1999 WR400



31 replies to this topic
  • Jekel

Posted November 10, 2002 - 02:10 PM

#21

I didn't mention the accel pump because the dealer supposedly looked at that item. i read on DRN tech about how pressure affects the resistance between the electrode and point of plug. With no resistance to speak of with the plug out you will get spark, and in the engine, especially high comp, the resistance will null the spark of the plug. This made sense to me from prior experience and lots of beating my head against the wall :D I agree with NH Kevin on the APJ. If you are sure the dealer went through it and cleaned it. I went the lazy route on my bike and bought a P38 :)

  • The_Missile

Posted November 11, 2002 - 04:55 AM

#22

Cleaning the apj is not enough. You need to check that that damn thing isn't shooting like a fire hose.

  • big_G

Posted November 13, 2002 - 11:43 AM

#23

"...well, actually, your engine compression ratio is constant and doesn't change with altitude, so the 12.5:1 on a..."

This is correct. Your comp ratio does not change. Only when valves or rings fail would your ratio fall. High elevation means thin air as mentioned, making your bike run rich. This is just less air per same fuel as regulated by the carb. Octane has nothing to do with this either. In fact its a bit opposite of what was stated above. At the point of detonation (knocking), higher octane will burn at a higher compression than regular, but the use and/or calculation of this is way beyond our needs here. The reason regular makes no difference in alot of cars is because the compression ratio is already low and higher octane does nothing to offset detonation, but adds just a bit to your mileage because it, yes only barely, makes more power per throttle than regular... make sense??

But to address your problem, air, fuel, and spark in that order. Start with the easy things like filter, boots, air leaks and loose hoses. If nothing, move into the carb, look for problems with fuel delivery, jets, pilots, air circuits, needle valve, accel pump and on and on making sure all that is setup properly. If you dont solve it there, start checking electrical and the more intrusive tasks last.

For you, it sounds like a clogged jet or accel pump.

I wonder though, my bike, which runs perfect, will do something like you describe if you really whack the throttle super quick. However, I have to assume I can't open the throttle that fast when riding, so its not really an issue... so I dunno, any of that help??

  • jwriott

Posted November 13, 2002 - 12:50 PM

#24

The higher the octane rating, the lower the stored energy rated in BTU's (British Thermal Units) contained in the fuel. You will not get more power by using higher octane fuel. The only reason race engines with high compression need high octane fuel is to resist pre-detonation. You want to use the lowest rated fuel you can without pre-detonation (pinging or knocking) to get the most hp out of your engine.

Here a site that explains this in detail.
Racing Gasoline Basics

The higher the octane rating, the more resistant the fuel is to detonation. This allows you to compress the fuel/air mixure higher before detonation. Lower octane fuel burns too easily under high compression.

"The reason regular makes no difference in alot of cars is because the compression ratio is already low and higher octane does nothing to offset detonation, but adds just a bit to your mileage because it, yes only barely, makes more power per throttle than regular... make sense??"

No this doesn't make sense and is not true. Your car will not make more power nor will it get better mileage by putting higher octane fuel into an engine designed with a low compression ratio.

I'll agree with you on the point that there is less air per the same fuel at altitude compared to sea level if the jetting is for sea level. That is why we have to rejet the carb. However, when the piston compresses the fuel/air ratio inside the cylinder, there is sigificantly less compression the higher you go. Because of this, it is equivalent to a lowered compression ratio. Using a fuel that is resistant to detonation means you could be exhausting unburned fuel out the cylinder. Essentially running rich eventhough the air to fuel mixture ratio has been corrected.

  • big_G

Posted November 14, 2002 - 09:34 AM

#25

No this doesn't make sense and is not true. Your car will not make more power nor will it get better mileage by putting higher octane fuel into an engine designed with a low compression ratio.




Actually you are wrong here, I've been tracking milage on my Jeep for 8 years. Not only does my mileage go up on super unleaded, it also goes up when towing. Why this is can be anyones guess, but for sure, the answer doesn't lie in any octane study's that don't include my engine. Further, I indirectly said that increased power can be produced by burning higher octane fuel. Personally, I'm not positive if this could be from the octane directly, or from the fuel just burning a bit cleaner or that the regular gas actually burns with a discrete amount of detonation and the super doesn't, or whatever. Honestly, my point was not to hijack this thread to rediscover the origin of octane vs. horsepower, but fair warning, you're arguing some basics here with an engineer, and I study internal combustion as a personal interest as well.

there is sigificantly less compression the higher you go. Because of this, it is equivalent to a lowered compression ratio.



