Uneven RPM

[Dave O]

Late this season I noticed that I am getting uneven RPM when in neutral at the dock.

After the engine warms up and still in neutral, engine speed periodically increases or decreases apparently "at will". The symptoms are exactly as if someone was adjusting the throttle up and down. The engine does not die ... it runs smoothly without hesitation. No roughness. Just as if an invisible hand was moving the throttle lever up and down somewhat. It is periodic.

I had the same problem last year. During the winter I did a complete tune up including adding a new carb. The problem seemed to be solved, but it has reappeared during the last 2-3 weeks or so.

I do not notice this when the boat is underway ... but that may simply be because engine speed it typically higher and I can not evaluate RPM as easily.

Today I took a look at the throttle connection at the carb and did not see any movement that might explain the situation.

I do not have a tach.

Does anyone have any suggestions?

Thank you

Dave
C&C29Mark I
Port Credit, ON

[Don Moyer]

Dave,

Assuming the problem occurs mostly at idle, this scenario is usually caused by the idle RPM being set by the end of the cable travel and not by the set screw on the carburetor throttle arm. You can check this by looking closely at the end of the idle set screw to be sure it seats firmly against the stop pin.

Don

[Dave O]

Thank you Don. I took the boat out for another run today and unfortunately the problem has worsened.

At idle in neutral at the dock I am getting the uneven RPM described earlier. I checked the idle screw and it seems to be positioned "OK".

However, now underway, I am experiencing the same thing and this is the first time I have experienced that.

I don't have a tach but:

1. Engine is fully warmed up
2. Running at about 2.5 - 3 knots consistent no wind
3. No change on my part to the throttle.
4 Am experiencing the same "up and down" engine speed I described earlier. Seemingly at will RPM will decrease, then increase (back to the original setting) and then decrease and increase with no change in the throttle position. It as become a bit of a cycle. During all this the engine is smooth.

Can anyone suggest further things to look at? The carb is new in the spring and I imagine that any blockage or dirt would cause roughness? Any restriction in the fuel supply would do the same?

Is this most likely a linkage issue? Or perhaps the mech. fuel pump is starting to fail and is thus delivering inconsistent fuel flow?

Thank you

Dave
C&C 29 Mark I
Port Credit, ON.

[Don Moyer]

Dave,

These RPM excursion episodes are the most difficult to diagnose of any
problem I can think of - thankfully, they don't happen very often.

If the throttle cable is not the problem (you could station someone at the
engine to actually observe the throttle arm on the carburetor to see if it
is moving), here are a couple other suggestions:

1) Vacuum leak. Vacuum leaks will sometimes come and go in tubing joints
as with a loose fitting on a scavenge tube, a defective PCV valve on an
Indigo Electronics crankcase venting kit, etc.

2) Also in the category of a long shot, a small flat piece of scale just
ahead of the main jet could sequentially block fuel as it is moving from the
main passage through the main jet. If you have access to the carburetor you
can remove the main passage plug and drain the main passage into a clean
glass jar (this is the lowest point in the carburetor) to check for dirt in
the fuel. While the plug is removed, it's also a good idea to flush a bit
of fuel through the carburetor by operating the fuel pump a few seconds.

3) Also, as a generic suggestion, always think back to the time a problem
began and try to figure out if anything else changed in the engine's life;
like some kind of maintenance action.

Don

[tmcdonagh]

RPM excusions

I am experiencing this same problem. Any new insight on the problem?

Thanks, Tom

Any

Dave,

These RPM excursion episodes are the most difficult to diagnose of any
problem I can think of - thankfully, they don't happen very often.

If the throttle cable is not the problem (you could station someone at the
engine to actually observe the throttle arm on the carburetor to see if it
is moving), here are a couple other suggestions:

1) Vacuum leak. Vacuum leaks will sometimes come and go in tubing joints
as with a lose fitting on a scavenge tube, a defective PCV valve on an
Indigo Electronics crankcase venting kit, etc.

2) Also in the category of a long shot, a small flat piece of scale just
ahead of the main jet could sequentially block fuel as it is moving from the
main passage through the main jet. If you have access to the carburetor you
can remove the main passage plug and drain the main passage into a clean
glass jar (this is the lowest point in the carburetor) to check for dirt in
the fuel. While the plug is removed, it's also a good idea to flush a bit
of fuel through the carburetor by operating the fuel pump a few seconds.

3) Also, as a generic suggestion, always think back to the time a problem
began and try to figure out if anything else changed in the engine's life;
like some kind of maintenance action.

Don

[tenders]

I’ve had mild symptoms that might fall into this category that were caused by occasional sticking valves. The solution was to put a few shots of Marvel Mystery Oil into the fuel, in the dosage recommended on the bottle (1 ounce every 10 gallons, I think, but don’t quote me on that).

I do this for all the fuel that goes into the boat’s tank, and the valves do not stick any more.

