bryanflood

bryanflood

Well Known Member
Hi, need some advice from the experts. I have an RV-9A with an O-320
and a MA-4SPA P/N 10-5217. I checked the fuel pressure and it is
normally between 5 to 5.5 psi. About 9 months ago I installed a
rebuild kit in the carb. This included a new float needle valve and
seat but not a new float. I set everything by the book and it worked
great for about 100 hours until this weekend. The plane was
impossible to start, it was flooded, couldn't figure out why. Then
found that the gas level in the bowl was at or above the carb split
line. Took it all apart inspected the needle and seat under a 10
power scope, still looked good. The float is metal and I cannot see
any evidence it contains any fluid. However even after making sure
everything is set correctly I was able to comfirm that that fuel was
being allowed into the bowl until the float was completely
submereged and then just barely stopped, this seems odd b/c the seat
and needle are really too new to fail and the float is dry. The float seat combonation was set to the correct height, so it became "solid" at the correct float setting, it's jsut that it took more pressure on the seat to fully shut off the fuel. The float part number is 30-766 with a date of 3 90. Can floats fail in some other way? or should I just try to throw another needle and seat at it? The whole thing seems odd to me.

Thanks for the help,

Bryan
 
Sticky float?

Bryan, if the float is still sound, (dry) then it might be something simple like dirt and/or varnish build up on the float pivot arm and holding the float down (open). Hit it with a good cleaner once or twice and see if it changes anything.
 
Been there

First, your problem and mine may be very different, but I had similar less than a year ago on the same setup. Replaced the needle/seat, then the float. Nothing except a big waste of money. My mechanical fuel pump had decided to increase it's output pressure from time to time. Never did figure out why, and five A&P's told me it couldn't happen, but replacing the FP did the trick. I would look very closely at the FP and at fuel pressure in general. Also, and I don't know if this could be a factor, but when fuel is warm, it expands, is less dense and might not exert as much upward pressure on the float valve. I don't know where you are, but I'll bet it is warm there.

Bob Kelly
 
More clues

So, here are some things I probably should have mentioned before. I have seen the bowl overflow both on the engine driven pump and on the electrical pump. Most of my testing however has been on the electrical pump. I tested the fuel pressure for the electrical pump and it was dead stable at 5 psi, a buddy who was also there said he saw 5.5 psi briefly, but I never did. I also thought the float was sticking in the bowl, so I suspended the top half of the carb in a large glass bowl with the float completely free and turned on the fuel. The fuel level went up until the float was almost completely submereged, like 99 to 100% and then the fuel level seemed to stop or slow to an undetectable level. So the problem is not likely to be that the float is sticking in the bowl.

I have not cleaned the float, maybe varnish buildup has made it too heavy or sticky on the pin as WilburD2 suggests, although it certainly seems to be free. I will give this a try today.

Thanks for all the suggestions...

Bryan
 
Ideas

Bryan,

After replacing the float bowl on my MA4-5 (why is another story) I noticed the float was rubbing the side of the bowl :eek:

There is not much room in there at all, at least on the 4-5's.

What I would do is with the carb dry of fuel I would pick it up and rotate it to the left all the way around until it was upside down. I could hear the float flopping over. When I did the same thing to the right the sound was very different so I was pretty sure the float was rubbing the bowl. I smeared a light coating of grease on the bowl and sure enough it rubbed off on the float. i just had to "tweak" (bend) it a little to clear. The hinge pin allows the float to have quite a bit of lateral play. This can also be "tweaked".

If the float is "catching" on the bowl it will stay down a liitle too long and flood. With the engine running it will be less likely to catch I would think due to the engine vibration.

Just something to check.

One more thing. Confirm that your seat part number is the correct one.
If the seat has too large of a hole the float cannot hold back the pressure like it should. Simple "hydraulics"

Mark
 
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Check the clip too

Check that the main valve "clip" is seated correctly too.
The clip that connects the floats to the main valve.
the hinge pin runs thru it for it to seat properly.
The clip can be pushed forward ahead of the hinge pin and look correct but it will not let the valve "seat" correctly.....
Ask me how I know..... :(
Good Luck.
 
Questions from previous posts...

Mark - Any idea how big of a hole the seat should have? Yesterday I confirmed that the "quick kit" I used 9 months ago was the correct kit... but that doesn't mean the hole in the seat is correct. Great idea something I have not checked.

