I’ll start that I’m a newbie to the site. Been submersed in the wealth of information and knowledge here. Being in VA and away from the airboat world of central FL it’s like finding a piece of home. Amazing! I have a fresh build on an 0320 (160) that hasn’t even been in the water yet. Boat had sat for 7+ years. Putting polishing touches on the boat now but the carburetor (ma4-spa) has started leaking like a sieve. Didn’t do it when first powered engine up assuming various seals /plungers dried out from sitting and the little bit of trailer operation (<1 hr) was enough. Leaking from accelerator pump and throttle shaft. Haven’t pulled spark arrestor to see if leaking from throat. Also mixture control lever does not cutoff fuel supply. Shaft turns, slight roughening of the idle, but no shut down even after 1.5 minutes. I’m fairly mechanical but have not worked on ac carb before. Does this all sound like strait forward seals/plungers etc that I might should take on or should I be thinking about sending to one of the guys on the site that obviously know how to rebuild these.???
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Carb rebuild ????
- Thread starter DHam
- Start date
if the carb was full of fuel when it was shut down and has set for that long most of the fuel circuits are most likely plugged with residue from the fuel and imo a full overhaul is most likely in order but taking it apart will tell ya what needs to be done ....it is the best way to be sure....if the l/r does not kill the engine at full lean the cable shaft is most likely twisted and out of position but once again taking it apart will answer all your questions....and make it easier to decide what to do
GMAC76. Thx for the encouragement. Tore it down today. Housing on the pump inlet strainer had a crack in it. Nothing else stood out. Gave it a good cleaning. L/R lines up correct. Had a little crud. Not terrible but I know with carbs in general doesn’t necessarily take much. Blew everything out. Soldered the crack in pump housing. Figured didn’t have anything to lose. Will see if it works. Will post tomorrow when I get a chance to try it. Thx.
GMAC. Got Carb Parts today. Can't thank you enough. Did engine run-in as per manual (i.e. 200 rmp steps @ 10 min each) and everything seemed to check out except mag drop. Lost 200+ on one mag @1800 and really rough. Going down the list.. plugs, wires, wire contacts in mag, points. Need to run to airport and get more fuel. If I can't find problem, I'll be posting ?'s. I was wondering on shooting CHT with IR gun. Where is the best place on jug to take reading? This engine was rebuilt in 2012 and has sat in shop since. Man that rebuilt has since passed so I don't have anyway to ask him questions on exactly what he did. Trying to make sure everything is right so I don't screw it up right off the start.
I think most guys shoot the cht with the IR gun at the base of the top sparkplug....shoot all 4 to find out which cyl is the hottest......200 rpm drop is too much.....on your check off list add a possible carbon track on one of the dis. block towers...them 4cyl bendix mags are famous for carbon tracking especially the black dist blocks....if ya need any parts let me know
Slidin Gator
Well-known member
I got a O-320 going back together right now. This pic shows the CHT port on the bottom of most Lycoming cylinders. From this years testing with CHT sensors in these ports I find that my IR gun hitting the flange just inboard of the bottom spark plug (Just below CHT port in pic) nets me a reading that is 100 degrees less than the actual CHT sensor in the port.
Pic super helpful. Will try that. I wasn’t sure and tried various places getting readings all over the place. Noticed when trying to get down there on bottom jug that readings were super sensitive to the exhaust. How much difference do u typically see between cht port vs upper spark plug region like GMAC said. I’ll find that aiming point while she’s off so when she’s running I can hit it without the header sending me off the chart. Got avgas yesterday and hope to play around with it some this afternoon. Nothing else to do it won’t quit raining up here this winter.
Slidin Gator
Well-known member
I get about 50 degrees cooler on top plug vs point I show in pic, which reads 100 low. So 350 on the top plug would be roughly 500 on the CHT sensors.
Bro,do yourself a big favor and do not let that boat idle until it’s seated the rings. Or you’ll be taking her back Down did a fresh 540 last summer that a guy idled up n down the idle zone. He swore the builder ripped him off. Not so. Man build him a fine motor,he broke it in wrong. Run the piss outta it. Just keep temps in check.
I understand what guys are doing with the IR guns etc...but wouldn't it be simpler to just buy a good quality FAA/PMA approved CHT gauge install it per the instructions and go ride........I guess I just don't understand all the second guessing of the aircraft CHT gaugesSlidin Gator said:
Slidin Gator
Well-known member
No second guessing on my part, Putting on the CHT’s was a big dose of reality. I always thought things were good given the IR readings but I found that was not the case. These cooler plugs are making a big difference for me, almost as much as you sorting out the carb. No loss of power from the plug change.GMAC 76 said:
X3 on the breakin procedure, I hammer everything I rebuild hard, best to break it early.
Yeah. I was headed down the wrong road at first thinking it was good to start the engine and let it at least get lubricated. Realized from reading so many posts on the forum that I was going to do more harm than good. Sure sounds like y’all are reiterating that here. That’s why I did the run in per the manual. Learning a lot from you guys. Any recommendations on specific cht gauge? I.e where to get? Is the sensor sold separate or does it come as part of the gauge set?
:thumbleft: :thumbleft: I just wanted to put this out there. I’m no A&P. But I’ve built a dozen did many topends. I usually do a run in on the trailer or ground. I don’t let a motor idle. But I always pull the plugs and spin it till I see oil pressure. I Crank the engine and run it 1/2 to 3/4 throttle but never really a steady rpm for long. I do that till she’s hot then shut it down to cool a bit. I repeat 2-3 times then I’ll run it hard checking temps. If you’ve built it with good parts there shouldn’t be a bunch of oil burn. That being said chrome r harder to break in than steel. I also find that atf does better on chrome than a/c oil and there’s less chance to get carbon behind the rings. Like I said I’m no pro. Just my method and it’s always worked for me. Hope it helps someone.