

[darrenposton]

I had already eliminated valve clearance issues cuz I checked it prior to this last ride and it was spot on. All the external vacuum lines, hoses, connections etc. are all in good shape and tight. I too first thought air leak so double checked to make sure the hot start hadn't started leaking as well but it wasn't. Both problems do indicate in my opinion a lean condition however.

I was talking to my buddy this morning and he said something similiar happened to his quad a while back and he found that a piece of trash had been sucked up into the main jet causing a lean condition.

I didn't pull the bowl plug or main jet last night, but after thinking about what he said, it seems to match up and could very well be the problem - will be checking that this evening

D

 

[holeshot]

This is probably not the problem, but -

The gas tanks on these 400's/426's often developed cracks (and leaks) at the rear corner of the tank. The result is that gas would be sprinkled on the air filter at various points in the ride, creating a rich condition that caused the bike to burble and cut out. Sometimes, it wouldn't leak and develop problems until later in the ride (when things got aggressive).

People often referred to this as Yamaha's "self cleaning air filter system". :laugh:

Just something to look for ...