Vire 7 - Bulletin Board

What settings do you use on your carburettor ?

10th February 2002 - Introduction from the Webmaster - Advice, even for the initial settings, varies. For the low-speed jet, Tillottson (the manufacturers of the carburettor) recommend that you open the idle adjustment screw only 0.75 turns; the earlier (1975) Vire 7 Owner's Manual recommended 1.25 turns, but a newer manual (1992) agrees with Tillottson and recommends 0.75 turns.

All the official sources are agreed that you should set the main jet initially by opening the high-speed mixture screw 1.25 turns. Final adjustment - which might be in either direction - is then made with the boat underway, at full throttle. However, Crinan Marine suggest an initial setting of only one turn for the main jet, and on the webmaster's boat the final setting is definitely less than 1.25 turns.

I've heard of plugs surviving as little as 20 minutes before dying and being extracted, covered in soot. It would be nice to know a range of final settings, as a guide for those who are struggling to get it right. So, please E-mail the Webmaster and tell us what settings you use. Tell us what grade petrol/gasoline you use too.

Received on 7 March 2002, Posted on 14 March 2002 - I always use unleaded 95 octane 50:1 and always mix by hand.

I find the slow running mixture screw very critical. Rule of thumb is right in then 1.25 turns out will always start. That won't give optimum performance though and you have to fiddle ( always further out ) to get even high speed running. From John, UK. E-mail

Received on 9 September 2004, Posted on 19 September 2004 - I've got a new Vire 7 that has only done about 30 hours running. At 1.5 turns on the idle screw the plug is getting sooted up fast. Tonight i've got it at about 0.75 turns and it seems ok. I'll test it further and also adjust the main jet shortly and let you know the result. From Pete C.

Received on 19 March 2004, Posted on 8 January 2005 - In my case there was a flat spot at part throttle. I adjusted the idle mixture screw until the flat spot was not evident. The mixture was on the richer side of optimum at idle and the engine 4 strokes. This gives way to normal 2-stroke firing as soon as the throttle is opened. The idle mixture screw has (should have) very little effect on the full power setting. If it does, the effect is indirect, caused by spark plug fouling at idle.

The full power setting was adjusted for maximum revs with the boat under way at full speed. The adjusting screw was opened about two and a half turns. This setting was on the weaker side of optimum when the engine was driving in full reverse, probably because the engine load was less as the blades of the folding propeller do not open fully in reverse.

Running the engine when it is too weak at full power is not good. My engine had an unhealthy appetite for spark plugs when I got it at first, probably due to weak ignition, resulting in strange carburettor settings just to keep the engine running. I went through the complete catalogue of spark plugs from J-4 to J-12. At the end of the day the J-10 worked out best. With good ignition and correct mixture the spark plug life is very long (>10 years). From Iain Forsyth.

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