For sure, your analogy of compression ratio and altitude is a valiant, but wrong conclusion. You could take that engine into outer space, with no air, and the volumetric difference inside the cylinder from TDC to BDC will always be the same. If you measure the compressed volume at one, you will find the expanded volume to be 12.5. You could run that engine on air, oil, cookie dough, or whatever, and that engine will still have a 12.5:1 ratio. I don't recommend the oil or cookie dough, but the math is still the same. That make sense??

But once outside the cylinder, most of your assumptions on jetting are appropriate.

Using a fuel that is resistant to detonation means you could be exhausting unburned fuel out the cylinder.



Well that theory there, I can't make heads nor tails of, you'll have to explain that a bit more.

But to get back to your bike problem, you might just want to go thru the accel pump routine describe in detail elsewhere on this site, as that could cause your throttle responce issue.

Good luck.

  • WR_Jason

Posted November 14, 2002 - 10:01 AM

#26

Compression ratio dose not change wiht altitude and at 12.5 to 1 apmospheric preasure from sea level to 7000 feet will be insignifagant to the compression or overall PSI of the motor at TDC, your talking about pennies on a $100 bill, you would be better served considering the extra blow by at lower RPMs to higher RPMs affecting the compression than the outside air preasure, both IMHO are insignifagant.
Highter ocatene fuel DOSE give better milage, just try a tank of cheap stuff and good stuff on your weekly drive to work and track the milage, the differance is apparent. It is NOT BTUs that drives the piston it is the expansion of the gasses. Heat is a by product of the reaction and heat = loss of effiency, this is true for the motor, higher octane fule burns cooler and improves the effiency of the motor.
The issue of running rich at altitude is realated IMO to less oxygen density and that alone. You get the same results when its hot and humid as well.

Back to the meat of the question though, :D , I would as said, change out your spark plug, check your hot start sytem and make sure its not open or any vacume lines are loose. A good way to do this is to spray a bit of eather down by the carb while the bike is ideling, if it revs up, its sucking air from somewhere besides the air box :) . Next I would remove the push rod from the ACP and see if she runs better, if so, look to do the ACP mods or just leave it un hooked. Next, check the jetting spread sheets and strat messing with the jetting. I would doubble check the jetting and reset all the screws to make sure you know what your starting with.
Good luck with it. Also make sure it warms up nice and hot before you jump on it and start ripping up trails :D

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  • jwriott

Posted November 14, 2002 - 12:20 PM

#27

First of all, compression ratio is used to obtain an internal pressure within the combustion chamber. I am not disputing the fact that the volume of the cylinder is reduced 12.5 times when the piston goes from BDC to TDC. That is a given.

This is not just a volume ratio problem as you are assuming. It is a volume & pressure ratio problem with volume as the given and pressure is the value we are solving for.

Let's go back to some basic engineering formulas and run through the scenarios.

P1*V1=P2*V2 assmuming a closed system. (no leaks through valves, rings, etc.)

P1 = 14.7 lbs/sq. in (1 atm, sea level)
V1 = 426cc
V2 = 34.08cc (426/12.5)
P2 = Pressure when compressed

At Sea Level:
((14.7 lbs/sq. in.)*(426cc))=34.08*P2
P2 = 183.75 lbs./sq. in at sea level

At 7,000 feet, P1 = 11.76 lbs/sq. in. (0.80 atm)
P2 = (11.76 lbs/sq. in)*(426cc)/(34.08) = 147 lbs/sq. in.


At 13,000 feet, P1 = 9.996 lbs/sq. in (0.68 atm)
P2 = (9.996 lbs/sq. in)*(426cc)/(34.08cc) = 125 lbs/sq. in


Now, if we work backwards and want P2 to equal 147 lbs/sq. at sea level we have the following:

P1*V1=P2*V2, V2 = 1 because it is the compressed volume
(14.7 lbs/sq. in)*V1 = (147 lbs/sq. in)*1
V1 = 10 or the equivalent compression ratio of 10:1.

Now, if we work backwards and want P2 to equal 125 lbs/sq. in. at sea level we have the following:

(14.7 lbs/sq. in)*V1 = (125 lbs/sq. in)*1
V1 = 8.5 or the equivalent compression ratio of 8.5:1

Does this make sense to you? Looking at the equations, altitude makes a significant difference on compression. To say it's "pennies on a $100 bill" is far from the truth.

A British Thermal Unit is referring to the amount of energy stored and waiting to be released. The higher the rating, the more energy during combustion to push the piston downward.

British Thermal Unit (Btu): The standard measure of heat energy. It takes one Btu to raise the temperature of one pound of water by one degree Fahrenheit at sea level. For example, it takes about 2,000 Btu to make a pot of coffee. One Btu is equivalent to 252 calories, 778 foot-pounds, 1055 joules, or 0.293 watt-hours.

