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Twins Technical Talk Technical Talk for Hinckley Triumph Twins: Bonneville, T100, Speedmaster, America, Thruxton, and Scrambler.

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Old 12-10-2012, 06:23 AM   #1 (permalink)
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Bonneville Backfiring

I had my 07 Bonneville in at the local Triumph dealer today for new tyres. Whilst the bike was there I asked if the carbys could be checked to make sure they were operating ok.
Upon picking up the bike and riding it home, I found that it was back firing quite badly from about 75 kms up when backing off the throttle.
I rang the dealer when I got home and spoke to the mechanic. He informed me that he had found the engine to be idling too high(which it had been) and had adjusted it accordingly and that he had also found the carbys to be out of balance and had also adjusted them.
He further claimed that all Bonnevilles backfire to some extent (and that the EFI models were worse) and if I wanted the back firing to stop, I would have to have the bike run on a dyno tune machine and possibly have the carbys rejetted to fix the problem.
My bike has only done 12,500 kilometres, it is stock standard apart from Staintune mufflers. It certainly has never backfired the way it did when I rode it home this afternoon.

Before I take the bike back to the shop, could some one explain to me what the mechanic would have done to "balance the carbys" and is it possible that the high engine idle speed may have been stopping or masking the problem of the bike back firing when I backed off the throttle. Is the mechanic leading me up the garden path when he now says the bike needs to be run on a dyno tune if I want the back fire to be fixed.

thanks in advance, Neil
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Old 12-10-2012, 08:43 AM   #2 (permalink)
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Because engines are set to produce as little emissions as possible by means of a carefully adjusted mixture and the use of an Air Injection device, they're prone to backfire to a certain extent. To alleviate this the CVK carbs are fitted with an ACV (Air cut-off valve). This richens the idle mixture on deceleration and minimises backfires. These valves sometime fail and have to be rebuilt.

The way they work is that the idle circuit is fed from one gas jet and TWO airways. The valve closes one of the airways during closed throttle deceleration and consequently a high vacuum, making the mixture richer.

Once you fit an aftermarket exhaust not only is the backfiring more easily heard but if the mixture is not enriched accordingly the backfire gets worse.

Another source could be air leaks at the exhaust joints, ensure these are well sealed.

If the joints are sealed, the ACV valve is working correctly and you still get it, you could try to eliminate it in two ways:

Richen the idle mixture a little at a time until the backfiring goes away or try and temporarily block the Air Injection system like this:

http://www.triumphrat.net/twins-tech...disabling.html

The reference to "balance the carbys" is a procedure whereby the mechanic ensures both carb butterfly valves open at the same time when the throttle is used. It's acomplished by the use of a pair of vacuum gauges connected to the manifold vacuum spigots, normally blanked off by rubber caps. An adjusting screw between the carb operating shafts is turned one way or the other until both gauges read the same. Properly balanced or synchronised carbs mean a steadier idle, cleaner pick-up from low revs and less vibration.

Last edited by Forchetto; 12-10-2012 at 01:22 PM.
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Old 12-10-2012, 10:15 AM   #3 (permalink)
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+1, what he said.
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Old 12-10-2012, 05:46 PM   #4 (permalink)
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My 2011 T100 has the TORS installed, and, ostensibly, the appropriate map, downloaded at the dealer's. Cackled like a sonofagun after I brought it home. Temporarily blocked the AI system per Forchetto's link above and the backfiring was decreased about 90%, which I can easily live with. Seeing as how I live in California, I'm going to leave it at that, as what I've done so far is easily reversible. Sounds good, runs good and warms the cockles of my ancient heart.

Thanks Forchetto!

Jeff
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Old 12-10-2012, 05:55 PM   #5 (permalink)
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to balance the carbs a manometer would have been attached to both intake manifold ports and vacuum measured. the idle on one carb is then adjusted independently until vacuum is the same in both intake manifolds

with the idle set at a higher rate there would be less popping on decellaration with the throttle closed. you will have to open your pilot adjust screws in order to allow a richer mixture and prevent popping. seeing you have staintunes larger main jets are probably required as well
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