What the tell-tale signs of the engine running lean?

I've heard that by looking at the spark plugs, you can tell if the engine is running lean. Is this true?

Also, I've heard that the engine may back-fire if the engine is too lean. If this true, would it back fire all the time or only during acceleration or when backing off on the throttle?

 

A lean bike can backfire out the exhaust. The pipes get hotter and they ignite unspent fuel. It usually happens on decel.

It's actually pretty common on stock bikes due to EPA regs.

 

My bike actually does "pop" when slowing down - so I'm guessing it is running lean. This is slightly frustrating considering I just had the carbs cleaned/rebuilt at a Yamaha shop. thought they would have checked for the bike running lean.

 

You can definitely tell if it's running lean by checking the plugs -- the insulator will be bright white, and the metal will all be bare with little or no colour at all.

Another give-away is being hard to start when cold, difficult to keep at idle, and having to leave the "choke" on a little bit to get good running condition.

Most of these can be fixed with a colourtune plug and some adjustments to the mixture screw on the carbs. That fixes the idle mixture.

For running, you'll have to do a plug-chop --- that's riding at wide open throttle (WOT) then kill then engine and coast to a stop, then check the plugs. That'll tell you how the mixture is when riding.
If it's lean, that actually requires re-jetting however.

 

For checking the lean condition at WOT, do you have to have the bike under load? For example, can you run the bike WOT on the center stand to do the plug chop or do you have to be riding it?

 

In THEORY, you could do it on the centre stand I suppose.... but unless you've got one of those runner-wheel setups for the rear wheel you'd have to be in neutral, and then at WOT you'd probably blow the engine!

Best to go out for a little ride to a highway, open 'er up, shut 'er down, and do a little checking on the roadside...

 

Have a look here for how to read your sparkplugs:

Reading spark plugs

Backfire under de acceleration normal cause lean backfire.

General backfire guide:

When does the Popping happen?

If you are slowing down and therefore backing off on the throttle does it pop then?

The popping is most likely a lean backfire.

If you get popping on acceleration check the exaust gaskets for leaks.

Popping on idle, bogging down, poor running (and hesitation or surging at even throttle) check plugs and coils. Breaking down under load.

 

Best done on a quite country road.
Have a couple of sets of spare sparkplugs on you.

Run the bike at the rev range you want to test, using care pull the clutch and kill the engine. Glide to a stop.

Remember on a bike you run the risk of loosing control when the motor is not running.