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  #1   IP: 67.186.97.244
Old 11-11-2017, 02:19 AM
Sam Sam is offline
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Engine RPM varies

I was motoring my 1966 Morgan 34 with original great running well maintained A4 for about an hour along Chicago lake front toward winter lay up in about 3 ft chop. Ultimately I would need to join a 20 boat flotilla on the Chicago river and endure 27 bridge openings in a 3 mile city stretch. I was at about 3/4 throttle when engine rpm started to become variable. It would drop down to a low idle then rev back up to speed without any adjustment to throttle or choke. It was cycling like this about 10 to 15 times a minute. When I lowered the throttle to 1/2 or less it would cycle to stop but would restart fairly easily but continue the rpm cycling. During the next 30 minutes I limped into a nearby harbor and as I was approaching a mooring slip the engine smoothed out and ran fine. Docked and as much as I tried I could not repeat the problem - checked spark, plugs points/gap, rotor, cap, wires, electric fuel pump/filter, banged the carb bowl, etc. Ran the engine in gear while tied to the dock - all was well and decided to join the waiting flotilla. One hour and 4 bridges later the problem returned and I am now a navigational hazard for barges, water taxis, kayackers, tour boats etc. I docked and the boat sat for 4 days till next flotilla. To be safe I used the great Boat US for a river tow. 20 minutes prior to final docking I easily started the engine and let it run to facilitate an oil change/fogging once we docked.
- I have a gas filter between tank and facet/purlator cube fuel pump [surprised to find it is 20 seasons old @ about 20 hrs/season]
- 2 weeks prior we were sailing very rough water and later the fuel filter became unusally dirty requiring changing.
- engine is winterized and in the spring I'll change everything to be safe.

Is this a carb/main jet problem or float sticking or carb cleaning/rebuilding? Is it electrical/ignition imitating a fuel problem? [engine never "sputtered out"]? Any insight or direction would be appreciated. Thanks all, Sam
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  #2   IP: 98.165.208.30
Old 11-11-2017, 08:23 AM
zellerj zellerj is offline
 
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Sounds to me that you have dirt in the carb that is periodically blocking the main jet. Drain the carburetor bowl into a glass and see if you have a spec or two. Even filters can have corroded housings that become the source of carb issues. Sounds to me you need to clean out your fuel system - drain the tank and give it a good rinsing. Nothing beats a clean fuel system for reliable engine performance.
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Last edited by zellerj; 11-11-2017 at 08:26 AM.
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  #3   IP: 72.194.223.97
Old 11-11-2017, 10:15 AM
JOHN COOKSON JOHN COOKSON is offline
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You may be running on three cylinders most of the time with the 4th cylinder cutting in and out. Sticky valve, electrical or ?. What does the inside of the distributor cap and rotor look like?

TRUE GRIT
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  #4   IP: 172.222.238.142
Old 11-11-2017, 10:47 AM
Dave Neptune Dave Neptune is offline
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If your cycling was cyclic (evenly timed) I would look to the fuel pump and or clogged filter. The old pump you have may be getting weak.

If the cycling was intermittent then I would consider inside the carb.

I am assuming the engine is running smoothly, is it when the rpm's drop? Electrical issues are usually off/on or the engine misfiring causing a rough running motor.

A simple fuel pressure gage will answer these questions.

Dave Neptune
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  #5   IP: 216.80.24.55
Old 11-11-2017, 11:59 AM
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Antibes Antibes is offline
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I second the carb cleaning, the filters only do so much and the carbs are very sensitive. I had a similar problem last year on my last trip.
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  #6   IP: 24.152.132.65
Old 11-11-2017, 12:09 PM
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ndutton ndutton is online now
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Yes, it sounds like the carb needs cleaning (filter replacement too) but if you don't solve the source of the contamination you're destined for a repeat episode. Even cleaning/replacing the tank isn't enough. Figure out how the contamination entered the tank in the first place so you can remedy the root cause.
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  #7   IP: 67.186.97.244
Old 11-15-2017, 05:05 PM
Sam Sam is offline
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First I would like to thank all for taking the time to reply. It has been very helpful in developing a spring game plan. My greatly varying rpm was somewhat cyclical and constant but in a narrow range, 10 - 15 times/min but running smoothly, no cough/sputtering etc. Like I said in the post the a few of times during the layup trip the A4 ran fine. Based on the forum input and other web stories I am going to concentrate on fuel delivery. I am going to check with the yard and see if they have a service to clean out the tank [unfortunately its almost full at about 18 gal] - its the original "monel" alloy used by Morgan [50+ yrs old, tank is on top of the engine under the cockpit]. [About 20 yrs ago I put a new copper pick up tube in and shorten it by 1 1/2 inches off the bottom]. I'll replace filter from tank to pump, install a new electric fuel pump [2-3psi], add another polishing filter from pump to carb, and completely clean the carb. I will also by a small pressure gauge to test periodicaly test fuel pressure. I never had a problem like this before in 40 + yrs of sailing this boat - please let me know if you think there is more I should do to solve this intermittent problem.
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  #8   IP: 71.118.13.238
Old 11-15-2017, 05:49 PM
Dave Neptune Dave Neptune is offline
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Sam, that would be a great start. You can just leave the FP gage inline between the polishing filter and the carb. Then all you have to do is look.

Also you could suck a bit out of the bottom of the tank through the pick up tube port. That way you can get to what the "tube" can't.

Dave Neptune
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