Light popping/burble on acceleration

[Lbxpdx]

If, as you seem to be indicating, you can induce surging by leaning the Brick onto its side stand, that is BIG news. Replication is a key element of proving an hypothesis. All you need to do is consider which elements could be affected by gravity, or G-forces (as in the case of occurrence at speed). Pick one after another—wiring, hoses, switches, for example—and wiggle or move them attempting to cause an effect while the engine is running in neutral on its center stand.

The pedestrian element of a defective fuel pressure regulator should be resolved. Verification of work claimed to have been done, such as valve clearance measurement, should be undertaken. Many owners seem to have been disappointed by BMW service centers.

Obstruction in the air snorkel should be investigated. Another element to consider is the air flow meter. Is there obstruction in its intake; is its air temperature sensor dirty—there is a dedicated cleaner for that; is its electrical connection compromised, does the movement of its internal vane appear to drag or is the movement unregulated?

Diagnosing an ailing Brick should be an orderly process of system-by-system investigation. Preconceptions and assumptions made without sufficient depth of experience can stall resolution. It can be tedious but it should be focused. Appearances can be deceiving.

So I decided to pull the fuel pump out and check the lines. Everything was fine. 

I pulled the air meter out and cleaned the port and made sure the cane moved easily. 

Still the same. 

Decided to spray carb cleaner around while idling and my idle dropped when I sprayed around the injector o-rings.  I had them rebuilt, so the o-rings are new.  They are rather easy to pull in and out.  Could I have gotten o-rings that are ever so slightly too small?
The OD measures .575, 14.65mm.

I feel as though I should not be able to see the yellow when they are connected to the rail and inserted.  I can get them a touch deeper when not connected to the rail.

[Laitch]

You found leakage.  That the o-rings you purchased were the wrong size is always a possibility. Try some OEM rings if correct measurement seems elusive.
Verify that all parts of the injection assembly are present and fastened securely. Use a manual and parts fiche diagram for reference.

[Laitch]

Which o-rings have the 14.65mm measurement?

[The Mighty Gryphon]

Are you seeing a loose fit on all the injectors or just one or two?  It is odd that you have an air leak at the head but no fuel leaking from the injector's rail end o-ring.

Pull the injectors and put a little Vaseline on the o-rings before putting them in the head.  They should be a snug fit in the bore when you install the injector.  +1 on Laitch's advice to take things apart and reinstall the injectors in the head.  I am pretty sure the rail should be bottoming the injectors in the head bores.

[Lbxpdx]

Part #7 in the partsfiche are the o-rings with an OD of 14.65


All of the injectors pop in and out with relative ease, though it took some work to get them into the fuel rail.  I pulled the whole unit apart last night, inspected everything and reinstalled it. 

Can anyone verify that I should be able to see the spacer above the o-ring when installed?

[Laitch]

Here's an entry from the parts list of that diagram. I don't understand why there's a discrepancy at your end.

[The Mighty Gryphon]

Where did you get your o-rings?  Were they included in the injector servicing by the outfit who cleaned them for you?  It's puzzling to me because the o.d. you measured is actually larger than the stock o-ring. 

Were the injectors a tight fit in the head before they were sent out for service?

Take one of your injectors to the local NAPA and ask for new o-rings.  I bought a box of 10 for something like $8.  It even included a tool to install them. 

[Lbxpdx]

The O-rings were included in the service. On my way to work I was thinking that I did not measure the ID of the O-rings, and that there might be a slight possibility that they are larger than necessary, causing the O-rings to compress and not make a perfect seal.

[Laitch]

The O-rings were included in the service.

When were the o-rings measured—before installation into the head, or after the leak was found? Did you moisten the injector o-rings before inserting the assembly into the head? Did you check torque of the rack's mounting screws while tightening them?
As far as o-ring dimension discrepancies are concerned, sometimes it only takes fractional differences to screw up an assembly's function.

[daveson]

It's normal if you can see the spacer above the o-ring when installed.

One of my bricks had a loose rail due to worn out rubber washers under the rail, it seems to be number 10 in the above drawing. From memory it didn't cause a problem but maybe it was small and not easily noticeable. I changed them cause it felt horrible having a loosey goosey rail.

The rail on my K100 is secured with bolts, not nuts as in that drawing, that makes me wonder a bit about that part of the drawing.

[Lbxpdx]

When were the o-rings measured—before installation into the head, or after the leak was found? Did you moisten the injector o-rings before inserting the assembly into the head? Did you check torque of the rack's mounting screws while tightening them?
As far as o-ring dimension discrepancies are concerned, sometimes it only takes fractional differences to screw up an assembly's function.

Measurements were made after the fact.  I did not lubricate them nor did tighten the rail bolts to torque spec.  Unlike the image, mine are bolts that thread into the head. 

I did just measure the ID and they are indeed larger, .325 vs .296(7.52mm), than spec.  I’m going to see what I can find at the parts store. 

Side note, what is this vent for?

[Laitch]

The rail on my K100 is secured with bolts, not nuts as in that drawing, that makes me wonder a bit about that part of the drawing.

To whom it may concern: There is no nut in the rail fastening assembly of that drawing.


#8 is the rubber grommet vibration damper that seats within the mounting bracket hole; #9 is a sleeve bushing that fits within the grommet to receive #11–the mounting screw (bolt); #10 is a washer that is placed on the outboard side of the grommet; and #12 is a spring washer that bears on the washer to provide tension on the mounting screw to keep it from backing out during engine operation and to supply resistance for a torque value that will secure the assembly but won't crush the grommet reducing its damping ability.


