Logo
FULL SITE MENU

Honda bf90 load drone / base noise

H

hairdresser100

Contributing Member
I've been working on this issue for 2 seasons now and won't type a book but I'm out of ideas and need some help from the best!

My twin 1997 BF90's started to make this extremely loud, drone, base sounding tone between 4000-4800 RPMs that even the best ear muffs can't block out.

I slow / speed up each motor to try to identify which one is doing it but they both seem to. They're tied together by a steering bar. I'm afraid to get up on plane and shut one motor down; it's a big boat but that may be next.

This all started mid last year and the only thing I did was drop both legs to install water pumps. I don't think this drone started instantly after the pumps but who knows.

The port motor shakes like a paint shaker but runs terrific. This may be part of my issue. I didn't see a counter balancer in the Honda book but this shaking should be addressed. The other motor is much, much smoother.

Is there any way the exhaust is being diverted from the prop hub out the top of the midsection at these RPMS? I swapped props from SS to new aluminum but that did zero.

What causes a Honda 90 to shake like a paint shaker?
 
This must be driving you nuts!

No balancer on the 90's.

You said you changed the props, so it is not a twisted prop.

Shaking like a paint shaker and running terrific seem to be contradictory statements...

The tie bar could be causing the good motor to vibrate sympathetically with the other ( like one tuning fork causing another one of the same frequency to vibrate)

Bent prop shaft?

Only running on three or less cylinders?

Have you tried swapping lower units to see if the shaking follows the lower unit?

As for the noise....if they make any of the noise when not in gear, run them one at a time at a rpm above idle and take a long screw driver to use as a stethoscope and touch the engine in various places top to bottom with the other end up to your ear. Sometimes using a short piece of hose and doing the same thing helps locate noises.

Look under the top (black plastic cover) for any metal shavings or loose items.

Just some thoughts....

Mike
 
Thanks for the reply.

The lower units came off and went on very easily. I didn't drop drive on the shaft but yes, it could be bent. I have a spare lower unit so I'll install it and measure the removed lower unit's drive shaft for run out.

The tie bar is transferring the vibration for sure.

I use the boat commercially and run a half hour then idle for a 1/2 all day. I shut them off when the shaking gets to me but they run with a twist of the key, idle for hours, run fast, strong, but that noise is making me mental. I do agree with your point.

I pull all the plugs a lot looking for a dead cylinder but all plugs always look clean and the same color. I pull wires off from time to time and engine reacts but I did think of this and I'm still searching for a dead cylinder. Engines are strong and boat jumps up on plane and I'm not pushing one throttle further than the other or more forward than in the past.

Just had both timing belt covers off and cleaned very well in there. I checked the rope pull disk on top of the engine to be sure it wasn't broken in half and greased the rust prone belt idler spring. I didn't see any filings or anything strange. Thought about removing the rope pull disk but thought it may act as a cooling fan so I didn't.

I guess I'll have to find the time to swap the lower units.
 
Agree with Mike - "Shaking like a paint shaker and running terrific seem to be contradictory statements..."

You need to find that problem in the port motor first. I suspect if that problem gets solved, the other problem will go away. I would start by pulling and inspecting plugs. They should all look the same. If not, replace with the proper NGK's. While plugs are out perform compression test - screw in type compression gauge, fully charged battery, throttle wide open, spin motor for at least 6 seconds for each cylinder. Also, test your coils per the shop manual.
 
I agree with you and have been working hard on it.

After thinking about this I will remove the lower unit and connect the garden hose right to the water tube and run outboard with no lower unit. If outboard still hakes, it's the engine. If it does not shake, I'll install my spare lower unit and test run.

It is a contradictory statement but my point was the engine isn't shaking because it's sputtering, popping, stalling, surging..... I could have worded that better. I do understand your point and appreciate your help because I'm at my boiling point with this drone.

Just pulled plugs, all looked exactly the same and I do have brand new proper NGK plugs installed. Used the Honda service manual to obtain plug number.

I tested compression last year on both motors and all 8 cylinders were very, very low (125-ish) but exactly the same so I though it was the gauge. I'll borrow another tester and do it again. I've been running tons of sea foam this season but it didn't help. I figured I would clean carbon and adjust valves at one point.

Can/will test coils plus I have spare coils on a parts engine if needed.
 
Last edited:
Not much to add except:
You don't say when the shaking is at it's worst. Idle? 4k to 4.8krpm? At all times? I'm guessing it is at idle? Since you say you idle so often.

