Hi all. I’ve been scratching my head over this so hoping somebody might be able to help.
Last time my K100 ran it started and ran perfectly but now doesn’t want to start, other than the fact it was hammering down with rain there was nothing unusual about its last ride.
The bike has spark on all four cylinders, there’s good compression on all cylinders and the injectors are working and spraying nicely. There’s plenty of fresh fuel in the tank and I’ve checked there’s no water in the fuel given how hard it was raining. The engine doesn’t want to run but makes some enormous bangs and has created some incredible flames!
As the bike was ticking over nicely when I arrived back from its last ride I can’t foresee any engine timing issues but I have already done the following: changed the fuel pump for a new one along with a new fuel filter, fuel hoses and fuel regulator. I’ve also replaced the water temperature sensor, changed the ecu under the seat and the air flow meter in the air box. The battery is in great condition and I have a spare which I’ve tried for the sake of it too.
After cranking over for a while I get fuel dripping from the joints in the exhaust - guess this is the cause of the big blue flames! If I unplug the fuel tank/pump the bike makes a small attempt to run but very quickly goes back to popping and banging but no running..... any ideas?

It looks like you are over fueling, you need to check the temperature sensor connector at the sensor and at the computer. There is also another connector under the fuel tank which is often overlooked. There are posts on how to check this. It is also important not to use thread tape or sealant on the sensor threads, it seals with the washer on the sensor body. Using sealant or tape can stop the sensor from earthing.
Regards Martin.

There is a check valve in the return line fitting on the fuel tank. There is a chance that it is stuck closed and preventing the fuel bled off by the pressure regulator from flowing back to the tank. This will cause a very high pressure in the injector rail and too much fuel being injected.

A quick way to check it is to disconnect the return line from the tank and add a few feet to it so you can stick it in the filler opening. This will bypass the check valve. Then try starting the engine. If it starts and runs, the check valve is the problem. Fortunately, it can be removed and cleaned, or you can replace it fairly inexpensively(in the U.S. it's less than $10).

Hi. I forgot to mention that I’ve already tried this but has the same result.

The fuel should be pulsing, not constant. With a screw driver between an injector and your ear, you should hear a tick, tick, tick, while cranking.

Have you tested the ignition control unit using the steps in the Portal's troubleshooting guide, or cleaned its plug's contacts, reinserted it firmly then tried restarting the engine?

Are the ignition coils bone dry?  I'm not sure, but your bike may have the early version coils that were replaced sometime around '86 or '87.  These early coils have been known to fail relatively often.  One of the failure modes is a weak spark that will work when the plug is out of the engine, but not under the pressure in the cylinder at the top of the compression stroke when it's needed to ignite the fuel.  Any moisture on the coil will make this loss of spark more likely to occur.

Another long shot is the spark plug wires.  Others have been known to mix up the wires for cylinders #3 & #4.  If you have factory wires, check the numbers on them and the cylinders they are connected to.

Ignition problems on the k100 usually result on the engine running on only two cylinders when it does run.  When you unplugged the tank and the engine ran, was it running on all four cylinders?  

When starting, you aren't opening the throttle, are you.  The fuel injection on these bikes can easily flood the engine if the throttle is opened before it's running.  If that is the case, unplugging the tank and cranking the engine to blow out the excess fuel can help the engine start.

daveyson wrote:The fuel should be pulsing, not constant. With a screw driver between an injector and your ear, you should hear a tick, tick, tick, while cranking.

The injectors are working as they should having tested them into a container

Laitch wrote:Have you tested the ignition control unit using the steps in the Portal's troubleshooting guide, or cleaned its plug's contacts, reinserted it firmly then tried restarting the engine?

is this the computer under the tank? If so I’ve tried a donor one and cleaned all the contacts which made no difference.

Point-Seven-five wrote:Are the ignition coils bone dry?  I'm not sure, but your bike may have the early version coils that were replaced sometime around '86 or '87.  These early coils have been known to fail relatively often.  One of the failure modes is a weak spark that will work when the plug is out of the engine, but not under the pressure in the cylinder at the top of the compression stroke when it's needed to ignite the fuel.  Any moisture on the coil will make this loss of spark more likely to occur.

Another long shot is the spark plug wires.  Others have been known to mix up the wires for cylinders #3 & #4.  If you have factory wires, check the numbers on them and the cylinders they are connected to.

Ignition problems on the k100 usually result on the engine running on only two cylinders when it does run.  When you unplugged the tank and the engine ran, was it running on all four cylinders?  

When starting, you aren't opening the throttle, are you.  The fuel injection on these bikes can easily flood the engine if the throttle is opened before it's running.  If that is the case, unplugging the tank and cranking the engine to blow out the excess fuel can help the engine start.

