WR450F Bogging Down at High Revs

I took my WR450F in for it’s MOT at the start of the month. The last time I’d taken it out (a few months previous) it was bogging down after 15 minutes of riding. It seemed it just wouldn’t provide the power is should over certain revs. I would have to change up through the gears very quickly to get any kind of speed, almost like a rev limiter was kicking in.

The guy round the corner cleaned the carb and said there was a little water in there.

It seemed to be fine. I’ve been out on it today and it was around 50 minutes in, after I’d been down a muddy, wet green lane that it seemed to be doing it again on the way home. I get to 13mph in 1st, 20 in 2nd and it’s starts bogging down, sometimes on then off, on then off. Out of gear, revving the engine it sounds fine.

I assume it is a fuelling issue? I’ll take it back round in the morning, just wondered about people’s thoughts on it?

It’s something to do with the burning not happening properly, generally, so I’d check the spark is sparking properly (and see what it looks like) that there’s no fuelling constrictions (carb, line and tap) and the air supply’s fine (filter’s looking good, nothing’s blocking anything). Head gasket’s a plausibility, too, so now all I’ve done is said “I think it’s in the engine” :slight_smile:

The mud and wet affecting things makes me tend towards something electrical - I’d clean out the hole the spark plug is in, check the contacts are fine on the end of the HT lead, the HT coil and stuff.

Does it smell funny when it runs?

filter is full of shite.

Almost certainly - I had similar issues when I put a K&N in the wrong way round…

Thanks guys. Hmm, a filter? I might even be able to do that myself?!

I don’t think there was a smell Big Red S.

I had a similar problem with some of the yam
trailies on the last off-road day, Barry said the
bikes were, “over charging”.

Then again I’ve had something not too different
on my own bike and it was a pinched fuel line.

couldn’t find a video for a fuel filter replacement.

If its cold when you ride it could be carb icing.

No, it seems to be when the engine is hot. From cold it’s been fine the two times it’s happened.

Someone help me understand why it only happens in motion. If I stop and rev, it flows up through the revs with no problem?

Here comes my complete lack of knowledge of a working engine, but is an engine sat stationary running at 6,000rpm different to an engine on a moving bike running at 6,000rpm? Logically to me they should be the same use of fuel and air?

I’m often wrong but I’d agree with that logic.

When the contents of the cylinder burns it releases energy, which is transferred to the piston, through the crank and gearbox and to the back wheel. This energy accelerates you at a rate proportional to the weight of that being accelerated (you and the bike) and the force with which the energy arrives (this is the newtonian mechanics you learnt in school, but in the real world).

It takes a lot more energy to propel a bike and rider to 20mph with an engine spinning at 4000 rpm than it does to spin an engine at 4000 rpm with no load - for the same rate of acceleration - so it’s possible to have incomplete combustion that causes very little energy to be transferred through piston and crank, perhaps enough to spin the gearbox but not enough to move you and the bike at 20mph.

The power output of a bike is entirely dependent on how well the fuel and air mixture in the cylinder burns, and with how much force it pushes the piston down. If it doesn’t expand with enough force to push the piston against the resistive force from the rear wheel the engine stalls. If it expands with more force than is necessary to simply propel you then you accelerate.

Okay, I think that makes sense. So if the ‘revs’ are the number of mini explosions in the engine each minute, the actual power provided by each explosion changes depending on the resistance (me and the bikes weight being moved). Therefore I pull in the clutch it can rev high as not much actual energy is being taken from each revolution, but engage the engine again and it doesn’t have enough fuel/air to make each explosion powerful enough to propel me?

Yeah, essentially - it’s very easy to spin an engine that has no load.

Your total power output depends on how fast the engine is spinning and how much power is gained from the explosion every second revolution. If an engine turns very slowly, or doesn’t burn fuel efficiently, it wont produce much power.

Okey dokey.

Just started it up, it’s bogging down right away, so the engine temperature is not a factor, which means it is more likely something has blocked or limited the fuel or air.

The air filter looks okay to me…

It was rather oily though.

That’s the limit of my investigatory ability. I’ll take it round the corner to the mechanic again I think.

There’s a quick and easy test to see if it is overcharging…

Give it 10 - 15 mins with lights on, while the engine is
not running, to drain some charge from the battery.

Then take it for a run, if it is the overcharging issue
it will run fine for a while until the battery is
fully charged again.

I seem to remember Barry strapping the front brake on
his bikes, so the brake light was on, to drain the battery,
those bikes didn’t have headlights, (which is probably
why they were overcharging)

Obviously, if you end up with a flat battery then it’s
not the “overcharging” issue, but you’ve got a kick-start
so that shouldn’t be a disaster.

I think Barry’s theory was that the increased voltage
generated by higher revs, while it had an overcharged
battery, caused the ECU to effectively limit the revs,
by reducing power, to bring the voltage down.

Doesn’t make much sense to me but the test is quick and
easy.

That’s interesting. My battery seems to be a bit of an issue though, I don’t think it charges properly and is dead within a few seconds of trying to start it with the electric starter. I had to kick start it this morning and it was suffering from the fault straight away.

It does feel like the rev limiter is kicking in when it shouldn’t though.

That doesn’t sound like the overcharging issue.
But there may be another electrical issue.
And it may be time for a new battery.

I had a similar issue with a 426 WR

I would suggest draining the float bowls of the carbs a few times and priming it between running it. If its water in the bowl then itll probably be only higher revs that it blocks the jet.

Check your main jet, bristle from a soft broom is perfect for the job.