My Kenmore gas dryer is cycling pretty frequently. The net effect of this is that it is not drying the clothes - at least not as quickly as it used to.

I took off the front of the unit and removed the drum so I could access the burner. Machine starts fine, but flame only lasts about 20 seconds, then no flame for about a minute or so. Ignition cycle keeps repeating: 15 seconds igniter heats up, then 15 seconds flame, then a minute or so with no flame. I'm thinking it's getting too hot maybe. I traced the elctrical back to what the electrical diagram indicates shoul dbe the Thermal Cutoff (#280010), but acts more like the running thermostat because it resets itself.
My flame is maybe 75% blue/25% yellow. This is natural gas, not propane. I have heard that yellow in the flame may indicate it is running too "rich". However, I do not have any gas odor that would indicate an incomplete burn. The flame extends the entire length of the shielding "cone" and the tips even get into the metal duct at the back of the machine. Should the flame be that long?
The burner assembly has what appears to be a regulator built-in. But adjusting the screw on top has no apparent effect on the flame.

Other info: I recently replaced the Thermal FUSE (#3392519). I know what you are thinking - but this is the one on the return duct - not the Thermal CUTOFF which mounts next to the thermostat on the supply duct. I also cleaned out a lot of link and junk at that time, so the machine is pretty clean.

So... Am I burning too hot? Can the flame be adjusted? Is there something else I should check?

Thanks in advance!

Simon,
Thanks for the quick response!

I forgot to mention that since I pulled it away from the wall to work on it, I actually HAVE BEEN venting it to the room when I observe the flame cycling. I don't have a load of clothes in it though, so I'm not sure if that makes a difference. Now my outside vent might be obstructed - I haven't checked it yet - but it still behaves poorly when not hooked up to it.

I have measured Voltage upstream from the burner assembly, because I am loosing the 120V power to it when the flame dies. So swimming upstream, the first component is the Thermal Fuse, which I replaced a few weeks ago. It was definitely bad (no continuity) and the dryer didn't dry at all before I replaced it. That checks out OK because I would be getting nothing at all if that were bad.
Next is the Operating Thermostat. I know you recommended replacing this, but I'm losing my 120VAC even before it hits this. I get nothing on either of the outside leads (red wires, not the purple ones) when the flame goes out.
The next component is what I suspect is the culprit. According to the electrical diagram inside the unit, this should be the "Thermal Cutoff Non Resettable 352 F". When the flame goes out, I see 120VAC on one side of this device and nothing on the other. The odd thing is though that this DOES reset itself because a minute or so later, I'll get my 120VAC to the burner again. This device and the "Hi Limit Thermostat 205 F" are mounted right next to each other on the hot supply duct with a short black lead between the two. So maybe the actual order they are wired is swapped from the way it is shown on the diagram. The replacement "kit" for the thermal cutoff sold by Sears looks like it contains BOTH the Cutoff and the Thermostat and the jumper wire - so I'm guessing they recommend you change both together. I have that on order.
My question about the flame was because that hot duct is getting pretty warm with that flame hitting it, so I imagine it could very well be hitting that 205 F limit. How long should the flame extend back and what should be the coloring of the flame? It just looks awefully big to me - extendng all the way back to the duct at the back.
P.S. Yes, the lint trap is clear.

OK,
Kit arrived so I replaced both the thermal cutoff and hi limit thermostat. Probably a waste of time and money because clothes are still not drying well.
I did observe a longer inital burn though, so maybe the thermo was starting to go. (trying to make myself feel better!)

I also thoroughly cleaned my vents and there was indeed some lint stuck in there so perhaps it was partially blocking. But no help to the wet laundry.

So I decided to take your advice: pop the peep hole cover off and have a look at the ignition system while a load was running. Put a full load of wet clothes in and set for 60 minutes on high temp (cotton) setting. I observed several misfires which did not happen at all when I had the unit opened up. And by "misfires", I mean ignitor glows for 15 seconds and then goes out - no flame in fact it doesn't seem to call for gas at all. So now, I'm thinking flame sensor. The unit fires up when starting cold every time, but subsequent firings are unsuccesful.

Here are my observations:
00:00 Start machine
00:10 Ignitor Glows
00:26 Flame. Ignitor off.
06:20 Flame Off (good 6 minute burn)
06:30 Ignitor Glows
06:45 Ignitor off - no flame
07:20 Ignitor On
07:35 Ignitor Off - no flame
08:10 Ignitor On
08:35 Ignitor Off - no flame
09:15 Ignitor On
09:30 Ignitor Off - no flame
I finally opened the door to stop it and let it sit for a minute or 2.
Then started back up and got flame.
It burned for about 3 minutes then went off. Then misfire cycle repeated again.

That makes sense and would explain the behavior I'm seeing. Parts are on order. I'll let you know. Thanks!