You are right that when the air outside is hot, your engine will run rich. Why? Because hot air is less dense than cool air. This reduces oxygen, hydrogen, nitrogen, etc. Again creating a lower pressure within your cylinder when fully compressed because you start with a lower pressure prior to compressing the volume 12.5:1 times.

Since all of these engines that we are referring to are water cooled. The temperature within the engine should remain constant, once warmed up because of your thermostat and coolant.

  • MN_Kevin

Posted November 14, 2002 - 01:30 PM

#28

can we get Carnot to build us an engine?

  • big_G

Posted November 15, 2002 - 08:45 AM

#29

lol at this whole thread... :)

ok i see your issue, you're arguing pressure, not compression, so obviously you can build a case using a pv=pv scenario, but if your wanting to caluclate compression, you're barking up the right formula for the wrong value...

if you were to simply assume any value for p, like one, you might see that the delta V is actually the comp ratio... like i said above, 12.5 Vbdc = Vtdc, you see it yet??

also, internal combustion is a process with alot of losses, heat being the biggest as its pumped out of the engine right thru the cylinder wall, so arguing power loss vs. altitude is a futile process for more than one reason.

but hopefully by now, you can see that compression ratio is a constant, not a variable in internal combustion, right?!?!

u in school or what anyway???

  • WR_Jason

Posted November 15, 2002 - 12:36 PM

#30

"Does this make sense to you? Looking at the equations, altitude makes a significant difference on compression. To say it's "pennies on a $100 bill" is far from the truth."

Yeah it makes sense to me, its nice to see the actual values for the altitude, but I would not consider a %20 differance between sea level and 7000 feet a "signifigant differance" cnsidering most of us ride between 200 feet and maybe 5000 feet, maybe some of you Pikes peek Calorado guys may see that altitude differance in one day, other than that,,, I dont think so. As far as 13000 feet goes,,, better bring your Oxeygen mask with you!!! LOL. Anyway, the differance in the PSI is no where near enough IMHO to affect the flash point of the fuel, like I said the issue is less dense air and less Oxeygen, do you agree or do you feel that the 40 or so PSI of almost 200 PSI is the factor at altitude?
As far as BTUs and Octane, so your telling me I should get less milage and performace with higher Octane becuase it has less energy density?
Our bike cooling systems are not as efficent as our motors are inefficent, unfortantly with weight and size constraints on our dirt bikes and the fact that we usally travel MUCH slower than other motor vehicles but at usally higer RPMs our bikes are extra sensitve to heat conditions IMO and in the boiling over coolant resivior on my WR on a hot day when Im following the Quads in first gear for an hour :) . Heat expands both the piston, block, valve train, and makes the motor tighter and less efficent. The motor is designed to perform at a given temparature but the cooler you can run it the better IMHO.

Thats my story and Im stickin to it!!
Great thread BTW!

  • jwriott

Posted November 15, 2002 - 12:54 PM

#31

I give up. I don't have the time or energy to continue explaining this subject to you.

For the final time, I am not disputing the fact that the ratio remains 12.5:1 at any altitude. I am trying to explain to you that as you go up in altitude the pressure within your cylinder goes down. This changes the dynamics of combustion.

FYI: I received a B.S. in Mechanical Engineering from Colorado State University in the Spring of 1990.

What type of Engineer are you and where did you graduate from?


WR Jason, Here is a pretty good link on octane issues.

Octane Notes

  • big_G

Posted November 15, 2002 - 01:43 PM

#32

Well the feeling is gettin pretty mutual, lemme tell ya...

Here are pieces of YOUR OWN posts as seen above...

I normally ride at 7,000-13,000 feet so my compression should be reduced considerably.
~~~
Your compression ratio is not the same at altitude.
~~~
V1 = 8.5 or the equivalent compression ratio of 8.5:1.
~~~
I am not disputing the fact that the ratio remains 12.5:1 at any altitude.
~~~
...as you go up in altitude the pressure within your cylinder goes down. This changes the dynamics of combustion.




I'm done.

Not only can't I tell whether or not you have a grasp on basic engine mechanics, I don't care anymore. If you had a point, I can't tell what that was anymore either... you're all over the map and look lost. You contradict yourself, hijacked this dudes thread while proving nothing.

Now his bike still ain't runnin, this work will teach no one how an engine runs, and frankly by trying to help, I think I may have actually lost some IQ points in this process.

Further, I wouldn't go challenging other folks, especially fellow engineers, unless you know what your talking about. I've been out of school for like 10 years and even I can remember that the CR of an IC engine is constant, its an air pump, remember??? I guess not.

None the less, good luck and good day sir.




 
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