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Honda BF40a intermittent rough idle.

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IslandKev

New member
Hello everyone,
I am new to this site and this is my first post so If I’m doing anything wrong please feel free to straighten me out. :)

I have twin 2003 Honda BF40a outboards on my 22’ C-Dory. I have owned the boat for 18 months and put around 150 hours on the motors during this time. As you can see by the attached videos, my starboard engine has developed an intermittent issue that is causing it to run/idle rough. The first time I experienced this problem I had been running the boat all day trouble free. I had shut down both motors for a few minutes and the problem began when I fired the starboard engine up again. This was when I immediately noticed it didn’t sound right. Not wanting to cause further damage I returned home on the other engine. At home I replaced all 3 spark plugs (purchased from Honda NKG DR7EA) and started the engine. The problem was resolved. Next trip out, I had 10 hours of trouble free operation. Again I shut down the motors for maybe 30-60 minutes this time and when I restarted the starboard engine it immediately sounded terrible again. I changed the plugs while at anchor but this time it did not solve the problem. The following morning I returned home on one engine again. During the 5 hour slow cruise home I started the starboard engine 3 times and still had the same issue so I shut it off. Once I returned home, I started the engine again in a large horse trough as this is how I flush my engines after running in salt water. To my surprise, the starboard engine ran normally and idled for 15 minutes in the bucket without any issues. It sounded normal. To my untrained ear I believe I may be only running on 2 of 3 cylinders intermittently but don’t know exactly what that would sound like which is why I have attached the 2 videos.

A few things to consider that may or may not help diagnose the problem:

Hours of engines are unknown. All of the following has been done when I purchased the boat prior to putting about 150 hours on it.

carbs professionally cleaned as well as the fuel pumps and lines Under the cowling.

New fuel filter

Fuel tanks (portable tanks) and fuel lines are new.

New fuel/water separators as well as the housings.

New spark plugs

Impeller checked and strong pre stream.


After the first time I had this issue I changed lower unit oil and engine oil and filters. I always purchase everything directly from Honda.


Any thoughts on this intermittent issue would be greatly appreciated. Currently, I have no problem but I expect it will return again and that makes me nervous to go too far.

The boat never sits unused for long. I have run it at least monthly since purchasing it. If I don’t use it for a couple weeks. I add stabilizer to the fuel and run the engines in their trough once monthly for 12-15min to help prevent ethanol issues. I avoid ethanol fuel whenever I can and always run premium. Here in Canada it is getting harder and harder to find ethanol free gas but I try. I also always buy my fuel on land when the boat is on the trailer to avoid water issues in my fuel.

Thank you in advance to anyone who takes the time to assist me. Please let me know if I should provide any additional information.
 

Attachments

  • MOTOR VIDEO 2.mov
    886.2 KB
  • MOTOR IN TANK.mov
    924.7 KB
Hi Kev,

Welcome to the forum.

After watching your videos, I agree with you that it may be running on only 2 cylinders.

You can prove that and, more importantly, determine which one isn't powered by doing a cylinder contribution test.

Here's how:

If you haven't smeared a thin coat of dielectric (tune-up) grease inside each spark plug boot, you should do that now.
The grease seals the boot from moisture and makes it easier to pull the boot off and push it back on for the test.

When the issue is occurring, engine running, you pull each plug wire off one at a time noting any change in cadence.
Push the wire back on the spark plug before pulling the next one off.

The cylinder that makes little or no difference is the weak/dead cylinder.

Actually, when the wire for a good cylinder is pulled, the engine will likely die as it probably won't run on just one cylinder and you'll need to reconnect the wire and re-start the engine to continue.

Special "plug boot" pliers are available for doing this but you can also use DRY, thick gloves or even a DRY rag or towel to pull the wires to prevent being shocked.

WARNING! If you use any medical device such as a pacemaker or have a heart condition DO NOT do this test. While an accidental shock from an ignition coil doesn't harm most people in good health, it's not worth risking for people with heart conditions.

If you can identify the misfiring cylinder, you can then begin investigating why it doesn't fire.

From what you've said that you've aready done, I would suspect an ignition problem due to the intermittent nature of the symptom and how you say new plugs helped at first. An inexpensive spark tester might be a good tool for verifying spark but you should also inspect the plug wires and the ignition coil connectors and terminals for cleanliness and tight fit.

