I'm about to replace the original (and most likely fooked) FPR on LFB for an adjustable one (like the one Martin used). Does it matter whether it is upstream of the injectors or does it have to be downstream as OEM? I don't think it matters, but better to ask, reason being that I have the gauge mounted very nicely on the top of the coil cover and the obvious place for the FPR is hanging off the side of the radiator. It looks as if that's what Bert did, but the pic ain't big enough for me to be sure.

Burt's FPR mount looks like it was attached to a threaded boss on the left frame downtube.  That put it off to the side of the radiator and out of the hot airflow from the radiator.  I think he also put an opening in the radiator surround to allow outside air to flow over the FPR.

It looked like he took the return line from the forward end of the injector rail and plumbed it to the FPR, and then put a nice loop of fuel line from the fpr to the return spigot on the tank.  Said loop was large enough to allow moving the tank back far enough to reveal the relay box. 

His setup is what the factory should have done in the first place.  Simple, clean, reduces fuel heating and makes servicing the regulator much easier.

You could mount it upstream and it would work just the same but, there might (or might not?) be an advantage to running fresh fuel through the rail with the downstream layout.
Cars that I can think of seem to follow the 'opposite end of the rail' layout.

I hate to be the voice of sanity (and claim none) but people all over the world have put oodles of miles on Ks with the factory FPR in the factory location without issue.

If you really want improved performance then you should spend your money on a 600 or 1000 CBR or something of that ilk.

Not to denigrate K bikes (i own way too many of them and love them to death) but you can't make a silk purse out of a sow's ear.

JMHO.

It’s the challenge Duck, the challenge…
I mean, that engine, that insane polygon of a tank….

The fuel pressure regulator needs to be downstream of the injectors or else the injectors would be on the return line where the pressure would be close to nix pounds per square inch.

I'm absolutely with duck about improving performance, if I wanted a bit more power I'd sell the brick and get a bike with the power I'd want, or more. But I do admire the skill and dedication of those who tinker. And the saying in this neck of the woods used to be "you can't make strawberry jam out of pig shit"

The idea is to get the regulator away from the hot airflow from the radiator.  This is to help reduce fuel temperature in the tank and the boiling that many have encountered when riding in hot locations.  If there is any power gain from running cooler fuel, it's just a side benefit. 

To me, the power gain is meaningless when compared to the reduced roasting of my thighs when I'm riding in places like Death Valley or the Arizona desert.  The best thing I ever did to my K75RT was to add a nice big fuel cooler out in the breeze.  The tank temperature reduction was dramatic.

The original FPR is old and toast; pressure is a smidgen below 30psi and buying a s/h one could lead to the same result, hence the replacement. I couldn't really be a$$ed about any perceived increase in power with an adjustable one - in fact, I didn't know that they are supposed to give a boost in power - I just wanted something that I could see was working and running at the correct pressure. As it happens, I got this new Malpassa half-price for £52 as opposed to £50 for a s/h OEM from Motorworks, although I did pay an additional £21 for a genuine Syco pressure gauge. Still cheaper than a new OEM at £129.

So, downstream of the injectors it is.

Ended up fitting it right where Bert did. Although the adjustible FPR is slightly larger in diameter than the OEM one, it still fits inside the fairing at that point (just!). The gauge now says the pump is providing 2.5 bar rather than the 2.1-and-a-bit that the OEM one was providing. I made a nifty wee sliding mount for the pressure gauge so that if I need to remove the coil cover, the gauge slides out and saves messing with the fuel pipes.

Edit: pics




The tongue slides under the plate at the back of the coil cover. The plate is sort-of bridge-shaped to hold the gauge securely. The socket on the right is a standard Hella socket for charging the battery.

Repeats to self "There's no such thing as a daft question" takes deep breath and asks:

"Are those readings with the vacuum hose disconnected?"

Neat installation btw.

Dai wrote:Ended up fitting it right where Bert did.

It looks like a hydra wearing a monocle—very nice!

Suzi Q wrote:Repeats to self "There's no such thing as a daft question" takes deep breath and asks:

"Are those readings with the vacuum hose disconnected?"

Neat installation btw.

Thank you. Readings were done with the hose connected, Chris, as that's how I checked the OEM FPR.

I couldn't find any web info that states you have to disconnect the hose. But, that's how it was always done with vacuum advance on distributors (remember them?)
Reason I ask is because there's a big drop in the indicated fuel gauge pressure with the vacuum hose connected to the fpr, at anything below open throttle. Obvious really, because the function of the vacuum line is to keep fuel corrected to the same pressure relative to the inlet manifold. If this didn't happen, then the injectors would be squirting into something less than atmospheric pressure which, when added to the uncorrected fuel line pressure of 36 psi, would mean that the injectors were operating at anything up to 50 psi. differential pressure between the injector inlet and the injector nozzle.
I set my Malpassi fpr (same as yours - good choice!) at 36 with the hose disconnected. The gauge indicates around 31 with the hose connected, on tickover. Fuel pressure rises as you blip the throttle (and inlet manifold vacuum drops), and then drops as revs rise (and inlet manifold vacuum rises again)
I've seen anything as low as 20psi on the fuel pressure gauge when riding - closing the throttle for heavy engine braking. Otoh, a hefty handful of throttle when riding sees fuel pressure rise to a 'normal' 36 psi.
This makes me think that 36 psi is the target pressure for the fuel rail, relative to the inlet manifold pressure. The inlet manifold spends most of its time in vacuum (that's what the 'throttles' are for - they're almost constantly throttling the poor old engine into a state of advanced asthma) so, although the fuel pressure gauge shows something less than 36 psi absolute, the fuel is more or less a constant 36 psi nett relative to the inlet manifold.
Phew?
What's anyone else think?

