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pghwrrider

1999 WR400

32 posts in this topic

I just bought a 1999 WR400 from a local dealer and about an hour into my first ride the problems started. When I twist the throttle hard from idle the bike just boggs out . It even stalls if I don't let off the throttle in enough time . I set the idle high to temporarily fix it just to get back . Well seeing as how I just bought it I took back to the dealer for them to fix , here's what they have done so far : complete cleaning of the carb , try new factory jets , and inspected the accelerator pump . Still not fixed . I just figured the pilot jet was clogged . Any ideas out there ?

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I'd change the plug and check the airbox. I put a Clarke tank/yz seat on my bike and bought the tank used. It had a tiny bit of dirt in the bottom that I found later and it partially plugged the pilot. The bike ran like total crap.

I took the pilot and main out and cleaned them completely with compressed air. Not just solvent. I also cleaned the air filter and tank. Put everything back and same problem. Bogged badly, missing, backfiring, etc.

I got back home and pulled the seat and tank and put a new plug in and it runs perfect. For some reason, mine seems to foul plugs to the point that they can't recover.

I've heard this happening with street bikes before and obviously 2 strokes but it seems weird to me that a 4 stroke would foul a plug beyond use but it is the case with mine. Anyone have input on this?

Hope this helps.

John

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here in Oz, i was told (and with some experimentation found to be true) that WR's foul plugs if you use premium unleaded(not sure what you guys call it but its 98RON fuel) only run standard unleaded fuel in your WR. Are you running a standard plug? you could try running CR9 instead of ten.

any further than that and im out of my depth

cheers

Lachy

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I am running the standard NGK CR8E plug and Super Unleaded fuel.

I am not sure what the CR9 or CR10 plug would do for me.

I am also at altitude so the higher octane fuel could cause problems as you suggest since it takes more compression to ignite. I normally ride at 7,000-13,000 feet so my compression should be reduced considerably. If my ratio is not high enough at this altitude, then running Super Unleaded would not allow all the fuel to burn causing a rich condition. These are things I already know, sometimes I just need to be reminded.

Thanks,

John :)

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well, actually, your engine compression ratio is constant and doesn't change with altitude, so the 12.5:1 on a '01 426 is the same at any altitude.. what happens is that there is LESS oxygen for the engine to utilize, and an engine tuned to run at sea level will be too rich at altitude. the rating on your fuel is to control detonation, the higher the number, the higher the resistance to detonation, because engines with high compression (yz's)may knock or ping (detonation)... what some of the supreme high octane fuels also add (to justify the expense) are cleaning agents of some sort, that may not like the plugs.

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Your compression ratio is not the same at altitude. You are assuming that only oxygen levels are reduced as you go up in altitude. The overall density of the air is reduced. This means that you have less dense air in your cylinder, which in turn means your compression ratio is lowered. The higher you go, the lower your compression ratio which in turn means you can run lower octane fuel because it will not predetonate as easily.

At 7,000 feet which is the low end of my riding spectrum, the air density is at 80% of sea level (10:1 ratio)

At 13,000 feet which is the upper end of where I ride, the air density is at 68% of sea level. (8.5:1 ratio)

Atmosphere Table Link

Compression Ratio Correction Link

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how correct you are, my mistake, i was thinking in a static sense of just the VOLUME of the cylinder, not the pressure, works opposite of going down (underwater is an extreme example, the deeper you go,the pressure increases..not sure if that was good analogy) but thank you very much for the info! :D at least i was thinking straight on the octane... :)

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No problem. I can't actually take credit for this information. I was on a site for my Vmax and these guys are major gearheads.

They do so much to those bikes it's incredible. Anyway, they had a lot on compression ratio/octane/altitude, etc.

I learned a ton and still had questions.

Later,

John

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I might be able to help a little...

Whatever the atmospheric pressure is outside, your cylinder head takes a volume of 400cc (12,5) at admission and reduces it to 32cc (1) as the piston reaches the top of the cylinder head (12,1:1 ratio) which means that the ratio is not variable because it describes the volume variation that occurs in the cylinder head over the piston during one stroke. Whatever the pressure outside is, the engine always takes 12,5 of what is outside (plus the gazoline) and brings it to 1.

If the ratio is constant, why does a motor breath less in altitude? It is just because the general atmospheric pressure is diminished which means that you have less nitrogen, less carbon dioxide and (very important) les oxygen in that starting 400cc (12,5) volume so less oxygen in the compressed 32 (1) volume. So the ratio is constant but the pressure outside and inside varies with altitude.

Is it useful to bring back that the utility of compression is to heat up these gaz (and the gazoline) and raise their energy level so the oxydation of gazoline (iniated by the spark) becomes possible.

Math

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ok, not being an engineer here, i think i understand here. both trains of thought are correct i believe. the combustion chamer size at TDC and TBC are set and do NOT change with altitude (although saab has a variable combustion chamber.. the head is raised on the block while the engine is running, something like that) but the EFFECTIVE compression ratio, not the ACTUAL ratio changes due to less oxygen/nitrogen.. so in a sense, the engine may have the 12.5:1 compression ratio and it stays the same, but as the altitude increases, it would start to produce less power, as if the CR is changing, which it can't physically do.. is that close, or am i way off again.. maybe i was too close to the gas tank while i was working on my bike....

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Sorry guys, I did not want to bother you :D, I'm not an engineer either...

It is just that I think that the ratio describes what the engine does with what it has access to.

