Outer Limits 4x4 Board

I've stumbled across this mod http://www.youtube.com/watch?v=Bk-W4EMvsBM
and its got me thinking, How can i tell wether my secondary is actually opening or not?
My Sierra struggles to crack 100km/h but i put it down to oversized tyres, and gearing. Im wondering if this mod may help at all?

Any opinions for or against this mod?

Thanks
Mike

Your secondarys only open when they are needed, they are vacuum acutated and open when manifold vacuum drops (indicating engine under load). If you want to test them test the vacuum acutator on the secondary linkage and make sure that your carb and vacuum hoses are not blocked and are leak free. There is a thread somewhere on this mod, search FTW.

Layto....

to test if your secondaries are opening when they should be, take the air snorkel off the top of the carby and start the engine. With the engine running, rev it and put your hand or a rag over the top of the carby to block air flow and create a vacuum. At this point (before the engine stalls) you should see the actuator operating. If it does'nt, you have a problem.