Logo

Lack of power top end

reds

New member
Hi,

I have a rib boat with 5,5m and a Honda bf90 from 2004. The engine is good but when I compare it to other boats with the same setup, I seem to lack power in top rpm.

At 5600 I get 30 knots but a friend of mine with the same boat and engine gets almost 35knots. I don't know the real hours the engine has but I bought it as 750 hours told by the old owner. I made a compression test and got all the cilinders with something like 10bar.

I can't get more than 5600rpm.

What can make this situation? The engine being old? When I bought it 3 years ago I remember getting something like 33 knots but the boat stands all the winter without being used.

thanks to all.

luis
 
The 3 things that come to mind are......... decarb the motor, adjust the valves, and then try different props. Also, make sure your throttle linkage is adjusted correctly so that your motor is getting max fuel.
 
Totally agree with Btravlin's advice. Do the throttle linkage first. If you get the engine/boat combo to 6000 rpm WOT, you likely will be at the 35 knot speed.

Having said that, you would be amazed at how much a dirty (or rippled) hull can slow down a boat's performance at the high end.
 
Hi,

the valves were adjusted last year and didn't notice anything different. The throttle linkage is fine and was checked by my request at my Honda agent.

It can be carbon, or even dirty hull but the hull looks very clean to me.


 
Let's back up a bit. If the engine was decently maintained it's not an "old" engine. My 2007 model BF 225 has over 1900 hours and still runs great and I can get 5950 rpm at WOT (with out riggers down and less than 1/2 tank of fuel.) Several folks on this forum have put well over 3000 hours on their Honda's.

Let's assume that the throttle linkage is correct and the valves are still within spec. (Personally, I would recheck both.) Obviously, it's firing well and you did not mention anything about missing at the high end. Furthermore, you said that previously you were getting 33 knots when you first got the boat, so let's assume that you don't have a (significant) prop problem at this time.

So what's left is compression, air flow, fuel flow, or a problem with the VTEC system.

You said all cylinders were "10 Bar." I don't understand that. What PSI or millibars were you getting? I don't know the exact specs for the 90, but I think it's around 210 to 215 psi. Compression test should be done on a warm engine, fully charged battery, all plugs out with throttle fully open. Engine should cranked to at least 300 rpm. There should be no more than a 10% difference between the compression on the highest cylinder and the lowest cylinder. If you are not getting that level of compression on the cylinders - at least 200 psi - then a good dose of Yamalube Ring Free (or a similar top quality decarbing product) might solve your problem. Try to hook up a small external tank. I believe the correct mixture for a shock decarb using Ring Free is 2 ounces per gallon. Warm up the engine, hook up the external tank and run enough mixture through the engine until you are sure it has saturated the entire fuel system. Then let the engine sit for several hours before restarting. Most folks will do two decarbs just to make sure they get as much carbon build up out as is possible. You'll need to change your oil after decarbing. Then, re-measure compression.

Btravlin2 just completed this process on the BF 225's that he is restoring, so he has a lot of recent first hand knowledge.

Air flow- I can't help much here. Just make sure that your air passages are all clear. I recently had a problem with the intake air bypass (IAB) control on my BF 225, but I have no idea where or how that works on the BF 90. If it is not working properly, you will lose top end performance because the cylinders are not getting enough air through the intake valves.

Fuel flow. Have the high pressure and low pressure fuel filters been changed recently? They should be changed at least annually. If you have an external fuel/water separator (which you should) has that filter been changed recently? On the Honda fuel injected engines, there are a couple of "hidden" fuel filter screens that tend to get clogged. On the BF 225, one is under the high pressure fuel pump and has been the source of several performance problems with BF 225 owners. I suspect the are also present on the BF 90, so they should be checked.

VTEC system. Again, I can't help you on the BF 90. On my BF 225, the VTEC kicks in at 4600 rpm. If for some reason it is not kicking in as designed, then you are really operating with a 75 HP motor. There are tests for determining whether or not the VTEC is working properly, but again, I'm not familiar enough with the 90 to give you any directions down that path.
 
Let's back up a bit. If the engine was decently maintained it's not an "old" engine.

The engine was sold as having around 750 hours, but is a carburated engine, so I don't know the real hours.


Let's assume that the throttle linkage is correct and the valves are still within spec. (Personally, I would recheck both.) Obviously, it's firing well and you did not mention anything about missing at the high end.

The engine sometimes sputters a little bit at 1000rpm. Could it be possible to have be missing a high rpm? I know that carbs 1 and 4 are burning a little richer than carbs 2 and 4, which may indicate something electrical???



You said all cylinders were "10 Bar." I don't understand that. What PSI or millibars were you getting? I don't know the exact specs for the 90, but I think it's around 210 to 215 psi. Compression test should be done on a warm engine, fully charged battery, all plugs out with throttle fully open. Engine should cranked to at least 300 rpm. There should be no more than a 10% difference between the compression on the highest cylinder and the lowest cylinder.

10 bar is around 145psi but all the cilynders have the same reading. It was done by a Honda agent with all spark plugs out and maximum throtle. The engine was run only 5 minutes before and only at 1000rpm so I think it hadn't time to warm up. I think it's low readings but there isn't any diferrence between cilynders, so it's a good sign.

The engine hasn't VTEC because it's the carburated model.All filters are good. I will try to do a decarb like you said.

Thanks for the help
 
If you get the engine/boat combo to 6000 rpm WOT, you likely will be at the 35 knot speed.

Do you think that those 500rpm will give me more 5 knots? I can't get to 6000 even alone in the boat because I always carry some weight. I would have to change the prop to a 15 inch. Now I have a 17 inch. If I change the prop, I'll get to 6000 but then I will loose top end speed, right?

Thanks
 
OK. I don't know when the Honda 90 went to fuel injected, but I think it was definitely a while ago. I think they were the last Honda outboards to go to fuel injection, but not sure when. Maybe around 2004.

Having carbs puts a whole new perspective on your performance issues. And clearly I have no experience with carbureted Honda outboards. Several other folks on this forum do have lots of experience with those, especially Mike, AKA Hondadude. Sputtering at 1000 rpm's and unequal burn tells me that the carbs probably need to be finely balanced, but I'll leave that up to the experts on this forum.

The 145 psi still sounds way too low. I would still consider doing a shock decarb of the engine.

Switching to a 15 pitch may or may not get you better speed, there are too many factors to consider. Possibly you could get a loaner and try it out.
 
The 145 psi still sounds way too low. I would still consider doing a shock decarb of the engine.

What can produce that low readings? carbon? Because they are the same on all cylinders and the engine seems to be very in strengh to pull the boat out of the water. It's only about top speed when I compare to other boats from the same brand/model with the same bf90 engine.

Thanks
 
If you are sure your throttle linkage is in specification, then a shock decarb may give you better top end performance by cleaning the caked carbon and varnish off the rings and valves. You should do a compression test before decarbing, then do it again after decarbing. Then change your oil and have the valves readjusted.
 
Back
Top