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Carburetor heat problems...

WindOnHisNose said:
Dave, no sweat for me regarding ruffled feathers. I want people around me to feel free to offer criticism and concerns, both professionally and personally.

Regarding life and death, I flew with the windows open all the way home Friday! :eek: Steady, fresh, crisp air. Chose my route carefully, too. I don't subscribe to the motto "all's well that end's well."

Randy
Click to expand...

Great Randy. There was no criticism meant to you or anyone else actually. More like participating in a meeting :roll: (can't believe I said that, I hate meetings).
 
Good to hear that the problem is fixed.
I had a plane with the same problem this summer. It was a NEW carb, never worked on. That eliminates the theory that the "reused" locking tabs were the problem. All the tab locks were still locked.
If the tabs look good, and after locking them in place, they are not broken, I reuse them. What are the odds of the tabs breaking off on all 4-5 bowl screws at the same time? I'd say slim to none.
I came to the same conclusion that Pierce did. The gasket had to shrink.
Has the material that the gaskets are made from been changed? I suspect it has.
I wonder if the same Co. that makes the new bungees make the gaskets? :angel:
 
Regarding the loose bowl half screws...not uncommon. And I am not convinced that the cause is merely the gasket shrinking. I have replaced the lock washers and torqued the screws on a carburetor with loose bowl halves just to have them come loose again in 50 hrs. I think that the threads were pulling out on that carb. Perhaps check the internal threads for damage?

Regarding the enrichened mixture...the air pressure on top of the fuel in the bowl is engineered to provide a certain amount of fuel into the nozzle, on more sophisticated carbs the air into the bowl can be controlled by the economizer valve (MA4-5). And splitting the bowl halves ruins the controlled air pressure atop the fuel in the bowl.

Some of the older FAA A&P handbooks have a good explanation of the economizer system in the MA4SPA and the NAS carbs, but the newer books have mixed up the systems and confused many mechanics.

Vickie
 
Vickie, Darrel Starr also echoes your concern that perhaps the internal threads may be stripped. I hope not...that would put the kabosh to me flying to TN, as I am not sure I can locate a new bottom half by Tuesday afternoon, when the airplane needs to be ready.

It is an impressive amount of wiggle, though, between the top and bottom halves. I don't think the entire answer lies with the gasket, either. I just hope the mechanics who worked on the carb in the past were believers in torque wrenches.

Will know more tomorrow afternoon, that is for sure.

Randy
 
Thank you Vickie for the explaination and the model numbers. Is the MA-4-5 what is used on Randy's engine? I take it then it is a mechanical connection to the economizer? Using your model numbers I found this on the net if anyone is interested: http://home.comcast.net/~r123rs/Documents/Carb Manual.pdf
My read on that (economizer) is the float bowl operates (vents) at outside air pressure except at full throttle. Didn't Randy's problem manifest itself at cruise rpm? And finally again if running lean/rough why doesn't turning carb heat on improve the situation? dave


Vickie wrote:
Regarding the enrichened mixture...the air pressure on top of the fuel in the bowl is engineered to provide a certain amount of fuel into the nozzle, on more sophisticated carbs the air into the bowl can be controlled by the economizer valve (MA4-5). And splitting the bowl halves ruins the controlled air pressure atop the fuel in the bowl.

Some of the older FAA A&P handbooks have a good explanation of the economizer system in the MA4SPA and the NAS carbs, but the newer books have mixed up the systems and confused many mechanics.

Vickie[/quote]
 
Dave
The loose fuel bowl will cause a rich mixture with carb heat. That is what causes the large RPM drop. Leaning the mixture with carb heat on will allow the engine to return to normal operation. I was on the haul road when my problem was at it worst. I had lots of time on my way back to anchorage to figure it out.
DENNY
 
:( Finally I get it. I thought someone early on said the loose carb screws made it run lean. The whole system through out RPM range running rich makes sense to me and allows me to line the ducks back up in my brain. I went back and reread all the posts and no one said "lean" exactly, just extra air. Randy never mentioned trying to lean mixture during rough running which in a way could have masked the problem. Not having a carb to look at, Vickie probably identified the economizer valve as the culprit. I wonder if it was bad enough to warrent looking at the plugs?
I learned a lot from all these posts. I have never dealt with an economizer valve in a carb before and did not know how the carb responded to full throttle enrichening for takeoff, I just knew it did because a long time ago my instructor said it did. Since several posters here have had this problem it must be very common. Does this happen mainly to the round air filters or is it common with the old small (lighter?) filter also?


DENNY said:
Dave
The loose fuel bowl will cause a rich mixture with carb heat. That is what causes the large RPM drop. Leaning the mixture with carb heat on will allow the engine to return to normal operation. I was on the haul road when my problem was at it worst. I had lots of time on my way back to anchorage to figure it out.
DENNY
Click to expand...
 
