2006 BF90 power restriction when motor is under load

Hi all, I've lurked around here for a while and have read many old threads in an effort to try and understand what the issue is with my Honda 90.

Thanks to everybody on this site that takes time to share knowledge and experiences with regards to all kinds of motor issues. There is a wealth of information on these boards.

The issue with my motor has been intermittent. The sea trial went great and the motor ran well as we cruised around for an hour or so at 4200 rpm's which is about 25-28mph depending on conditions and how much weight/gear is in the boat. Fast forward to a trip or two later and the boat fires right up, idles well, and does fine under low load up to planing speed somewhere around 12+ mph. The motor trys to plane at 3400 rpm's give or take but when this issue is happening it just digs, bow lifts and won't climb over the bow wake. It doesn't stall, sputter or sound like it is missing. I back off the throttle at this point because it is clear that something is not right. I don't know how to describe it other than, "the motor gets louder than normal but doesn't have power". However, I do not believe the culprit is cavitation.

For example, we head out of the bay and the boat struggles to plane. We call a few friends, look some things up online to no avail. Motor is still running fine at idle and low rpm/load. Out of nowhere my buddy pumps the fuel bulb and then boom we are good to go. Boat gets up planes really easily and we run trouble free 5-6 miles up the coast to fish and the boat ran well the rest of that trip.

I was thinking there must be a fuel restriction sense when he hit that bulb it "appeared" to make a difference that time. Next trip the motor ran well for about 4-5 hours until the end of the day when the wind picked up and it was time to head in, lol. 10mph back to the ramp and unfortunately priming the bulb did not fix the issue this time. I thought it was a spun hub so bought a new prop. The following trip with the new prop on the boat ran well again. I was thinking "cool must have been that old rubber hub was worn out/heating up" etc.. Went to a Flo-torq II hub kit with a quick silver prop. Boat ran well out of the bay so we made bait. After making bait we ran a couple miles north to some fishing grounds and made a few drifts. Probably fished for an hour or so with motor idling. Once it was time to head in...same thing, no power to plane and heading in at 10 mph.

I was thinking maybe some kind of fuel restriction so I swapped the fuel water separator, checked fuel lines and vent from tank to motor. They aren't cracked and when I pulled off the fuel lines they weren't deteriorating on the inside. Fuel itself looks good from what I can tell. I had my buddy drive the boat while I primed the fuel bulb but this didn't work either. I pulled the plug wires, while the boat was in the bay idling, one at a time and all 4 cylinders responded the same. Plugs don't look fouled. Was thinking maybe heat issue but 2 trips ago we trolled for 2-3 hours offshore at 5-7mph. Trolling didn't produce so we decided we'd be better off to look inshore. We were 8.5 miles off shore at that time and we ran back in at a comfortable 25-26 mph 4200 or so rpm's. If it was a heat issue wouldn't the motor have already been hot by then? Same day after fishing inshore for a little bit we had the power restriction again on the way in.

Sorry for the long post, just trying to illustrate how the issue is intermittent.
Anyway, thank you very much for any ideas or advice you all can share.
-Tim

Just seen this post. Have you done a cyl drop test under full load to see of all cylinders are firing when problem is present. You have two fuel pumps and two ignition coils. Fuel pumps feed top two and bottom two cylinders each and top coil runs one and four and bottom coil runs two and three. I would seem a cylinder is dropping out. BTW, I am assuming it's the last of the carburetor engines and not an efi. ?

Is the engine trimmed all the way down every time you take off and try to plane? I have a BF90 on a 18 footer and I think the engine is set too high. If I trim the engine up to level on the gauge and take off from a standstill, it will cavitate and not plane well, and possibly not plane at all. If I trim it all the way down, it planes fine. At speed while on plane, if I trim it up to level, it will cavitate, engine RPM climbs, and the boat will slow down. If I trim it all the way down while at speed on plane, it stops cavitating and drops RPM, and increases speed. Trimming it up to about 25% above full down is the max speed for the boat.

Maybe your fuel bulb pumping was coincidental? I did have some fuel pump problems, and swapping them fixed the problem, but they had a different failure symptom than yours.
http://www.marineengine.com/boat-forum/showthread.php?448176-BF90-needs-fuel-pumps

What you can try (after ordering new orings), is to remove the pumps and see if the inlet ports produce a vacuum pulse and the outlet ports produce a pressure pulse. Basically put your fingers over the applicable ports and press in the plunger as fast as possible. You will feel the pulse of vacuum or pressure momentarily. The vacuum or pressure does not remain, it is just a short pulse. My pumps only pulsed the vacuum side, not the pressure side. Only pulsing the pressure side may produce a different failure mode.

