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Convert OS 7mm twin needle carburettor to 3mm barrel for Upshur 6cc engine

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IanM:
Others may have done similar, but I tried out advice I had received, converting a 7mm venturi 2 needle OS carb to 3mm by sleeving the barrel. I did this because I could not buy a sufficiently small venturi carburettor. I have tried making one but also I wanted to see if I could adapt a larger OS brand purchased one. The OS carb when stripped down has a cylindrical barrel with clearance holes for the jet needle and opposite that, the idle needle. A brass insert, in the original 7mm barrel cross drilling, machined to match the barrel cylindrical dimensions and drilled out to 3mm, with top and bottom drillings to remake the clearance for the two needles - was tried out.

Engine ran and it managed good throttle response. My conclusion is that such an adaptation is worth attempting and gives decent results.

I hope this may help or be of interest.

Roger B:
An interesting conversion  :ThumbsUp: Can you post some pictures?

Most commercial carbs are designed for high reving methanol fueled two strokes and don't match slow running petrol (gasoline, benzine) powered engines. I have modified the jet assembly on a commercial carb to get better control in the range I needed.

IanM:
Roger - have (I think) posted pics but as yet they don't show. I'll look in a while and see if they appear.

IanM:
Maybe I tried to post too many pics. Here are two, incl the modified barrel with brass insert 3mm hole. Lets see if this loads..

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