The coils I believe are sound. They other a very strong spark when testing the plugs against the block. When/if it starts when the tank is unplugged it runs on all four.
The leads are in the correct order.
No throttle is used when trying to start like you say I know it doesn’t like. Even with the throttle closed fuel starts dripping from the exhaust pretty quickly followed but some bog shotgun style bangs and blue 2ft flames. Goes to show it’s got a spark I guess

Point-Seven-five wrote:Are the ignition coils bone dry?  I'm not sure, but your bike may have the early version coils that were replaced sometime around '86 or '87.  These early coils have been known to fail relatively often.  One of the failure modes is a weak spark that will work when the plug is out of the engine, but not under the pressure in the cylinder at the top of the compression stroke when it's needed to ignite the fuel.  Any moisture on the coil will make this loss of spark more likely to occur.

Another long shot is the spark plug wires.  Others have been known to mix up the wires for cylinders #3 & #4.  If you have factory wires, check the numbers on them and the cylinders they are connected to.

Ignition problems on the k100 usually result on the engine running on only two cylinders when it does run.  When you unplugged the tank and the engine ran, was it running on all four cylinders?  

When starting, you aren't opening the throttle, are you.  The fuel injection on these bikes can easily flood the engine if the throttle is opened before it's running.  If that is the case, unplugging the tank and cranking the engine to blow out the excess fuel can help the engine start.

The coils I believe are sound. They offer a very strong spark when testing the plugs against the block. When/if it starts when the tank is unplugged it runs on all four.
The leads are in the correct order.
No throttle is used when trying to start like you say I know it doesn’t like. Even with the throttle closed fuel starts dripping from the exhaust pretty quickly followed but some bog shotgun style bangs and blue 2ft flames. Goes to show it’s got a spark I guess

Clean and dry your battery and brick.

I had an overfueling problem once, so it wouldn't start at idle speed, but it did start at revs because at revs it needs a high fuel flow.

If throttle isn't the cause of flooding, but helps with starting, that might show that flooding is the problem.

bigminiandy wrote:. . . there’s good compression . . . There’s plenty of fresh fuel . . .  changed the fuel pump . . . a new fuel filter, fuel hoses and fuel regulator . . . replaced the water temperature sensor, changed the ecu . . . and the air flow meter. The battery is in great condition . . . The coils I believe are sound . . .The leads are in the correct order . . .  the computer . . . I’ve tried a donor one and cleaned all the contacts . . .The injectors are working as they should having tested them into a container . . .

We're approaching ancestral curse territory now. Have you done a thorough geneology?

A coil with failing insulation will spark when the plug is out, but can have problems with voltage leakage if it is wet and trying to fire into the high pressure in the cylinder. It's the one part of the equation that can sometimes look good when you test it, but not when you try to start the engine. A failure to ignite the fuel on the first couple compression strokes will soak the plug tip, and then it will be very hard to get a good spark.

You don't mention changing the spark plugs. How old are they? Make sure the gap hasn't opened up. Reset the gap to spec or even a little less. That will reduce the amount of voltage needed to spark.

You mention the heavy rain. I would try to do a thorough drying of the coils. Pull the wires off and hit the plugs and the terminals with a hair dryer. Clean any dirt on them as well. You want them spotlessy clean and dry.

If you have access to replacement coils, it might be worthwhile to try swapping them.

Point-Seven-five wrote:A coil with failing insulation will spark when the plug is out, but can have problems with voltage leakage if it is wet and trying to fire into the high pressure in the cylinder. It's the one part of the equation that can sometimes look good when you test it, but not when you try to start the engine. A failure to ignite the fuel on the first couple compression strokes will soak the plug tip, and then it will be very hard to get a good spark.

You don't mention changing the spark plugs. How old are they? Make sure the gap hasn't opened up. Reset the gap to spec or even a little less. That will reduce the amount of voltage needed to spark.

You mention the heavy rain. I would try to do a thorough drying of the coils. Pull the wires off and hit the plugs and the terminals with a hair dryer. Clean any dirt on them as well. You want them spotlessy clean and dry.

If you have access to replacement coils, it might be worthwhile to try swapping them.

The spark plugs are brand new and gapped correctly. I have those flashing caps fitted between the ht lead and the plug for testing so know the plug is getting the power but I haven’t got a donor set of coils to test with unfortunately.
The last time it was rode was back in May so whilst it was very wet at the time it’s had plenty of time to dry out as it’s kept in a heated garage.

MartinW wrote:It looks like you are over fueling, you need to check the temperature sensor connector at the sensor and at the computer. There is also another connector under the fuel tank which is often overlooked. There are posts on how to check this. It is also important not to use thread tape or sealant on the sensor threads, it seals with the washer on the sensor body. Using sealant or tape can stop the sensor from earthing.
Regards Martin.