[pknier]

Do not count out a developing "water in the fuel" problem...from my experience with our motor...at high speed it would change slightly in RPMs and sound slightly different...Did not make much of it until it stopped idling at low RPM and then hard starting. During the troubleshooting process, found water in the sediment bowl and then in the separating filter...cleaned and replaced and we are fine...back to excellent low RPM idle - no variation in the high-speed RPMs and no variation in sound

[sdemore]

Two cents on my recent experience. I was seeing a similar thing, more in cruise than at idle and more the harder I pushed it. Turned out that the reversing gear needed to be adjusted. It was never really hitting a good neutral position and the clutch was slipping intermittently. Readjusting the reversing took care of it on mine.

[tmcdonagh]

RPM excusions

I routinely experience RPM excusions. This weekend I noted the amperage meter reflected the RPM excursion. When the RPM dropped, the AMP meeter went high, when it returned to "normal", the AMP meter needle returned to the zero position. My chartplotter (B&G) can display voltage and I noted high voltage (15.2-15.8) when RPM dropped and 13.2 volts when RMP dropped. I have always had poor battery charging with my boat. I am on the 2nd house battery after 5 years. Perhaps my Alternater and/or voltage regulator is bad? The alternator appears to be stock/original and there are no markings or model numbers that I can see. Any suggestions?

[roadnsky]

When the RPM dropped, the AMP meeter went high, when it returned to "normal", the AMP meter needle returned to the zero position. My chartplotter (B&G) can display voltage and I noted high voltage (15.2-15.8) when RPM dropped and 13.2 volts when RMP dropped. I have always had poor battery charging with my boat. I am on the 2nd house battery after 5 years.

•How long do these episodes last?
•What kind of batteries?
15+ volts is a likely reason for your toasted batteries.

Perhaps my Alternater and/or voltage regulator is bad?

Very likely.
Our resident DC experts will be along soon and can suggest some testing for you to perform.

[tmcdonagh]

Thanks for the reply Roadnsky.

High voltage periodicity is anywhere from 10 to 60 seconds followed by random 10-60 low voltage. I have standard flooded Lead acid batteries installed.

•How long do these episodes last?
•What kind of batteries?
15+ volts is a likely reason for your toasted batteries.


Very likely.
Our resident DC experts will be along soon and can suggest some testing for you to perform.

Pics of the unlabelled alterator attached.

Thanks,

Tom

[roadnsky]

That's an original stock MOTOROLA 35A alternator.
I think, given the short durations, that you have a regulator issue.
I can give you a few suggestions...

BUT, let's wait for the guys who know more about this to help us along.

Paging Dr's Neil and Joe...


EDIT - Could you post a pic showing the area of that alt where the wiring is connected please.
The opposite side of where the pulley is. Also we're looking for a tombstone looking casing

[ndutton]

BUT, let's wait for the guys who know more about this to help us along.
Paging Dr's Neil and Joe...

Nah, you don't need me. Roadnsky is all over this. The regulator is done, 15+ volts is murder on your batteries and heaven forbid you have electronic ignition. Instead of replacing the regulator on an old, low capacity alternator it might be an excellent time to improve your charging system. If you agree, be sure the new alternator is ignition proof.

[sastanley]

Nah, you don't need me. Roadnsky is all over this. The regulator is done, 15+ volts is murder on your batteries and heaven forbid you have electronic ignition. Instead of replacing the regulator on an old, low capacity alternator it might be an excellent time to improve your charging system. If you agree, be sure the new alternator is ignition proof.

+1 what Neil & Jerry said. If you have had issues in the past, now is the time to come up with 2.5 good-old-boat bucks ($250). (New boats a boat buck is $1000)!

Jerry, that sideways pic is good enough to tell me with 95% certainty that is an OEM regulator attached to the outside. I am not sure how it has survived this long and still be serviceable at all!!

Common options are replacement of the entire thing with a proven unit like what Moyer sells (55A), or if you are intent on keeping the 35A Motorola, take it to a local shop for a rebuild..they will test the alt. and then can probably get you a regulator, which I agree is most likely suspect. The stock regulator that bolts on the outside of the alternator is 13.8V output..adequate for its time, but a little low by today's standards of 14.2-14.4v for lead acid batteries, along with the low by today's standards 35A output. I suspect that cost to be over $100, but probably less than $200 for a rebuild.
Below is a link to Moyer's offerings, which is my recommendation also. The 55A is about $230 and should solve all of your problems, unless you have extreme battery loads (I do find it very interesting you have B&G instruments on your T-30!! ). I would heartily consider new over rebuilding the old one..but that is your call. Taking yours to a rebuild shop for an estimate may help you decide. If you don't have a reliable shop around and want to try to fix the existing, we can discuss further about finding some regulators on the internet for your 35A, but then we are in experimental land.

https://moyermarine.com/product-category/electrical/

[CajunSpike]

Hoping this is useful...

i used to have a shrimp boat with a diesel engine.
I found and installed a NAPA single connection alternator to power all the electrical winches and accessories.

https://www.napaonline.com/en/p/RPS7152PS

Besides the mount, there was only 1 wire connection required, the hot to the battery.

Would this be appropriate(assuming the mounting works) to put on an A4?