Hello!
Ive been doing a complete overhaul of my gas tank, did a test start tonight, everything that I had repaired and replaced inside the tank worked fine after a few moments of hesitation (engine has not run for a year)!

But... Behind the plenum chamber I saw a nasty fuel leak, in the first few minutes of engine running at idle, I saw a jet of fuel splashing (probably) out from the fuel return hose, close to the FPR.

I think the fule return hose is broken (and was the initial problem, not the indeed very poor condition of the parts inside the fuel tank) and I will replace both of the hoses to and from the FPR.

Would it make sense that the FPR at the beginning of engine startup will receive a fuel overpressure, and let it off through the fuel return hose? Or would it be more probable that the FPR itself could be broken or leaking?

Else everything looks and sounds ok, fuel return check valve is cleaned and is in good working order as far as I can determine.

If I replace the fuel hoses and the fuel leak is gone and engine is working ok, can I presume that the FPR is ok, or what do you think?

Thanx!

/ Örjan.

If it doesn't leak and the engine runs well, I would tend to believe the fuel pressure regulator is okay.

If the fpr diaphragm is leaking, it will probably be an internal leak that can't be seen.  It is connected to the rear cylinder by a vacuum hose at the throttle body area. If there are signs of fuel or dampness there when removing the vacuum hose,  that would point to a leak,  especially if the cylinder was on the intake stroke at shutdown.

If leak in the diaphragm is large enough, fuel can be sucked into No 4 cylinder. This can result in a hydraulic lock and a bent conrod.
regards Martin.

MartinW wrote:If leak in the diaphragm is large enough, fuel can be sucked into No 4 cylinder. This can result in a hydraulic lock and a bent conrod.
regards Martin.

Got me there.... Just replacing an engine, because the cylinder was damaged too...

Hello, thanx for answers!

I will check (and replace) the hoses and hope for the best.

How does the FPR work? In what phase of engine operation does it let fuel through the return hose? Should I be surprised to see fuel (leakage) at the return hose at all, or does the fuel (at high pressure) pass through this hose on a (very) regular basis?

BRG/ Örjan.

The return hose should not have any pressure.
It's just a hose with open ending dumping the fuel back into the tank.

The pump pumps at around 6 bar of pressure the fuel to the injectors and the FPR.
The FPR is set to 2,5 bar, any more will pass.
The injectors will receive 2,5 bar.

The FPR has a internal diaphragm and a spring with a vacuum inlet on the upper side of the diaphragm this counters the spring pressure when the need for high fuel flow dictates high fuel pressure is needed.

MartinW wrote:The FPR has a internal diaphragm and a spring with a vacuum inlet on the upper side of the diaphragm this counters the spring pressure when the need for high fuel flow dictates high fuel pressure is needed.

Actually, the vacuum, in working against the spring lowers the fuel pressure when the throttle is closed.  Since this is usually done when decelerating, this reduction in fuel pressure reduces the amount of fuel injected into a coasting engine and helps to prevent backfiring.

The K75 I'm working on has a leaking FPR, sending the fuel through the vacuum hose to cylinder 3.
Unfortunally, injector at cylinder 3 did also leak.
Double the fun

Point-Seven-five wrote:

MartinW wrote:The FPR has a internal diaphragm and a spring with a vacuum inlet on the upper side of the diaphragm this counters the spring pressure when the need for high fuel flow dictates high fuel pressure is needed.

Actually, the vacuum, in working against the spring lowers the fuel pressure when the throttle is closed. . . .  

In the L-Jetronic, the FPR's sole function is to maintain consistent—not fluctuating—fuel pressure. Vacuum connection aids in stabilizing pressure.

High fuel demand and low fuel demand are satisfied at the same rate of fuel rail pressure. Closed-throttle fuel delivery rate is governed by the throttle switch signal's effect on fuel injector firing.

From the Bosch LE Jetronic Manual:

"The pressure regulator keeps the pressure differential between the fuel pressure and the manifold pressure constant."  

ie. The pressure in the rail will be reduced by high vacuum.

"The spring chamber is connected by a tube with the intake manifold downstream of the throttle valve.  This has the effect that the pressure in the fuel system is dependent upon the absolute manifold pressure.  Therefore, the pressure drop across the fuel injection valves is the same for any throttle position." 

ie. The vacuum acting on the regulator spring causes the regulator bypass to open at a pressure lower than the spring rate, thereby reducing the actual pressure in the rail as measured by a gauge in atmosphere.

