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TECHNICAL MOTOBRICK WRENCHING In Remembrance of Inge K. => The Motobrick Workshop => Topic started by: Doingitdeeper on February 26, 2026, 03:25:29 PM
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Hello. Long time lurker, first time poster.
I have nut and bolt restored an 84 K100, and I have a final hurdle. The bike turns over fine, but won't fire with the cold start on or off. It spits fuel from the 4to1 part of the exhaust.
To lay out the current setup, all new fuel and air filters, compression and timing correct. All new fuel lines and new FPR. New coolant temp sensor showing 3.6k ohms at 15 Deg c.
Not new injectors but the bike ran fine about four months ago before the strip and rebuild.
Fuel pump age not sure. It looks newer than the rest of the bike did. Completely steam cleaned tank with new mesh on the fuel pump bottom.
Coils wired in correct as per the stickers for wiring colour. New coil earths to chassis. Proved spark. Plugs are wet but not black. It's not even trying to fire.
Ignition pickup plate in the same place it was when I stripped the bike, I marked it.
Any ideas? Stuck injectors or injectors not operating properly?
Thanks!
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- The forward coil's secondary wires connect to cylinders 1 and 4 spark plugs; the rearward coil's secondaries connect to cylinders 2 and 3 spark plugs.
- The fuel return line connects to the forward pipe on the floor of the fuel tank at its left front end; the fuel pump delivery line connects to the rearward pipe in that area.
- Verify that fuel is entering the tank from the return line. It will not be returning at much pressure; that's okay. If the return line is clogged somewhere along its length or inside its tank fitting, fuel pressure will rise excessively at the injectors and flood the cylinders.
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Hi thanks for your reply. My fuel pipes are the correct way round at the tank, I'm going to triple check my secondaries orientation tomorrow. I'm pretty confident it's alright, but a check won't hurt.
The line is not blocked, but as you say the tank fitting might be. Is it easy to see the return line in the tank? I've never actually noticed it.
I'll check tomorrow and report back. I had intended to pull the injectors rail of and see if I can rig up a leakoff type bottle test. But I need to buy four bottles first haha
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If your Brick's tank is its original and the return fitting is unmodified, the returning fuel comes from the return fuel assembly opening below the tank's fuel level, so all that might be seen is a slight agitation of the fuel supply by the returning fuel. That return system was was replaced by a metal tube brazed to the tank floor inlet opening which conducted fuel to the top of the tank where it then spilled into the fuel supply.
If your Brick's tank has a fuel return fitting screwed into the tank floor, it could contain a check ball within it to prevent fuel spillage when the rubber return hose is removed. Sometime that ball can seize in the flow-stopping position. Sometimes, previous owners have removed that ball after it started vibrating with an audible buzz. There is no effect on engine performance if it's missing.
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Good afternoon, I ran some checks today based on your advice.
Tested all injectors, all firing. Plugs still wet.
However I am getting zero fuel return when the return hose is off the tank connection. My return in tank does still have the little ball though, and is clear. I understand this should be about 500ml for every 15 seconds of cranking after reading some old posts. I gat absolutely zero.
Am I right suspecting that the FPR is kaput, or possibly the vacuum line to it is kinked or not playing?
With no fuel return at all it seems that's why it's dumping into the cylinders.
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The original post indicates the fuel pressure regulator is new. Do the injectors indicate they are pulsing? Did you verify you connected the coils to the correct cylinders?
The return line needs to be checked its entire length for leaks and clogging. When fuel pressure increases beyond spec, flooding of the cylinders starts. Carefully remove the vacuum line from its attachment to its throttle body and see if fuel leaks from it. That would be a symptom of a bad regulator diaphragm but unlikely to stop fuel returning to the tank, and also unlikely in a new fuel pressure regulator.
Excessive fuel pressure cause by a clog, flooding from injectors stuck open by electronic or mechanical faults, and spark plugs not firing correctly because their wire connections are incorrect or corroded, or their coils are faulty would allow excessive fuel to accumulate in cylinders and subsequently be blown down the exhaust headers to the muffler joint.
This system could possibly have more than one fault impeding starting.
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Hi there I'll add some replies to your points to clarify.
Brand new fuel hoses throughout, no blockages. Yes the FPR is new however I think the vacuum hose was poor, it could have been kinking. Im going to try and replace it today. I suspect it is not OEM and is too thin, it may have been kinking under vacuum or collapsing - it was a cheap silicone one I had lying around.
