Engine Hesitating after 2 hours of Running

[Amphibiographer]

Hi all!. Our A4 is hesitating, stuttering, after about 1 1/2 to 2 hours of running! Starts fine, runs great at idle, and at high revs under load, then starts hesitating after awhile.

It started out doing it when putting into gear after warmup. We replaced connection on igntion wire to the coil and it seemd to help, but then it started hesitating after running for a couple hours.

We worked on the carburetor then, taking out main passage plug and spraying carb cleaner in it. We cleaned oil and dirt off the air filter screen and inside the intake throat. While running we adjusted idle screw and the engine purred like a sewing a machine! We ran for about an hour, turned it off and floated around looking and listening for humpback whales, then started up and headed home with no issues until another 45 minutes of running when the motor started hesitating again! Kept running and we made it home.

Anyone have ideas about what this could be?

We're running on raw water cooling so engine runs very cool. We recently changed fuel filters, including the in-line after the fuel pump. And the carburetor is only a few years old. New Alternator and disributor cap, rotor and points. Could this be a dirt in fuel tank issue (but we have a 1 micron in-line fuel filter), or an electrical?

Thanks, Terry L. Brown

[Al Schober]

Two thoughts..
Fuel tank vent might be clogged (spider nests). See if runs better with the fill cap open. I also recommend a fuel pressure gauge at the carb inlet.
Coil. Could be getting hot and trying to fail.

[Amphibiographer]

Thank You Al! We did try unscrewing the filler cap and it didn't make a difference. Have not heard of a fuel pressure gauge, how does that work? Do we need a new coil then if this one is overheating?

[Peter]

New coil - maybe you should do a little trouble shooting before buying parts? It will save you some $ potentially but more importantly, give you the confidence that the parts you have replaced actually were defective and that you have solved the problem.

The engine needs fuel and spark to run.

Immediately after the next shutdown, check for spark.

You could do this directly by pulling a plug and cranking the engine with the ignition on and the plug grounded to see if there is a spark.

If there is no spark at the plugs, pull the lead off the centre of the distributor and hold it close to the head while cranking with ignition on to see if you can draw a spark.

CAUTION - do not leave the ignition on for prolonged periods during these trouble shooting procedures as that can damage the coil. Also, if you have a water lift muffler you will want to shut the water intake thru hull to avoid possibly flooding the engine.

Indirectly - give it a shot of starting fluid. If it shows signs of life, it is a fuel delivery issue. No sign of life, a spark issue.

Fuel pressure gauge - installed in fuel line between fuel pump and carburetor. They are inexpensive (be sure you get one that is metal and not plastic) and easy to install. See the incredibly elegant photos of an install in this post

https://www.moyermarineforum.com/forums/showpost.php?p=122287&postcount=3

HTH,

Peter

[JOHN COOKSON]

Do you have an EI or points?

ex TRUE GRIT

[roadnsky]

Fuel pressure gauge - installed in fuel line between fuel pump and carburetor. They are inexpensive (be sure you get one that is metal and not plastic) and easy to install.

Thanks Peter!
Here is a close up view.

[Amphibiographer]

New coil - maybe you should do a little trouble shooting before buying parts? It will save you some $ potentially but more importantly, give you the confidence that the parts you have replaced actually were defective and that you have solved the problem.

The engine needs fuel and spark to run.

Immediately after the next shutdown, check for spark.

You could do this directly by pulling a plug and cranking the engine with the ignition on and the plug grounded to see if there is a spark.

If there is no spark at the plugs, pull the lead off the centre of the distributor and hold it close to the head while cranking with ignition on to see if you can draw a spark.

CAUTION - do not leave the ignition on for prolonged periods during these trouble shooting procedures as that can damage the coil. Also, if you have a water lift muffler you will want to shut the water intake thru hull to avoid possibly flooding the engine.

Indirectly - give it a shot of starting fluid. If it shows signs of life, it is a fuel delivery issue. No sign of life, a spark issue.

Fuel pressure gauge - installed in fuel line between fuel pump and carburetor. They are inexpensive (be sure you get one that is metal and not plastic) and easy to install. See the incredibly elegant photos of an install in this post

https://www.moyermarineforum.com/forums/showpost.php?p=122287&postcount=3

HTH,

Peter

Thanks Peter for your suggestions and the photo of the pressure gauge!!! There is spark and fuel getting through for an hour and half or more so what would cause it to start hesitating after that? I should mention also that while running the engine I got a shock from the distributor while turning it to set timing, and from the rubber spark plug cap (#4 rear one) when I pushed it down to make sure it was tight!

[Amphibiographer]

Do you have an EI or points?

ex TRUE GRIT

Points which we recently replaced. And rotor as well. New distributor cap last year. We timed the engine ourselves without a timing light, looking into the spark plug chamber for TDC then turning distributor to get a good rpm. Could a not quite perfectly timed engine cause the hesitation problem? Perhaps overheating the coil?

[Amphibiographer]

I should mention also that while running the engine I got a shock from the distributor while turning it to set timing, and from the rubber spark plug cap (#4 rear one) when I pushed it down to make sure it was tight!

[JOHN COOKSON]

Take voltage reading at coil + when you first start the engine and another reading just before a expected shutdown - maybe after an hour and half or if possible when the engine starts hesitating.
The alternator output may be dropping off.
BTW is the alternator drive belt tension correct?

ex TRUE GRIT

[ndutton]

Nothing is off the table yet. The timing of the symptom does not fit the normal overheated coil model but that's no guaranty either way. The fuel pressure gauge recommended by others is an excellent diagnostic tool in this case but in the meantime I'd really like to know how Al's suggestion plays out.

See if runs better with the fill cap open.

[Amphibiographer]

Thanks to everyone for your help with this!!

We finally found out what was wrong when we had a meltdown (with LOTS of smoke!) of an ignition wire which was being rubbed by the throttle control!! Replacing that solved the intermittent hesitation problem we'd been having for months!! Seems that the wire was being shorted out at times.

No hesitation now and the engine runs beautifully.