I suspect it WILL work with SPI-just not very well...! As I understand it (and I don't have an example to check) the throttle is obviously a simple barrel-which can obstruct the air intake tube-which will only regulate the amount of air taken in, not fuel-so such throttling as is achieved occurs largely by richening up the mixture considerably when the throttle is operated-and to a slight degree reducing overall mass flow-so a smaller charge is inducted. SPI will lessen the effect because it allows another route for air to reach the crankcase other than through the reed induction system-thereby acting in opposition to the throttle. Depending whether or not there is a fixed bleed hole in the throttle barrel, some degree of idle optimisation may have been achieved-which in turn would be upset by the presence of SPI. If not then I would expect the effect of SPI to result in poorer throttling-a reduced throttle range and higher low speed throttle revs.....I hesitate to use the term idle as these small engines usually 'idle' at about 6,000 rpm.....-or putting it another way, crude throttle setups like this usually result in halving the full speed revs- a 2:1 ratio as opposed to a decent throttle with fuel metering as well as mixture control (however achieved) where a 5:1 high speed:low speed ratio or better- is often achievable...

'ffkiwi'