Engine RPM varies

[Sam]

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

[zellerj]

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.

[JOHN COOKSON]

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

[Dave Neptune]

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

[Antibes]

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.

[ndutton]

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.

[Sam]

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.

[Dave Neptune]

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

[Sam]

Dave, thanks for your past comments. I'm planning my spring commissioning and am going to install in line a fuel pressure gauge. A lot of different brands are out there in the 0 - 15psi range with reviews all over the map - one brand's lens would even fog up if a drop of gas accidently got on it?? Can you give me some input as to what brand/model you use or recommend. As I am finish writing this I am thinking Moyer may even have one? Let me know. Thanks

[tenders]

https://www.amazon.com/gp/product/B000BPZ4SW/

I don’t believe there is an inexpensive 0-5 psi fuel pressure gauge on the market. What you’re looking for on the gauge is steady pressure at about 2-4 psi and the 0-15 range is quite adequate for this.

This is a useful and inexpensive device regardless of what the current problem is so I’d suggest installing this first between the pump and the carb. It will probably confirm the pump is working, so you can go ahead and clean out the carb. If it isn’t either of those things or a clogged filter it could be a failing coil or sticking valves. If the coil is 20 years old I’d replace it as a matter of course. If the problem persists after all this I’d pour two cups of MMO in the gas and see if that frees up the valves.

[JOHN COOKSON]

I went with a JEGS brand 0-15 PSI fuel pressure gauge because they have stainless steel cases and glass lenses. You can do the research for yourself. It's all online. Another thing to decide is do you want a liquid filed gauge or not so include that in your research.

TRUE GRIT

[zellerj]

If the issue is a piece of dirt in the carb blocking the main jet, will a fuel pressure gauge indicate an issue? My question is: Will a fuel pressure gauge detect an issue downstream from the gauge, or only detect issues up stream (towards the tank) of the gauge?

[JOHN COOKSON]

The fuel pump will supply gasoline to the carburetor at the pump's rated pressure. If it doesn't do this you need a new (electronic) fuel pump -assuming the rest of the rest of the fuel system is in good condition and there is free flow of fuel to the pump.

TRUE GRIT

[Sam]

It is late spring here in Chicago and I would like to update all after my winter layup as a teaching moment to newer readers. Following the advise received I first drained my tank by jury rigging the old electric fuel pump, hose and filter directly through the cockpit fill inlet. I was shocked by the black gunk particles off the bottom captured in the filter and the last two gallons were a brown liquid something other than gas. [I used a glass Specter "see through" 40 micron filter and clogged two of them. The black gunk later dried to a brown powder - any ideas]. I re added some filtered gas back into the tank a couple of time and repeated the process, then finally added new fuel. I installed a new Facet cube pump [1 1/2 - 4psi, same as old which served me well for 20 seasons], new A-1 low perm fuel hoses, installed a "Marshal " liquid filled pressure gauge [very cool] and gave the carb a through cleaning including the jets. Here is where I easily started the engine w/ether spray and it ran fine at the yard dock for over an hour [3 psi on the gauge]. Since my fall rpm problem was intermittent I checked the coil resistance @ 4.6 Ohms which seemed high so I changed the coil with a new one @ 4 Ohms. Couldn't remember exactly how old the cap, rotor, condenser and wires were so they were changed as well. Just completed a 6 hour run to the harbor, half through some medium swells, and my virgin 52 yr old early model A4 ran flawlessly [steady temp @ 140 F w/Dole setup, oil pressure 35 psi]. A great little engine and I would like to thank all again forum responders for your help and input. I used it all.

[Mo]

Dirt or water.

"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."

Sure sounds like water in fuel to me...or as mentioned, dirt...water in fuel, depending on how much, can cause low rpm under load, sporadic poorly running engine, stalling and it restarts, run good for a bit and then start symptoms again. This is more frequent in choppy conditions as well as the water mixes with good gas within the tank and is usually why it will keep running. I had that occur about 8 years ago...filled up with two Jerry Cans and there was water from the fuel station. What you describe above is what I put up with for about 2 hrs one day on an excursion.

If you can find fuel that doesn't have ethanol in it that's what you go for. So far here in Nova Scotia there is no ethanol in high test fuel / high octane fuel / or super (whatever we want to call it). That's what I use in everything from the gas cars, lawnmower, snow thrower, boat etc...no issues.

So, ensure you buy the gas with the least ethanol; ensure a good seal around the fuel fill and change filters frequently.