Jump to content

R1150RT Twin-Spark Lower-Plug Problem

roger 04 rt

Recommended Posts

roger 04 rt

Recently my R1150RT began starting less crisply than it usually does. It only occurred on the first start after sitting overnight. Although the engine turned over briskly and started quickly (~1.5 seconds), it idled anemically for the first 10-15 seconds. If I blipped the throttle, the idle was restored more quickly but I wanted to figure out the root cause.


It turns out that the culprit was the lower plugs, in particular, a fouled left-side lower plug. Although there is plenty written about the symptoms of failing stick coils on twin-spark 1150s, there is not a lot written about the symptoms of a failing lower plug on those bikes so I’ve added some narrative of the symptoms.


Because I’ve got an LC-1 controlling Closed Loop lambda on my R1150, I’ve also got the ability to log AFR from the moment I start cranking. It’s also possible to set up a GS-911 to begin recording data before starting so I had the possibility to try and capture the weak idle conditions. Below is a chart of exhaust AFR (actually exhaust oxygen content converted to AFR).


Looking at the chart you can see that during cranking the AFR starts at about 19:1, which is very lean—too lean for a cold engine. Then after the engine catches, the AFR slowly richens over the first 15 seconds to normal fuel ratios of about 13:1 for the 20C ambient temperature at the time. If the bike was running perfectly, the AFR would instantly drop to about 12.5-13:1 and then slowly get leaner until fully warmed up.


Most AFR problems are fueling related but my fuel pressure and fuel volume were good, and the fueling was correct at all other times—just not during some cold starts. And a leaking injector would most likely have led to a richer starting mixture not a leaner one. Compounding the problem, although the idle was anemic (~1000 RPM with the Fast Idle Lever at mid-detent position), the engine wasn’t misfiring.


Problems like this can be tough to find—intermittent and marginal—so I hoped that the data would point to the solution. The answer was there but I had to look at the leanness in a different light. Rather than seeing the lean AFR as the result of too little fuel or too much air in the combustion chamber, I went with the idea that not all the fuel and air combined during combustion, resulting in unburned O2 in the exhaust, which the LC-1 reports as a lean AFR.


Since the stick coils on my bike are only a year old, I decided to consider that the lower plugs were part of the problem. So I disconnected (and grounded) each plug wire from the lower spark plugs. As it turned out, a single lower plug disconnected at cold start gave the same symptoms. Two lower plugs disconnected resulted in a very slow, rough idling engine for 15-20 seconds. After the bike had warmed up for a minute or so, it ran fine without the lower plugs.


Spark advance also seems to factor into the problem. One of the benefits of a twin-spark ignition is that the mixture in the combustion chamber burns faster. As a result, the ignition advance curve for a twin-spark engine is less than for a single-plug version of the same engine. My R1150 has 5 degrees of spark advance during starting on a 20C day but the single-spark R1150 has 15 degrees of advance under the same conditions. That leads to too little spark advance when the lower plug(s) doesn’t fire, apparently resulting in unconsumed fuel and oxygen in the exhaust, which could explain why I was seeing excess O2 (lean AFR) without misfiring.


After getting this far, my next step was to remove the plugs. Both were dark but the left plug seemed oily-wet. DR suggested that oil may be seeping past the rings when the bike is parked on the sidestand. I’ll be checking to see if that’s the cause of the fouled lower plug.


Bottom Line

The anemic idle and lean AFR were caused by a non-firing lower plug that caused the stock spark advance to be too short for complete combustion in a cold engine. I’ll figure out if the plug is collecting oil and then replace the lower plugs.


The consequence of losing a lower plug seems to be harder starting and (perhaps) poorer top end performance both due to too little spark advance without the lower plug(s) firing.



Link to comment

I have had success with NGK Iridium plugs in similar plug fouling situations. In my twin spark that is what I am currently running in both positions. Perhaps a hotter plug would also help.


Have also seen an engine that due to it being in for lengthy diagnostic testing ending up with the plugs being fouled enough to cause further problems.


As always interpreting the data is key to solving the problem.

Link to comment
roger 04 rt

I've tried a couple more checks on the lower plugs.


1. After sitting for several hours on the side-stand and then overnight on the center stand the lower left plug was removed and inspected. There wasn't any oil pooling inside the plug well but I noted that the threads and seat of the plug were oily. (My bike consumes only a small amount of oil between oil changes.)


2. Started the bike after an overnight cold-soak with the left lower plug wire pulled and grounded. The bike had a very hard time starting and running. It was as if I had pulled the wire from both plugs. (Turns out the right plug had an anemic spark so it was starting on just the main plugs.)


I think it's safe to conclude now that the twin-spark R1150 needs all four plugs firing to start quickly and idle well when the engine is started fully cold. This is consistent with the short spark-advance (4 degrees) and the lack of a second flame front when one of the plugs isn't firing.


3. Next the bike was started and run until the engine was hot, 6 bars (no plastic, external fan). The plugs were removed and inspected. The tips of the insulators had started to self-clean but both plugs had an accumulation of black oily residue between the body of the plug and ceramic insulator. The right plug was visibly worse.


Then the plugs were tested one at a time, on each side for spark, with the same results when tested on either side. The left plug had an acceptable spark but the right plug had only a faint spark--the culprit, and seemingly confirmed by test 1 above.


A new plug was also check and has a better spark than either of the old plugs.


While making the plug check, I tested how far I could pull a spark from the new plug body to the engine block and got a great strong spark at about 1/4".



--The secondary coil and wires seem fine

--Both plugs had black build up, the right to the point of having a compromised spark.

--Will replace with a couple new YR6LDEs (stock plug) that I have and check cold-starting tomorrow

--In a few days I'm going to install a couple hotter plugs and see how they do.

--The short spark-advance of the twin-spark R1150 requires all four plug to fire well for a clean, strong cold-start.

Link to comment


This topic is now archived and is closed to further replies.

  • Create New...