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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Annoying 289 Carburettor Leak
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<blockquote data-quote="druridge" data-source="post: 96448" data-attributes="member: 3754"><p>I've been going through similar problems on my Rapide with (quite new) 276's. Issues mainly on the rear (lower) carb with petrol from the top of the float chamber and around the base of the float chamber holding bolt (below the main jet). I found a few particles of almost powder size rust in the bottom of both float chambers but this wasn't the problem. Also changed both float needles without improvement. The new ones supplied by the club included a 'wrong' rear (top closing) needle, quickly sorted by the Spares Team - wrong part bagged up by their supplier.</p><p>I suspect my problem centres around the carb / fuel lines set-up. On my bike the petrol is supplied from the LHS petrol tap to both pots via petrol pipes to an inverted 'T' plan. The RHS petrol tap is for reserve.</p><p>The petrol is fed to the base of the front carb and to the top of the rear carb. Hence the float needles are totally different with the front pot needle sealing at the base of the float chamber and the rear pot needle sealing at the top (or not). With the petrol turned off, there is nothing to stop the fuel in the front/upper float chamber and the contents of the fuel lines running down towards into the rear/lower float chamber other than the seal on that rear/lower float needle. On inspection (after a wait) I found the front float chamber empty, the rear full to overflowing. There is also a problem with the throttle closed on long downhill runs with popping & banging in the exhaust and eventually firing from the exhaust (big bangs); with the throttle reopened the bikes seems to run on one pot until it clears itself. Otherwise the bike is a good starter and ticks over/accelerates fine etc. I am unsure if I now proceed to try to:</p><p>(A) produce a mating surface of the needle seat / needle taper with fine paste (I can see a faint mark with a glass on the seat).</p><p>(B)Try to locate a needle that works. There used to be needles made with a hardened rubber (?) taper to improve the seal.</p><p>(C) Put up with this by managing symptoms rather than fixing the real problem, eg I now turn off the fuel a few hundred yards before stopping and have tried turning off the fuel for periods from the tops of downhill runs. Both plans work, no drips in the garage, and no pops & bangs on descents. </p><p>After talking to a few people and thinking about this, I'm still not full of confidence in the needle/seat set-up on the rear (top sealing) carb. The potential sealing face seems quite small and its very easy to imagine with vibration and road bumps etc that the 'seal' must often be allowing fuel to pass. It does seem possible to rig up separate fuel feeds to individual carbs from the Left & Right fuel taps, then the feed on the rear pot could be switched to bottom feed - which might help.</p><p>I guess I'm not the first to have this problem, and I havent searched this site for other threads yet.</p><p>Any thoughts?</p></blockquote><p></p>
[QUOTE="druridge, post: 96448, member: 3754"] I've been going through similar problems on my Rapide with (quite new) 276's. Issues mainly on the rear (lower) carb with petrol from the top of the float chamber and around the base of the float chamber holding bolt (below the main jet). I found a few particles of almost powder size rust in the bottom of both float chambers but this wasn't the problem. Also changed both float needles without improvement. The new ones supplied by the club included a 'wrong' rear (top closing) needle, quickly sorted by the Spares Team - wrong part bagged up by their supplier. I suspect my problem centres around the carb / fuel lines set-up. On my bike the petrol is supplied from the LHS petrol tap to both pots via petrol pipes to an inverted 'T' plan. The RHS petrol tap is for reserve. The petrol is fed to the base of the front carb and to the top of the rear carb. Hence the float needles are totally different with the front pot needle sealing at the base of the float chamber and the rear pot needle sealing at the top (or not). With the petrol turned off, there is nothing to stop the fuel in the front/upper float chamber and the contents of the fuel lines running down towards into the rear/lower float chamber other than the seal on that rear/lower float needle. On inspection (after a wait) I found the front float chamber empty, the rear full to overflowing. There is also a problem with the throttle closed on long downhill runs with popping & banging in the exhaust and eventually firing from the exhaust (big bangs); with the throttle reopened the bikes seems to run on one pot until it clears itself. Otherwise the bike is a good starter and ticks over/accelerates fine etc. I am unsure if I now proceed to try to: (A) produce a mating surface of the needle seat / needle taper with fine paste (I can see a faint mark with a glass on the seat). (B)Try to locate a needle that works. There used to be needles made with a hardened rubber (?) taper to improve the seal. (C) Put up with this by managing symptoms rather than fixing the real problem, eg I now turn off the fuel a few hundred yards before stopping and have tried turning off the fuel for periods from the tops of downhill runs. Both plans work, no drips in the garage, and no pops & bangs on descents. After talking to a few people and thinking about this, I'm still not full of confidence in the needle/seat set-up on the rear (top sealing) carb. The potential sealing face seems quite small and its very easy to imagine with vibration and road bumps etc that the 'seal' must often be allowing fuel to pass. It does seem possible to rig up separate fuel feeds to individual carbs from the Left & Right fuel taps, then the feed on the rear pot could be switched to bottom feed - which might help. I guess I'm not the first to have this problem, and I havent searched this site for other threads yet. Any thoughts? [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Annoying 289 Carburettor Leak
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