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Author Topic: Flames in the carbs?  (Read 2053 times)

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Offline Hush

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Flames in the carbs?
« on: July 17, 2009, 01:06:39 AM »
Anyone know why carbs would flame everytime I try to start a bike?
I've never struck this kind of problem before!
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Offline bucky katt

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Re: Flames in the carbs?
« Reply #1 on: July 17, 2009, 03:04:11 AM »
bad intake valve adjustment, cam might have jumped time, timing error. the mustang i had caught the carb on fire one night with a backfire after i had a bad ground on my MSD and i wasnt getting full voltage to the ignition.
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Offline mcpuffett

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Re: Flames in the carbs?
« Reply #2 on: July 17, 2009, 04:32:10 AM »
Timing issue?  :-\
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Offline themotoworks

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Re: Flames in the carbs?
« Reply #3 on: July 17, 2009, 04:42:34 AM »
it's because your bike is powered by satan

no, probably one of the above
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Offline punch455

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Re: Flames in the carbs?
« Reply #4 on: July 17, 2009, 04:54:00 AM »
That sounds so damn awesome I don't think I would bother fixing it. Post a vid!
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Drew75

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Re: Flames in the carbs?
« Reply #5 on: July 17, 2009, 06:09:43 AM »
better yet how about flames out of the pipes a la Raising Arizona?

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Offline BobbyR

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Re: Flames in the carbs?
« Reply #6 on: July 17, 2009, 07:32:09 AM »
As in an Automobile with a carb, it can be as simple as a lean condition, or it can be timing issue. How much have you done on your carbs, is your choke working properly, do the butterflys close all the way. Have you checked your timing. I would do these simple things before I messed with the valves or engine internals.
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Offline TwoTired

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Re: Flames in the carbs?
« Reply #7 on: July 17, 2009, 09:22:58 AM »
Quote from: Hush on July 17, 2009, 01:06:39 AM
Anyone know why carbs would flame everytime I try to start a bike?
I've never struck this kind of problem before!

Yes. It is because there is a correct air/fuel mixture and a source of ignition at the same time present in the intake runners.
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Lloyd... (SOHC4 #11 Original Mail List)
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Offline Hush

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Re: Flames in the carbs?
« Reply #8 on: July 17, 2009, 04:15:10 PM »
TT are you saying the inlet valve is open when the plugs ignite the fuel/air mixture?
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I think the thing I most like about motorcycling is the speed at which my brain must process information at to avoid the numb skulls who are eating pies, playing the ukulele, applying make-up etc in the comfort of their airconditioned armchairs as they make random attempts to kill me!!!!!!!

Offline TwoTired

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Re: Flames in the carbs?
« Reply #9 on: July 17, 2009, 05:05:25 PM »
Quote from: Hush on July 17, 2009, 04:15:10 PM
TT are you saying the inlet valve is open when the plugs ignite the fuel/air mixture?
I'm not saying that yet.  I'm just stating the root physics/chemistry requirement.  I think that's a first step toward understanding the issue. 
I don't know your cam specs as to when the intake valve is supposed to close with respect to BDC.  (could be on part of the compression stroke which could push some fresh charge back into intake runner at very low RPMS.
I don't know what your spark ignition timing is with respect to TDC (or if it is set *to* specification.)  This can be BTDC and if there is intake valve leakage, fire gets to the intake runner.  I also don't know if you have some spark energy cross coupling between spark leads, so that spark can occur near BTDC.

I don't know if there is an ember in the intake runner.

I don't know if your valves are sealing properly.

I don't know if there is cam lobe overlap where the intake valve may open during the exhaust stoke, pushing some unburnt air fuel mix into the intake runner.  Then, due to the wasted spark system, the plugs fire between the exhaust and intake stroke, igniting the ripe mixture.

This later possibility seems the most likely on an engine in otherwise good health.  But, there are lots of things I don't know.   :-\

So, what are your cam specs, and is your spark timed correctly?  What is that spark timing relative to the valves and piston positions?

Cheers,

P.S.  Remember, no one has given me a 650 to play with, yet.
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Offline Hush

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Re: Flames in the carbs?
« Reply #10 on: July 17, 2009, 08:14:58 PM »
All fixed thanks, it was my sons bike I was playing with and after shooting some ether into the carbs she fired right up and I left it running until all the crap in the system cleared, think TT could have been right on his original diagnosis of running extremely lean.
Anyhow that bike has been sold and will be on a transporter to Auckland this week so I can concentraite on the CBR400RR.
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I think the thing I most like about motorcycling is the speed at which my brain must process information at to avoid the numb skulls who are eating pies, playing the ukulele, applying make-up etc in the comfort of their airconditioned armchairs as they make random attempts to kill me!!!!!!!
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