BTW, I had a suspect OE fpr, same as you. It wouldn't go above 31 psi (same as yours) whether the hose was connected or not. Charlie also noticed the same reading with a fpr that he had an issue with. I do wonder if the OE fprs weaken - maybe the spring oscillates a gazillion times in its life and weakens?

With the hose connected, give it a handful and the needle flips up to about 2.7 bar.. At idle, it's rock-steady on 2.5 bar. Haven't done any on-road testing yet as it's missing something called an MoT.

Edit: Big chunk removed because it was totally wrong. See below.

Suzi Q wrote:I couldn't find any web info that states you have to disconnect the hose. 

What's anyone else think?

About the spring, I agree, I'm guessing it gets weaker over years and after much use, the rubber diaphragm would also fail eventually. That's one of the benefits of computers, less mechanical items with moving parts are needed, and moving parts wear over time. Many unneeded parts continued to be used when computers started being used, due to convention.

On page six of the BMW Testing Instructions for K Models it specifies a fuel pressure of 2.5 bar with the vacuum hose disconnected, that's about 36 psi.

Take this with a grain of salt, you'll see why, the K1100 fuel pressure regulator doesn't have a vacuum hose, I think, because the computer has enough inputs to know what it is, without an unneeded input which will eventually be unreliable.

Similar to the carby days, the early K100 bricks had a vacuum switch to alter the advance at mid throttle, I assume advancing it, but I've never tested that yet. I've read it was deleted cause it didn't do much, but maybe they realised the computer could work it out, making that input obsolete. I guess the computer part number didn't change, but I've also read that BMW sometimes did that, since it's probably backwards compatible as well, phew too.

As spannerman used to say (Motorcycle Trader, another victim of COVID, I miss that magazine) we might be overthinking this, but here's another bit about the spring. Maybe the low pressure is because of a weak or broken spring, not in the fuel pressure regulator, but the relief valve in the pump, I realise this doesn't apply in your case. If the return line is slowly pinched until the relief valve opens, then that would confirm that the pump is good, and it's the fuel pressure regulator that wants to be replaced.

daveyson wrote:On page six of the BMW Testing Instructions for K Models it specifies a fuel pressure of 2.5 bar with the vacuum hose disconnected, that's about 36 psi.

Ahhh.... Good job I haven't replaced the lower fairing yet. Thanks for that

Useful post from Daveyson, thankyou. Glad it's resolved because I've looked and couldn't find the answer. 

Dai, I'd say your manifold setup for the vacuum line is a plus. I have the same Malpassi fpr and the fuel pressure gauge oscillates at tickover, whereas yours is steady. And that's even after I teed all three vacuum stubs together to try and even things out. That did reduce the oscillation, but didn't get rid of it. Maybe being a triple adds to the issue as well?

SQ, did you just tee the vacuum ports together or tie them through a manifold as Dai did?  I have been looking at his arrangement and wondering if that manifold also acts as a vacuum reservoir to help smooth the vacuum pulses in the ports.  I have seen vacuum accumulators added to vehicles that had vacuum operated devices to prevent loss of vacuum when the devices were operating.

Just plastic tees and 4mm hose. I was just concerned that, after fitting the Malpassi fpr, the fuel pressure gauge (which was already installed) seemed to oscillate a good deal more than it had with the OE fpr. I was feeling a bit concerned for the fpr diaphragm jiggling itself to an early death at the hands of a boisterous triple that's all.

Reset the FPR to 2.5 bar without the vacuum tube and didn't forget to block the tube off before testing (I knew 4.5mm drills were good for other things apart from drilling holes). I'm now getting the same readings as Chris reported, but the needle is still rock-steady. My only comment regarding a vacuum reservoir is that the commercially available ones all appear to have some form of reservoir. The person to answer the question re. the needle behavioir is (whistles) @MartinW because IIRC, Martin has a Syco gauge, Malpassi FPR and an AliBaba commercial vacuum manifold.

Dai, I think your arrangement qualifies as a vacuum accumulator as well.  I suspect any chamber with a volume will be enough to tame rowdy vacuum pulses. 

I am going to install some tees and 3/16" tubing since I obtained all the parts for a miserly $6.  If I experience any issues, Plan B will be to fabricate something using copper pipe and brass tube.

I had a nother good look at dat der needle today. It it flicking but only just - you have to stare long and hard at it to see it.

Come to think of it, staring long and hard at the needle is probably making it uncomfortable and that's what's causing the tick.

Re. vacuum hose connected/disconnected when testing/setting fuel pressure:

This is the picture after a few hundred miles with the adjustable fpr set at 36 psi with vacuum hose disconnected - the indicated fuel line pressure is about 31 psi on tickover, and about the same on steady throttle cruising at sixty or seventy. Indicated pressure rises to 36 when the throttles are quickly opened.



I'm happy with that (can anyone remember the generic 'picture of rich/lean/okay spark plugs' that used to be in every Haynes manual lol )

Dang!  Those plugs are perfect!