Rear cylinder switches off when hot.

[Roaming Wolf]

Hello,
2018 K-9 VFI. Rear cylinder switches off when hot and the bike is idling. It happens after about 30 min riding. No check engine light, just the bike looses power. Checked for the spark - nothing. If the front cylinder wire is disconnected the bike dies. The coil is relatively new -2 years old I believe. The wires are the same age, Screaming Eagle. If I switch the bike off, then start again and ride on 1 cylinder, after about a minute the second one switches on and the bike runs fine. I understand the rear cylinder gets hotter due to its location, especially now as the heat is coming back over here. The ECU that I have which was installed by Big Dog is locked and I can't change any settings there. I contacted S&S, they confirmed this and offered me a new one that can be adjusted. It is quite expensive though. So, before I buy it I want to check if anyone has had a similar issue. I understand most of the bikes here a crab ones. I have been thinking of downgrading it to a carb. This will require quite a lot of mods though, according to S&S. In such case buying a new ECU might make more sense. Can there be an issue with anything else? So far I have: coil, wires, ECU. O2 sensors a very unlikely. I changed one to original Bosh and they usually through a check engine light. Also, the spark should remain on the second cylinder.

 

[HMAN]

Wow, thats nutz. Id swap out the coil with a known good one for a test ride. That plastic can crack and can be hard to see. When it warms the crack expands and no rear cylinder. Thats MHO tho.

 

[Jwooky]

Hello,
2018 K-9 VFI. Rear cylinder switches off when hot and the bike is idling. It happens after about 30 min riding. No check engine light, just the bike looses power. Checked for the spark - nothing. If the front cylinder wire is disconnected the bike dies. The coil is relatively new -2 years old I believe. The wires are the same age, Screaming Eagle. If I switch the bike off, then start again and ride on 1 cylinder, after about a minute the second one switches on and the bike runs fine. I understand the rear cylinder gets hotter due to its location, especially now as the heat is coming back over here. The ECU that I have which was installed by Big Dog is locked and I can't change any settings there. I contacted S&S, they confirmed this and offered me a new one that can be adjusted. It is quite expensive though. So, before I buy it I want to check if anyone has had a similar issue. I understand most of the bikes here a crab ones. I have been thinking of downgrading it to a carb. This will require quite a lot of mods though, according to S&S. In such case buying a new ECU might make more sense. Can there be an issue with anything else? So far I have: coil, wires, ECU. O2 sensors a very unlikely. I changed one to original Bosh and they usually through a check engine light. Also, the spark should remain on the second cylinder.

Do you have a laptop with Protune on it? If not, this would be the absolute first step.

You can see any faults but also read all the parameters in real time, such as cylinder temperature. You can also log data and review it after the ride and capture an event such as cylinder deactivation.

I can share, over 10 years of riding even idling in traffic, I can’t recall once the rear deactivated.

There is a temp sensor in the rear head, I did have to replace mine years ago, so that could be a suspect. You can read it with Protune.

Would not even consider downgrading to a carb, would be an extremely costly downgrade.