Kerosene propellant buildup advice

[Sciencek]

I'm running an install that has Neartea's cryogenic engines and kerbal atomics installed, with the LH2-NTR option active. So the stock LV-N has been switched over to using hydrogen. 

Curious to use LF in an NTR engine, I have been fiddling around with a modded engine that attempts to emulate an issue that an NTR using kerosene propellant would probably encounter: carbon buildup. To attempt this, I have made the engine bimodal.

• One mode is high thrust, ~600 isp, uses LF and produces carbon buildup. (actually ablator, as I'd rather use existing resources)
• The other mode is low thrust, ~200 isp, and uses Oxidizer and the buildup/ablator.

My intent is to have an engine that can use the dense LF, rather than LH2, but suffers because of it, having less stellar isp, and requiring periodic cleanings/flushings using Oxidizer to burn away carbon-rich buildup from thermally broken down kerosene.

I have used a method to cause buildup that I know to be faulty: both modes have ablator as a propellant, but in the LF mode, its ratio is negative. So as the engine burns LF, the amount of ablator increases, as it burns Oxidizer, it decreases.

However, I have run into an issue. Once the ablator reaches the maximum amount storable in the engine, it doesn't shut off, as I would like. I know that this is because using a negative ratio for propellant is a no-no, and it's not checked when a propellant runs max, only when it runs out.

Does anyone have any tips on how I can get the engine to run only until the buildup is at the maximum amount, and shuts off once there?

[Sciencek]

Does anyone have any suggestions?

If this is the wrong forum for this sort of discussion, then I will happily make a new post in the correct place.