Plug leads all on the right coils, wiring correct the coils.
No fuel in the fpr vacuum line.
Brand new plugs all sparking well.
I have not yet checked for pulsing, I'm going to do that tonight. I have the fear of atomsied fuel everywhere so I need to nip to the shop and get some pop bottles to fire them into.
Thanks for your reply! I really do miss forums. They're a dying breed.
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Ok, some progress. New vac line, no difference.
I removed the rail and rigged up a leakoff kit to catch the fuel from the injectors. Not one of them is pulsing, just shooting that precious fuel directly into the bores.
Funnily enough the bike started fine with risidual fuel in the bores. So that's nice!
Onwwards and upwards to find out why the bike chooses not to send pulses. The injectors are closing and opening fine so I am going to say they're operating technically correct.
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Funnily enough the bike started fine with risidual fuel in the bores. So that's nice!
It started because there wasn't an excess of fuel available to flood it. If it ever idles and accelerates, it will be really nice.
The fuel injection control unit is beneath the seat within a box that has the tool box above it. Remove the battery ground strap from its terminal on the transmission and don't allow the cable terminal to contact metal. Decouple the electrical plug from the fuel injection control unit. Spray the pins of the unit and the sockets of the plug with a high-quality electronic cleaner like DeoxIT D5 (a highly effective cleaner found in musical instrument shops selling electronic gear or online), CRC electronic cleaner or other equivalent. Let the components dry then install the plug verifying that it firmly latches and cannot be swung outward. Reconnect the battery ground cable to the transmission. Start the engine to determine the effect of cleaning the fuel injection control unit contacts.
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Well, I found my issue. I tested the injectors and they were dumping fuel with no pulsing. On and off fine when cranking.
Thinking this may be an earth problem as they are earth activated, I tried with no ECU connected and they didn't fire. Which is good.
Thinking the ECU was at fault, or maybe some components, I opened it up to find this:
New ECU on order. Happy I have a fault identified at least. The rest of the bike showed no such damage.
(https://i.ibb.co/p6cPtqmt/20260228-144256.jpg) (https://ibb.co/p6cPtqmt) (https://i.ibb.co/gbGmwwRp/20260228-144302.jpg) (https://ibb.co/gbGmwwRp) (https://i.ibb.co/1fyvb2ty/20260228-144310.jpg) (https://ibb.co/1fyvb2ty) (https://i.ibb.co/ycxj3v7J/20260228-144316.jpg) (https://ibb.co/ycxj3v7J)
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103123
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flood victim?
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Not entirely sure. The condition of the rest of the bike doesn't match the ECU, perhaps it was an addition before I bought the bike.
It was not on the roads since 2011. The guy I bought it off said it was half restored by a gentleman who died, and he ended up with it. The bike was in pieces in Nov 25, so I stripped it all down. Powder coated the frame and engine covers. Replaced every bolt with stainless.
There was some rust sure. But nothing like this.
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is it fine dirt, like dried silt or mud? I wonder if it's a replacement ECU from a parted flood bike.
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No dirt at all actually, I had a second look. Just water, rusty water. It's clearly been flooded for a long time.
Interesting though it works enough to turn the injectors on. Just not enough to turn them off!
I am hopeful my replacement comes quickly and works as intended.
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After installing the replacement give this one a good clean. I bet you'll be able to find the issue for repair. Rusty residue tells me you might have a simple short or a culmination since rust is conductive.
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It's likely this Brick was cleaned more than once in its history using water from a high-pressure hose like those found in do-it-yourself car washes. Even soapy water at high pressure could have been used. BMW specifically warns against using high-pressure cleaning for this reason—moisture could infiltrate electronic units and electronic connectors eventually causing oxidation that interferes with engine operation.
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Hello again, little follow up. I fitted a replacement ECU and retested the injectors. They now all pulse and fire well, with a nice spray pattern. I also for my own sake tested their resistance and got a healthy 16 ohms.
Bike still wouldn't start.
So I tried various things, tested the trigger plate, spark, all good.. etc etc.
So I wondered if the injectors were gummed. I pulled the rail and turned the bike over.. and it started! Ran fine for four seconds or so with the fuel in the bores.
So where does this leave me? I must presume over fuelling still. The only thing I can think of is I have no coolant in yet, so the CTS is in air. But again, this doesn't make sense as if the bike thought it was 15 degrees (air temp) and not probably 6 degrees (water temp) it would run LEAN not rich.
So. Kind of back to square one but I have fixed one issue. I need to find a fuel pressure gauge I think.