Bob - Looking at the service manual the figure on page 9 and figure 13 show the clip in front of the float arms or another way to look at it is between the float arms. But then figure 10 of the service manual shows the clip behind the arm of the float in front of the hinge pin. I'm not sure how to get the clip to go around or behind the hinge pin b/c the float bracket that holds the hinge pin and the float travel stop are in the way. We very carefully made sure that the clip did not contact any area that would casue the float to hang. I'm probably missing something in your response... could you explain it again to me. I really hope this is my problem, would love it to be an easy fix. BTW- My clip is behind the flaot arms in front of the hinge pin and bracket, like in figure 10. You know, sort of off to the side pointing right and slightly aft.
 
Bob - Looking at the service manual the figure on page 9 and figure 13 show the clip in front of the float arms or another way to look at it is between the float arms. But then figure 10 of the service manual shows the clip behind the arm of the float in front of the hinge pin. I'm not sure how to get the clip to go around or behind the hinge pin b/c the float bracket that holds the hinge pin and the float travel stop are in the way. We very carefully made sure that the clip did not contact any area that would casue the float to hang. I'm probably missing something in your response... could you explain it again to me. I really hope this is my problem, would love it to be an easy fix. BTW- My clip is behind the flaot arms in front of the hinge pin and bracket, like in figure 10. You know, sort of off to the side pointing right and slightly aft.[/QUOTE]

First off, mine is a MA4-5 for my 0-360, so not sure exactly how your 0-320 carb looks. I don't have the parts manual at home here either. But I'm trying in attach a picture for you to see my part.
The SS clip that connects the floats to the valve hinges on the same pin the floats do. You can push the clip farther forward, towards the bowl and it will work but not correctly. It would let the valve seat 100%....sometimes, so it gives the illusion it is working. But it isn't. I hope the picture helps explain this. The picture shows the clip installed correctly.
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bryanflood said:
Mark - Any idea how big of a hole the seat should have? Yesterday I confirmed that the "quick kit" I used 9 months ago was the correct kit... but that doesn't mean the hole in the seat is correct. Great idea something I have not checked.
Click to expand...
Bryan,

Sorry, But I don't know the answer to this question. Maybe a call to Precision will get you the answer. I've never called them so I don't know how helpful they are.

Precision Airmotive phone: 360-651-8282

Mark
 
Thanks Bob

Yeah, my clip is made of bent wire, different than yours. Great picture though, wish people could always send pictures it makes things so easy. Nobody seems to have pictures of the inside of the carb (probably not a big suprise).

Thanks for the help.

Bryan
 
Bob K Question - Fuel Pressure

This question is for Bob Kelly, but also all the other knowledgeable people out there. After no luck fixing my carb problem including a new seat, needle, proper seat tightness and washer + new float, I hooked up a mechanical fuel pressure gauge right at the carb and see up to 5.5 psi on the mechanical pump and up to six "normally" on the engine driven pump. Once I saw 7.5 psi on the engine driven pump, and a buddy of mine said he saw 7 psi while I was in the cockpit. The lycoming engine manual seems to say 8 psi is max 0.5 min, while the carb people seem to want 0.5 psi to 6 psi. The lycoimg fuel pumps seem to indicate 3-6 psi is normal. So my question for Bob and Others is what high fuel pressure was causing your problems? Is 7.5 psi too high?

Thanks,

Bryan
 
Bryan,

When I was having the overflow problem, we also checked with a "calibrated" gauge and saw a normal of around 6 psi. It would just go up from time to time, as I recall to a little over 8. I think it over-pressures the float valve at that point and won't go any higher as it has a relief. The way we proved the FP was the culprit was when we bypassed it by connecting the carb directly to the outlet of the gascolator and using only the electric FP. I used a union behind the FP to connect the gascolator line to the line to the carb. No overflow in about 20 minutes of running. We replaced the FP and the pressure is always steady and have not had any overflows in 75 hours of flying. NO idea why or how a fuel pump could do that. I even took it apart before sending it in as a core exchange and found nothing obviously wrong. Couldn't dig too deeply into it, however, and didn't really care at that point. I think 7.5 is too high, but I can't speak for Lycoming. Hope that helps.

Bob Kelly
 
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Bryan,
If you are certain that the carburetor is assembled correctly with the right parts, the only other thing that can make it overflow, is too much fuel pressure. A restricted engine breather can make the fuel pressure rise or the engine driven pump can cause it. By process of elimination you have either of two choices; a different carb or a different fuel pump. At this stage of the game, with all the carb work you have done, if it were me, I would try the fuel pump first.
Never measured the exact pressure that the float starts to leak at, but the fuel pressure you are seeing, is higher then what is normally seen.
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."
 