Have you tried looking for a miss fire condition using an inductive timing light? It's not all that easy to do but since you don't seem to be getting any "definitive" results pulling the plugs and inspecting them, I would give that a go.

As per Mike, using a stethoscope for listening is a good trick. I like a sawed off wooden broom handle or dowl. The rounded part of the handle fits my ear nicely.

Try running the engines with the covers off and observing the "rust prone belt idler spring" and compare the "action" of the two belt tensioning systems. If one moves or jumps around more than the other, that might be a clue.

During my automotive/truck tech career, I found that, as the manufacturers added more and more plastics under the hood, we would experience a greater degree of noises that could be described as a "droning" sound set up by some designs of air intake systems. It might be worth looking at that part of your outboard to see if anything has come loose in that area. Those engines move quite a bit of air and one missing clip or screw or a loose flame screen could set up some screamin' harmonics.

For that matter, you might try running without the covers for a "sound check".

Just a few things that popped into my ugly head this morning. Good luck.
 
Good question but it’s hard to tell if the engine is shaking at high speeds because I can’t get to it. The cabin is in the way and can’t drive and feel the outboard at the same time. Inductive timing light would be good but still can’t get to the engines that well at high speeds…but anything is possible. If the coil was bad that would shut down 2 cylinders and I would feel no power at all on such a big boat....right?

I’ll take a good look at the tensioner when running. Good point.

I’ve been removing and bracing things in the boat looking for whatever part could be shaking like a drum but have exhausted that method. I’m chasing the motor end of deal now so I’ll remove the hoods and timing belt covers to see what I learn.
 
Yeah, you really need to have someone that you TRUST drive the boat for you while you look for problems. Be VERY careful with the covers off and make sure it is calm water with no traffic. One little lurch of the boat and you need to brace yourself and you put your hand in the wrong spot and...well...you know. Not to mention getting spray up there and making things even worse.

I don't have a lot of experience with the big Hondas but I can tell you that the small twins run INCREDIBLY well with one cylinder completely dead! They start and will actually run fairly smoothly. So smoothly, in fact, that they have fooled me more than once that a cylinder wasn't firing.

So, with that in mind and the fact that you have another outboard pushing right along side, I'm not sure what sort of noise or power loss you would "detect" if one or even two cylinders were dropping out.

And, based on what you just said, it IS shaking at idle so that is where I thought the induction timing light would be a good thing to try to see if you have a cylinder "skipping". At the higher turns, it would probably only be useful to let you know there is just no spark at all.

Standing by and waiting for your next report.
 
Sorry I was typing the last and didn't see the question.
Answer: Yes, exactly. You will need to clamp each plug wire individually, one at a time. and observe how the light flashes for several seconds per cylinder. You are pretty much looking for either a cylinder that looks WAY different from the others or no light at all. It is not a good tool for checking spark QUALITY or amplitude and you really shouldn't draw any negative conclusions if the flashing looks a bit erratic because that can be normal. Your just using it to identify something much different than what is taking place on the good engine/cylinders.
 
No problem, done that a million times with my Merc.
If after I pull the leg, run it, and still find it shaking I’ll dig out my timing light and see if a cylinder drops out of sparks differently. Stay tuned......thanks again. Much to check.
 
Mike,

I looked here with a flashlight and all was clean and in great shape. Will run engine and look again.

"Look under the top (black plastic cover) for any metal shavings or loose items."..

What was it you were looking for?
 
Under the starter pulley, they is a stator and pulsars. If one of them worked loose, they could be rubbing on the rotor that is under the pulley.

Also pull on the pulley and see if it might be loose itself.

Mike
 
I removed both starter gears and verything under them was tight.

To let other's know the run out on your 90 prop shaft it 0.006" measured towards the case side of the shaft. (Away from the 55 ft.lb prop nut.) Mine were arguably within spec.

Next comes driveshafts and ignitions.
 
HAPPY 4TH!

Back to working on this drone / paint shaker motor and found something.

I tested the coils on this paint shaker motor and both coils had primary resistance values not to spec. Spec calls for .35 to .43 ohms and I was 1- 1.3 ohms. Secondary resistance was fine. 23-34k ohms.

After connecting the timing light to both motors and studying the blink pattern, I certainly have cylinders that skip a blink and blink much slower. (I did this at idle...not sure if I needed more RPMs. )

Both motors had blink issues. Of the 4 coils only one was 0.4 ohms.