I can hear the fuel returning when cranking so that side of things is all working. The bike starts fine with the fuel pump connector removed and some good old easy start.
Very odd.
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Did you mess with the TPS?
Also, I'm not sure if you mentioned having cleaned the fuel pump connector. Personally, I changed the OEM connector to a SuperSeal connector from cycleterminal.com. Check the wiring for any breaks, given that it's an active, maneuverable part of the harness. You might have an intermittent electrical problem. Overall, since finding the rusty stains, checking and cleaning each connector would be a priority. On the side: sometimes upgrading a connector is necessary, given the age of the bike. Even replacing wires where necessary is important. You can't expect or assume proper current flow with age.
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Yes. It's been off and back on. I put it back to where I thought it was.. and it's internal switch is 'off' when I am not twisting the throttle.
I do not have a functioning lunchbox at the moment, can I test and set it's position somehow through electrical testing?
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Modified my prior post, added stuff for good measure.
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Thanks Edward, would you suggest TPS may be a culprit here?
The bike starts and runs when the injector rail is not connected.
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Test the fuel pressure on the delivery line using a T-fitting for the gauge.
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I suspect there might be several factors. I had already mentioned the possible fuel pressure regulator and the TPS. For the TPS, try to slightly rotate the throttle a smidge, allowing for more fuel. The pressure regulator is behind the airbox. I've replaced three so far and now keep one as a spare. Yes, they do fail even though many think they last forever. They don't, and no one wants a pool of gas on top of the engine while out and about. Check the associated hoses to the fuel system; I can't recall you saying that they are all new. The vacuum hose is often forgotten about. Did you thoroughly inspected inside the tank? Is the fuel filter new, not just assumed to be new, and installed in the correct direction? Is it the right part for the job? If it's an assumed new filter, there could be a restriction. One issue could be water. Yes, water again... Did you test the pump with the fuel injector feed hose leading into a deep container? All the hoses need replacing after a few years. I give mine around 5 years, including the internals. Did you pop out the fuel pump to inspect the screen and connect it to a 12V source to see if it spins? Detaching the fuel line from the rail and activating it does the same. I just like to check when I'm inside the tank. All spark plug wires going to the correct plugs (bike does start for a wee bit)? You have the manual, correct? Maybe this should be a numbered list so others can chime in.
So after all the basics, as mentioned above, the Hall Effect Sensor might have gone glitchy on you. Thats avail at Euromoto 12 11 1 459 033. Its located behind the T-Plate on front of the engine. Hex keyed. Hall makes the plugs spark at the right time.
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. . . I wondered if the injectors were gummed. I pulled the rail and turned the bike over.. and it started! Ran fine for four seconds or so with the fuel in the bores.
So where does this leave me? The bike starts fine with the fuel pump connector removed and some good old easy start.
It leaves you with an engine that idles and runs without fuel pressure but not with fuel pressure. One way to check that is by leaving the fuel supply connected to injectors mounted in the engine and checking the fuel pump pressure as recommended with a gauge fitted into the delivery line and cranking the engine. The result will indicate whether the fuel pump is functioning correctly. If the pressure is at spec, you can turn your attention elsewhere to determine the fault.
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You can always connect a 9v battery to each injector independently or have them sent off for sonically cleaning which is a smart thing to do regardless. Mr. Injector does a fine service. Iridium plugs are good to have as well. Spray aerosol injector cleaner through them backwards from a fresh can. Then compressed air. Simple. Frankly sending them off is the proper method so they can be tested and visualized with a graph. Its all apart of the process, no worries. Step by step.
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I've been following this thread and thought I would add my own experience with this issue.
The only time my bike would not start and was spitting fuel out the exhaust the culprit turned out to be a bad temp sensor. It was telling the computer that the temp was extremely low and needed to dump fuel like crazy to overcome that situation. I see you checked the resistance of the sensor, but could the problem lie in the wiring of that part of the system or the computer itself?
Frank
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I rectified this issue so thought I would comment for closure.
I ensured all the points above were right.
Playing with the injector timing plate on the front of the engine, turning it clockwise led to the bike starting but revving high.
This was then rectified by resetting the TPS as it was not only in the wrong place it was also holding the idle open.
Both were tweaked for the best possible running and now the bike idles and starts fine.
To clarify my non firing injectors was down to the ECU being fried. The non starting after this point was spark timing and then TPS.
Thankyou everyone for all your inputs.
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icon_cheers
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cool.