What this means is that the gauge pressure in the rail will vary for different throttle/loads.  It does this to compensate for the changes in intake manifold pressure which will vary from 0psig to fairly high vacuum readings.  This is important in overrun situations with the throttle closed where the high intake vacuum would result in excessive fuel delivery causing increased emissions and decreased fuel efficiency, and in the case of our bikes, backfiring. 

Under heavy load, the intake vacuum goes to near 0psig, and the effect is to raise fuel rail pressure to deliver the proper amount of fuel for combustion.

Operation without the vacuum connection is virtually unaffected at high loads and wider throttle openings.  At smaller throttle openings, the engine will run with a richer mixture, with the amount of enrichment dependent on how high the intake manifold vacuum is.  Maximum enrichment occurs when closed throttle in high vacuum engine braking situations.  Rich mixtures under these conditions leads to backfiring.

Point-seven-five wrote:From the Bosch LE Jetronic Manual:

Neatly copied!

Our understanding seems divided by a common language questionably translated. Perhaps we can agree that the throttle switch has a greater effect on closed throttle performance than the FPR, based on how irritated some riders get by throttle response at low speed.

You are right, we are both talking about the same thing from different points of view.

I agree that the TPS works on a closed throttle, but the vacuum applied to the Fuel Pressure Regulator prevents overfueling in situations where the throttle hasn't been totally closed and when the owner has disconnected the TPS because of notchy throttle response at low speeds.

Replace the hoses going to the Fuel Pressure Regulator (FPR)and see if that fixes your problem.  I'm in the process of doing that to mine and also going to relocate the FPR. The hoses & the FPR are in a very high heat area, so they do fail.

With respect to the vacuum comments on the workings of the FPR.  On our bikes is there an advantage of drawing vacuum from all the cylinders, similar to manifold vacuum in a car/truck engine?  I've read several comments about those who have made stuff that allows the FPR to get vacuum from all 4 cylinders on our K's. It is supposed to be a benefit.  That actually makes sense to me as you wouldn't have the large drop in vacuum at the FPR when under acceleration, when you are drawing vacuum from only 1 cylinder. Drawing from all 4 would smooth out the vacuum drop.

That being said, I doubt the engineers who designed the engine and its components ever felt it was necessary, and the engine ran fine with just getting vacuum to the FPR from just one cylinder.  It also wouldn't matter which cylinder you got the vacuum from, #4 ( K100 ) was easiest to route the vacuum line, but when moving the FPR you can take the vacuum from what ever cylinder is closest.

From the Bosch LE Jetronic Manual:

Can some one give the official figure for the pressure regulator release pressure?

My flaky manual does not seem to give a figure but I had 2.5 bar in memory from somewhere. Someone above mentioned the same figure.

I have a K75 which is failing to start. It seems to run fine once it's going. I'm not that sure the regulator is the problem but I'm just checking everything at this stage and it is blowing at 3bar with the vacuum line open to air. In fact I saw it go to almost 4 at one point so it may be a bit rusty and sticky.

I was going to just buy a new one until I saw they cost about $150 and availability looks doubtful now.

I'll give it a blast in the ultrasonic clearer and a bit of WD tonight but I'd like a reliable control value to see whether it is doing what it should.

Thanks to anyone who has the official value.

36 PSI and you can still get the regulator try EME BMW seehttps://www.euromotoelectrics.com/product-p/fpreg-451e.htm.
Regards Martin.

OK that's about 2.6 bar near enough what I thought. At least I have a test figure now.

Thanks for the link but I'm in EU and the regulator + postage is $106 and will probably get hit with customs duties on entry. Seems like a lot of dough for valve and a spring.

My ultrasonic is making grating noise in the corner. I have two units in there. I'll see what happens tomorrow. 

Thanks for the  quick reply.

If you're willing to go Chinese Aliexpress have adjustable universal regulators, from $20.00 up. I've been running a Chinese rear master cylinder and a Chinese copy fuel pump for 4 years. However I don't know if I would go with a Chinese regulator. There are also other brands of adjustable regulators.
Regards Martin.  https://www.aliexpress.com/item/1005002951670764.html?

Thanks for that suggestion.  Sadly the chinese never even properly show what they are selling. The photos don't match box and it's not clear how many inputs there are. Does it have vacuum input ?? I may dig around and see if there is something clearly described.

The rear master cyl may be handy, do you have a link for that?

Actually , I need new rear brake reservoir. All my originals have got sun-burn and are falling apart if I look at them too hard. I found a Brembo remote reservoir and tied it on but it's lash up.  Any ideas for that ?

Thanks.

