Will a Four Stroke Motor Damage with Lean Condition?

Ruggybuggy

Ruggybuggy

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So here's the question, if you lean out a four stroke motor will you do engine damage? I work as an auto tech and this was debated in the lunchroom today. My FZ1 recently had a problem with the Juicebox Pro fuel controller and I called Ivan about continuing to use the bike (I had his reflash done) and he said with the modes and the reflash you could do engine damage from a lean condition and to not use the bike. Now jumping to the marine community I also own a Princecraft boat with a '08 Merc 75 outboard. Mercury makes a 75, 90 and 115hp versions of this same motor. All three of these motors are identical including the ECM's. The only change is a air restrictor on the throttle body to reduce the air available. Remove the restrictor and boat performance improves. Top speed before was 38mph and now it's 43mph. The holeshot also improved with better fuel mileage. Everyone in the marine community says you will lean out your motor and cause damage. Some of the guys in the lunchroom said "it's a four stroke and uses engine oil to lubricate the pistons and if you lean it out two much it will just perform poorly" and others would say "leaner mixture will produce more heat and higher cylinder pressure that can lead to a damaged pistons". So what's the consensus here? My personal feeling it would be fine and I've never seen four stroke engine damage from a lean condition. :popcorn:
 
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Go ride your bike, worry less.

Unless you are pulling a zillion point of fuel trim out, you will not harm your motor.

How many points of fuel did his map add in the cruise range?
 
Of cause you can do damage from too lean a mixture in a 4stroke,seen plenty of motor`s nip up[to hot] hole piston`s etc,But saying that I believe the FZ1 has forged pistons and you did`nt sieze up the motor so unless you are suddenly using more oil,blowing smoke or have low compression you should be fine
 
Hellgate said:
How many points of fuel did his map add in the cruise range?
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Up to 40 but not worried about the bike. Just threw out the question can you lean a four stroke out and cause damage. I just asked the question because it's come up twice for me (boat and bike) and some say yes and no. But back to the bike you say no but Ivan says yes. Not trying to stir the pot, just want some discussion on the topic.

Let's say you take any given four stroke installed in auto, boat, bike or whatever, can you cause engine damage from a lean condition. Two stroke motors guaranteed but because 4 strokes cool and lubricate their pistons with engine oil would this prevent a meltdown?
 
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Of course you can. If you make it too lean you will get detonation instead of a burn of the fuel and damage the piston head and more.
 
My brother burnt holes in the pistons of two Hondas many years ago. Caused by perforated paper air filters causing a lean mixture, not pretty. Both stock untouched.
 
Yes you can. 4's are much more tolerant tha 2's but yes it can and will cause damage if too lean. With the boat its probably fine since you took out some restriction.
Let's put it into prespective: would you add a turbo without changing fueling? Of course not you will eat a motor in no time. All you are doing is leaning it out by adding more air...
 
Absolutely.

I have changed many pistons and cylinders.

If you are truly concerned, replace your rings and hone the cylinders. It really isn't that bad of a job. Invite a buddy over on a Friday or Saturday and drink some beer in the garage.

I have replaced many pistons and rings in two stroke and four stoke dirt bikes. even if a 4 stroke runs lean, it may have decent compression but you can still see blow by on the piston.
 
E10 str. Fuel ratio is 14.1:1 compare to the old 14.7:1

I tune my ride to 13.5-13.8 in the cruising range. 12.8 to 13.0 run good on the bottom end and make the throttle a little smother.

7% fuel trim in the PCV is 1 fuel ratio. (14.0 to 13.0:1)

The worst part of running lean is detonation. Espacially when you start cranking the ignition timing advance...


Sent from my SAMSUNG-SGH-I337 using Tapatalk 2
 
Chandler said:
Absolutely.

I have changed many pistons and cylinders.

If you are truly concerned, replace your rings and hone the cylinders. It really isn't that bad of a job. Invite a buddy over on a Friday or Saturday and drink some beer in the garage.

I have replaced many pistons and rings in two stroke and four stoke dirt bikes. even if a 4 stroke runs lean, it may have decent compression but you can still see blow by on the piston.
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He I asking if running the bike without the fuel controller until he gets a new one will damage the bike since it will be running lean. So why do you want him to change the rings and hone the cylinder. I don't see the connection here.
 
