115hp speedo tube

Can anyone tell me how the speed tube picks up it feed? mine appears to come from the back of the speedo and dissapear into the bottom casing, if i blow down the tube it appears blocked and if i blow toward the speedo it registers fine. before i strip the bottom casing off am i looking in the right direction for the problem, i've tried looking at parts manuals but they don't really show much detail

Thanks

[boats.net]

It picks up by way of a small hole in front edge of lower unit(below cavitation plate), use a 1/16 drill bit to clean out hole, just spin drill bit in by hand do not use drill.

I'll give it a go in the morning thanks for the quick reply

Best regards

Worked a treat, the hole is so small and was so blocked i didn't even see it as it must have been blocked and then painted over before i bought it, that's a 4 year old problem sorted in 1/2 hour, superb advice thanks again

Best regards