I’ve replaced the sender and checked the wiring. To be honest I can’t remember if I used PTFE tape of not as it was a few months ago. It’s easy enough to test tho, guess I could temporarily earth the sensor to the block with a carefully placed piece of wire

Laitch wrote:

bigminiandy wrote:. . . there’s good compression . . . There’s plenty of fresh fuel . . .  changed the fuel pump . . . a new fuel filter, fuel hoses and fuel regulator . . . replaced the water temperature sensor, changed the ecu . . . and the air flow meter. The battery is in great condition . . . The coils I believe are sound . . .The leads are in the correct order . . .  the computer . . . I’ve tried a donor one and cleaned all the contacts . . .The injectors are working as they should having tested them into a container . . .

We're approaching ancestral curse territory now. Have you done a thorough geneology?

Yup, I agreed it seems cursed. I’ve tried everything I can think of over the last few months, I'm hoping somebody will pop up and say have you tried......

bigminiandy wrote:Yup, I agreed it seems cursed.

I was implying that you might be cursed but you've raised an interesting point. Perhaps you should replace it with a Moto Guzzi and see how that goes.

You believe the coils are sound. Does that mean you've tested their resistance, or looked for sparking around them and their wires by running the starter in darkness?

The K100 coils especially the old black/dark grey ones are a big problem. I have only ever seen one set that was truly good and tested perfectly.

I have had that coils issue and everything 'looked' good but not the case. No arcing or sparking along the coil, but definitely a sparking issue under compression.

Having more Ks a change of coils took place, sure enough one dodgy one. 

Rain does not really access any of the places that can kill a K engine except for the Hall sensor cover if the gasket is not good. 

Testing components is fine too BUT is does not necessarily identify if a corroded or poor connection to or from it is the issue.

Plug bobbins ok too?

Yes I’ve tested the coils, couldn’t tell you what the results were as it was a couple of months ago and I’m not at home to retest them plus my memory has slipped. I remember the results were the same for both coils and guessed that both wouldn’t have failed/deteriorated to exactly the same level at the same time so they must be good! However I’ll be very happy if I’m proven wrong. I’ll find some donors and report back. For the info they are grey in colour so guessing they could be the originals.

Sounds like a problem I had a couple years ago. I had fuel dripping from the exhaust pipe after only a few attempts at starting, banging etc. Too much fuel for the amount of starts I attempted. Had me scratching my head for a while. I went through all the usual suspects such as temp sensor, fuel pressure regulator etc etc. I checked the injectors and they 'appeared' to be working fine. Upon closer inspection I realized it was more of a constant spray than a pulse. (I can't remember exactly but I have a feeling only one of the injectors was not pulsing. It was open constantly. Letting way too much fuel through the injector). Luckily a mate of mine recently acquired a K so I  borrowed his ECU and plugged it in. My bike started almost straight away. I subsequently bought another ECU and all has been good ever since. 

I realize that you stated that you changed your ECU already. I would have put money on it that it was your ECU that was the problem but it appears it may not be.... 
Maybe try another? Very quick and easy to try if you know somebody with one that you can borrow to try from a working bike.  

For fuel to be dripping out of the exhaust, something is allowing an injector or injectors to stay open longer than necessary and let way too much fuel through. A lot of fuel. Maybe try pulling injectors out while still attached to the rail and note how much fuel is coming out of each one. Should be a fine mist not a long heavy spray. Are they putting out the same amount of fuel? That's what I noted on mine anyway....

Other than that, I have no idea for a fix but the problem seems very familiar to what I experienced. 

Hope you find the problem and I'd like to hear of the fix out of curiosity.

I’ve replaced the coils and have exactly the same problem - no change.

I had the same problem on my K75. It turned out to be a faulty fuel injection computer. You say you changed the ECU. Do you mean the one under the tank for the ignition or the FI computer on top of the battery? You might also try pulling the fuse for the fuel pump and if the bike starts quickly put the fuse back in.

Jsfr80

If you still have your faulty computer don't throw it away. Believe it or not, with the help of riders on this forum you might be able to fix it.

Bigminiandy

I can't seem to tell from your replies how much fuel is being supplied. My money would be on Martin's suggestion about tape on the temperature sensor.

There are quite a few other causes of overfueling not mentioned yet. A quick easy test is to listen for a ticking sound from an injector while cranking.

If you hear it ticking, ignore my post. If it doesn't tick, you might for example have an injector earth wire earthing out, causing overfueling. I know that sounds weird, an earth wire earthing, but I've had that problem. In this case the problem would remain, even after replacing the fuel injection computer.

Did you remove tank recently. Last time I had same symptom I broke brown ground wire under the tank. Didnt notice that. Its only one wire and check it carefully. After quick connect bike started as usual even it was flooded with gasoline.
Best regards