1988 K100RT, 92,000 miles. Good starter hot or cold but below 2000 rpm, any throttle position, the engine stutters as if on 3 1/2 cylinders. Once 2000 rpm is passed, it clears and runs very well and remarkably smooth with minimal vibration at 70 mph. Suspected an air leak so dismantled the fuel injection side but found all rubbers in good order including manifold bodies. All replaced with new 'O' rings for the injectors. Throttle butterfly's and air screw synchronized, no difference detected. Checked the resistance readings for the plug leads, caps and coils but all to speck. All four, new, plugs light grey in colour. Have moved the TPS switch so does not affect low speed running. Engine does not back fire on the over run. It has a new fuel pump and filter. Am running out of ideas. What would you suggest? John

Look at your ignition coils, measure them, clean the contacts and look at your spark plug cables and spark plugs. You have bobbins on your spark plugs, right? :-)

Did check leads and coils and all resistance values were as per Haynes book, but can easily check again. If there was an ignition problem in this area I would have expected some backfiring issues in the exhaust, yes? but it never does this. Yes plugs have bobbins fitted. John

Test if TPS is functioning properly - I'm not sure if it could do this, but just thinking that if it is stucked in the full throttle position?

Did you remove the manifold rubbers and reseal with 3 bond and leave it for 24 hours to set.  They do have a real bad habbit of leaking where they are supposed to seal on the head

I used Hylomar non setting blue. The head face and manifold rubbers were in good order and before and after removing them I carried out the test with a flammable aerosol which had no effect on the engine idle. What I did try before balancing, when it was idling, was to screw in and out each air bypass  screw trying to pin it down to a particular cylinder, but again, no obvious indications. Annoyingly whatever I have checked/tried has neither improved or worsened its running, would a tight valve do this?  John

A slightly tight valve will have a effect on the ballance but not general running. It would need to be riding all the time, that's what I would expect

It feels a bit like a leak in the crankcase breather tube in the way that at low speed the air gets sucked in and disturbs the 'round' running, but at higher engine speeds there is enough vacuum being pulled to basically pull the leaking bit closed (or part closed) and therefore not important enough anymore to upset the balance. Is the elbow between the AMM and the plenum properly fitted?

temp sensor ?

could be beano !!! 

not all bad, but on the way i guess

cheers

"Any suggestions chaps"

Don't eat the leftover salmon pâté.

3 to 400 miles later the bike runs very well above 2000rpm, in fact its not really a problem below 2000, it just doesn't sound right  but am no further forward in resolving its stuttering issue. Double checked all the ignition bits, all to spec. on resistance readings etc. What I want to do is try and pin it down to a particular cylinder. knowing the dangers of shorting plugs out to both the machine and my wellbeing I'm wondering, is if feasible to pull off of the electrical plug to each injector in turn or will this cause problems within the electronics.  John

Could you find a spare injector to connect up to the removed plug?  It would provide a standard load for the circuit but no fuel path.

What is your voltage below 2K RPM?

Can I check this whilst the bike is running? I have power there when I press the starter button with engine stationary but didn't check voltage. John

8 valve motor or 16 valve john ?

8 valve motor, does that make a difference? John

If the TPS is a tiny bit out they run like a dog down low on a 16v

as "bean" suggested test the temperature sensor

pull the connector off the underseat computer ...count 10 sockets from the left of the cable (as it sits on the computer )...red probe on the connection . black probe on the earth bolt at the gearbox ....multimeter on 20,000 ohms

and tell us the value ...and the motor temperature when you measure it ... preferably when cold ....but if it is warm  thats ok too if you have a temp guage fitted

there is a diagrame on the "efi electronic ingnition troubleshooting "  on the portal page  you will have to scroll down to see it

this unit, if found too high a value will make the bike run too rich at lower revs 

its an easy test to do and good to discount this as an issue first off

obviously it is working to a degree ...but how good ?

before going in and sealing off the throttle body rubbers

i found issues with both of the things i suggest on what i thought was a good running engine .....now a lot better after fixing it ...erm...some little while ago

Thanks Charlie, will try that tomorrow when cold, been out for a run on it, can't keep of it. The damned thing runs really well otherwise. Come back BSA C11! John

Cold engine, 14deg C air temp, 3000 Ohm's. Think that's ok? Holding cold engine at 2500 rpm there is a definite misfire. Going to make up some new plug leads. NGK caps, copper cored wire. John

If the CO mixture was out, could that cause it to be running weak at low rpm?

There is a method of setting the CO without a gas analyser - 'The Lean Drop Method'

Also, bear in mind the following extract from Bert's excellent Fault Finding Guide about plugs & leads:

The K100 is originally fitted with Bosch X5DC (or can be fitted with the NGK D7EA). Both are non resistor spark plugs and here is the reason: The original BMW spark plug cap already integrates a resistance of 5KΩ. Similarly, the connector at the other end of the original ignition cable also includes a resistance of 1KΩ (The total resistance is 6KΩ). If you replace the wires with after market non resistor wires and caps and keep the original spark plugs, you will run into troubles and take a chance to damage the output transistors of the ignition module (resistance of the wire/spark plug will be close to 0). It is possible to fit a K100 with after market plugs and wires, but the spark plugs have to be replaced with a resistive type (Bosch XR5DC or NGK DR7EA). These spark plugs have a built in 5kΩ resistor.

Mike

You can get 5K resistive plug caps from NGK

k100virgin wrote:Cold engine, 14deg C air temp, 3000 Ohm's. Think that's ok? Holding cold engine at 2500 rpm there is a definite misfire. Going to make up some new plug leads. NGK caps, copper cored wire. John

sounds good to me john

good that you did the check,  you can discount that bit for now

Hi John, I've been chasing a 'misfire' around 1500 rpm recently and last week I found a split in the hose between the AFM housing and the plenum chamber. Replacing the hose has cured it.

Another possibility?

Paul.

Yes Paul, mine was shot but sadly replacing it did not cure my problem. Waiting for a kit of parts to arrive to make up new plug leads. I honestly don't think it will be the cure but the existing one's are looking a bit tired. John