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TOPIC: Backfire
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Backfire 10 Years, 4 Months ago
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Hi folks,, well after taking the fuel pump off, installing a larger needle valve assembly and also went one step richer on the pilot jet.., I have picked up some backfiring on decel.. I'm thinking it may be the float set too low..the only thing I had off the bike was the carb, didn't touch the pipes... could an air leak where the carb connects to the mani cause this? Idle is good and smooth,, and it runs good thru-out the entire throttle range..the temp here was in the 50s, and very dry, don't know if that would be a cause either...  |
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Gram (Admin)
Been there, wrecked that.
Admin
Posts: 4797
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Re:Backfire 10 Years, 4 Months ago
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Does the backfire only happen on full throttle close, or does it happen anytime you slow down?
Backfire is always ignitable fuel in the exhaust, exhaust leaks allow air in that makes the remainder of unspent fuel ignitable, but I can induce backfire on almost any bike (with jetting settings) so air leaks in the exhaust are not the only cause. When that unspent fuel gets to a point in the tail pipe where there is fresh air and still in contact with the hot tailpipe it will ignite.
If it happens primarily on full throttle close, it can probably be reduced or eliminated by turning in the pms to reduce the unspent fuels in the exhaust. |
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Re:Backfire 10 Years, 4 Months ago
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When ...
- Backfiring once when backing off a hard acceleration
- Backfiring numerous times when backing off the throttle at high rpms
- Popping at around 2000 rpm after backing off the throttle
Doc |
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Gram (Admin)
Been there, wrecked that.
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Posts: 4797
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Re:Backfire 10 Years, 4 Months ago
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Sorry, read your post wrong. An air leak in the manifold spigot would tend to cause it throughout the throttle range, not just on decel. But would still be a good thing to look for.
GRAM |
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Re:Backfire 10 Years, 4 Months ago
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It seems to happen from almost any rpm from approx 2000 and up if I close the throttle completly, if I leave the throttle open just a touch it won't do it..oh and it is numerous backfires..I have a bad habit of changing too many things at one time..  I think it may have bit me this time..actually I am thinking the float level,,as moving up one on the pilot should not have caused this..as if anything, it would have made the mix more rich..right..  |
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Gram (Admin)
Been there, wrecked that.
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Re:Backfire 10 Years, 4 Months ago
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The change on pilot size to larger would make the mix richer, as you said. When the throttle is fully closed the pilot is the primary circuit, and with the larger pilot jet is now putting more fuel in. Probably more than the engine can fully combust with the small amount of air coming in at throttle close position, so might be leaving too much behind in the exhaust..... So now it hits fresh sir in the tailpipe, the mix values are right for combustion again....
Might want to just try adjusting the pms before taking the carb apart...
Just my $.00000000000002
GRAM |
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Re:Backfire 10 Years, 4 Months ago
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Yes you have made the carb richer and yes that's your problem.
If you get popping (muted backfiring) on decel at about 2000 rpms then you can usually eliminate this by turning the PMS out (enriching the circuit).
If you have numerous backfires that the whole neighborhood can hear, then you have an excess of unburnt fuel (see Gram's post). When you close the throttle the pilot regulates the gas .... and you have too much gas and not enough air (slide in the carb throat is closed). This symptom disappears if you "leave the throttle open just a touch", as the slide is partially up and so you have enough air to overcome the excess fuel problem.
I would suggest you go down on the pilot.
Doc |
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Re:Backfire 10 Years, 4 Months ago
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Last Edit: 2007/10/27 15:01 By DocShadow.
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Gram (Admin)
Been there, wrecked that.
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Posts: 4797
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Re:Backfire 10 Years, 4 Months ago
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It seems we are cross posting
Pay attention to Doc's 2000 rpm test too.
GRAM |
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Re:Backfire 10 Years, 4 Months ago
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Just my B.A.G. but you already have the 45mm on there. Going to a bigger piolt is drowning her.
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