Radne Raket 85, User Manual

10

Users Manual

10

How to check the correct opening pressure for the needle 

valve.

Connect a small air pump with a manometer to the fuel nipple of 

the carburettor. Drop some fuel on the needle valve and pump up 

some pressure. At 1.0 kp/cm2 the needle valve shall open – and 

you can see that it bubbles around the needle valve. Now reduce 

the pressure down to 0,5 kp/cm2. Check with a few more drops of 

fuel that the valve is tight – no more bubbles. If the valve is leak-

ing all the time, clean everything again and change to a new valve 

and spring. 

If the opening and closing pressure don’t match the values given 

in the text, you have to change the spring and lever until it is 

correct. Finally, take a last check of the main membrane and 

the gasket, there is very little that can go wring with these. If the 

carburettor is carefully cleaned and you have checked everything 

according to the instructions and the H and L needles are OK 

– the carburettor works.

The only problem that can remain now is if the engine is very dif-

ficult to start and don’t react on adjustments of the L-needle. Then 

there could be dirt under the expansion washer, which is located 

under the main membrane. The fuel that passes the L-needle is 

also passing a small chamber on its way to the venturi. In very 

rare cases this chamber can be blocked. To solve this problem 

you have to drill a 2 mm hole in the middle of the washer (3020) 

and pry a thin tool into the hole and bend up the washer. Clean 

and check all channels to and from the chamber carefully and seal 

it with a new washer. To do this, place a new washer in correct 

position and expand it with a gentle blow of a small hammer. That 

is all, now your carburettor shall be working. Normally this mem-

brane carburettor will give you very few problems, but sometime 

you get a carburettor that doesn’t give good results. Then try to 

borrow a carburettor that you know works well. If the difference 

between the two is big, you have to buy a new carburettor. It is 

very difficult to change a band on into a good one.