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Discussion Starter · #1 ·
So we took the exhaust off of my buddys 2011 250 today and wow. It was loud and sounds pretty throaty. He wants to keep it this way but he heard something about it damaging the valves. Does it hurt anything else? Can anyone provide any input on this? Thanks.
 

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Valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean.

If you remove the exhaust can, you need to rejet. Also If you like straight pipes, is better to switch to a 2 in 1 without compensator, like the OEM has.

Wear earplugs. Cheers.
 

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Discussion Starter · #3 ·
Thanks for that good info. Very helpful. One more question though...what is rejet? Sorry, Im new to the bike world. Lol
 

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Theorically speaking, rejetting is changing the quantity of fuel and how is delivered into the engine by the carburetors.
Basically, you adjust the quantity by changing the jets and the "fine" adjustement by changing/shimming needles. When you remove things as OEM filters or exhausts, the engine works without restrictions = it "breaths" better = burns more fuel (needs more fuel) = more power.

If you buy a complete jet kit, such as dynojet, etc.. it comes with instructions on how to do it, adjust...

Cheers
 

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Discussion Starter · #5 ·
Thanks for that man. I appreciate it. Any idea on a price for that kit?

Does it add any performance qualities?
 

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Valve burning doesn't occur from a lean burning engine. I did this writeup some time ago about the "myth" of burning exhaust valves from removing your exhaust:

http://www.kawiforums.com/off-topic-discussion/153031-vex-motv8s-random-daily-factoid-will-no-way-benefit-anyone-2.html#post1956189

You know what happens when you lean out EXCESSIVELY?




You melt pistons. THAT engine had a sna-fu when the fuel pump seized under a heavy load and the piston literally melted inside the cylinder. The head? Perfectly fine and is still in use. The valves are all kosher. In that write-up I did about burning up valves I explained where the myth came from and why it doesn't apply (any longer). Now a days if you want to burn up a valve, just neglect doing your valve clearance checks and eventually when your valve lash goes to zero your valves will burn up because they aren't experiencing enough seat time to cool down.

And removing the muffler on a 250 is going to increase flow so minusculey that needing new jets isn't necessary. Most people with full exhausts (which flow significantly better than the stock header and muffler) still don't need to rejet the 250s. Only shimming the needles is really necessary.
 

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You know what happens when you lean out EXCESSIVELY?


your motor turns into a single-cylinder yamaha rig?
 

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Valve burning doesn't occur from a lean burning engine. You melt pistons. THAT engine had a sna-fu when the fuel pump seized under a heavy load and the piston literally melted inside the cylinder. The head? Perfectly fine and is still in use. The valves are all kosher. In that write-up I did about burning up valves I explained where the myth came from and why it doesn't apply (any longer). Now a days if you want to burn up a valve, just neglect doing your valve clearance checks and eventually when your valve lash goes to zero your valves will burn up because they aren't experiencing enough seat time to cool down.

And removing the muffler on a 250 is going to increase flow so minusculey that needing new jets isn't necessary. Most people with full exhausts (which flow significantly better than the stock header and muffler) still don't need to rejet the 250s. Only shimming the needles is really necessary.
Well, in first place, is good to know that somebody can give a "real" experience of what happens with a mono engine with lean setup on the intake. Mi appreciation is based on A/F readings on board (I have a wideband connected into my exhaust) to check the stoich relation with my intake/exhaust setup since OEM.
Out from the dealer, no changes made, I had 12.8 at idle and 13.5 at full accel (sniffer at dyno) fair little 26.9 HP at that run
No exhaust - straight pipes (I had the same feeling like you Vex -No rejetting needed) : 12.2 at idle, 12.9 full accel.
So, no valve burning, OK. BUT if you are running a not well valve clearance checked bike and if you run straight pipes, things are gonna be hot there, pistons, exhausts and valves too..
Ok then, is not just your valves are going to be cooked, but its on the melt-combo too.
Now I'm running with 13.2 at idle and 15.6 at full accel (112 main, custom made needles + 2x0.7mm shims)
Off topic : when running with 30% meth mixture : 12.8 idle 14.8 full, I need to go up on Mjets I know, but working with ignition right now...
 

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This is all really good info. I have a 650r and have been searching the forums to find the answer to this question:

"slip on" vs. "full exhaust"

What is the difference other than the obvious. And what are the pro's and con's of each?
 

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Well, in first place, is good to know that somebody can give a "real" experience of what happens with a mono engine with lean setup on the intake. Mi appreciation is based on A/F readings on board (I have a wideband connected into my exhaust) to check the stoich relation with my intake/exhaust setup since OEM.
Out from the dealer, no changes made, I had 12.8 at idle and 13.5 at full accel (sniffer at dyno) fair little 26.9 HP at that run
No exhaust - straight pipes (I had the same feeling like you Vex -No rejetting needed) : 12.2 at idle, 12.9 full accel.
So, no valve burning, OK. BUT if you are running a not well valve clearance checked bike and if you run straight pipes, things are gonna be hot there, pistons, exhausts and valves too..
Ok then, is not just your valves are going to be cooked, but its on the melt-combo too.
Now I'm running with 13.2 at idle and 15.6 at full accel (112 main, custom made needles + 2x0.7mm shims)
Off topic : when running with 30% meth mixture : 12.8 idle 14.8 full, I need to go up on Mjets I know, but working with ignition right now...
It doesn't matter how many cylinders you have. Stoichiometry is fuel dependant (unless you're doing forced induction then you need a little more fuel). We run a wide-band actually, but that ECU didn't have a provision to shut down the ignition if the O2 readings went too high (the new ECU does). The engine we used to use? A ZX6r engine on E85. Tuning that is identical to tuning the single. IDENTICAL.


