VansAirForceForums  
Home > VansAirForceForums

- POSTING RULES
- Donate yearly (please).
- Advertise in here!

- Today's Posts | Insert Pics

  #21  
Old 08-31-2016, 11:19 AM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 2,369
Default

Quote:
Originally Posted by snopercod View Post
That's exactly what I thought as well, but couldn't figure out why, if the mixture control has no effect below 1,000 RPM, we're supposed to pull the mixture control out during ground operations to keep from fouling the plugs. Then I found this: Precision Airmotive's MSA MSA Float Carburetor Handbook. If I read this correctly, the mixture control DOES effect the idle mixture (as well as the idle adjust screw). Words just confuse me, but this link has lots of diagrams I've highlighted the important stuff:
In simple terms, the mixture control is a valve that sits BEFORE all of the other fuel delivery circuits in the carb. All it can do is reduce total flow into the carbs various delivery circuits, allowing you to choke off flow and thereby over-ride the carbs metering function the the lean side.

You can use mixture to lean at idle. However, it is a very specific point in the travel and difficult to get to. If you lean properly with mixture control at 800, you will not be able to run at 1200 RPM, as you would have choked down the flow to a rate for 800, which would be too lean for 1200.

However, there really is no need to do this at idle, as this can be fully covered by the idle mixture setting. If the idle mixture is properly set, manual leaning can only make you idle TOO lean. If your idle is set too rich, manual leaning can fix it, but why do that? However, manually leaning will help during taxi as you will likely be getting on the transition circuit or even the main circuit and running rich without overriding it. The carbs tend to run on the rich side in that area to help with transition from the idle to main circuit.

Larry
__________________
N64LR
RV-6A / IO-320, Flying as of 8/2015

Last edited by lr172 : 08-31-2016 at 11:27 AM.
Reply With Quote
  #22  
Old 09-01-2016, 12:39 PM
snopercod's Avatar
snopercod snopercod is offline
 
Join Date: Aug 2016
Location: Asheville, NC
Posts: 808
Default

Well this is embarrassing. While looking for a good place on the carb to attach a thermocouple, I looked up at the flange where the carb bolts onto the oil sump and said to myself, "What the heck is that hole doing up there?" It turns out that somewhere along the way, the "Idle Drill Plug Screw" had fallen out, leaving a .20" hole all the way into the barrel of the carb. I confirmed this by sticking the butt end of a #10 drill bit into the hole and observing it inside the throat of the carb, sticking out above the venturi and below the throttle fly. That can't be good, but I'm not smart enough to predict what effect it would have. You would think it would make the engine run lean, but it seems that adjusting the mixture control would compensate for the extra air. At idle, the throttle fly is mostly closed so it wouldn't make much difference; There's lots of air coming up from below anyway. All I know is that the replacement plug/screw is supposed to arrive tomorrow by FedEx. I'll know more when I install it (with Loctite!) and fire up the engine.


Last edited by snopercod : 09-01-2016 at 03:43 PM.
Reply With Quote
  #23  
Old 09-02-2016, 01:40 PM
snopercod's Avatar
snopercod snopercod is offline
 
Join Date: Aug 2016
Location: Asheville, NC
Posts: 808
Default

Here's a photo of the replacement plug. $2.50 for the plug and $14.50 for shipping. I don't know if MA-4SPA carbs have one of these or not:

Reply With Quote
  #24  
Old 09-03-2016, 01:43 PM
snopercod's Avatar
snopercod snopercod is offline
 
Join Date: Aug 2016
Location: Asheville, NC
Posts: 808
Default

The part pictured above doesn't fit. It's threaded 1/4-20 and the hole in the carb is (apparently) 12-24. My carb is probably 60+ years old and somewhere along the way, Marvel Schebler changed the design. I'm going to have to build my own plug, apparently.

