Fuel Pressure Regulator? - Honda RC51 Forum : RC51 Motorcycle Forums
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post #1 of 23 (permalink) Old 03-26-2013, 08:44 PM Thread Starter
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Fuel Pressure Regulator?

Hey all!

been having an issue lately with my SP1 motor has 15k.

Leaking a fuel/oil mix from the rear manifold. Also running very rich, which tells me the something in the throttle body is dumping fuel into the cylinder and washing the oil out with it which can destroy rings so no riding for me for now

This leads me to my question. It seems like there is a known issue with the FPR on the RC's? I could be wrong but does anybody have any advice before I replace the entire Throttle Body assembly!!

Thanks in advance!
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post #2 of 23 (permalink) Old 03-26-2013, 08:54 PM
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Originally Posted by enviousmedia View Post
Hey all!

been having an issue lately with my SP1 motor has 15k.

Leaking a fuel/oil mix from the rear manifold. Also running very rich, which tells me the something in the throttle body is dumping fuel into the cylinder and washing the oil out with it which can destroy rings so no riding for me for now

This leads me to my question. It seems like there is a known issue with the FPR on the RC's? I could be wrong but does anybody have any advice before I replace the entire Throttle Body assembly!!

Thanks in advance!
Well, you just nailed the whole scenerio of a failed fpr. Much cheaper than replacing the throttlebodies.
Pull the small vac hose off the fpr and then turn the key on to prime the fuel pump. If your fpr is bad, it'll leak, drip, or squirt gas out of that nipple. So yeah, when the vac hose is hooked up, it's dripping all that unmetered gas into the throttlebody. The front one if you have SP1.
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post #3 of 23 (permalink) Old 03-26-2013, 09:15 PM
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Jondog is right on however my FPR went cruddy but I didn't get the text book "drool" I got an uber rich bike that would die at idle like a flossed motor. I tried the test but for some reason couldn't get a drip. Replaced the FPR and had a perfectly running bike- best its ever been.

So with your symptoms I would do as mentioned and replace it whether you have a drip or not

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post #4 of 23 (permalink) Old 03-27-2013, 12:05 AM Thread Starter
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Ok. So I tried exactly what Jondog suggested (removing vacuum line from FPR and priming the fuel pump with the turn of the key) there was a very steady stream of fuel that lasted about 3 seconds. I think I found my problem!

So I need to purchase FPR, Gasket set, and new "joint set"? What do you guys think?
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post #5 of 23 (permalink) Old 03-27-2013, 07:07 AM
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You should just need the FPR. There are two o-rings on the tube that it slides over. On mine they were in good shape and no leaks. My problem was that the "new" FPR was defective. Still waiting for the replacement.
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post #6 of 23 (permalink) Old 03-27-2013, 11:26 AM
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I had one go bad on the F4i and the result is it filled the #1 & #2 cylinder up to to the throttle bodies with fuel! It was only pure luck that I noticed it while doing maintenance and prior to starting the bike otherwise it would have most likely hydrolocked the cylinder and bent a rod or two.
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post #7 of 23 (permalink) Old 05-01-2013, 09:52 PM
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both my rc51

both of mine had bad fuel pressure regulators. ran like crap at high speed, dies at idle. you can pull off the vacuum line and plug it put a line on th fpr and block that for a quick temp fix until you get a replacement. did that to mine and worked fine until i replaced it
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post #8 of 23 (permalink) Old 05-03-2013, 11:31 PM
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Originally Posted by garylkendall View Post
both of mine had bad fuel pressure regulators. ran like crap at high speed, dies at idle. you can pull off the vacuum line and plug it put a line on th fpr and block that for a quick temp fix until you get a replacement. did that to mine and worked fine until i replaced it
Seems like that would alter the fuel pressure?
How does the vacuum, or lack of, effect the fpr?

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post #9 of 23 (permalink) Old 05-04-2013, 01:22 AM
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Here is how she works.

How does a fuel pressure regulator work?
The fuel pressure regulator is able to maintain proper fuel pressure to the vehicle it has been designed for because inside the regulator housing there is a spring pushing against a diaphragm, the spring pressure has been pre-set by the manufacturer for the desired fuel pressure, so the fuel pump has to pump enough fuel and enough pressure at the same time to overcome the spring pressure.
The extra fuel not needed is sent back to the fuel tank through the fuel return line.
When the vehicle is at idle, there is less pressure against the fuel coming inside the regulator because the fuel pressure regulator has a vacuum hose attached to it, this way the fuel pressure will be lower ( from 5 to 10 psi depending on the system) due to the fact that the vacuum is forcing the diaphragm inside the regulator housing to have extra pressure against the spring, resulting in a lower fuel pressure when the car is at idle because there is high vacuum inside the intake manifold; when you accelerate and the vacuum drops, the fuel pressure increases to allow the engine to have more fuel as it needs it.

My guess is either the diaphragm material fails after so much contact with fuel and or spring failure....whoever has an old one, A cut away would be nice



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Last edited by madbuyer; 05-04-2013 at 01:24 AM.
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post #10 of 23 (permalink) Old 06-22-2016, 09:59 PM
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Smile FPR cut away.

Quote:
Originally Posted by madbuyer View Post
How does a fuel pressure regulator work?
The fuel pressure regulator is able to maintain proper fuel pressure to the vehicle it has been designed for because inside the regulator housing there is a spring pushing against a diaphragm, the spring pressure has been pre-set by the manufacturer for the desired fuel pressure, so the fuel pump has to pump enough fuel and enough pressure at the same time to overcome the spring pressure.
The extra fuel not needed is sent back to the fuel tank through the fuel return line.
When the vehicle is at idle, there is less pressure against the fuel coming inside the regulator because the fuel pressure regulator has a vacuum hose attached to it, this way the fuel pressure will be lower ( from 5 to 10 psi depending on the system) due to the fact that the vacuum is forcing the diaphragm inside the regulator housing to have extra pressure against the spring, resulting in a lower fuel pressure when the car is at idle because there is high vacuum inside the intake manifold; when you accelerate and the vacuum drops, the fuel pressure increases to allow the engine to have more fuel as it needs it.

My guess is either the diaphragm material fails after so much contact with fuel and or spring failure....whoever has an old one, A cut away would be nice
Thats a prefect explanation on how a regulator works. Great job. I have a few mills at my house. If anyone is willing to send me an old FPR i can make the cutaway and post it up for everyone.
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