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The original Honda 90hp carberated - bogging down under accel

majbach

New member
Hi, everyone. Need some pro help with this one:

1999 Honda 90hp with 700 hrs and very well taken care of but probably only 100 hours of the 700 are from the past decade.

Issue: Outboard bogs down under hard acceleration - a common problem I have now read. Cause still eludes me.


I noticed this first trip out this year. None of the symptoms except for the odd spitting are noticeable when I have it hooked up to the muffs in the driveay. The motor seems to accelerate fine when I advance the throttle slowly but if I advance to WOT at any point during acceleration, there is an instantaneous change in sound and power, as if it lost one cylinder. There is also a shutter/vibration. The effect is most pronounced when I do this from idle. If I advance it slowly (or even if I let it just ‘go’ when I give it WOT) it eventually revs up and smooths out. However, I’m down quite a bit in top end – 31 knots when I used to be able to squeeze out 40. I tried toying with the mixture screws. Factory is 2 ¼ turns out and I tried every point between on one turn out to 3 ½ turns out. I may be imagining it but when set to rich it seems a tiny bit smoother and more responsive.
While I had the hood off to adjust the carbs, I noticed a new sound when the bogging down occurred – almost identical in sound to what you would hear from a grounded spark plug arcing. That got me to thinking that it may be an electrical problem and not fuel. The spark arcing sound only was noticeable (but fairly distinct) while under hard accel and of course I was at the wheel all the time so couldn’t pinpoint it. But, I also noticed that the sound was faintly there when it had maxed out top end– over the sound of the engine. The hood must muffled this. Even when I advanced the throttle slowly to get to wide open without bogging down, the noise appeared as it was approaching full out. The sound was instantaneous when I tried to hard accelerate too. Btw, I can’t say for certain what the rpm was topping out as as the tach started acting funny too this year. But it’s the same prop Ive used for awhile and it used to top out at 5400-5600 rpm. It sounded like it was not quite reaching that rpm but not enough to drop it down 9 knots. However, I was in salt water for the first time too so maybe increased drag?

Okay so there is a bit of a background leading up to this issue that I didn’t want to state up front for fear of influencing your first diagnosis: I rebuilt the carbs this spring.
Yup, last year I learned the hard way what happens when you store an outboard and forget to drain the bowls. When I launched for the first time, it couldn’t hold idle then I quickly noticed she was spewing gas from two carbs – needles were stuck. So, I replaced every bit of brass on each carb and new gaskets and O-rings on everything. Only thing I didn’t screw with were the needles and float adjustment. I was meticulous not to alter anything here and the needles were in new condition.

After rebuilding the carbs, she DID NOT run very well in the test tank. Initially only on three cylinders. In the process of determining which cyclinder, a different one dropped. Turned out I had a lot of water in my gas. This was gas I added to an empty tank from a jerry can that had about 1/3 water in it. Naturally, I got all of it out of the tank before going any further. I ran a bit of methyl hydrate in it after filling it completely up and siphoning all but a ¼ tank to ensure I got all the tainted gas out. Motor ran well in the driveway but the problems appeared first time in the water.

The only other symptom that persists that wasn’t there before (or nearly as bad) is the odd sputtering. Seems to occur at anything above fast idle and below say 2500 rpm but, it’s not very bad, just nagging – a constant reminder that all is not perfect in the world.

I also never sync the carbs after the rebuild – don’t have the adapter fittings. But, the motor runs smoothly at idle so I’m not inclined to believe there’s an issue there.

So aside from the book I just made you read, what are your thoughts? Can this be a resistor cap on one of the spark plug leads (had that problem on a motorcycle many years ago). I looked for signs or arcing or any type of tell-tale signs the wires for the plugs start – nada. Plugs themselves are probably two years old but less than 25 hours.

Thanks a heap for taking the time

R
 
I would clean the carbs again, and pay attention to the intermediate jets, sounds like they may be partially or completely blocked. It often helps not to over think these issues , you almost certainly have a fuel issue.
 
You already know hesitation off idle is pretty typical of the 90's. AND I've discovered, like you did, that running the low speed idle adjustment screws on the rich side (3+ turns) can be a big help in reducing that issue (kinda proves they are keeping them on the lean side). The only time it's of any consequence for my own use is when pulling somebody on a tube. We don't do that often, so I tend to just baby it while getting it moving. Otherwise, I'd just run it rich. I don't see that hurting a thing.

The "spark arcing" noise you hear, could that be spark knock maybe? I'm not sure I would blame that on timing, but being lean can darn sure cause it, as well as garbage gas or possibly even over heating.

I find the big power loss particularly concerning. Between that and the hesitation it sounds like it's running pretty lean. Could there be a big vacuum leak?

My action plan would be to try some fresh gas first, maybe using a portable 6 gallon tank. Then the carbs would come off for a good going over. While they're off, it might be a good time to have a look at all of the fuel lines downstream of the fuel pumps - to see if you aren't sucking air at high flow rates.