My presumption is that your engine is carbureted.

iang6766 said:

Just seen this post. Have you done a cyl drop test under full load to see of all cylinders are firing when problem is present. You have two fuel pumps and two ignition coils. Fuel pumps feed top two and bottom two cylinders each and top coil runs one and four and bottom coil runs two and three. I would seem a cylinder is dropping out. BTW, I am assuming it's the last of the carburetor engines and not an efi. ?

Click to expand...

Hi, thanks very much for the response.
Yep I have done a cylinder drop test while motor was idling in the bay last time out. All 4 cylinders responded the same.
i put in a new racor filter and fuel filter under the cowling that sits below the fuel pump carbs.

Spark plugs seem okay but maybe on the lean side.

I also pumped the fuel bulb while running but it didn’t seem to have an effect. I pulled the cowling off and looked for anything unusual while trying to plane/experiencing the issue but didn’t see anything odd.

Thinking possibly anti siphon or fuel pick up issue? I’m going to try to run off a separate tank this weekend and see if the issue is still there or not. I don’t believe it is the vent because filling up is not an issue. Also, I don’t believe my fuel hoses are deteriorating or collapsing because I pulled the deck and checked them out the other day and they seemed okay.

thanks again,
-Tim

Do you drop test under full load, not at idle, so you can work out if it's dropping a cylinder or not. Spark plugs should look nice and lean, that is normal.

jrude said:

Is the engine trimmed all the way down every time you take off and try to plane? I have a BF90 on a 18 footer and I think the engine is set too high. If I trim the engine up to level on the gauge and take off from a standstill, it will cavitate and not plane well, and possibly not plane at all. If I trim it all the way down, it planes fine. At speed while on plane, if I trim it up to level, it will cavitate, engine RPM climbs, and the boat will slow down. If I trim it all the way down while at speed on plane, it stops cavitating and drops RPM, and increases speed. Trimming it up to about 25% above full down is the max speed for the boat.

Maybe your fuel bulb pumping was coincidental? I did have some fuel pump problems, and swapping them fixed the problem, but they had a different failure symptom than yours.
http://www.marineengine.com/boat-forum/showthread.php?448176-BF90-needs-fuel-pumps

What you can try (after ordering new orings), is to remove the pumps and see if the inlet ports produce a vacuum pulse and the outlet ports produce a pressure pulse. Basically put your fingers over the applicable ports and press in the plunger as fast as possible. You will feel the pulse of vacuum or pressure momentarily. The vacuum or pressure does not remain, it is just a short pulse. My pumps only pulsed the vacuum side, not the pressure side. Only pulsing the pressure side may produce a different failure mode.

My presumption is that your engine is carbureted.

Click to expand...

Yeah thats correct it is a carbed motor and I do indeed have the motor trimmed all the way in when trying to plane. If I feel I need to adjust the trim I do it once up and cruising. The last time the motor ran well, my friend and I ran a couple miles then made a few drifts. An hour or so of idling later and the motor sounded slightly different to me so I checked the fuel bulb. The bulb seemed full of air or fumes but almost no liquid at all in it. I know they get soft after running for a bit but it didn’t seem to have much fuel at all in it. I took the boat out for a test run after changing the racor and motor still struggled to plane so I didn’t push it. Thank you for the link and the information about testing the fuel pumps. But yeah you’re probably right it was probably just coincidence that day.
thanks again,
-Tim

iang6766 said:

Do you drop test under full load, not at idle, so you can work out if it's dropping a cylinder or not. Spark plugs should look nice and lean, that is normal.

Click to expand...

Thanks for the heads up. I’ll give that a shot this weekend. Going to run on a separate tank so I can confirm that it is the motor or fuel tank side causing the issue.
thank you,
-Tim

Hi all, checking in with an update.

I found a little time to get the boat out this weekend and ran the motor off an external tank with a separate hose, primer bulb, fittings etc.. the problem still persists.

I did a compression test with a cheaper gauge I ordered from amazon and each cylinder read about 180 psi. One cylinder was closer to 175. I ran the motor on muffs for 10-15 minutes before the test. Then disconnected fuel line, pulled plugs and pushed throttle to wide open with prop disengaged before cranking motor to get compression readings.

To recap, so far I have changed the fuel water separator, fuel filter, fuel pumps and plugs. I have inspected fuel hoses and they appear to be fine.