You may want to ohm test each coil primary winding looking for a large difference among the three. Sorry, I don't have the shop manual to provide the specifications.
 
jgmo,

Thank you for your prompt and thorough reply. This makes perfect sense to me. I will try to get out on the water next week and put some hours on the engine. Hopefully the problem resurfaces so I can perform the test you recommended.

In the meantime I will check, clean and grease connections as you recommended.

As soon as I learn any additional information I will be sure to update this forum right away.

Once I narrow it down, I will likely need some further guidance. I am not very mechanical however I do have a good understanding of electricity.

Thank you again,
 
Yes, an intermittent problem such as this is the hardest to diagnose. You can't find the problem if it's not acting naughty. So, your plan is a good one and I look forward to hearing back about what you find.

I know that you probably don't need to be told but I have to say it anyway for clarity and to ensure that we're on the same page. Don't use the dielectric grease on electrical connectors. Only on the inside of the spark plug boots! And use dielectric grease only...not marine or other lube grease.

Good luck.
 
Yes, intermittent problems can certainly be a challenge. Fingers crossed that I won’t be chasing this one for too long.

re: dielectric grease. I am aware that it is very different from regular marine grease that would be used to grease nipples, prop shafts, etc. I was however considering applying some to any exposed electrical connections to prevent/slow further corrosion. I had intended on checking connections, cleaning thoroughly, re-connecting, then applying some dielectric grease to the connection after it was tight. My theory here is that I would not be affecting the continuity of the connection but possibly helping future corrosion as it is my understanding that dialectic grease is not conductive. Any additional thoughts on this? If not dialectic grease do you recommend any other product to help minimize corrosion on connections or just leave them bare?

Thanks again in advance, I sure do appreciate your prompt reply’s.
 
Personally I would leave them bare. Grease melts and runs in heat if applied too thickly and will eventually attract dust and dirt when exposed.
I don't think there's any great harm in trying some as a protective covering. I know mechanics that do just that.

3m Corp, though, makes Scotchkote Electrical Coating that dries quickly after applying and I prefer that as a protective coat for connectors. It  is expensive though.
So, it's really a matter of preference and what works for you.
 
If you are in Canada, I use the red or green can of rust check. Or Fluid Film. Just a very light squirt on anything like say a battery post, electrical connections, any fasteners that may rust. Wipe off any drips. Rust Check red is non conductive, not sure on the other 2.

I use this on my vehicles on all those spring hose clips and bolt/screw heads everywhere, I live in salt country, water and highways.

With your screen name, around here you’d be from Prince Edward Island. But I’m guessing you’re likely left of here?
 
NSDON

Thank you for your input. I will look into the products you suggested.

Yes, you are correct that I am on the west coast. Vancouver Island.

MY CURRENT PLAN:

I have not yet had a chance to get on the water but I thought I would update the thread on my current status and plan moving forward.

I pulled my ignition coils off of the troubled engine and removed the spark plug boots. Checked the resistance of the primary and secondary windings and they were all within the specs shown in my service manual. There was very little difference between the 3 coils on both the primary and secondary windings.

I am in the process of removing the 3 ignition coils, spark plug leads, and spark plug boots from the other engine and intend to swap them side to side. Port coils will go onto starboard engine and starboard engine coils into Port.

Once complete I will sea trial ASAP and hopefully reproduce the problem. (Fingers crossed)

The theory here is that if the problem swaps sides to the other engine, I have an issue with either a coil, lead, or boot. If the problem stays on the starboard side, I have eliminated these 3 parts and it is something else and I’ll have to keep going.

Furthermore, as soon as the problem resurfaces, I will immediately do a “cylinder contribution test” exactly as jgmo originally suggested. This should narrow it down to which cylinder I am having problems with.

Fingers crossed that all I need to do is replace an intermittently failing coil as this is what I think is most likely causing the problem. Again fingers crossed.