Interested in how you make out. Best of luck!
 
Well thanks for that info. I am exploring a new area of engine mechanics. Seems people are in agreement that there is a fuel issue. I will start by pulling the manifold off again. I'm curious if this can in any way be related to the fuel pump. The more I reflect, I realize that since I left the fuel in the system over the winter (that was a full years), I have not had the engine running properly. Can gas going bad cause issues to the fuel pumps? What about running water through them? I don't even know how serviceable those fuel pumps are.
The issue I keep getting hung up on is how immediate and consistent the symptoms are. It bogs down at a certain point in throttle, although the symptom weakens as the rpms climb. It is 100% consistent and reproduce-able. Air leaks in my experience show more erratic symptoms - especially as the engine warms up. And fuel starvation...well once the fuel is in the bowls, how can it starve for fuel so instantaneously. I'm guessing only a clogged jet? I'll update you as I work on it.
Thanks
 
There are at least 3 different paths (that come to mind) fuel can take through each carb. There are low, medium and high speed circuits, all with their own fuel paths/jets. Witch path the fuel takes will depending on the amount of vacuum, rpm, and throttle opening. It's easy for me to see that it's not necessarily the common idle jet problem, but more likely one of the other circuits haunting you.

If you have the correct "tamper proof" screw head bit, you can take the fuel pumps apart for cleaning if necessary, but none of those parts are available separately (tread carefully!). They're serviced only as the entire pump assembly. If they aren't leaking, they're probably fine in my experience. Too, there are 2 pumps. One feeds the top, the other the bottom. It would seem to me you would notice 2 cylinders dropping out easily/big time if one of the pumps were headed south.

NORMALLY, you would see fuel starvation issues start at a certain throttle setting or rpm, and get increasingly worse as throttle/rpm increase. When you back off to just prior to where the problem starts, you can continue without issue. That's commonly how a fuel filter issue throws a red flag. It can pass so much, and when demand increases beyond that, it shuts down or becomes erratic.

Just tossing food for thought/ideas...... -Al
 
Well, thanks, Al. I do appreciate your in depth and well thought out answer. I don't want to sound like I am challenging your logic but I am hoping you can elaborate on something - for the sake that having a better understanding will help me to diagnose better going forward.
As it happens, I did change the fuel filter and I think it was OEM.
Where I get hung up is understanding how the fuel filter not allowing for adequate flow would matter since it is upstream of the fuel pump and carbs. Doesn't the amount of fuel contained in the bowls of the carb act as a buffer for any interruption in flow? In my mind, if the motor is running fine at idle, that means the fuel bowls are full and any immediate a significant demand for fuel is taken care of for several seconds or more.
While we're on the topic, can someone help me to understand how the level the carb float is when the needle valve fully seats impacts the mixture? In the shop manual, they go to great lengths to describe the Honda tool for measuring the static level of the float in bowl. And, the tolerances seem pretty fine too. The tool required to measue it isn't your everyday item either. How/why does this make so much of a difference? Isn't the float's function to simply allow the bowl to remain at a certain level of fuel? Doesn't the mixture screw take care of how lean/rich the gas is?
Thanks again for all this insight.
R
 
Seems like you are mixing several issues together, thinking they may be related - when that may not be the case at all. My suggestion would be to work on/discuss one issue at a time.

Like the hesitation off idle. That's a low speed idle circuit issue, very common on most any Honda 90, and we know that we can take care of that by running rich idle mixture screw settings. I doubt seriously that has ANYTHING to do with your other issues. Like the spark plug arcing problem you mention, or the big loss of power on top end. Those are NOT typical. The fuel flow affecting your top end takes a totally different path through the carb, AND there could be a few other things that might contribute to something like that going on.

So your question regarding the fuel filter would not likely be affecting the idle issue, but it could surely have something to do with a big power loss on top end. Follow?

And I'm with you regarding float level. I'm not near as picky as they suggest I should be, and turn out great running motors regularly. -Al
 
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Sounds like you have an air flow problem in one of the carbs you need air fuel and spark i can suggest do a simple test, with a vacuum pump put one cylinder at TDC on the compression stroke with the inlet and exhaust valves closed .Connect the vacuum pump onto the side of the manifold where the nipple screws in and check each cylinder this way with the butterfly closed and one to three eights open the vacuum readings should tell you more or less what carb has a air blockage in its passage way the highest reading will indicate a blockage of air flow i never done this try it it may work.
 
You may also just have a faulty dashpot replace it with one of the newer models one.

You need to read the service bulletin of september 2000 Honda did find some problems with the carbs and they said their techs had to change ,valve dashpot,diaphram assembly,assist spring ,throttle opener cam ,and fuel tube to improve the carb problems some where along the line they picked it up coming from the factory.i have all the new part no,s if you want them
 
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