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Discussion Starter #1
I have a 2004 that has been trouble free for nine years. M4 pipes, Kyle block off but no other mods. It backfires at about 5000 rpm, starves for fuel till you close the throttle then goes again until you hit around 5000 rpm again. I had to nurse it home this am. I checked for codes... none. I followed the manual and used the jumper method to check again... no codes. I will check the plugs tonight, but it starts and idles just fine.
Any help is appreciated.
 

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If you don't get an FI indication you should start looking the parts that the ECU cannot check.

1) Fuel pressure regulator malfunction (fuel leaks out the small vacuum hose coming from the throttle bodies if you disconnect it off the regulator).
You should have the strong fuel smell around the bike if that's the case.

2) Regulator/Rectifier doesn't charge your battery enough and the engine starves off spark (which sometimes can be mistaken for fuel starvation).

 

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Does the power fall off dramatically before you get to 5000 RPM?
If so, you may be running very, very lean at that point.

When I first picked up my RC51, at 5800 RPM the power fell off dramatically.
And if I pushed it further, it would even start to misfire until at 6200 RPM, it passed that point and took off like in after-burner.

Apparently, the power commander map was totally out of whack, with a major lean spot from 4800 to 6000 RPM.
A little time on a dyno to get a freshly minted map, and all was well.
 

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Discussion Starter #5
It just suddenly backfires really loud at around 5000 rpm, may be a lean miss. Then it starves for fuel I believe. I could nurse it back running and it was fine under 5000 rpm but at 5000 here we go again. I am thinking fuel pump or pressure regulator. I must say this bike owes me nothing this is its first problem ever and it is 9 years old.
 

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Any exhaust leaks?
 

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It just suddenly backfires really loud at around 5000 rpm, may be a lean miss. Then it starves for fuel I believe. I could nurse it back running and it was fine under 5000 rpm but at 5000 here we go again. I am thinking fuel pump or pressure regulator. I must say this bike owes me nothing this is its first problem ever and it is 9 years old.
No PCIII.....FPR check is in order. If you haven't replaced the plugs in some time, throw a new set in and read what the plugs tell you about your fuel burn (color)
 

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Discussion Starter #8
Thanks for all of the previous advice on this. I checked for fuel leaking from vacum side of pressure regulator... none. When I went to ride it again it was better. But it backfired and stumbled at 8000 rpm not 5000. It did it a lot less though. When I got home to check the regulator again I saw that I had been riding with the vacum hose unattached.... my mistake. Does this lead you to believe it may be the regulator rather than the pump?
I want to be sure before I buy a pump. (full disclosure I put a fuel pump on a jeep years ago that was merely out of gas.)
 

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Don't rush yourself and purchase anything before you're sure of what that is.

At least buy a pressure gauge and some high pressure tubes and make an adapter for that to measure the pump's output fuel pressure on the injectors' pipe outlet as the manual describes. It will cost you a lot less that a pump and it can narrow it down for ya..
 

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Discussion Starter #10
Can anyone explain how the backfire and stumble are at 5000 rpm with the vacuum tube hooked to the regulator but it moves to 8000 with the vacuum unhooked. This is odd to me.
 

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Did you check closely all the vacuum tubes attached on the throttle bodies for cracks or slashes?
Maybe a vacuum hose has a tear and the intake sucks extra air from it. That could definitely cause backfiring.

Removing the vacuum hose from the regulator, leads to a very rich fuel mixture in low rpm so if you already have a vacuum air leak, it will balance the mixture and not backfire.
In higher rpm though, extra incoming air can become more effective in turning the fuel mixture to leaner proportions.

Also check closely the throttle bodies' rubber boots for cracks or air vacuum leaking.
 

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You know, I accidently found that the boots under my throttlebody had loose clamps. I mean very loose. Finger tight. So I used a flashlight and long extension with a 6 or 7mm socket (can't remember the size) to tighten both upper and lower clamps on those throttlebody boots.
I guess this condition could allow unmetered air into the cylinders, causing a lean condition. I don't think it was happening to me, but....
 

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....... (can't remember the size)....
8mm in the TBs.....6 & 7 is usually used on the coolant hoses' clamps.

But it's better to use a long philips head screwdriver cause that makes you tight them just enough and NOT too much, otherwise the clamp can cut through the boot after a while due to the V-Twin's vibrations.
 

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Misfire Musing.

Hello, nctuck.
Did you get to find out what the problem was ?
It'd be great to get the diagnosis.
By the way. I've lived for a year in Reidsville, NC, back in '97. Is it close to where you are ? I sure wished I had my RC with me then. The roads & scenery & of course, the Blue Ridge Parkway, are absolutely gorgeous.
Take Care.
 

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