Fuel pressure gauge bouncing around (need a regulator?)

[mathias]

Here's another one in my search for the cure for my off-idle hesitation.

Yesterday I picked up a cheap fuel pressure gauge from Harbor Freight. I hooked it up on the line between the tank and the fuel rail (spilling fuel all over, before I got all the connections tight). When running, the fuel pressure needle hops around all over the place! Not in a random way - it quickly fluctuates between about 28 and 40, faster than I can focus on the needle. The speed of the needle seem to be directly related to engine speed. When I turn off the engine, it settles nicely around 38.

I would worry that it's perhaps just how these cheap gauges behave, but the video on top of this Motobrick thread seem to use the same gauge, and it's very steady:
http://www.motobrick.com/index.php?topic=4157.msg26026#msg26026

So what do you think, do I need a new fuel pressure regulator?

[JamesInCA]

I have the same HF gauge, and mine behaves about like the one in the video. Still, their quality control isn't the greatest, and you could have a bad gauge.

Also, maybe the vacuum hose to the FPR? It's just a soft rubber hose; they dry and crack over time. $0.99/foot at NAPA for 3/16" vacuum hose.

[mathias]

You might be on to something with your vacuum hose suggestion! Once I had removed the hose from the throttle body and capped it, the needle was much more sedate.
I'll try to get to the store to pick up some new hose tomorrow, the old hose didn't have any huge cracks, but it was starting to get fairly crumbly.

[Grim]

Mine bounced maybe 2psi when I blipped the throttle but recovered to 40psi with a twitch as the injectors fired.

My guess is the pressure regulator. It's vacuum operated. When manifold pressure drops it raises pressure. Sounds like that's not happening or the fuel pump is dying and unable to keep up with demand.

[mathias]

The fuel pump seems to be okay - I hooked the pressure gauge straight to the tank, and when running the starter I got a strong 70 psi (static pressure, clearly the engine wouldn't run).

Once I get a fresh vacuum hose on there I'll check the fuel pressure again. If the needle still bounces around, it really has to be the regulator, doesn't it?

[mathias]

Okay, quick update - I got a fresh vacuum hose on there. The old one was sticky and flaky, but apparently not leaky. There was no difference, the meter needle still jumps around between about 30 and 40 psi.
I would suspect a flaky meter (after all, it's from Harbor Freight), but the fact that disconnecting the vacuum hose from the rail calms the needle down makes me think that the meter works fine.

So I just ordered a new (used) fuel pressure regulator from eBay. I guess there's always the risk of a used one being faulty as well, but for $20 it's worth the risk. The hard part is waiting for the part to arrive.
An update to come in about a week, once the part is here and on the bike.

[wmax351]

The way the FPR works is that it is differential pressure between the manifold and the fuel. Since it is an individual throttle body setup, the manifold vacuum is going to be quite jumpy, especially at idle. I wouldn't worry about it. As the RPM increases, the pulses become more frequent, and the inherent damping in the FPR takes over, and provides a steady fuel pressure.

[mathias]

You might be right, but that wouldn't explain why somebody on another K75, with the same brand and model meter, has a nearly rock steady pressure. And his bike revs freely, while mine hesitates, and sometimes even dies.
The needle doesn't settle at higher revs, other than that it can't keep up with the speed quite as well, but there is still a 10 psi fluctuation.

I'm thinking that the spring in the regulator might have gotten weak, so the vacuum influences the fuel pressure too rapidly. Or perhaps the regulator should have a restrictor in it (but doesn't), like the in-line restrictors in the hoses for balancing your throttle bodies. A bit of a long-shot, I know, but I'm running out of things to try. Other options might be to try a different AFM, even though the one I have appears to be working fine.

[wmax351]

You might be right, but that wouldn't explain why somebody on another K75, with the same brand and model meter, has a nearly rock steady pressure. And his bike revs freely, while mine hesitates, and sometimes even dies.
The needle doesn't settle at higher revs, other than that it can't keep up with the speed quite as well, but there is still a 10 psi fluctuation.

I'm thinking that the spring in the regulator might have gotten weak, so the vacuum influences the fuel pressure too rapidly. Or perhaps the regulator should have a restrictor in it (but doesn't), like the in-line restrictors in the hoses for balancing your throttle bodies. A bit of a long-shot, I know, but I'm running out of things to try. Other options might be to try a different AFM, even though the one I have appears to be working fine.

