A4 Carburator gas leak - SailNet Community

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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Gas > Atomic 4
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  #1  
Old 11-30-2008
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A4 Carburator gas leak

OK, so a bit of background is in order.
We have owned our Tartan 27' #328 (built 1967) for 6 seasons so far. In this time I have become pretty good at changing the oil twice a year, changing the plugs, ignition wires, distributor cap and rotor, monitoring and changing the water pump impeller among others.
Over the years we have owned the boat I have to say that I have become grateful that this old engine (late model or hybrid) keeps on ticking but there is always something to worry about. One thing I have noticed is that our plugs do tend to get a bit sooty and a quick change of new plugs seems to restore the normal rumble and power to the engine. This season was no different with one exception.
About mid-season I began to notice an intermittent loss of power in forward and decided it was time to look at the Moyer engine manual I have. I also decided change the racor fuel filter and to use some carb cleaner which I sprayed into the flame arrestor/intake valve. I also gave a slight turn to what I thought was the rich/lean mixture adjusting screw on the carb. It seemed to run a little better. Then we changed the plugs and it seemed to run like a champ.
I should add that we usually only use the engine to motor out to the races and back which I know is considered a bad practice.
Around the same time during the season we began to notice a gassy smell in the cabin so I took some nice clean, white oil rags and put some under the carburator. Lo and behold, there was a light brown stain on the cloth and a healthy gasoline smell down below. During this seasons winter layup I closed the fuel intake valve, upstream of the Racor filter so the whole tank does not end up in the bilge.
I did some research over on the Moyer A4 forums and found a reference to a leaky float valve that could cause this unsafe condition. There is also a reference as to how to clean the carb float valve by removing the main passage plug and drawing some gas through it until it comes through clean.
So my questions to the experts on this forum are as follows:
1) Since I have done no maintenance to the carburator since owning this boat should I just bite the bullet and take the whole mess off and give it a good cleaning (servicing where needed) even if I've never done this before?
2) Should I try the method of removing the main passage plug and running some fuel through first? This method does not require the removal of the carb unit.
3) Is it likely that by using the carb cleaner through the flame arrestor/intake that I inadvertently got junk into the float valve OR is it just as likely that this would happen eventually?
4) Is using carb cleaner a good idea in the first when the engine is running?
5) Am I missing something?
Any and all knowledgeable comments are welcome as are snide remarks, ridicule and humor.
Thanks.
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Old 11-30-2008
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Quote:
Originally Posted by CalebD View Post
I did some research over on the Moyer A4 forums ....
So my questions to the experts on this forum are as follows:
With no offense to my fellow Sailnetters: I suspect you'll find more A4 expertise over in the Moyer Marine forums than here.

Quote:
Originally Posted by CalebD View Post
1) Since I have done no maintenance to the carburator since owning this boat should I just bite the bullet and take the whole mess off and give it a good cleaning (servicing where needed) even if I've never done this before?
Depends. Are you mechanically adept?

Quote:
Originally Posted by CalebD View Post
3) Is it likely that by using the carb cleaner through the flame arrestor/intake that I inadvertently got junk into the float ...
No. Not possible for something to get from the air intake into the float bowl when the carb is operating.

Quote:
Originally Posted by CalebD View Post
4) Is using carb cleaner a good idea in the first when the engine is running?
Other than potentially fouling the plugs: None of which I know. I never bothered, because I don't believe simply spraying carb cleaner down the throat(s) does all that much good.

That's all I have. I'm not an A4 guru and haven't had a carburetor apart in over 30 years. I wouldn't hesitate on ours, but I used to wrench on "old tech" engines all the time, back in the day .

Good luck.

Jim
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Old 11-30-2008
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Many thanks to both Jim and Paul for replying so quickly.
Jim,
While I know that there are probably more 'hands on' A4 types on the MM forums I find that the pace of responses is a bit faster over here. Over at MM it can get to be a bit like listening to crickets. I may very well post this same question over at MM and probably should but I was mostly fishing for guys like you and especially Paul to reply as I gather by his posts that he seems to know the A4 quite well.
I never did that much actual wrenching on my first (older 60's era) cars but my boat partner did some monkeying and he seems reluctant to tackle the project of taking apart the carb and re-building it which gives me some slight pause before just digging in on this project.
OTOH, I was a software IT guy for 20 years and really love doing hands on mechanical work. I even entertained the idea of becoming a marine surveyor like Paul and took a course in 'Marine Investigations' using the David Pascoe text. Between my boat partner and I we have most, if not all the necessary tools to attempt this. My instincts tell me to do it so that I can learn more about the A4 and at least become more acquainted with the A4 carburator unit (no complete re-builds yet, once I do that I hang up a shingle saying 'A4 mechanic' for sale).