Somewhere in there is a ZX6r engine :evil6:

At idle/low load with GAS 14.7 is the ticket. Anything richer is a waste. I think you mean you're doing 13.2 under full load and 15.6 at idle? Even then that's too lean.

That and we run E85. You're only looking at lambda. It doesn't matter what mixture you're running (say 15% ethanol, 60%, 85%, etc.) the sensor will only output a lambda signal. You can run full E100 and if your gauge is set for gasoline it's going to read "14.7:1" perhaps at idle even though it's REALLY at 9:1. The lambda sensor is just outputting "1". I tune for any decent ethanol mixture for a lambda of 1 for no load/light load, 1.1 for decel and .82 for heavy load. Then I fine tune the O2 correction map around those values. You can literally toss a completely different fuel in and theoretically it will trim your pulse widths to match (although most ECU's will throw a code because that's over a 10% trim which is considered "questionable")

And I tune engines ON a dyno, but I don't measure power. I only increase the load and tune to the A/F ratios I'm looking for. You will make maximum power and/or efficiency that way. I don't actually play with measuring torque until I'm setting my ignition maps. Because contrary to popular belief advancing your timing until it detonates and then backing off a couple of degrees doesn't necessarily net you the most power. Getting a consistent A/F ratio of 12.7:1 (for gasoline) for your fuel mapping does.

As for the valves typically your valves are steel (or titanium in our case). The valves can take a lot more heat and not expand like aluminum. If you burn up a valve it is 95%+ of the time due to insufficient valve clearance. I've never seen a valve cook from leaning out. Your aluminum piston will melt or your aluminum head will warp long before the valve follows suit.

This is all really good info. I have a 650r and have been searching the forums to find the answer to this question:

"slip on" vs. "full exhaust"

What is the difference other than the obvious. And what are the pro's and con's of each?
Slip-ons pro:
Save weight. Factory cans are pretty heavy.
Different tone: Especially with the 250. The factory muffler on the 250 sounds like a sewing machine.
Different appearance: Some of the aftermarket slip-ons look a lot cooler than the 250's factory can.
No tuning necessary. You literally slip it on and go. You don't need to remap or rejet.

Con: Expensive. For the money of a slip-on you'd expect SOME sort of benefit.
Noise: A lot of aftermarket slip-ons are LOUD. Like not just louder, but HOLY SHIT THAT'S FUCKING LOUD.
Often times you need to plug the Kleen air system as you get afterfiring.

Full Exhaust pro: Even more weight savings
Tuned header: While the exhaust isn't "freer flowing", the primary sizes (length and width) along with the collector style allows you to move your exhaust's power-band. Factory exhausts typically are tuned for better scavenging at lower RPMs to give the bikes torque off the line and it kind of chokes things at higher RPMs. Aftermarket headers usually tune for ideal flow ranges around red-line.

Power! You will get more power.
Looks: Especially with Ti exhausts they look seksy.

Cons: Expensive. Holy crap are full exhausts pricey. You want a nice Ti one? For supersports they cost north of $2,000!
Tuning: You NEED to tune your maps. Since the bikes now run like shit at low RPMs you usually have to trim your maps at lower RPMs and then enrich them at higher RPMs. Your exhaust will literally change the behavior of the bike.
Loss of low end torque. Since scavenging is crap at lower RPMs you lose torque. This will make the bike kind of a dog below 6,000 or so RPM.
With a lot of bike you have an exhaust diverter valve which you either need a servo-eliminator or have to leave the stupid valve in place (although no longer connected to anything). Luckily the 250's don't have this.
The Kleen air system needs to be dealt with. Without catalytic converters in the exhaust you will get afterfiring on decel.
 

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Alright, so i have a 650r and really don't do track and don't really feel like putting $2k in an exhaust system. So a slip on is plug and play from what you are telling me, and no tweaking neccessary.

And because I am completely new to riding I have no clue what plugging the Kleen air system is. Lol

You have already been a massive help. Thank you.
 

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There are multiple threads in the 650r section in regards to plugging the Kleen Air system. Two jump to mind that actually SHOW you how to do it.

The Kleen air system is similar to a smog pump system used in cars. Fresh air is pulled from the airbox through a reed valve in the valve cover directly into the exhaust ports. By adding oxygen to the exhaust gas it allows the unburnt gasoline to continue to burn. The trick is slip-on pipes have lower gas velocity and larger volume so the added air causes the gas to rapidly ignite and make "popping". It's not harmful, but on deceleration especially you'll get a lot of afterfiring and it's annoying. Full exhausts really compound this and you get crazy popping.

And for a lot of bikes like the 250r's and 650r's full exhausts START around $600. So you can usually find used setups for less than $500, but like I said a little more work is required :D
 

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So what is the better slip on exhaust for a 250r? I'm leaning towards two brothers because of sound, made in usa, and good reviews. But daym, they are expensive, my cat back on my truck was cheaper. So does anybody now a good site that sells them at least a little cheaper than msrp?
 
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