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Discussion Starter #1
Hello gang!
Went for my first mountain ride on my '01 EX-500 this weekend and had a minor problem. I have never noticed this before in town commuting to work. When climbing a few long hills at about 50-55 mph in 4th, engine is spinning about 6-7K RPM, and I'm starting to lose speed. I dip into the throttle a bit more and I get very little response, and what I do get is a rapid hesitation and surging pulse, about once per second, and no rise in RPM. The first time it happened I pulled to WOT and got nothing, no more power at all. Dropping a gear and getting the RPM up to about 8K and the surging is gone and RPMs climb as expected.

I noticed this a couple more times, only when climbing hills that were fairly steep, 6-7% grades at close to 10,000 feet elevation. As I said, I've never run across this in town at 5-6K feet of elevation.

So I took out the carbs today to see if they needed a cleaning or adjustment. The carbs were pretty much spotless, no gumming or residue at all. But, the left side carb's pilot screw was maybe 1/2 to 1 turn out from its fully seated position. I was dumbfounded! :eek: The stock pilot screw covers were still there, so how on earth did it end up so far in? It was so high the top of the pilot screw was sticking way into the venturi. The right side carb was just about right. I pulled them both and the passages were completely clean and clear. I reinstalled them both and set them to 2.5 turns out.

Afterwards, the bike seemed to be running great! Nice snappy throttle response and quicker to rev. I haven't been able to go for a proper ride yet, and I am anxious to try it out. Do you think that one carb being set that rich would have caused that surging sensation I was getting? Thanks!
 

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It seems the most common problem of getting that "hesitating" sensation is due to plugged pilot jets. When you had the carbs taken apart, did you remove them [the pilot jets] and make sure they were unobstructed (shine light through an end)? Even if the carbs appear spotless upon glancing over them, tiny particles or even varnish from old gasoline can plug those tiny holes up and give you problems exaclty as you described.

Maybe you know this already, but the Pilot Screws and the Pilot Jets are two separate things. You can use the attached image as a guide. You should remove all jets and soak/clean them when you have your carbs taken apart. And to help minimize a relapse of this problem, buy a $4 brass fuel filter.

[attachment deleted by admin]
 

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Discussion Starter #3
Yes, I know that the pilot screw and jet are different items. Most threads I have seen talk about adjusting and clearing the pilot screws, so I started there. They were very clean and I can easily see light through the holes. I set them back at 2.5 turns out and rode into work this morning. The throttle response seems better, but maybe I can keep adjusting to get it even smoother.

I'm going to pick up a small fuel filter, and maybe I'll also pull the pilot jets tonight and give them a cleaning. Any other thoughts on the issue?
 

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are the pilot screws even cleanable? When i cleaned my carbs i cleaned the bowls, main jets, and pilot jets. Was there more i should have cleaned? I have surging like that, but i just assumed it to be due to using to high (i.e. more towards 6th) of a gear when demanding more power by opening the throttle. If i downshift a few gears and accelerate it seems to pull well. Maybe i'm wrong, but since it's "just" a 500...i expected some stuttering and surging when applying high loads (in my case opening the throttle to accelerate) I'm "only" 170lbs tho, so maybe this isn't normal, lol.

Thanks,
Ken
 

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If I remember correctly, the pilot screws aren't "cleanable." The tip is just a very fine needle point, I don't remember there being any holes like you would find with the jets. I replaced my pilot screws, so I have my old ones laying around somewhere. I'll check, and if I'm wrong I'll revise my statement.

Anyhow, nearly all cases that you guys are describing with the said bogging when opening up the throttle are due to plugged pilot jets. You should make sure those are immaculate, then see where that leads you.
 

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Discussion Starter #6
No, there's nothing on the screw itself that gets cleaned, I was referring to the passage in the carb body that the screw goes in to.
 

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Pilot jets have no real effect in the upper rpm range. your condition sounds to me like lean surge- or lack of fuel and too much air. By turning the pilot jet out, the mixture is enrichened due to increased vacuum pressure applied to main jet which pull more gas to be mixed with air basically. If you hae emissions testing for bikes where you live (we do here and it is asinine I think as I watch some beater Chevy nearly dump raw gas from the tailpipe!) be careful as the pilot screw (aka the air/fuel screw) can be turned far enough to fail the test very easily. Also- you didn't say if you had any exhaust mods or jetting changes- but either way it sounds as if you are on the right track. Let us know!
 

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Discussion Starter #9
Things have improved a lot since I set the idle mixture screws at only 2 turns out and I did the FOG airbox mod. I haven't been up at higher elevations yet, so I may still need to change the main jet down a size, but around town here at 5-6K elevation it is running really well, the hesitations are gone. I was surprised how much a difference the airbox mod made. Thanks FOG!
 

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As stated in the Airbox Mod instructions, It's purpose is to lean out the overly rich mixture up high in the rev range and restore some driveability down low with the pilot screw adjustment. Your problem is caused by the altitude, Your mains are too large. if you dropped one size and re adjusted the pilot screws you may be able to achieve smoother running. You will not however regain the lost power. That lies at a lower altitude along with the Oxygen.

FOG
 

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Discussion Starter #11
FOG said:
As stated in the Airbox Mod instructions, It's purpose is to lean out the overly rich mixture up high in the rev range and restore some driveability down low with the pilot screw adjustment. Your problem is caused by the altitude, Your mains are too large. if you dropped one size and re adjusted the pilot screws you may be able to achieve smoother running. You will not however regain the lost power. That lies at a lower altitude along with the Oxygen.

FOG
Yes, understood about the power loss, it happens to all cars and bikes up here. I'm just shooting for smooth operation, so I will probably drop down to a 128 main jet and see how that goes. It already runs much nicer with the airbox mod and leaning out the pilot screws. Thanks FOG.
 
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