Paul,
Gee. That is exactly the kind of highly technical advice I was looking for that does not often come up on the MM website by someone who has been there and done that: "whack the side of the bowl (between gently and semi-gently... those are technical terms) with the handle end of a decent sized screwdriver." I will try this first. Question: should I whack the carb with a decent sized screw driver while the engine is running? I would think so but I had to ask.
Thanks very much indeed for responding to my questions. I will adjust my next moves accordingly.
Muchos gracias a todos.
CalebD
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Old 12-03-2008
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Clean the carb

CalebD,
- You could likely battle various symptoms for a long time, but I think it best to remove and clean the carb so that you know it is OK. A carb that hasn't been cleaned in years will have a layer of varnish deposits on all fuel surfaces. In cold climates there may be significant varnish/sludge buildup. This varnish will slowly diminish performance over the years by reducing the size of the carbs jets, nozzles and passageways. The Atomic 4 carb is an easy style to haul apart for cleaning. Most shops will be glad do this for a fee, but after you've done it once yourself you'll forever be outraged at what they charge. There are many things that can affect a carburetor's performance, but usually it is a combination of several minor problems. I always start with a complete disassembly and cleaning so that I know I am starting with a good carb.
- There are two types of spray carb cleaner. One is for spraying into a warm running engine and the other is for cleaning disassembled carb parts. The first type has very limited benefits. The second type is great for breaking down crud and varnish.
- When you sprayed into the dirty intake screen (backfire suppressor) it may have sent a lot of crud into the carb. There is a small passage in front of the choke butterfly that leads to the float bowl. It is used to equalize the air pressure inside the float bowl with the air pressure at the carb intake. This keeps a vacuum from forming in the float bowl. By spraying carb cleaner into the intake it is possible to force dirt through the passageway and into the float bowl, but highly unlikely that it could cause the float to stick open.
- Spraying into a dirty intake screen could cause crud to pool in the bottom of the carb. In the bottom of the carb body there is a scavenge tube that sucks any overflow of gas up and into the intake manifold. If your engine had been slightly overflowing gas for some time it would have been sucked up and burned in the combustion chambers. This is one possible cause for the soot as it would be quite rich. Dirt may now be plugging the scavenge tube and not allowing it to suck up the extra fuel which could sit in the bottom of the carb body. When running it will try to draw it in with the air flow but after shutting off the engine any remaining fuel could leak out.
- The fuel/air mixture is delivered by jets & nozzles. Fuel flows through different jets and nozzles depending on engine speed and loads. However, if one jet becomes blocked the vacuum will draw fuel via another jet. If the small idle jet is blocked the engine will draw the extra fuel it needs via the larger main jet. This will soot things up as it will be a rich mixture. Drawing idle speed fuel through the main jet could also cause slight amounts of fuel to pool in the carb body.
- Fuel can leak past the float valve and overflow the float bowl. Usually this is caused by a bit of dirt holding the valve open. This valve seat and valve may have a coating of varnish deposited over the years by aging gas. If a bit of this varnish flakes off it will leave a small gap that gas can seep past. Once disassembled, the float valve the seat is best cleaned with a small pointed wooden pin. Spin it between thumb and forefinger while pushing it into the float valve seat. This will burnish the seat smooth and wear away the fine layer of varnish.
- Dirt can enter the float valve from any point in the fuel delivery system. Your Racor water separating fuel filter is an excellent choice between the tank and pump. Also install a small polishing filter between the pump and carb. My polishing filter is a small metal disposable in-line automotive type. When reinstalling a cleaned carb always replace the polishing filter and the short fuel hose between it and the carb.
- The static adjustment of the idle mixture is one full turn out from being seated. After the engine has warmed up slow it down to idle and slowly turn the mixyture screw counter-clockwise until the revs start to drop. Then turn it back in until the revs peak.
- Improper ignition timing can also cause the engine to soot up and lose power. A distributor overhaul is relatively straight forward but at a minimum you should check that the timing is correctly set. Warm the engine up and, with good dock lines, place it in gear at high revs. Loosen the distributor and rotate it slightly back and forth looking for the highest revs possible.
Good luck with your Atomic 4. Hopefully after a few tweaks she'll give you another six trouble free years.
Cheers,
Dana, S/V Kiskadee
