Bad Stumble of Idle

Well I’m embarrassed, I’m finally doing an update to this thread! Better late than never!

Have had problems mapping the volumetric efficiency (VE) table in the lower RPM registers as well as having a really bad stumble just coming of idle. I thought this might have been just an Acceleration Enrichment (AE) problem, but no matter how much or little AE I gave it, it still stumbled. The VE table really looked weird, having a very large hump around the 1500 RPM mark, and then rising progressively to the max HP This really irked me especially trying to driving in start/stop traffic!

Moving on to the present, I have decided to as a temporary measure to remove the FI from the carb and go back to the std HIF44 SU fuelling. I’m currently running a BDL needle. It’s still running the MegaSquirt on the ignition side of things though. I just want to be able to use the car!

I’ve put on a K&N cone air filter today. Besides the slightly noisier intake noise, the top end is very noticeable better! My son Hamish just loves the noise it makes. It reminds me of my first MGB when I put a weber on it back in the 70’s.

I still haven’t abandoned FI though. I like the fuel economy and increased torque that EFI gave me. My thoughts are leaning two different ways, (that’s assuming you can lean two different ways!), they are:

  • Replace the HIF44 with a ‘proper’ throttle body such as a Jenvey TB along with a single 3 Bar injector. This would mean I would have to replace the Rover SPI fuel pump (1 Bar) with  an MPI fuel pump (3 Bar +);
  • Change from the current speed/density (MAP & RPM) algorithm to Mass Air Flow (MAF). MAF has the advantage of not having a VE table to tune, as well as allowing increases in tune of the vehicle without having to retune the fuelling side of the things, but maybe the ignition only.

I’m leaning towards the MAF route. I would be able to utilise the existing carb/TBI setup, pump etc. Would just need a Bosch MAF sensor along with some inlet manifold fabrication and wiring to the MAF.


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