Honda 90hp maxes out at 4500 rpm's, need ideas....

I upgraded the 2003 Honda 50s on my SeaCat Sl1 to identical 1998 Honda 90s this spring and so far so good. The only issue that I have is that the port engine gets 6000+ rpms while the starboard engine can only get 5500 rpms. After putting 60 hours on the boat so far the max rpms have now dropped to 4500 rpms on that starboard engine. I'm trying to figure out what the issue is and know its either spark, air, or fuel (haha). Its interesting because I've had other carb'd engines that have bogged before (usually carbs), but this one just seems to hit the max rpm without making any noises or sputtering and just won't go anymore.

This is my assessment to date:
1) Both engines are mounted at the same height and have the same props.
2) I confirmed that the throttle linkage is roller is moving all the way to the open position at WOT as compared to port engine, so it doesn't appear to need adjustment. Although its possible the carb butterfly valves aren't fully opening?
3) I've replace the onboard fuel filter, external fuel water separator, and drained carbs several times.
4) The carbs may also need a rebuild. The previous owner said he had rebuilt the carbs himself in the last few years, but I know the honda carbs are a PITA to tune. I did run some Seafoam spray to decarb the engine but I couldn't get the airbox off fully and the engine didn't seem to sputter or choke like it should have been as when I've done this on other engines. There doesn't appear to be an issue with the airbox.
5) Is there any way to test the fuel pump performance?
6) Is there a simple way to test the stator/alternator with the voltmeter for static resistance and/or make sure its putting out full voltage under load? I've noticed on the dash that the starboard engine is putting out slightly less (0.25 - 0.5) volts than port engine, but not sure if that's just error in the 20 year old gauges/wiring harness.
7) I do have a spark plug tester and that is also on the list to test...

Any other ideas would be appreciated!

Make sure you don't have something going on in the way of mechanical drag like a binding bearing in the transmission. Check gear oil and make sure it's clean and full.

You should also take a look at timing to make sure it is correct and isn't changing.

It could very well be the carbs. Have you tried giving it some choke to see if it will pick up or get worse? Tried squeezing the primer bulb when under way? A clogged tank vent could cause this symptom too.
Squeezing the primer bulb will usually help an ailing fuel pump deliver enough fuel and give you an indication the pump might need replacement.

You can hook a gauge up but I don't know what delivery pressure should be. But you could measure output of pump on port engine and compare the two.

Hopefully you'll get more/ better suggestions from others here.

Good luck.

jgmo said:

Make sure you don't have something going on in the way of mechanical drag like a binding bearing in the transmission. Check gear oil and make sure it's clean and full.

You should also take a look at timing to make sure it is correct and isn't changing.

It could very well be the carbs. Have you tried giving it some choke to see if it will pick up or get worse? Tried squeezing the primer bulb when under way? A clogged tank vent could cause this symptom too.
Squeezing the primer bulb will usually help an ailing fuel pump deliver enough fuel and give you an indication the pump might need replacement.

You can hook a gauge up but I don't know what delivery pressure should be. But you could measure output of pump on port engine and compare the two.

Hopefully you'll get more/ better suggestions from others here.

Good luck.

Click to expand...

Thanks for the input. Gear oil is full and clean. I do have a timing gun and could check that.....I'll add to list.That's not a bad idea measuring output on fuel pumps between engines to compare.

I forgot about priming or choking it while underway, I'll try that next time I'm out! I did replace all the fuel, vent, and fill hoses this spring too so I would imagine that its venting ok.

Update. I primed the bulb while underway with no change to RPMs and then I tried choking it, which caused the engine to loose RPMs because it was getting flooded. So maybe its not a fuel starved condition? I then tested the spark plug leads in the driveway by running the engine and pulling them one plug at a time and watching the RPMs drop. On 3 of the 4 cylinders when I pulled a lead the engine didn't loose much RPM and sounded ok. When I pulled the #2 lead from the top, the engine lost considerable RPMs and sounded like it was going to die. So I'm wondering if only one cylinder is getting full spark and if I have bad leads or coils? I have a good working set on the other engine, so I think next I might just swap them out and see if that's the issue. I can also test for spark, but this might be easier. I'll report back.

