Honda 40 4 stroke RPMs?

[MinUph]

What should to RPMs be on this size motor? I'm seeing 45000. 4500 sorry:facepalm:

 

[cp]

Re: Honda 40 4 stroke RPMs?

What should to RPMs be on this size motor? I'm seeing 45000.

Wow! you must be flying at 45000 rpm!

Sorry, couldn't resist. Honda recommends that you be propped to achieve between 5000 and 6000 rpm at WOT. You're 500 to 1000 rpm below (not 40000 rpm over) where they recommend, if the rpm you report is at WOT. That indicates in general that your prop is too high pitched, if nothing else is wrong. What prop do you have on the BF40 now? What top speed does that prop get you?

 

[MinUph]

Re: Honda 40 4 stroke RPMs?

Yea I guess I added to many 00's huh. Sorry bout that. I don't have the boat yet just test drove it today. Will be picking it up on Monday and will check the prop then. I'll be having the motor serviced also so I'll know more. It ran nice and smooth. Did shift a bit hard but I will have that checked out also. Is that a possible adjustment?

 

[cp]

Re: Honda 40 4 stroke RPMs?

No problem about the extra zero - wish I could do that to my paycheck - just had to rib you about it a wee bit.

Good to see that you say the motor ran nice and smooth. What kind of boat? What was top speed at the 4500 rpm?

If the boat/motor is brand new, the dealer should swap out props to get you into Honda's recommended WOT rpm range.

If the boat/motor is used, then you're kinda on your own. Once you're sure the motor is operating as it should - a proper servicing should determine that - then you'll have to find the right prop to allow getting into that 5000-6000 rpm range at WOT. WOT rpm of 4500 is an indication that the motor is over-propped, and that's not a good thing since the motor will then be overloaded and lugging, putting extra wear and tear on it.

When you get a look at the prop, note how many blades it has, its diameter, and its pitch (diameter and pitch may be stamped or engraved on the prop). The number of blades, diameter, and pitch coupled with the WOT rpm and speed will give a benchmark for determining the best guess as to what prop you should try to get into the recommended WOT rpm range.

Having the right prop should get you the best all-around performance and economy while prolonging the life of your motor. Keep in mind that not all props are created equal so there may be some trial-and-error to find the best one.

The 'clunk' when shifting into gear may be a function of rpm when you shift and the weight of your prop. Is the prop stainless or aluminum? The heavier stainless will usually cause a harder 'clunk'. As long as you're shifting at specified idle rpm then, IMHO, there's not much you can do about the 'clunk'.

Aloha.

 

[MinUph]

Re: Honda 40 4 stroke RPMs?

Well I picked her up today. The prop on the motor is a C3 12.1 x 9
What do you think I should try. The boat is an 18' Godfrey.

 

[cp]

Re: Honda 40 4 stroke RPMs?

Well I picked her up today. The prop on the motor is a C3 12.1 x 9
What do you think I should try. The boat is an 18' Godfrey.

First off, let me say that I have no experience with 'toons. So, I have no idea what top speed you'd expect to get but (from the Godfrey website) with a 'toon weight of about 1420# and max passenger weight of about 1270# and just 40 HP (max rated 65HP) I'm thinking about 17-18 mph based on Honda performance tests of a similar 'toon with a BF50 (see link).

Your BF40 should be propped to operate between 5000 and 6000 rpm at WOT to achieve that. Verify that your tach is reading correctly and the motor's throttle linkages advance all the way to the stops before chasing after a new prop. Did you try different motor trim angles? Different props have different characteristics from their design so keep in mind that it may take some trial-and-error to find the 'just-right' one. If you have a nearby prop shop that will let you try different props, that'd be ideal. If you have buddies with different props you can borrow, that's good too.

OK, so, if you're good to go with your engine and tach and want to try a different prop to get into the specified 5000-6000 rpm range at WOT, the general rule of thumb is that a change of 1" in pitch will give a change of 200 rpm. Since you have a 3-blade 12.1"-diameter by 9"-pitch on the BF40 now, you'd want to come down to a 6"-pitch but there doesn't seem to be any props made with pitch less than 7" (that's why I asked that you verify your tach reading correctly and your motor linkages going to the stops).

Honda sells an aluminum prop made by Solas with 3-blades, 12?"-diameter, and 7"-pitch which should get you up to about 4900-5000 rpm. If you're willing to spend a little more for a stainle$$ prop, here's a link to one specifically designed for 'toons (note that it's available down to a 7"-pitch). A stainless prop will be thinner and more rigid and the one in the link may have cupping and other design features that will help you get more 'oomph' out of your BF40.

Good luck.

 

[TD_Maker]

Re: Honda 40 4 stroke RPMs?

My Dad has a 18 1/2 ft Bennington Pontoon with a 1999 BF40 Honda. This little motor is really comfortable at 5500 RPMs at WOT. I have never really clocked this boat for speed, but I know it is not a speed demon either. But what pontoon really is?

I have serviced this little engine (for my dad in Kentucky) since it was brand new. What a fantastic dependable little motor that has never let the old man down. I change the oil and lower unit fluids once a year. Plugs have been replaced twice (not that they really needed it) I have changed the impeller twice since new. There is not a Honda dealer within 150 miles of dad's house, so it is important to me that the motor is trouble-free for him. Great little motor for a pontoon boat.

 

[MinUph]

Re: Honda 40 4 stroke RPMs?

I have serviced this little engine (for my dad in Kentucky) since it was brand new.

TD,
We took her out today for our maiden voyage, The motor runs well over 2000 rpm anything under and it is rough. At idle it goes from 1500 to 2100 rpm. Never idles down properly. When I test drove the boat I didn't run at idle speed. Live and learn right. Anyway this is also the cause of the banging when I put it in gear. What could this be caused from? I do know some about outboards from a maintenance point of view. I changed the oil, gear lube, and plugs before today. I know the gas in the tank was probably not fresh but the guy did add Sta-Bil to all the gas he used. In reading the manual it is OK to run on up to 10% ethanol. I believe the tank is about 12 gallons, I added 6 to fill it. The only Honda dealer around me is not very reliable and I'm not confident in him. I know other mechs but I would like to understand the issue. Maybe adjust myself or have it done. But I want to know what it might be. The carbs were rebuilt not long ago. I think they need some tweaking according to the seller.
Thanks for any input.
Paul

 

[TD_Maker]

Re: Honda 40 4 stroke RPMs?

As far as the low speed operation, it sounds like you have some gunk in the carbs. The low speed jets could be clogged and not letting the little motor idle down like it should. I would guess that with the high ilde speed, you are forcing the engine into gear; thus, this is perhaps the "CLUNK" you are hearing.

I would pull off the carbs, and clean/rebuild them again. You say the carbs were rebuilt not long ago; however, just one trip with bad (dirty) gas could screw them up again. What did the seller mean by "tweaking?" That could mean anything really, so I would start from scratch and make sure the job is done correctly. Also, make sure you replace the fuel filter which is mounted inside the cover on the BF40.

Here is one other thing to check. Dad had a problem with a rough running engine two seasons ago. I noticed the engine was leaking a little gas right from where the fuel line hooks to the engine. I replaced the fuel line connector and the motor straightened right out. I think the motor was sucking some air from the leaking fuel connector. Simple $5.99 fix.