I ordered two new coils and I'll put them on each motor to see the difference.

What else besides the coils would cause the spark to not fire, fire slow, or fire when they shouldn't? Plugs are new and the correct NGK model.

Again, motor shakes buts always gets me home and idles, runs, starts, and WOT with no issue. It does not lose RPM, speed, sputter or anything like that....
 
Last edited:
Well, you may be on to something with those primary resistance readings. The 1.3 ohms one has TRIPLE the resistance and probably gets worse as it gets good and hot.

Like I said before though, some skipping and erratic behavior of the inductive light COULD be normal. Those things are only made to illuminate a spot on a crankshaft pulley or flywheel and ARE NOT precision measuring devices by any stretch of the imagination. They will skip and "short blink" because they are picking up magnetic flux lines not all of which may be coming from the wire you have it clamped to. So, to be used more as a GO-NO GO indicator of secondary ignition activity.

I think you should at least try one new coil to replace that high resistance one. With one or two new coils in hand, you can move them around on the engines to see if they make a difference for "suspect" cylinders. I don't know how this system is "triggered" but I suspect that it is basically a solid state CDI type system that typically either works or don't work. However I would try to get MIKE to expound on that subject about what to look for in firing those coils.

It ussally goes like this though: a coil of wire picks up a signal from a rotating magnet on the crank or flywheel and a voltage produced in that coil signals a transistor in the CDI module to remove ground from the coil primary causing the secondary to fire. That is pretty simply stated but that's how it works.

If the rotating magnet gets crap on it (rust, oil, grease, dirt) the coil of wire might not see the magnet go by and not generate it's own voltage. Or, the coil sees the magnet but not clearly enough, generating a weak voltage. The coil of wire can start to exhibit high resistance just like your coil primary. The connections at the CDI to the coil of wire can be corroded. And on and on.

Ignition coils NEED good contact on the ground side of things or they will not last as long as they should. Leaving spark plugs in for many seasons or not cleaning the threads in the head leads to high resistance and that makes the coils work harder, eventually breaking them down.

I think you're on the right track but it could be only one track of two tracks or more.
 
Last edited:
Without getting as technical as Jimmy can...when you turn on the key switch, 12v goes to the CDI and internally it multiplies the voltage to a much higher voltage by a dc to dc converter. There are two pulser coils on either side of the flywheel, which alternately send a signal to the CDI to fire the higher voltage to the coils. Cylinders 1 and 4 fire at the same time. Cylinders 2 and 3 fire at the alternate time.

So to answer your question What else besides the coils would cause the spark to not fire, fire slow, or fire when they shouldn't? " Defective pulsers, crossfiring CDI, defective coils/wires, and of course spark plugs (should be NGK DR7EA...no substitutes). The other variable could be the lead that goes to the kill switch to turn off the spark. If it a intermittantly shorting to ground, it will screw your spark up big time.

Hopefully, your new coils will fix it.

If not, switch the CDI units from one motor to the other and see if the trouble switches too.

Mike
 
Ok guys great advise! THANKS!

100% have the DR7EA plugs, two new sets have been used with no luck.

I found the paint shaker motor's kill switch not working so I replaced it. The other motor's kill switch worked 4 out of 5 times so I exercised it and now it's working great. (These switches haven't been used in 10+ years.) The wire harness......hope I don't have to mess with that...my extra harness is a mess.

I have a spare CDI.


Thanks! Stay tuned....coils should be here on Monday! I'll measure their resistance to be sure the Honda manual is right.
 
Last edited:
I was on the boat today thinking and respectively have to question how my timing light isn't precise? By nature it has to be exact to time my Mercruiser engines an illuminate the crank shaft mark at the exact time. Are you saying the Honda systems are hard to read by a timing light?
 
Last edited:
Received my new coils and the resistance measured high but then the fluke meter slowly started dropping to 0.4 ohms. Hopefully they do the trick. I'll get the timing light on the new coils ASAP to learn more.

On a side note, been through HELL working with Lowrance to install a LMF-400 gauge and two 60 fuel flow sensors....I don't recommend Lowrance at all. Two defective meters, 22 phone calls and 222 headaches. But, I did get the 3rd meter working and my twin honda 90 carbs burn 11 GPH at 4400 RPMS. They burn so little under 2200 RPMs that the meter just reads zero. I do tons of so this is a bummer......o well.
 
Last edited:
Back
Top