I’ll update once I have more data. If anyone has any additional thoughts please speak up and thanks again. IMG_0451 copy.png
 
Last edited:
Kevin,
If you get this thread, I would like to know your resolution of your rough idle. I also have twin Honda 40s on a 22foot C-Dory with a rough idle on one engine.
Jim S
 
JamesSShelby said:
Kevin,
If you get this thread, I would like to know your resolution of your rough idle. I also have twin Honda 40s on a 22foot C-Dory with a rough idle on one engine.
Jim S
Click to expand...
Jim,
After swapping all three coils from the port to starboard side, I have put around 50 hours on the engines and have not had a reoccurring issue. This is disappointing as I keep expecting it to resurface. All my electrical connections looked good when I was doing the swap so I expected the problem to either stay on the starboard side or move to the port with the possibly failing coil. Since then I have just been enjoying C-Dory life. Just got back from 6 days trip in the southern gulf islands. Wish I could provide more info for you. Let me know what you find out. Does the engine sound like mine in the video? I know my Hondas run rough if the idle site too low. I often have to adjust the idle screw as over time they tend yo idle lower. I shoot for around 800RPM.
 
IslandKev said:
Jim,
After swapping all three coils from the port to starboard side, I have put around 50 hours on the engines and have not had a reoccurring issue. This is disappointing as I keep expecting it to resurface. All my electrical connections looked good when I was doing the swap so I expected the problem to either stay on the starboard side or move to the port with the possibly failing coil. Since then I have just been enjoying C-Dory life. Just got back from 6 days trip in the southern gulf islands. Wish I could provide more info for you. Let me know what you find out. Does the engine sound like mine in the video? I know my Hondas run rough if the idle site too low. I often have to adjust the idle screw as .
Click to expand...
My
IslandKev said:
over time they tend yo idle lower. I shoot for around 800RPM.
Click to expand...
Kevin,
Thank you for getting back. Your solution is simple enough and I will try it.
My boat has been out of commission for two seasons. It started with surges/missing at cruising speed and became worse with surging and stalling, both on my starboard engine. Reading the posts here I have had the carbs apart two times, then once by a “professional” outboard mechanic shop, all to no avail. Valves adjusted, compression 185, 190, 185. Timing checked, timing belt changed. The cid coils ohm check and within specs. I do not understand the vacuum enrichment system on acceleration. Idle is high for 15 or so seconds, then drops down to a quite rough idle and often stalls. Excessive high idle or stalls. Idle mixture on all three makes little effect, carbs have been synched, low vacuum of 5” on all three. I have 3 slow idle jet tubes coming but I am positive the originals are open, but can not see the shape of the very small interior jet. I bought the boat with new engines 26 years ago. I have scoped all 3 coils, primary and secondary. Both types of patterns are irradic. An in line coil light tester shows a consistent spark on all three. The cdi must produce a fairly low “high” voltage, the coils will not activate my timing light. For 23 years very happy with the engines, last 2 1/2 yrs not so. Any one with other thoughts?
 
James,
I agree with swapping out the low speed emulsion tubes (jet sets) because they ARE difficult to clean and verify that the internal orifice is clear. Hopefully that's a fix.

But while you have the carbs off, take the opportunity to THOROUGHLY hose out the low speed and high speed air jet passages. Those are accessed through separate drillings on the forward mount flange. The outer passage is the low speed passage with the passage closest to the carb throat being the high speed air jet feed.
Liberal amounts of carb spray through both ensures the air jets are clear and will allow for maximum "draw" of fuel from the float chamber into those circuits.

The enrichment system basically consists of the enrichment dashpot check valve (item 18 in link immediately below) and the throttle linkage activated "dashpot" diaphragm.


The dashpot diaphragm is item 5 in the link below.


Simply stated, when the throttle is quickly moved to full, the linkage strokes the diaphragm valve and forces a rush of fuel through the check valve for a quick burst of additional fuel.

As the dashpot reaches maximum stroke, the additional fuel stops flowing through the check valve and a spring inside the check valve closes the valve to prevent any "backflow" of fuel from the carburetor.

The poor idle condition you describe occurring immediately after acceleration and/or slowing might indicate that there is something leaking taking place inside the check valve. Sadly, it appears that those check valves are no longer available and the valve itself isn't repairable.

At a minimum you can inspect/test the check valve to see if it seals. A Mityvac hand held pump is useful for doing that.
I don't know of any source for a replacement though if you do find that the check valve is bad.

You could try contacting
Tim's Outboard Repair
In Hackensack Mn.
(218) 682-2331
To see if he can scrounge one up for you.

Good luck.
 
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