That given, with altitude, the starting pressure (outside) is lowered so the pressure in the cylinder after compression is also lower which causes the bike to breath badly.

Anyway, I think we understand each other very well :) but just saying things differently...

Math :D

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Hey Math, you are in no way bothering me with more input. It helps me think about this stuff more. I should have said that the compression is lower at altitude, not the compression ratio. You guys are right. It is taking what is available and compressing it 12.5:1 no matter what the altitude.

I'm pretty sure though, that when the manufacturers determine what compression ratio they want the engine to have, they are using atmospheric pressure at sea level (1 atmosphere or 14.7 pounds per square inch). When you go up in altitude, atmospheric pressure goes down so you have less pressure within the cylinder at the time of combustion.

With lower pressure inside the cylinder, you don't need as high an octane fuel because the detonation level is lower. For that reason, you can use lower octane fuel which actually has higher potential energy.

Our standard unleaded fuel here is 85 octane, mid is 87 and premium is 97. I know at sea level, standard is 87 octane, mid is 89 and premium is 91 octane.

Anyway, I'm going to experiment a little with lower octane fuel and see if I notice a difference.

Thanks for all the information and help.

Later,

John

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Effectively, I think that saying that the final air pressure after compression is lower is more correct than saying that the ratio is lower. So we agree on that point.

Your precisions concerning the octane level regarding different altitudes is interresting. I guess that if I was a mechanic I would have know it. But in fact, I'm not a mechanic at all and I'm at sea level so I had never had the occasion to ruminate about that. Lower octane in altitude makes sense since you don't have to prevent a premature (can I use this word here) detonation: pressure is too low.

Ok! Now that beeing said, when do we start that engineering compagny? :)

Math :D

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This air pressure stuff is freat but what does is have to do with the original post about the bog on the bottom when he hits the gas. Isn't someone smarter than me supposed to tell him to adjust the FUEL SCREW ?

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I'm not sure about the Engineering company. I think the bikes have become way to complex for the average guy to modify easily. Even my local dealers can't seem to figure it out.

I realize that the air pressure stuff got off track quite a bit. My fault but I was originally referring to fouled plugs.

I wouldn't touch any carb setting until the plug is changed. Then go from there. Start with the simplest items first, after they are ruled out, move to adjustments.

A dirty air filter could have the same effect that messing with the air screw could since you will run rich from lack of air.

pghwrrider, have you changed the plug and checked the air filter to see if it's clean?

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Sounds like the dealer did the carb good. Did you check fuel flow? Next, change the plug, even if you have spark while testing it on the engine. I read on another site about the plug being able to have spark, but under pressure it is harder to have the spark, hence in a high compression engine-less spark performance. As always check the filter and clean properly. Next I would ask the dealer to check the electrical for you, Or swap parts with a buddy who has one also, to see if any items fix the problem. You can also ask your shop to run a comp test for you also? Nag your shop to get this fixed before they say you had the bike to long and it is your problem. I had a shop do that to me. Bought a new bike and took it back within the week toi fix a fork adjuster. They kept it to fix and returned it day before 30 day warranty up. Then wouldn't fix what they broke while working on the fork :) good luck on gtting your biker worked out.

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Sounds like the dealer did the carb good. Did you check fuel flow? Next, change the plug, even if you have spark while testing it on the engine. I read on another site about the plug being able to have spark, but under pressure it is harder to have the spark, hence in a high compression engine-less spark performance. As always check the filter and clean properly. Next I would ask the dealer to check the electrical for you, Or swap parts with a buddy who has one also, to see if any items fix the problem. You can also ask your shop to run a comp test for you also? Nag your shop to get this fixed before they say you had the bike to long and it is your problem. I had a shop do that to me. Bought a new bike and took it back within the week toi fix a fork adjuster. They kept it to fix and returned it day before 30 day warranty up. Then wouldn't fix what they broke while working on the fork :) good luck on gtting your bike worked out.

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The other thing I forgot to tell you to check is to make sure that the spark arrestor screen is not plugged. This will also have an effect on how the bike runs.

The reason plugs foul to the point of not being useable again was explained to my by a mechanic at work. He told me that once there is enough carbon down inside the plug, you basically get a path for some of the spark to travel back to ground. No matter what you are doing with jets, if the plug is fouled, it will not run right if part of the energy is being grounded.

Jekel, why do you think it is harder to have spark under compression? I haven't heard anything about this so any info would be helpful.

pghwrrider, I had one more question. You said when riding in the first hour, things went bad so I'm assuming that it ran okay when you first started riding. If you shut the bike off and were messing with the throttle, you may have fouled the plug. When I first got mine, my boys were playing on it and twisting the throttle. It would start and die over and over until one of my buddies came by and told me to change the plug. Ran great after that.

Later,

John

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what's up TT'ers?

Maybe I missed it, but no one has mentioned the accelerator pump jet (APJ)... :)

The 98/99's carb can be modified to mimic the infamous BK mod. Do a search for KL Mod or Taffy mod. 2 different approaches to the same problem.

When I checked my APJ, the squirt duration was > 3 seconds!! :D

>> The carb is being deluged w/ excess fuel resulting in the bog.

W/ my KL mod, my squirt is down to ~ 0.3 seconds.

BTW: The 99 is NOTORIOUS for crapping up the accelerator pump. Crud goes down the accel pump actuation rod and seizes up the diaphragm and the AP.

You can also do the gray wire mod and blue wire mod. These mods are for improved ignition performance.

Do a search on these as well.

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