Honda BF40 EFI losing RPM slowly

[goof008]

Hi, I have a 40hp EFI that starts off fine at about 5200 RPM. After about 5 minutes into running at WOT the RPM's slowly decrease to about 4000...still at WOT.
Any think I can look at to try to maintain the 5200 RPM?

It has brand new plugs and has had an oil change. Lower unit oil was changed and prop is brand new 11 3/4 x 10.
it's on a 20 foot pontoon.

Thanks

 

[goof008]

A follow up. I cleaned the bowl and screen on the fuel filter. There was some sand. This time the WOT was only 4400 and it dropped to 3800 as I went around the lake.
Before I left the dock I revved it past 5500RPM in neutral and it was responsive and crisp. When I came back it was hard for it to hit 4200-4400 and it felt very labored to get there.
The water coming out of the tell tale is warm, but not hot and is flowing well.

 

[goof008]

One more thing I noticed. As I raised and lowered the trim, the RPM's dropped 3-400 RPM and seemed to really put a load on the motor.

 

[pvanv]

Never Ever revv the motor like that in neutral. Never. Ever.

 

[Sea Rider]

Never Ever revv the motor like that in neutral. Never. Ever.

Right on, or risk blowing the engine. One story is at load while pushing a boat, a different story is at neutral...

Happy Boating

 

[goof008]

I never rev it that high, but I wanted to make sure it could get to full RPM.

Any thoughts on why its dogging so badly?

 

[ahicks]

First, not to beat a dead horse, but you did say you revved the engine to 5500 in neutral. That's hard on the engine anyway you look at it, and tells you nothing about the engines condition or what the engine can/cannot do. With no load any info you received doing that is meaningless.

If you have them, are you keeping an eye on your red and green idiot lights? It sounds like it's going into "limp" mode to try and keep from self destructing. Its not just water temp that will do that. Oil pressure will do that as well.

If your filters are clean, the oil looks good on the dipstick, and it's peeing luke warm water, it's time to take it to the doctor. Unless you are very good mechanically, where you could obtain and understand troubleshooting guides, you've gone as far as you can.

 

[goof008]

the idiot lights have not shown anything and no alarms have sounded. It had issues with having water in the gas. I have replaced the tank, but not the hose. I will change the hose and see.

 

[Sea Rider]

Is the tach in excellent working order ? Seems it's an electrical issue, component heat us, shorts and engine misses or revs way low. Intermitent electrical failures are the worse to diagnose spot on.

When there's fuel hose issues external air enters the fuel line due to connector o'rings being already worn, dried, doen't seal well. In such case the engine will miss specially when revving middle to max wot rpm range. Report if the same issue continues with a new hose.

Happy Boating

 

[goof008]

I switched out the line., no real improvement. I am running injector cleaner through it now also.
I will bring my Tach out to check the boat tach. i think the tach is a little off as it rests at 900rpm with the engine off and reads a constant 1000 rpm when started. It should be around 850, from what I've read.
The throttle is fairly responsive from low to mid RPM's and throttle position. The end sounds normal. When you go WOT it gets a fairly loud howl and hesitation. It's the same sound my gas blower uses when it's not warmed up or it's on old gas.

 

[Sea Rider]

Check current installed tach with other one in excellent working state, probably with reading issues at wot ?

Happy Boating

 

[goof008]

digital tach reads about 250 RPM below one on the boat, Not enough to be of concern.

 

[Sea Rider]

-250 idle revs should be a concern, if digital tach is reading right indicates that the other tach is bit faulty. Test with fresh fuel and warm engine at least 5 minutes before going full hammer down...

Happy Boating

 

[schematic]

check fuel pressure while testing on lake. see if its dropping.....