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Discussion Starter #1
Running into an issue with the late model Atomic 4 in our Pearson 30 and looking for some suggestions....

The engine starts reliably and without hesitation and runs smoothly. After what I believe is a satisfactory warm-up period, the engine will sometimes stall when shifting into gear. It will stutter very briefly before stalling and I can sometimes ‘catch’ it before it stalls by playing the throttle a bit. Once she’s in gear, she runs fine though I recently had a stall coming into the work dock at our club when backing down to neutral and shifting into reverse.

What I’ve checked thus far… I checked my plugs (new this season) for fouling and found only light soot. Since I use the engine pretty infrequently and it tends to run cool, I switched over Autolite 437s since they run a little hotter. I adjusted the fuel mixture screw a quarter turn clockwise (i.e. a little richer) – not much difference, but an additional quarter turn caused it to run a little rough so I backed it to its original position. I adjusted the idle a bit higher this morning, but am not convinced this is the underlying cause. Sitting on the mooring and backing down/moving forward on the pennant didn’t result in a stall, but I’m looking for some additional insight on this.

Could this be as simple as an idle adjustment? Thanks in advance for any thoughts or comments. (BTW, I'm cross posting this over at Moyer Marine's forums as well.)

-Chris
 

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the pointy end is the bow
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seems like it's a fuel mixture problem to me, probably on the lean side. Automotive carbs have a little diaphragm pump on them that squirts extra fuel in the carb when you step on the gas with a load. I don't know if your carb has that. If it were me, I would start by choking the motor a bit before putting it in gear to see if it makes a difference. If it helps, I would adjust the mixture a bit more rich.
 

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erps, A4's don't have an extra jet like that. but, you're probably right about the fuel issue.

Chris, you're likely running short of fuel. I'd replace the fuel filter(s) and make sure to drain the carb. If there is a little water in the carb, you can run into issues when the engine needs fuel.

Bad fuel can also cause the motor to feel like it's starved. For example, this season I had a spate of bad fuel and the motor would run fine choked, but then sputter once in a while. I pumped a quart of fuel out that looked varnished and old, no problems since.

I would pump a little fuel out of the pump into a clear container. Have a look and see if it looks good. If it's dark or you get a bit of water, get the intake to the lowest point while heeling, and pump until you see good fuel.

If your tank has a top feed, with a pipe running to the bottom, you likely have a screen at the end of that. If convenient or nothing else works, I'd check that out.

A4s are pretty easy going and the only problems I've ever seen are basic mechanical or fuel related.
 

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Discussion Starter #4
seems like it's a fuel mixture problem to me, probably on the lean side. Automotive carbs have a little diaphragm pump on them that squirts extra fuel in the carb when you step on the gas with a load. I don't know if your carb has that. If it were me, I would start by choking the motor a bit before putting it in gear to see if it makes a difference. If it helps, I would adjust the mixture a bit more rich.
Good idea, Erps. I should point out that when it *does* stall, it will restart only if it is NOT choked (mind you, the engine is already warmed up and may not have anything to do the reason for the stall). Regardless, these things are best solved by a process of elimination, and this is a good next step.

Thanks.
 

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Discussion Starter #5
erps, A4's don't have an extra jet like that. but, you're probably right about the fuel issue.

Chris, you're likely running short of fuel. I'd replace the fuel filter(s) and make sure to drain the carb. If there is a little water in the carb, you can run into issues when the engine needs fuel.

Bad fuel can also cause the motor to feel like it's starved. For example, this season I had a spate of bad fuel and the motor would run fine choked, but then sputter once in a while. I pumped a quart of fuel out that looked varnished and old, no problems since.

I would pump a little fuel out of the pump into a clear container. Have a look and see if it looks good. If it's dark or you get a bit of water, get the intake to the lowest point while heeling, and pump until you see good fuel.

If your tank has a top feed, with a pipe running to the bottom, you likely have a screen at the end of that. If convenient or nothing else works, I'd check that out.

A4s are pretty easy going and the only problems I've ever seen are basic mechanical or fuel related.
Definitely worth a check. My Racor filter is only one season old and I probably put less than 12 hours on the engine. For spring start-up I checked the sediment bowl on the fuel pump (mechanical) -- nothing suspect there. On a cold start, she'll catch right away on full choke and smooths out to a purr after pushing the choke in.
 

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The point of operation requiring the most intense spark is right at off-idle. If a car is developing ignition problems the driver will always complain that it stalls on take off.

Your carburetor actually has three holes where fuel enters; idle; off-idle; and run. If the engine can rev up normally, then all three of them have proved that they can operate correctly and make the transition from one to another

1st I'd take a vacuum gauge, attach it to the engine and set the fuel adjusting screw for maximum vacuum and leave it there. That's the proper way to adjust idle mixture. Don't try to do it by ear.

Then I'd go after the ignition system. You already visited the spark plugs, but what about the points, distributor, and plug wires? Somehow, some way you are losing ignition energy just when you need it the very most.
 

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A sudden increase in load can create a cylinder 'stall" requiring a hotter than normal spark to generate ignition. While it may be points related, the utility fours I've worked on (and the A4s too) would stumble when either the points had a bit of corrosion, the rotor discharge tip was burnt, the cap contacts were burnt or the condenser was on it's way out. Start with the points, Hold them open with the ignition off and pass a short strip of emery cloth between the closed contacts, look at the rotor tab and buff the tip with the same emery cloth, finally look at the cap contacts inside the cap, if there's any sign of corrosion or a dark line buff them clean. Put the rotor back on, screw the cap down ad crank her up. If this doesn't solve the problem then it's either the condenser or the coil. Replace the condenser next (cheapest first).Fair wind and a clear course!
 

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First check for a leaking head gasket. An A4 will run quite well with one cylinder at low compression and at idle, ... until you apply throttle and load. The head on an A4 should probably be re-torqued every 400-500 hrs. of run time.

Secondly (and quite common), disassemble the carburetor and inspect/clean the emulsion tube ... the emulsion tube (a hollow tube that is approx 1/8" dia X 1.5" long and has several teeny 'holes' bored into the side of the tube) controls the air/fuel ratio for the engine at 'above idle speed' and if one of the tiny 'holes' in the side of the tube becomes clogged with debris will drastically change the fuel/air mix when the engine is 'above idle'. Just a wee bit/spec of crud in one of the 'holes' will result in drastic air/fuel ratio changes when you attempt to apply 'power'; the emulsion tube is NOT 'active' when the engine is at idle it only comes 'on' at above idle.

If you dont already have one, get an adjustable main jet for the carb.
 
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