Randy asked me to post some pictures of his carburetor. But unfortunately (as they say in England) I have the pictures in a computer in the hangar so I will have to post them tomorrow.
The gasket definitely was compressed. The original thickness on parts of the gasket not between the carb halves was .052 in. thick. Between the halves the thickness was .025 to .035 in. thick. So the average compression and resulting gap between the carb bodies was .022 in. Apparently Marvel Schebler Engineers never anticipated an air leak above the float of that magnitude.
Darrell Bolduc and Kevin, the Bolduc Shop Mgr, both said that this is a common issue that they often see!!!!
Darrell Bolduc provided a new gasket (that looked just like the same material as the old one), screws longer than the original ones and new locking tabs. Larry installed the new gasket and torqued the screws to 45 lb-in. All of the screws held torque and the threads had passed a visual inspection so we all feel the problem will not repeat anytime soon.
So the main takeaway is – On Every Walk Around, Grab Hold of the Air Cleaner Dome and Give It a Vigorous Shake, If It Moves Look Closer!
Randy & Larry took Randy’s Super Cub for a ride this afternoon and it ran great. So it looks like Randy will be on his way to Tennessee on Wednesday!
Darrel
 
What Darrel said goes for me, too:)

The engine ran great, the carb heat pulled the rpm down by 100 rpm, the mags ran great!

Darrel's photos will be pretty demonstrative.

fyi, a fellow supercubber was kind enough to send a pm to me this morning, asking me to call him asap. He knew my carb was going to the shop this morning, and he wanted to point out that he has had three different aircraft with the same problem as I experienced, and with each the halves had separated. While the gasket separating these two halves had become compressed to the point that a substantial leak occurred, he said that the vibration that occurred with the loosening actually caused the threads in the lower housing to become partially stripped. His advice is to buy a new carb and be done with it, rather than overhaul the carb. I understand his point, and will consider that in the future.

In the meantime, baby, I am on track for the Great State of Tennessee! Yeeeeeehaaaaaw. Many thanks to you for sharing your thoughts and expertise. Darrel, thanks for all your help.

Randy
 
I heard about this awhile back(SB)---quite common up here. I heard about it from Mad Mike or BJ engines. They had seen quite a few of these. I have had this happen to two of mine and personally know of a couple more. Hope this helps. I just wish I could remember the rest of what they told me they did with these carbs---I think I am forgetting something important on the fix;

http://www.lycoming.textron.com/support/publications/service-bulletins/pdfs/SB366B.pdf
 
Would it make sense to put a dab of Glyptol or similar marking paint on each of the bolts? That way you could see whether or not the bolts were backing out.
 
Here are pictures of Randy's Carb gasket as removed. The area not squeezed is .052 in. thick. The squeezed down area is .025 to .035 in. thick.
Darrel
P1110157.JPG

P1110156.JPG
 
Whats with the gasket just to the left of the lower left screw in the pictures? Rhetorical question there. Appears that screw not tightened or body is warped. Actual question: is the discoloration of the venturi area in the pictures normal on aircraft carbs with the number of hours? Bikes never see that many hours I guess and always look beautiful. Also thank you for inside carb picts. Drawings are a little confusing so it's nice to see a picture. dave
 
I think you might be looking at an area near the lower left screw that is not squeezed in the joint. The gasket in the picture isn't quite lined up with the body. I don't know about the venturi discoloration, perhaps someone can answer that.
All of you guys who had the correct answer to this problem and posted it should get a prize or collect an appropriate beverage from Randy when you see him at one of the future get togethers. Personally I was just sure it was an exhaust leak.
The metal float was in good shape and Randy wanted this back together so he could make it to Tennessee so Larry didn't change the float. However, the annual is coming up in Dec. and the airbox is wallowed out around the shaft so the plan is to ship the airbox out to Randy Ruppert to have the airbox rebuilt with Delrin bushings -- and install the new float also.
Darrel
Delrin is good stuff!
http://plastics.dupont.com/plastics/pdflit/americas/delrin/H76836.pdf?GXHC_locale=en_US
 
The carb was never designed to support the long Super Cub filter. It is common with a new carb that the gasket compresses, just retorque it every 25 hours or so till the screws stay tight, then you can mostly forget about it. A good Super Cub preflight should always include grabbing the filter dome and giving it a good shake to see if it moves.

The rub mark on the venturi in the picture is probably from the accelerator pump tube. You have to bend this tube in the correct position when assembling a carb so it works properly.
 
Randy,
I'm sure it's in the book, don't have time to look it up...It's a screw going into a aluminum casting....I'll admit that I just do it by feel with a little bit of experience.
 
Hey Mike

Works when I click the link---anyone else??? Did I store it on the computer??

Hell I don't know

Mark
 
AkPA/18 said:
Hey Mike

Works when I click the link---anyone else??? Did I store it on the computer??

Hell I don't know

Mark
Click to expand...

I get the pdf,

but it just says don't tighten screws yourself, take carb off and take to a shop... :( :( :( :(

no torque values, which I assume is what you reffed to?
 
I've found that when I torque something on farm equipment to airplanes, if I go back a few hours later and do the same torque, the nut or bolt will turn a little more. So, was it really torqued the first time?
 
While building my engine I was surprised that some bolts are required to be lubed prior to torquing. That was my first exposure to that. But the issue here is with the gasket's inability to maintain thickness after initial torque, not with the screws or initial torque values, right? That the gasket material hasn't been improved is startling.

SB
 
StewartB said:
That the gasket material hasn't been improved is startling.

SB
Click to expand...

SB...Its pretty startling the amount of airplane items that have not been improved due to high certification costs and lawyers.

Tim
 
But the issue here is with the gasket's inability to maintain thickness after initial torque
Click to expand...
Last night I checked the screws on my new MT prop spinner. They have some kind of fiber washer and I was able to get just about everyone to turn a little more. That is the second time since June so those fiber jobs must squish a little over time also.
 
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