Based on your drop test it sounds like it's time for a compression test and leak down test. Also, the same results could be caused by seriously unbalanced carbs.

Well, that sounds like a plan but if you do that swap and nothing changes then you might want to adjust the valves and then do a compression check.

You said you had a timing light but these outboards have timing belts. There are marks that need to line up and the condition of the belt and belt tensioner should be assessed.

You should have the shop manual for reference if you want to tackle this yourself. If the timing is off, continuing to run it could cause damage to the engine. By the same token, getting the timing wrong can cause damage too.

With outboards of this age, if the timing belts and tensioners haven't been replaced, it might be time to do both.

I'm not trying to alarm you but just pointing out what could be things that might need attention.

Hopefully it is just the wires and all else is good to go.

Good luck.

Good point about the carb balancing chawk! I didn't see your post before or I would have said this in my last reply. Once again, you point out something I whiffed on ;>).

Thanks for the input guys and I do have the full shop manual. A compression check was conducted last winter when I purchased the engines, and it was well within spec. Haven't done a leakdown test although I know that's more complicated. My plan moving forward will be to test resistance on the leads/coils and then swap them to see if it makes a difference.

I will also check the timing belt alignment since that's easy and I did that on the old Honda 50s that were on it before. I actually have the new belts & tensioners as I know that this needs to be done on an engine this age, but haven't done it yet as I had a way bigger project installing these engines this season. Plus the procedure looks to be more labor intensive than I want right now based on the youtube video I watched with straps and big wrenches everywhere. I had adjusted valves on the Honda 50s that I had on there previously, so I know I can do it but was hoping to take care of that in the off season.

It could be unbalanced carbs, but I don't have a carb tuner/balancer so maybe its time to get one. If the carbs need a rebuild I will likely have a friend who moonlights and is a Honda tech so I might have him just take care of it.

Update. I have completed the following items:
-I checked the timing marks and they line up correctly. I even checked versus the other engine to confirm too.
-I swapped coil packs between the starboard & port engines with no change in performance of either engine.
-I then had the carbs professionally rebuilt including all new screws/valves/ports etc. and the carbs were balanced as well, but there was still no increase in RPMs.

So I am still trying to figure out my issue. Could valves that are out of spec really cause that dramatic a loss in RPMs? the procedure looks straight forward in the shop manual, and I did adjust valves in my old Honda 50hp so that might be next. I'm thinking a leak down test should be one of the next steps as well.....does anyone have a procedure for doing this and what equipment I need?

Are you certain your tacho is reading correctly, I have know people to chase problems like this that are not there?

iang6766 said:

Are you certain your tacho is reading correctly, I have know people to chase problems like this that are not there?

Click to expand...

I was thinking this could be the case, and was going to look to see if I could swap the quick connect fittings on the wiring harness to swap the tachs and eliminate this. Its hard to tell by feel if that engine is getting full rpms or not as its a cat with engines on either corner so it tracks straight at WOT.

Good idea to swap them

A single bad coil or plug will reduce your RPMs by 500 or so. Try the easy stuff first.

Coil are Siamese, two coils each firing two cylinders, bad coil would result in drastic rev reduction. Set of fresh plugs wouldn't hurt, I would still be checking correct RPM readings.

So I ended up swapping the control panel wiring harnesses between the engines and it was pretty easy with just 4 quick disconnect fittings on each. I ran the boat and and at WOT that same tachometer was now reading 5,000 instead of 4,500 rpm! However, it was still 500 rpm below the port engine that was reading 5,500 rpm. So it seems that the tach is the problem! I have one of those mini spark plug wire tachs and want to confirm actual rpms while under load. These are the original 1998 Honda gauges as far as I'm aware, but does anyone know if there any way to adjust or calibrate them? Thanks for everyone's input here and now I feel humbled that it was something so simple.

Quote: "I feel humbled that it was something so simple." Anyone who tries to diagnose outboard problems knows the feeling well. Join the club.