YZ450f Longetivity gains with richer jetting?

Planning on riding this 08 YZ450 in the dunes and was considering richening for slightly lower elevation and WOT runs. I used to always do this for my 2-strokes for obvious reasons, but I know 4- stokes get the same lube via direct engine oil to the cylinder. Can lean jetting on these 4-strokes burn up valves or melt pistons?

Thanks for your input!

In most 4-strokes, jetting lean enough to pose any danger to the engine's health will cause performance problems enough to get noticed before any damage is done. Pinging or irregular misfiring due to lean mixtures are the first signs.

Jetting a 4T excessively rich is detrimental to good cylinder lubrication.

In most 4-strokes, jetting lean enough to pose any danger to the engine's health will cause performance problems enough to get noticed before any damage is done. Pinging or irregular misfiring due to lean mixtures are the first signs.

Jetting a 4T excessively rich is detrimental to good cylinder lubrication.

How so, GR???? Not trying to show you up or disagree, I'd just like to know why.

Excess fuel fuel ends up diluting the oil, and washing out embedded anti-wear compounds.

I believe the excessive fuel can wash the rings out and then cause excessive carbon build up...... There are probably other reasons too.

I need to type faster....:smirk:

"Washing the rings out" is a little bit of a misnomer. The compression rings are actually primarily lubricated by fuel (this is another dirty little secret about ethanol that no one likes to talk about; it's very "dry", and really needs a "top oil" additive). But when the jetting is too rich, the amount of unburned fuel pushed past the rings increases, and this has a very negative effect of the oiling at the piston skirt.

You are very correct about carbon, though, and in particular, the carbon that builds up around the piston top ring lands. This can cause cylinder wear, and cause the rings to bind in the grooves, increasing the blow by, and further damaging the piston skirt.

my old lawnmower that had a Brigg and Stratton engine was always run on my premix. That thing lasted years. I know its a 4 stroke but I cant help but think that Yamalube R had to help keep that thing running. I abused that motor and never changed the oil. Id like to hear your opinion Grayracer. Maybe I just got lucky

lol. Some small engines will run off anything. I had a Craftsman push mower (tecumseh motor) and I'd use anything in it. I ran 110 octane leaded race fuel, stale Pro 101 octane race fuel, premix, and once I even topped it off with kerosene (didn't know it at the time).

Now I bought an expensive Toro (with personal pace :smirk: ) and I try to run regular 87 in it. :)

my old lawnmower that had a Brigg and Stratton engine was always run on my premix. That thing lasted years. I know its a 4 stroke but I cant help but think that Yamalube R had to help keep that thing running. I abused that motor and never changed the oil. Id like to hear your opinion Grayracer. Maybe I just got lucky

You're comparing apples to a wax zucchini. A YZ450 produces over 110 hp per liter under a constantly changing load state ranging to over 11,000 rpm. The B&S is a steady state implement engine that produces barely over 20 hp per liter running no more than 4500 rpm. You could probably run a B&S on camp stove fuel and lube it with Crisco and still get 10 years out of it.

You're comparing apples to a wax zucchini. A YZ450 produces over 110 hp per liter under a constantly changing load state ranging to over 11,000 rpm. The B&S is a steady state implement engine that produces barely over 20 hp per liter running no more than 4500 rpm. You could probably run a B&S on camp stove fuel and lube it with Crisco and still get 10 years out of it.

LMAO!!^^^^

haha I bet your lawnmower will smoke my 450.. literally.

You're comparing apples to a wax zucchini. A YZ450 produces over 110 hp per liter under a constantly changing load state ranging to over 11,000 rpm. The B&S is a steady state implement engine that produces barely over 20 hp per liter running no more than 4500 rpm. You could probably run a B&S on camp stove fuel and lube it with Crisco and still get 10 years out of it.[/QUOTE]

I 2nd that. That's hilarious.

You could probably run a B&S on camp stove fuel and lube it with Crisco and still get 10 years out of it.

Quick, somebody call Mythbusters.

Getting back to the original topic, how lean is too lean? My YZ426 is bone stock with the exception of a pipe. The previous owner never rejetted after installing this pipe. My understanding is that with the aftermarket pipe and dense cold air, the stock jetting is not providing as much fuel as I need. It runs great though: no backfiring, no sputtering, gobs of power. The only thing I do notice is that the top end feels a little weaker than what I was expecting (I'm new to stroke mx bikes, not sure what it's supposed to be like). I'm not sure if it's the lean condition causing this, or the fact that I'm expecting a bigger top end hit like the 250 smoker I'm coming off of.

You are probably expecting a hit more like a 2t and that is making you think that you are weak on the top.

Like GR said, while you can jet a 4t to be excessively lean and damage the engine in the process, you will notice a serious drop in performance before that happens.

... how lean is too lean?
16.5:1 or so is where the trouble really starts. At that point, the engine will begin to misfire from being unable to ignite the fuel charge. Maximum power is developed at around 13.5-14:1, and true stoichiometry (the chemical point where there is precisely the right amount of oxygen available to completely oxidize all of the fuel, no more, no less) is 14.7:1. A 4-stroke will typically put up with AF ratios as rich as 11:1 before it starts to show obvious signs of excess richness such as black smoke or misfire, but it won't be as powerful at 12.5:1 as it was at 13.5:1.

The thing is that the engine will tolerate being fairly well off of optimum and run quite well. When the engine is leaner than 41.7:1, the excess oxygen simply goes unused, and doesn't really add anything to the combustion chamber temps unless it can find something besides fuel to react with, like maybe carbon deposits, etc.

Unless you use a dyno and a gas analyzer, you just have to try a couple of different jets and try to get a feel for how they work. To use my '06 as an example, I run a 165 in it most of the time. I have run a 170, which is actually too rich, but it doesn't cause any really obvious symptoms of poor performance, and the bike "feels" pretty strong. There are just subtle things about the way it runs with that jet that make the engine feel less crisp and sharp. So, if I were to install a 175, that might start acting up to a noticeable degree, or it might not, but the jet that I do run is 6% smaller than that in size, and 3% smaller than the jet I tried that was only subtly too rich.

Did that help any?

I would love it if my bike had EFI and a Air/fuel ratio gauge!! I have a turbo 4 stroke snowmobile and A/F gauges make it easy to tune!!

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