Yeah, checking with a new regulator could be worthwhile. Or possibly the fuel pump: It can't keep up with the injectors firing. Try pinching the vacuum line, which approximates a restrictor. If it still bounces too much while the vacuum hose is disconnected, that would suggest an excessive pressure drop with the injectors firing.


That video is from a K1100: They don't have a vacuum referenced FPR. [size=78%]http://www.k11og.org/forum/viewtopic.php?p=80040[/size] The only bounce there would be from the injectors firing.

[mathias]

Ah, crap, you're right, that video is of an 1100. I had assumed it was a K75, since it was posted on a K75 thread.
So the bouncing needle might be normal for a vacuum modulated FPR? I might have ordered a replacement one for no good reason, we'll see if I decide to switch them out anyway.

But here's a development - I looked at the timing setting. I had looked at it before, noting that the notches didn't line up (which seems to be a way to set the timing roughly to stock), instead it was rotated about 3 degrees. I had made the assumption that a previous owner had advanced the timing a bit, something I read about a lot. But with a second look, the plate had been rotated COUNTER clockwise - if I understand it right, retarded the timing. Perhaps somebody had misunderstand the directions for advancing the timing? So I set it to stock.

Big improvement! When cracking the throttle, instead of going whu-beaurgh-whoom! (or sometimes whu-beaurgh-click...), it now goes whu-whoom! Now I probably need to rebalance the throttle bodies, which might improve things further. Perhaps one day I'll get it to do a straight whooom!, but that might be too much to hope for.

[wmax351]

Ah, crap, you're right, that video is of an 1100. I had assumed it was a K75, since it was posted on a K75 thread.
So the bouncing needle might be normal for a vacuum modulated FPR? I might have ordered a replacement one for no good reason, we'll see if I decide to switch them out anyway.

But here's a development - I looked at the timing setting. I had looked at it before, noting that the notches didn't line up (which seems to be a way to set the timing roughly to stock), instead it was rotated about 3 degrees. I had made the assumption that a previous owner had advanced the timing a bit, something I read about a lot. But with a second look, the plate had been rotated COUNTER clockwise - if I understand it right, retarded the timing. Perhaps somebody had misunderstand the directions for advancing the timing? So I set it to stock.

Big improvement! When cracking the throttle, instead of going whu-beaurgh-whoom! (or sometimes whu-beaurgh-click...), it now goes whu-whoom! Now I probably need to rebalance the throttle bodies, which might improve things further. Perhaps one day I'll get it to do a straight whooom!, but that might be too much to hope for.

Yeah, the bounce should be expected, and a lack of it would be an indication of failure.

If you use a timing light off of the number 3 coil, at idle, the top of the notch on the wheel should line up with the mark. Though it sounds like you tracked down your problem.

[mathias]

Yes, after trying a whole bunch of red herrings (which nonetheless were worthwhile, especially replacing old rubber and adjusting the valves), I finally found the problem!
After I (roughly) set the timing right I balanced the throttle bodies, and there it is - whoom! No hesitation!

I have a basic timing gun, so I might as well check the timing that way. Question: where do I look for the mark to line up with the top of the notch? Do I peek through the notch?

[wmax351]

The notch is on the rotating portion, behind the hall sensor plate. There is an arrow cast into the timing chain cover. Its at 3 o clock. When the arrow is lined up with the notch in the base of the shutter wheel, it is tdc for cylinder 3. I believe it is 6 degrees btdc for the idle timing. The ibmwr.org tech section has a very good guide, with exact instructions.

Yeah, all of the things you have done are essentially maintenance on an old bike.

[mathias]

Great, thanks! At first I thought that the notch you referred to was the cutout in the hall sensor plate.
Even though it runs great now, any bike can always run a tiny bit better!

[mathias]

Alright, for the archives, in case anyone has the same question - yes, it seems that the fuel pressure does hop around all over the place.
I tried a different fuel pressure regulator, and the results were the same. It's the vacuum hose from throttle body #3, which we would expect to function in pulses, that makes the pressure all squiggly. I guess it's part of the design. Pinching the vacuum hose to restrict it (simulating the restrictors that come with carb balancing rigs) settles the pressure right where it should be.