Side note: This hole is where a carb temperature probe goes, if you have one.
Reply With Quote
  #25  
Old 09-03-2016, 05:25 PM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 2,369
Default

You can fill the hole with JB Weld. Just form a piece of thick tape on the inside of the carb before filling and make sure to form an oversized head on the outside and be sure to clean with acetone first for good adhesion.

Larry
__________________
N64LR
RV-6A / IO-320, Flying as of 8/2015
Reply With Quote
  #26  
Old 09-03-2016, 06:48 PM
snopercod's Avatar
snopercod snopercod is offline
 
Join Date: Aug 2016
Location: Asheville, NC
Posts: 808
Default

Larry-- I'll use J-B Weld as a last resort, but I might want to install a carburetor temp gauge some day and that's the hole I would need to use. I found a 12-24 brass machine screw at Ace Hdwe. this afternoon and will try to plug the hole with that tomorrow.
Reply With Quote
  #27  
Old 09-03-2016, 09:54 PM
lr172 lr172 is offline
 
Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 2,369
Default

Quote:
Originally Posted by snopercod View Post
Larry-- I'll use J-B Weld as a last resort, but I might want to install a carburetor temp gauge some day and that's the hole I would need to use. I found a 12-24 brass machine screw at Ace Hdwe. this afternoon and will try to plug the hole with that tomorrow.
I would put some red thread locker on it as well.

good luck.

Larry
__________________
N64LR
RV-6A / IO-320, Flying as of 8/2015
Reply With Quote
  #28  
Old 09-04-2016, 05:30 AM
snopercod's Avatar
snopercod snopercod is offline
 
Join Date: Aug 2016
Location: Asheville, NC
Posts: 808
Default

Quote:
Originally Posted by lr172 View Post
I would put some red thread locker on it as well.
I plan to use the blue "medium, removable" since someday I will want to install a temperature probe in the hole. As an extra precaution, though, I'll drill a small hole in the 12-24 screw head and safety wire the screw. It ain't gonna' be pretty, but it should work.

Thanks to you (and everyone) for all the good advice. Hopefully today I can fire up the engine and see how it runs without the air leak
Reply With Quote
  #29  
Old 09-04-2016, 07:31 AM
bret's Avatar
bret bret is offline
 
Join Date: Feb 2010
Location: Gardnerville Nv.
Posts: 2,595
Default

Yip! that will mess with the whole metering process, glad you found that! so did the problem just show up one day?
__________________
7A Slider, EFII Angle 360, CS, SJ.
2017 gladly supported
Reply With Quote
  #30  
Old 09-04-2016, 02:02 PM
snopercod's Avatar
snopercod snopercod is offline
 
Join Date: Aug 2016
Location: Asheville, NC
Posts: 808
Default

Quote:
Originally Posted by bret View Post
Yip! that will mess with the whole metering process, glad you found that! so did the problem just show up one day?
I don't know when that plug screw fell out, but I hadn't noticed any symptoms. A few days ago the engine was running just fine both at cruise and when taxiing. One would certainly think that hole in the barrel of the carb would mess things up, but maybe not so much. Today I plugged up the hole and ran up the engine. Amazingly, nothing much changed. The idle was smoother (natch!), and the idle mixture test gave me 100+ RPM rise at idle cutoff rather than the 50 RPM rise a few days ago. Those were the only differences on the ground. So I adjusted the idle mixture screw 1/4 turn in the lean direction, and re-performed the test. I got 50-60 RPM rise that time so I left it and put the cowling back on. I would have expected to have to enrichen the idle mixture to make up for the extra air coming in that hole, but that wasn't the case. Anyway, that problem is fixed, but I have no idea whether the heat soak problem is still there or not. This missing screw may have been just a detour on the road to finding the original problem. OTOH, maybe it has something to do with it. Time will tell. Here's my redneck fix. I used blue loctite on the threads and J-B Weld on the head in lieu of safety wire.

Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -6. The time now is 03:55 AM.


The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.