Gas Furnace Flame-Out

I'm having problems with a 10-year-old Rheem Centurion forced-air furnace. The unit uses natural gas and is rated at 100K BTU.
I'm sure I'll use a lot of improper terminology trying to descibe this, but here goes:
Gas is fed to the combustion area through an electrically operated valve. The output from this valve goes to pipe that stands vertically. The pipe has four hex-shaped jets that each feeds into a horizontal venturi chamber where, I assume, the gas mixes with air for combustion. The flame from each venturi chamber is drawn into a horizontal, tubular combustion chamber about 12" long. Inside each combustion chamber, there is a pair of interlocking angle-brackets that, in effect, divide the length of the tube into four areas.
Here's what's happening with my unit: -- The ignition sequence goes fine. The impeller motor starts, the igniter glows, and the gas valve opens, all per the timing specs on the control-unit flowchart. -- During original combustion, with just the impeller motor running, I get blue flames racing neatly down the length of each combustion chamber. -- However, as soon as the main blower motor kicks in, the flame in the upper-most combustion tube starts losing its 'conformation.' A few seconds later, I'm seeing yellow flame mixed with the blue, and the flame starts to burble in the area between the venturi chamber and the combustion tube. If I don't shut the furnace down manually at this point, the out-of-chamber flame will trip a limit switch. -- This problem is unique to the top combustion tube; the other three continue to burn as expected after the blower motor starts.
Yesterday, I took out the venturi assembly and cleaned it. I also cleaned the angle-bracket 'separators' that run inside the combustion tubes. (I did not clean inside the tubes because I was afraid to poke something that didn't want to be poked.)
I noted that the bottom three 'separators' were entirely straight, while the top one (where my problem is) was bent about 6 degrees approximately four inches from the deepest end. This bend was somewhat crude; but I think it had to be intentional: there's no chance for something in this area to get whacked by mistake, and when I tried to put one of the straight separators into this tube, it didn't seem to fit.
Anyhow, after the cleaning, I put things back together and fired up -- but the problem was still there.
In the 10 years I've had this furnace, I've replaced one control unit (bad relay), one igniter, and one impeller motor (bad bearing). Other than that, the unit has worked pretty well.
Can anyone suggest why I'm getting this flame-out when the main blower motor kicks in? Solutions? Any hints on how to clean the inside of the combustion tubes? The combusion-tube 'separators' can be rotated about 45 degrees, to sit like the letter 'X' or like a horizontal-vertical cross; does this make a difference? Can anyone confirm that just one of the four separators on this model furnace is supposed to be bent? Also, the hex-headed jets on the gas feed pipe appear to be adjustable; any tricks-of-the-trade for these?
Thanks for any help.
Regards, Heatless (and clueless!)
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snipped-for-privacy@dodgeit.com wrote:

<SNIP>
Sounds like a hole has burnt thru the heat exchanger, allowing the (main) blower to force air back into the combustion area. Better crack open the piggy bank...
Jim
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You =are= speedy! (I don't think my post was up more than 5 minutes before you replied.)
Hole in the heat exchanger makes sense. Would this be DIY fixable, a pro-only job, or are we talking about a whole new furnace?
And now for the dumbest question of all: What would happen if I blocked off the problem burner (at the jet) and ran on 3 out of 4?
Thanks, Heatless
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snipped-for-privacy@dodgeit.com wrote:

As for blocking off one burner....you won't get me to endorse that in writing. <bg>
BTW, 10 year life on a heat exchanger doesn't say much for Rheem, does it? Or is it Ream...
Jim
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LOL. I don't know what's considered normal life for any of the components I've had to replace: MCU, IDB, igniter, and now this.
I saw the suggestion that my furnace had been abused. Let me say that I've been very good about changing filters and keeping the unit clean -- and very bad about calling a pro for 'regular maintenace.'
Maybe one of the pros here can explain just what this 'regular maintenance' involves and how it would have prevented the hole in my heat exchanger.
Regards, Heatless
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Holes in the HX can ge caused by many things, but flame impingement is one that comes to mind....
We are trained to look for problems that may lead up to HX failure.
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Hi Speedy, hope you are having a nice day
On 19-Oct-04 At About 09:32:30, Speedy Jim wrote to All Subject: Re: Gas Furnace Flame-Out
SJ> BTW, 10 year life on a heat exchanger doesn't say much for Rheem, SJ> does it? Or is it Ream...
I don't sell rheem but how do you know that the unit wasn't abused? maybe it didn't get the needed filter changes or regular maintainence.
-=> HvacTech2 <=-
.. "See this? This is a copy; I want you to make me the original."- s.w.
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More inserted:
--

Christopher A. Young
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Hi snipped-for-privacy@dodgeit.com, hope you are having a nice day
On 19-Oct-04 At About 15:46:17, snipped-for-privacy@dodgeit.com wrote to All Subject: Gas Furnace Flame-Out
h> From: snipped-for-privacy@dodgeit.com
h> I'm having problems with a 10-year-old Rheem Centurion forced-air h> furnace. The unit uses natural gas and is rated at 100K BTU.
what you have there is a classic sign of a cracked heat exchanger. you need to shut this unit down and have it checked by the hvac company you normally use. immediately!!
-=> HvacTech2 <=-
.. "Yes Officer, It Did LOOK Like I Was Driving Fast, but it was only 80"
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