Bryan,
Also make sure your primer system is not leaking. I had my carb off twice only to find that I had left my primer unlatched causing fuel to flow through and dripping out the carb. try bench testing your carb off the engine using various pressure readings.
Good luck
 
Bryan,

Re-reading your last post, the only thing I didn't see mentioned was float level. I assume you have the float level correctly set. Don't remember the dimension, but my A&P used a drill bit of that diameter to check the clearance/level. He also set it a bit "wide" as a test. That didn't work, either.

Bob
 
The solution

Okay, so I thought after screwing around for something like a month with no flying I should probably post what I have found for the benifit of others. I eventually ordered a new metal float from spruce and tried it out using the top half of the carb over a glass jar to check the fuel level at 5.5 psi. The new float was actually worse than the one I already had, and by this I mean that when the fuel finally shut off the fuel level was actually higher than with the old float. Both were set at 7/32 per spec. What really scared me however was that both floats were almost COMPLETELY submereged in fuel when the fuel level stopped rising at 5.5 psi. The entire float was submereged, there was just almost no float left exposed and thus I reasoned not much margin for error for increased pressure. I had seen up to 7.5 psi on the engine pump. Not good. Then I talked to one of the local guys at the airport, who had been involved in a project using a much higher fuel pressure (10 psi) with the same carb. According to him the metal floats were inadeqaute at this pressure and he found he had to switch to a composite float, that he believed was more bouyant. Of course you always hear horror stories about composite floats. Apparently the old ones absorb fuel and eventually sink also not good.

So I checked the precision web site and apparently there is a new composite float available that may still be used in this carb, so I ordered one of those. I also ordered a new engine driven fuel pump. While I was waiting for parts I decided I didn't much like how the overlowing fuel always leaked out of the airbox and straight into the cowling (Seems bad). So I modified my air box to drain into a tube that runs down the nose gear leg to drain that is flush with the nose gear fairing and about half way down the gear leg. Incidently I later tested this and it worked great, the leaking fluel just streams right out of this hose onto the gound and not into the bottom of the cowling near the exhaust pipes.

Anyway, when the parts arived I checked the new composite float. There seemed to be two benifits: 1. It appears to be more bouyant than the metal floats, thus the fuel level does not rise as high in the bowl at 5.5 psi as it does with the metal floats and at this level, less of the float seems to be submereged. So my conclusion is that the composite float has greater room for error in terms of increased pressure from the fuel pump not causing an overflow of the carb bowl. 2. The composite float is physically smaller than the metal float (narrower). So when you align it with the gage there is MUCH more room in the bowl for the float to move and thus appears to be less chance of a hang-up of the float on the bowl.

So I assembled the carb with the new style composite float and then replaced the engine driven fuel pump with a new one. The new engine driven fuel pump only puts out about 6 psi absolute max (instead of the old 7.5 psi) and frequently as little as 5 psi, this seems much better (or at least in spec). The engine seems to be very happy with the new combo and no more overflows have occured with this set-up. I also like knowing if another overflow ever occurs the fuel will drain away from the hot exhaust pipes and be dumped overboard.

Bryan
 
Float type

Just to clarify, the new float is a 30-864? in the 666-1000 kit which is not technically called a "composite" float but, I believe, is called an advanced polymer or solid foam with smooth skin float. Hope that clarifies any confusion.

Bryan
 
Another float bites the dust...

At 45 hours ...
very rough operation on final and after landing.:eek:
Fuel in airbox ...
Called Lycoming and they had us send the carb to G&N for inspection/repair.
They called this am and said that they found the float flooded when they opened the carb up.
Supposed to be back tomorrow....

If anyone else doesn't have a new float installed, they should check this out:
SB-2 A - http://www.tempestplus.com/SB-2_A RPLmnt of Foam & Hollow Floats 04-16-09.pdf
Instructions http://www.tempestplus.com/vE1000 Word 97-2003.pdf

SB-5_A - http://www.tempestplus.com/SB-5_A MSA-13 Supecedure 06-01-09.pdf

http://www.tempestplus.com/Lycoming_service.htm

SB-4_B - http://www.tempestplus.com/pdfs/SB-4_B Bowl Clearance.pdf

http://www.allpromedia.com/clientaccess/volare/

BTW: MARVEL-SCHEBLER is back...
http://www.tempestplus.com/pdfs/Tempest press release name change 8-10 Rev 1.pdf
 
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