At this time, it could behoove Lbxpdx to take a census of those parts to verify all are present, in good condition, and installed in the correct order. The rubber grommets should not be soft but they should be replaced if they are soft. Moistening the o-ring allows the ring to overcome resistance the head casting might create during installation. Most fasteners should be torqued to spec unless you've developed a sense of the correct torque from experience.

[Laitch]

Side note, what is this vent for?

That vent was part of an emission scheme. A hose attached to a port beneath the fuel tank transmitted fuel vapor from the tank into the crankcase. That vapor would be transferred to the air box via the z-tube and would be part of the combustion chamber air stream. The hose had a one-way valve in it that would open when fuel tank pressure reached a set level. Vapor would travel down the tube, and Bob's your uncle, only Bob turned out not to be your uncle, but instead became a distant, demented, inbred cousin, so the scheme was abandoned and BMW issued a service bulletin with instructions to cap that port in the crankcase and redirect those vapors from the fuel tank port into a hose that would exit behind the right footpeg plate, or go directly from the port into an air accumulator mounted on the frame beneath the port.

For those owners who are emotionally disabled by the threat of tedium, frankenduck has created this useful page concerning the art of air accumulation.

Stick a piece of hose on the crankcase port, stick a tight-fitting bolt into the hose, and clamp the hose to the port so it won't fall off, or just double over the hose and clamp it to the port, or put a tight-fitting rubber cap on it.

[Lbxpdx]

Well, I stopped at O’Reilys and they had a set of cometic injector o-rings in the correct size.  The o-rings that came from the rebuild are rather pliable compared to these.  The new o-rings snugged up much better in the ports.  I’m happy to say that the majority of the surging feeling is gone. 

I would still like to change the clamps from the TB to the plenum as I could rotate, not super easy but with one finger, the oetiker clamps.  I’d also like to change out the exhaust gasket and the copper header washers. 

I’m not sure what to attribute the overall feeling of the bike, it is smoother and revs easier.  I’m sure it is a healthy combination of the o-rings, plugs, plug wires and a cleaning of the air meter path. 

[daveson]

The bolt is shown on the wrong side in the drawing, and also on the Clymer drawing (page 232) so somebody has been copying from somebody.

Capping off that vent will also be an improvement, if you haven't already.

Do you get any backfiring on overrun?

[Laitch]

The bolt is shown on the wrong side in the drawing, . . ..

The source of the drawing I posted was MaxBMW's parts fiche—one of a couple of differing BMW parts fiche arrays at various vendors that have the same drawing. At this hour, some reasons I have developed for that anomaly are:
a) When the drawing was exploded, the dashed line was flopped and the rail inlet was displaced by the concussion.
b) Technical editors and proofreaders had not been hired before the date of publication.
c) All are fallen from the Garden and none is without sin.

It certainly could account for complaints by people who used it as assembly instruction that the inlet hose was too short to reach the inlet and their injector assemblies kept falling onto the garage floor because there were no threads for the screws to engage, but I haven't read any of those yet. 

[daveson]

My theory is that they were drawn in a dark room after a spiked black Russian, and there was nooooooooo bottom.

[Laitch]

. . . a set of cometic injector o-rings . . ..

Did they look like the Hale-Bopp without its streamers?

[Lbxpdx]

The hunt continues.  Since I last posted on this thread I have checked the valve clearances, everything is perfect and today I replaced the Cooper exhaust crush gaskets.  Everything I have done so far seems to have helped incrementally each time, though the problem is still present.  The only part in the air system that I have not looked at or replaced so far are the rubber boots between the plenum and the head. 

I think the crush washers helped quite a bit though.  With my exhaust off, I did notice my exhaust valve stems were pretty white.  Is this normal?

[Laitch]

They look okay for exhaust valves.
Did you balance the throttle bodies after you confirmed the valve clearances?

[Lbxpdx]

I checked the sync a few hundred miles ago.

[Martin]

There are a few reasons 75's backfire. Intake air leaks, exhaust leaks at the header to muffler junction, overly rich idle mixture, incorrect injector spray pattern, TPS adjustment incorrect and failure to close the throttle on overrun. And even with all those things corrected it may still have the occasional pop or burble. Regard it as a cat purring, it's happy.
Regards Martin.

[Lbxpdx]

I don’t mind the popping on deceleration, I’ve started to pull on the clutch and coast. It’s the popping on acceleration that bothers me.  I feel it is directly related to the intermittent and relatively small feeling of power loss at speed.  It almost feels like the system is adjusting the fuel mixture, but it does not seem to affect the rpm or the speedometer. 

[Laitch]

While the engine is running, carefully spray with Mapp gas all the intake components from air box to intake manifold at the engine side of each throttle body and also spray the z-tube connection between the engine and the crankcase. If you did it before, do it again. Have a piece of pie, too.

[Lbxpdx]

Well I ended up taking the TB's off the bike and cleaning the crap out of them.  They looked ugly from below.  Slapped everything back together, synced the TB's and while my idle is steady at 1000 rpms, I still have the slight hesitation at random speeds.  I did not take the rubber boots off of the TB's as I did not have the Oteiker clamp tool, fresh clamps or new boots.  Now that it is getting colder, I'm not sure I want to commute in 40 degree temps 30 miles each way to save $5 in gas a day.  I guess it's time to order some parts and get some new rubber bits.