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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 11-29-2012, 05:04 PM   #1 (permalink)
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Backfire through carbs, what's the real deal?

Did a search, found it both ways. Some indicate this is due to lean mixture and some say rich. This happens to me occasionally when starting the bike, I'll feel it fart through my side covers. I've got the usual ark, dominator tourings, 42/145. What do you guys think it is?
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Old 11-29-2012, 08:27 PM   #2 (permalink)
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It is generally a sign of extended lean burn.
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Old 11-30-2012, 03:56 AM   #3 (permalink)
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Quote:
Originally Posted by tsx123 View Post
Did a search, found it both ways. Some indicate this is due to lean mixture and some say rich. This happens to me occasionally when starting the bike, I'll feel it fart through my side covers. I've got the usual ark, dominator tourings, 42/145. What do you guys think it is?
Well the thing is both are correct. Lean or rich can cause backfiring. Good you qualified that it is a backfire and not an afterfire(exhaust ).

Things to check:
Vavle clearances
Timing! (doubt it)
Carb balance the CV need this if played with.
Personally i cant see the need for 42 at your level of tune. Itried 45, 42 and the standard 40 was the best.

Have you fitted shims?
Also check for any air leaks around carb seals.

My plan of attack.
Go back to the 40 and balance the carbs see how you go from there
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Old 11-30-2012, 08:25 AM   #4 (permalink)
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I have never experienced carb farts from an overly rich mixture.
OP said it's happening on start up, when extra rich mixture is needed, & probably not happening.
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Old 11-30-2012, 10:12 AM   #5 (permalink)
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I appreciate the suggestions, to clarify, this only happens on warm/hot restarts. Never on cold starts using the choke, all is fine there.
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Old 11-30-2012, 01:41 PM   #6 (permalink)
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OK, that helps narrow the issue.

BANG-type detonation in the pipes on hot restarts is generally a sign of an overly-rich mixture.

However, if I understand correctly your bike is "chuffing" (farting)through the carbs, correct?

I personally can't explain that, unless there is also a vacuum leak somewhere.
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Old 11-30-2012, 02:41 PM   #7 (permalink)
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Correct sir through the carbs. I will check for a vacuum leak. It only does it once in a while though so the problem is not consistent.
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Old 11-30-2012, 02:58 PM   #8 (permalink)
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Happens to me as well. It actually has since I got the bike- very effing annoying. I suspect it may be the manifolds.
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Old 12-01-2012, 09:08 PM   #9 (permalink)
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I remain a "doubter" on this one. All of your description except the part of definitely hearing and feeling it through the carbs is a description of a loud bang through the exhaust on warm startup--not that unusual, especially if giving a bit of throttle.

A backfire occurs only when ignition occurs with the intake valve open, partially open, or burned enough to leak. Fuel mixture should have nothing to do with a backfire, IMHO. Most people incorrectly call any popping sound from a motor a backfire. If you are certain that the "bang" is back through the carb, I would suggest a compression check and a valve clearance check. Also, as rich as many run the pilot circuitry of these carbs a fire is very likely on a backfire. If I had backfiring on my motor and the valves checked good, I would be purchasing another igniter.

Good luck,
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Old 12-02-2012, 02:27 AM   #10 (permalink)
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we have to know how many turns your mixture screw is turned out. 70% of carb issues are caused by wrong setting of mixture screws. You can put different jets inside the carb, without the right setting of this screw the engine will not run correctly.
look what I found out, perhaps ist helps you.
http://www.triumphrat.net/twins-tech...-main-jet.html
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