So if a lean condition will cause engine damage in a four stroke at what air/fuel ratio would it start to occur. Everyone knows 14.7:1 is the target for best combustion but if you started to lean it out to say 15.7:1 would you notice a performance drop? I agree detonation will kill a engine quick but couldn't you have a lean condition and not have detonation?

As auto techs we rarely see damage from lean conditions because automotive fuel injection is a closed loop system. With knock sensors detonation is a rare occasion. One engine that comes to mind were the older 4.0 Ford motors installed in Rangers. The mass air flow sensor would build up dirt on the sensor wire and the system would run lean. Detonation would occur and that motor would rattle up a storm to let you know it's happening. You would replace or clean the sensor and problem was fixed without any ill affect. Anyone every hear their motorcycle motor detonate?
 
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Kpd1031 said:
He I asking if running the bike without the fuel controller until he gets a new one will damage the bike since it will be running lean. So why do you want him to change the rings and hone the cylinder. I don't see the connection here.
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This discussion was never intended to answer a question about my own bike. I just wanted to listen to a discussion on 4 stroke lean conditions and engine damage. I do agree with you on your post "So why do you want him to change the rings and hone the cylinder". Bikes has very low mileage and runs great, why the hell would I take it apart? I could bore scope it if I wanted to look for damage.
 
Ruggybuggy said:
So if a lean condition will cause engine damage in a four stroke at what air/fuel ratio would it start to occur. Everyone knows 14.7:1 is the target for best combustion but if you started to lean it out to say 15.7:1 would you notice a performance drop? I agree detonation will kill a engine quick but couldn't you have a lean condition and not have detonation?

As auto techs we rarely see damage from lean conditions because automotive fuel injection is a closed loop system. With knock sensors detonation is a rare occasion. One engine that comes to mind were the older 4.0 Ford motors installed in Rangers. The mass air flow sensor would build up dirt on the sensor wire and the system would run lean. Detonation would occur and that motor would rattle up a storm to let you know it's happening. You would replace or clean the sensor and problem was fixed without any ill affect. Anyone every hear their motorcycle motor detonate?
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It was likely pre ignition vs detonation. According to the ASE manual I have detonation can blow a hole in the piston. I had an old 4.0 explorer detonate and blow the ceramic right out of the plug base.

Why with all the safety sensors etc if lean fuel condition was no big deal will it trip a CEL on a car?
 
Sebastian F7 said:
Why with all the safety sensors etc if lean fuel condition was no big deal will it trip a CEL on a car?
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Some will and some won't. On the old Ranger 4.0 if won't turn the light on. It'll just knock away. When your plug blew out detonation contributed to that but most likely an old plug or a defective one. I'll have autos coming in with the porcelain blown out because rust starts to weaken the bond.
 
Ruggybuggy said:
Up to 40 but not worried about the bike. Just threw out the question can you lean a four stroke out and cause damage. I just asked the question because it's come up twice for me (boat and bike) and some say yes and no. But back to the bike you say no but Ivan says yes. Not trying to stir the pot, just want some discussion on the topic.

Let's say you take any given four stroke installed in auto, boat, bike or whatever, can you cause engine damage from a lean condition. Two stroke motors guaranteed but because 4 strokes cool and lubricate their pistons with engine oil would this prevent a meltdown?
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If you are within normal tuning parameters you're fine. If you take out 100 point of trim you can harm your motor.

15.5 is fine if it is what is called for. 12.0 is fine if that is what is needed. Each rpm, and throttle position requires it's own ratio or CO% setting. It is what it is. There is no "magic" AFR.

This is kind of a stupid thread.
 
Ruggybuggy said:
This discussion was never intended to answer a question about my own bike. I just wanted to listen to a discussion on 4 stroke lean conditions and engine damage. I do agree with you on your post "So why do you want him to change the rings and hone the cylinder". Bikes has very low mileage and runs great, why the hell would I take it apart? I could bore scope it if I wanted to look for damage.
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I apologize, I had just read many of your other posts about your bike incident recently and even though you were not going to worry about your bike - I assumed this was just for an informative answer for kicks and grins. - either way I do not have enough knowledge to give you a ratio that damage will occur - I would think it would depend on a lot of other variables such as how the piston, cylinder, and rings were manufactured.
 
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