Logo
FULL SITE MENU

Honda BF75 question

M

Maynard530

New member
Hi everybody, I'm new to this forum, hoping to find some help. I have a late 70's/early 80's Honda BF75. Runs like a top. While adjusting the idle, I noticed steam coming off of the lower end unit when water would hit it. I have a great stream coming from the pee hole, but just not sure why the lower end would be hot like that, never noticed steam coming off. I did hit some rocks coming in to shore, but the prop still works. Could a seal have gone out somewhere? Thanks.
 
alumarine said:
Is it steam or exhaust?
Click to expand...

I can smell exhaust, but I think its just from the relief holes. When I gave it full throttle was when I really started seeing it. Half throttle or lower I never saw anything... I dont think its smoke because it seems to be right where the water is touching the lower unit, which is warm to touch.
 
I think what you're seeing is perfectly normal. The majority of your cooling water actually exits the engine INSIDE the exhaust pipe itself. This is to cool the exhaust gasses as they enter the extension case. If not for this, the case would get localized hot spots and would be hot to the touch. The paint might bubble and blister in places too.

So, you can understand that some of the water will be superheated and turn to steam. And, the amount of vapor present will also change with atmospheric conditions in your area.....dew point and all that.

So, as long as the engine is running well and not overheating, I'm not convinced there is a problem. Keep an eye on it sure, but unless it looks wildly out of the ordinary, I wouldn't be taking it to a mechanic at this point.

Good luck and welcome to the Forum!
 
jgmo said:
I think what you're seeing is perfectly normal. The majority of your cooling water actually exits the engine INSIDE the exhaust pipe itself. This is to cool the exhaust gasses as they enter the extension case. If not for this, the case would get localized hot spots and would be hot to the touch. The paint might bubble and blister in places too.

So, you can understand that some of the water will be superheated and turn to steam. And, the amount of vapor present will also change with atmospheric conditions in your area.....dew point and all that.

So, as long as the engine is running well and not overheating, I'm not convinced there is a problem. Keep an eye on it sure, but unless it looks wildly out of the ordinary, I wouldn't be taking it to a mechanic at this point.

Good luck and welcome to the Forum!
Click to expand...

Thats good news! I ran it saturday at the lake with no problems. I was curious about trying to adjust the throttle, Im not convinced that its running at full speed. I put a garbage can underneath it while on my boat, thats when I noticed the steam. Maybe I just never noticed it because that part of the motor is under water? It on a 10' jon boat, so its pretty low to the water.
 
If your still worried test motor with a infrared heat gun.place hand in telltale stream,should be able to keep it there indefinitely. Place finger on head of motor for a six second count,no burn she's good to go!
 
johnnygjr said:
If your still worried test motor with a infrared heat gun.place hand in telltale stream,should be able to keep it there indefinitely. Place finger on head of motor for a six second count,no burn she's good to go!
Click to expand...

Thanks johnnygjr. I did hold my finger in the stream yesterday for awhile, kinda fluctuated between cool and barely warm. Thats a good thing correct?
 
That doesn't sound "too far off" but my experience is that the stream should stay warm to MILDLY hot to the touch. If you're feeling it go cool it MIGHT indicate that the thermostat is cycling open prematurely. If that is happening, your engine could be running too cool.
You will not get max performance and the cylinders will carbon up if it runs too cool for an extended time.
Might be time to put in a new T-stat if the one in there is more than a couple of seasons old.
You want the engine to run around 140 to 160 degrees Fahrenheit and stay there after it runs about 4 or 5 minutes.
Good luck.
 
Last edited:
jgmo said:
That doesn't sound "too far off" but my experience is that the stream should stay warm to MILDLY hot to the touch. If you're feeling it go cool it MIGHT indicate that the thermostat is cycling open prematurely. If that is happening, your engine could be running too cool.
You will not get max performance and the cylinders will carbon up if it runs too cool for an extended time.
Might be time to put in a new T-stat if the one in there is more than a couple of seasons old.
You want the engine to run around 140 to 160 degrees Fahrenheit and stay there after it runs about 4 or 5 minutes.
Good luck.
Click to expand...

Where is a good place to measure the temperature on these motors?
 
Yes, you should be able to get reasonably accurate readings at the head with a decent quality infrared temperature gun. The Harbor Freight stores sell inexpensive ones but I don't know how accurate they are. I use Ray-Tek guns and bead type thermocouples with my Fluke multimeter and the temp probe adapter.

You will want to run the engine up off idle speed....say 1/4 throttle..... for at least 3 minutes before measuring.
 
Back
Top