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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 11-10-2012, 01:15 AM   #1 (permalink)
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Mixture issues. Backfiring through intake, hard to start

Can anyone help me out there. My current game plan for the symptoms below is to remove the restrictor plate in the airbox and bump the pilot jets up to 42.

Right now this is my setup:
-AI disabled
-Snorkel removed
-Unifilter
-NH Togas
-Running at sea level
-130 main jets
-stock pilot jets
-2.5 turns out on the mixture screws
-1 shim under stock needle

Basically this is my problem:

"Also if I just barely hold the throttle, something like 1200rpm, the bike will start idling very poorly, fluctuating irradically- probably ossilating between 800 and 1200 rpm, and will click and spit and backfire out the intakes like how I described earlier and stall out if I don't back off or grab a ton of throttle. I video'd this and sent it to Mike at TTP and is convinced it is not the tune. I sure wish I could post it here so it can be seen. "

I took this quote from this thread: http://www.triumphrat.net/twins-tech...-and-tune.html

The backfiring through the intake rubbers is the most worrying symptom. There's a little puff of smoke that comes out when this happens.
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Old 11-10-2012, 01:58 AM   #2 (permalink)
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The spitting and backfiring through the carbs usually indicates a weak mixture.

Check for the presence and state of the rubber manifold vacuum spigot caps, the carb rubber adaptor tubes and manifold to head joint, although that one doesn't usually give trouble unless it has been disturbed.

Usual procedure: Check vacuum caps visually for cracks and splits, spray something combustible like cleaning alcohol, WD-40, etc around the rubber adaptor joints while the engine is idling.

Listen out for any change in speed or exhaust note as the combustible fluid is sucked in by the vacuum leak.

If the adaptors are the original 2001 parts they could be due for replacement. Check them visually for cracks and splits.

See if tightening the clamps improves things, but there's a limit as to how much more those clamps will go. The rubber tends to collapse with time and the clamps might need to be supplemented with strips of rubber, like strips of old tyre inner tube, etc.
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Old 11-10-2012, 03:25 AM   #3 (permalink)
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Thanks for the help! So by weak mixture you mean a lean running condition?

I've done the spraying with WD40 around the intake manifold rubbers to no avail. I do know that the left side rubber has a crack in it but when I sprayed it the idle did not increase.

I've replaced the 3/16'' vacuum caps as well when I did the evap system removal. I'll be doing the AI removal tomorrow and capping up those holes as well.

I'll pick up some intake rubber boots tomorrow if I can and replace them. I'm worried, however that the bolts on my intake manifolds themselves are either too long or that the seal between the manifold and the engine head is bad. Again, spraying with WD40 did not induce any increase in idle. Does it require constant spraying?
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Old 11-10-2012, 04:58 AM   #4 (permalink)
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Check the carb diaphrams to make sure they're not split / perished, and they're seated properly.
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Old 11-10-2012, 06:00 AM   #5 (permalink)
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If you're sure that there are no vacuum leaks causing the lean mixture, try turning the idle screws anticlockwise a little at the time (1/8 or 1/4 turn) and see if things improve.
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Old 11-10-2012, 01:34 PM   #6 (permalink)
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I've adjusted the idle a few times. The problem is is that the idle adjusted after a long ride is too low when I start the bike up after it's cooled down. Meaning, the stumbling and coughing and spitback stops after the bike is warmed up.
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Old 11-10-2012, 01:35 PM   #7 (permalink)
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Quote:
Originally Posted by denbow View Post
Check the carb diaphrams to make sure they're not split / perished, and they're seated properly.
Carb diaphrams being the rubber boots from carb to intake manifold correct?
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Old 11-10-2012, 02:01 PM   #8 (permalink)
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Quote:
Originally Posted by luckyduck View Post
I've adjusted the idle a few times. The problem is is that the idle adjusted after a long ride is too low when I start the bike up after it's cooled down. Meaning, the stumbling and coughing and spitback stops after the bike is warmed up.

I don't mean the idle speed screw, I'm referring to the idle mixture screws. These regulate the idle and low-end mixture, although they have an influence over the overall mixture strength right up to over 1/2 throttle.

The diaphragms Denbow is referring to are the rubber bellow-like things attached to the slide/needle assembly. They're under the top chromed covers.

Last edited by Forchetto; 11-10-2012 at 02:06 PM.
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Old 11-10-2012, 02:40 PM   #9 (permalink)
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Ah yes. The diaphrams under the carb tops are not cracked at all. I made sure to seat them correctly when shimming the needle.

I've adjusted the mixture screws quite a few times now. They haven't made all that much of a difference in starting or in curing the stumbles and weak power during warm-up.
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Old 11-10-2012, 03:17 PM   #10 (permalink)
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Luckyduck.
These bikes take 20 - 30 minutes to reach full operating temperature before they'll tick over properly, especially in cold conditions.
They need full choke (no throttle) to start and half choke after a minutes warm up. The choke can then be gradually pushed home after gentle riding for a few hundred yards but the throttle may need goosing for a while longer to prevent stalling when negotiating junctions etc.
If the choke is pushed in too soon you're sure to get the coughs, splutters and carb spitting you describe.
I'm not saying you're not carrying out the correct cold starting and warm up procedure correctly, only that it would be folly to tinker with the carbs needlessly if you're not.
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