I can't find the store I got mine from. However there are heaps around this is one.   https://www.aliexpress.com/item/1005001557183914.html? You might want to look at this http://www.motobrick.com/index.php?topic=9364.msg128997#msg128997 Also do not use the supplied reservoir to cylinder hose it is not rated for brake fluid.
Regards Martin.

G'Day, a regulator set at 2.5 bar delivers pressure at about 36.37 psi, and 2.6 bar about 37.83 psi. So either regulator with those ratings should be fine.
Cheers Tom

Beamer wrote:I found a Brembo remote reservoir and tied it on but it's lash up.  Any ideas for that ?

Do have a photo of that lash up? I'm bursting with suggestions; all I need is a topic. 

Thanks Tom.  The accuracy of these regulators will not distinguish 2.5 from 2.6 , it's the same unit. IMO

These are german Bosch units certainly designed and spec'ed in bar. Any differences are probably due to translating this figure to psi and rounding to nearest whatever. 

@Laitch

I found a cylindrical Brembo reservoir and tied it in place with a zip tie. It's bodge but does the job. I was not wanting suggestions on fixing it, I was wondering if there were any better solutions for replacing the odd triangular reservoir on the Ks when they rot out.

Beamer wrote:@Laitch
I found a cylindrical Brembo reservoir and tied it in place with a zip tie. It's bodge but does the job. I was not wanting suggestions on fixing it, I was wondering if there were any better solutions for replacing the odd triangular reservoir on the Ks when they rot out.

I was not wanting a description; I was asking for a photo. 

The odd reservoir—a photo would help here because I'm certain our personal definitions of odd could differ—can be replaced by the updated cylindrical reservoir that is held by a spring clamp attached to the frame. Your bodged reservoir might fit that clamp. Motobins or Motoworks might have one.

Beamer wrote:OK that's about 2.6 bar near enough what I thought. At least I have a test figure now.

Thanks for the link but I'm in EU and the regulator + postage is $106 and will probably get hit with customs duties on entry. Seems like a lot of dough for valve and a spring.

My ultrasonic is making grating noise in the corner. I have two units in there. I'll see what happens tomorrow. 

Thanks for the  quick reply.

Tills.de is a good source.

After the ultrasound and a night soaked in WD40, I gave them a blow through to chase out the solvent. That sounds them like a horn and gives a good shake too.

I now have two serviceable units but they blow off at 2.8 and 3.0 bars in a repeatable fashion.

Even 2.8 bar is a bit high but I don't think  that is my main problem so I'll leave it there for now and remount it.

A quick check of ign. timing next but it looks like I'm in for a head strip to check bore and valves.

Voila, a new  triangular rear reservoir for 20 eurobucks
https://www.tills.de/fluid-reservoir-square-corners-k75-k100.html

Thanks for the tip Olaf.

On the rear brake reservoir the original triangular early one is readily available and cheap, I replaced all mine.

Well , much to my surprise it's fixed.

I put the 2.8 bar regulator on after a good clean as above, changed all fuel lines which, while not split, were pretty hard and stiff.

All the starting problems disappeared and I was able to correctly set the air flow meter by-pass screw ( which was not possible before ), tweaked the injector screws on the vacuum columns and it's running better than it has for a while.

Unless I accidentally fixed something I didn't notice in the process, it looks like it was the FPR getting a bit stuck an maintaining too much pressure at the ramp.

I may even give 'er bit of engine oil as a treat now.

My guess is you had a vacuum leak at a connection and you have fixed it.

Beamer wrote:Well , much to my surprise it's fixed.

If it's a surprise, start the party after a hundred miles or so. 

For the rear MC, you can use almost any reservoir.  As you might have found out, the used Brembo reservoir you are using works perfectly well.  On my bike, the original reservoir broke into pieces so I purchased a used Yamaha reservoir, pictured below.


I located this reservoir under the right hand battery cover and used a long hose to connect it to the MC.  The advantage to relocating the reservoir is that it is shaded from direct sunlight and therefore protected from UV light that kills the plastic (the original reason for the deterioration of the OEM reservoir).

Yes, my lash up was a very similar one but at least Brembo. Hiding it is  a good idea. That's what I meant by "sun-burn" destroying my original.

Now I've found the original part for 20 eurobucks, I'll order a pair.

Laitch wrote:

Beamer wrote:Well , much to my surprise it's fixed.

If it's a surprise, start the party after a hundred miles or so. 

We could have a surprise party

Laitch wrote:

Beamer wrote:Well , much to my surprise it's fixed.

If it's a surprise, start the party after a hundred miles or so. 

Quite agree. No parties announced so far.