I’m not sure how to replicate a cylinder drop test while the motor is under load.

I have yet to go through the carbs. The motor idles fine after I crank it and give it a little throttle with the prop disengaged for a sec then dial it back to neutral. Having said that, should I go through the carbs? I ordered the gaskets and seal kits to do them sense they are the last stop as far as a fuel issue goes.

From what I have read, when carbs are gummed up there is usually an issue at idle or going into gear and staling. My motor has yet to stall unless slammed to full throttle and that only happened once.

Do carbs cause issues on the high end sometimes? Today the motor was fine up till about 3800 rpms then it would surge but still not plane. The rpms will surge from 4000-5500 or so but no power.

Does this his sound like a bearing issue with the Lower unit or something? I have heard of lower unit Corrosion issues with Honda’s used in Salt water.

I still have a hard time understanding how the tach can read high rpms yet the boat does plane out... what is lost in translation between tach reading and actual propulsion?

Anyway, sorry for another long post, just trying to provide as much info as possible to generate ideas.

Thanks very much in advance,
-Tim

Edit: I meant to say I have a hard time understanding how the tach can surge and read high rpms but still NOT plane. I accidentally said “the tach can read high rpms yet the boat does plane out”...that was a mistake and just clearing that up.

Are you sure your prop hub isn't slipping?

I thought the hub was the issue so I put a new prop on. Mercury quicksilver prop with their "Flo torq II" insert hub kit. Supposedly that hub kit is less susceptible to spinning the hub. I read that the ethanol In fuel these days creates hotter exhaust and apparently that can be a factor with the pressed in hub kits. Not sure how applicable that is.

Do lower unit issues sometimes come and go? This motor performed well 3 trips in a row, not counting sea trial. One of those trips it was having this issue at the beginning of the trip then it straightened out for whatever reason and ran well the rest of the day. However, the next two trips the motor ran well right out the gate and for the duration of the trips until it was time to head in, haha. This was after idling for some time will drifting.

Now it seems the problem is here to stay until I figure out what it is, if I do. Anyway, thanks again!
-Tim

so now do a drop test under load, pull plug leads off one at a time at full throttle under load to see if all cyls are firing.

iang6766 said:

so now do a drop test under load, pull plug leads off one at a time at full throttle under load to see if all cyls are firing.

Click to expand...

What’s the best way to go about this?

Avoid falling off the back of the boat or getting a shock. Get a ignition coil bench tester that will stimulate coil function and the output i know Ford make one that's called a Innova coil bench tester .I dont know if Honda makes something like this for load testing ignition coils.

I usually tie it up securely or run at 3/4 throttle on a trailer, never out on open water

Tegweni said:

Avoid falling off the back of the boat or getting a shock. Get a ignition coil bench tester that will stimulate coil function and the output i know Ford make one that's called a Innova coil bench tester .I dont know if Honda makes something like this for load testing ignition coils.

Click to expand...

Thanks for the heads up about the ignition coil tester. I'll see what I can dig up.

iang6766 said:

I usually tie it up securely or run at 3/4 throttle on a trailer, never out on open water

Click to expand...

Yeah I was curious what kind of stress may or may not be put on the motor running it in a barrel on the trailer to accomplish a drop test under load. Thanks again.

Finally have an update for anyone that may run into similar issues down the road.

Turns out to be a prop issue after all. I thought maybe my lower unit was slipping out of gear or something but it appears the after market props I was trying were simply ventilating and blowing out. I went back to an OEM Honda prop and got the boat on plane and up to about 30mph yesterday. However, the boat still isn't propped quite right. I can't really cruise at a speed that would be good for fishing with a bait tank and gear in the boat. With the 13.25x17 prop that is on the boat now the boat has some top end but when I back off to cruise at about 4k rpms it wants to fall off plane.

Anyway, just wanted to update. Thought I had engine problems and turns out I have prop problems. I may end up lowering the motor and seeing if that helps. Maybe my transducer is what is causing the ventilation problems.

Thanks again for all of the help.

After doing more research I called Ken over at propgods. I went with the prop he recommended, Powertech RXB stainless 4 blade 13 1/4x14. Was able to get the boat out for a test run with the new prop today. This prop stays hooked up way better throughout the rev range. I was able to get up and plane consistently and cruise at 4000-4200 @ 20-23mph on the gps. Thanks very much to Ken at propgods for taking time to hear about the problem and diagnose it best he could over the phone.