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Old 12-03-2008
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Thanks Kiskadee and Paul.
I have not idea when our carb. was last cleaned so I think it is time to bite the bullet and just do it. Besides, if I really butter finger something I can always ship/take it to someone who can 'theoretically' fix it for pizzles of money. It is the theoretical part that makes me want to do it myself so I know it is working properly even if I have to struggle a bit the first time. Between this forum on the one over at MMI I can get a wealth of information from folks like you who have been there and done that (in some cases many times before).
I have an uncle who was a fly-boy in WW II who taught me that the pilots of that era used to use ether a lot to help minimizing the varnishing of their carburators as they really depended on their engines. For this reason I keep a can of starting fluid on our boat which I rarely have to use (spring or really cold startup). I also spray this into the flame arrestor/intake which could also be complicating matters.
In short, you have all convinced me that it is time to take the carburator off and clean the flame arrestor and entire carb and clean/service/re-assemble as this is the time to do it.
It seems as though I also need to check the exhaust system for blockages and back-pressure which can also cause sooty plugs and poor performance. My gut feeling is that our engine works quite well when the hull and prop are clean and the plugs are fresh. It is just all these old systems that are supposed to work together to keep the engine healthy that start to worry me a bit when I think about going on a longer distance cruise.
After I remove and fix the carb. I need to put in a new centrifugal bilge pump as we used to have a diaphragm pump that crapped out. I tried cleaning and applying the service kit for one of these Jabsco pumps (cost more than a Rule pump) with no success to report. That was discouraging but I really think I can do the carb. clean/service/rebuild thanks to your encouragement. Just because you have not done something before does not mean that you cannot do it - it just means that you have not tried.
Thanks again for all your input and I will be typing with black crud under my fingernails soon!
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  #6  
Old 12-05-2008
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OK. So after getting a late start on this project in a light rain I managed to get the carburator disconnected from the engine despite the Moyer Marine Manual. The MMI A4 manual tells you how to dis-assemble the carburator but not how to disconnect it from the engine so I:
- disconnected the choke adjusting wire from the lever
- disconnected the throttle adjusting wire from it's lever
- disconnected the fuel input from the fuel pump, AND
- removed the two bolts that hold the whole assembly in place and it came apart quite easily (no gasket at this intersection!).
I wrapped the whole assembly in an oil pad and put it in a 5 gal. bucket and took it to my friends garage to follow the dis-assembly instructions in the MMI A4 manual.
I followed the instructions and managed to get the upper and lower sections separated but not without killing the old gasket. There was some varnish in the carb and some build up in a few low spots but it seemed pretty clean to me after spraying that carb-cleaner through all the jets, passages and cleaning as many surfaces as I could find. I was looking for a blockage of some kind but could not find anything obvious.
The only section of the dis-assembly I had trouble with was removing the choke butterfly valve and shaft. The screws for this are so up the throat of the carb that it seemed rather difficult to undo the screws and replace them. I did not have this problem with the throttle butterfly valve as the access is quite good. I decided to leave the choke assembly alone as it was not the likely source (I thought) for a gas leak.
The flame arrestor did seem to have a black line at the bottom as if much of the 'varnishy' gas had been leaking out through it. The 'varnish' I did find was of a dark gray color which matched the stain on the flame arrestor bottom area. I can now say that I have cleaned my carburator but I can't say that I have successfully found the source of my gas leak. I will have to next inspect the fuel pump as long as the carb is still off the engine waiting for a new gasket before being put back on. I could still apply the MMI service kit and replace the jets/screws/nuts etc. but they all seemed to be in pretty good shape.
My hands do smell like gas a bit, even after a couple of washings but I was glad that I did remove and clean the carb. Thanks to all who have been so encouraging and supportive. I appreciate all you have said.
(pictures to follow).
CalebD
Attached Thumbnails
A4 Carburator gas leak-engine-prior-remove-carb.jpg   A4 Carburator gas leak-carb-pre-removal.jpg   A4 Carburator gas leak-carb-throttle-clutch-disengaged.jpg   A4 Carburator gas leak-carb-bench.jpg  
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Last edited by CalebD; 12-05-2008 at 12:49 AM. Reason: adding photos
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