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Author Topic:   Flushing 90-HP Engines (Yamaha c.1989)
gil carpenter posted 04-21-2004 01:20 PM ET (US)   Profile for gil carpenter   Send Email to gil carpenter  
I'm a first time boat owner, having a great time learning. Can I flush my 1989 yamaha 90-HP motor using rabbit ears? Or do I have to put it in a tank like the manual says? If I can use the rabbit ears, where are they attached? On the lower unit small vertical vents near the prop? Or on the vents at the anti-cavitation plate? Because of the "plate", I don't think they'll mount there and get a good pressure seal.

I'm hoping to get it in the water this weekend.

Bigshot posted 04-21-2004 02:10 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
you can do it 4 ways:

1) In a tank but tank has to be big or sometimes water will not get sucked up. Not common unless you own a marina.

2) Earmuffs($5) which go on the [water intakes] AHEAD of the prop on the lower unit. Just above the hump where the prop shaft and gears are. Most common way.

3) There MAY be a small screw above your anticav plate that says FLUSH. If so you can buy a flusher from the dealer that screws in there...earmuffs are easier.

4) Buy a backflushing kit from fellow member Steve Leone that goes where your tell tale is now. He sells them on e-bay.

gil carpenter posted 04-22-2004 05:36 PM ET (US)     Profile for gil carpenter  Send Email to gil carpenter     
Thanks Bigshot. I got the earmuffs so I'll go that route for now. Should I do anything with the vents at the anticavitation plate while flushing? Thanks for the lead too, I'll check it out. Hoping to put in this weekend.
gil carpenter posted 04-22-2004 08:27 PM ET (US)     Profile for gil carpenter  Send Email to gil carpenter     
I assume I need to run the engine when using the muffs, right?
Legobusier posted 04-22-2004 10:47 PM ET (US)     Profile for Legobusier  Send Email to Legobusier     
Yes Gil, you will want to run the engine with the muffs - that will circulate fresh water through the water pump and all other areas that have been contaminated by the salt water. Idle is fine. Personally, I run about 10 minutes or until the engine is nice and warm.

In your case however, there's probably a lot of dust in all the cooling channels since you STILL haven't gotten that thing wet slacker. :)

Seriously, it's simple. I'll come by and show you if you want me to. Just make certain once you start the engine you have a steady stream of water coming out of the confidence stream nozzle. If you don't - something is wrong and turn off the engine quick like.

Legobusier posted 04-22-2004 10:51 PM ET (US)     Profile for Legobusier  Send Email to Legobusier     
A follow up on my note about looking for the water out of your confidence stream nozzle. As noted here:

some engines do require the thermostat to open. Don't know about yours. Bigshot does I'm sure.

jimh posted 04-22-2004 11:55 PM ET (US)     Profile for jimh  Send Email to jimh     
I own a similar vintage Yamaha 70-HP engine (which is virtually identical to the 90-HP save for displacement). This engine produces water from the confidence stream nozzle almost immediately upon starting.

I often run the engine at idle or slow speeds using water muffs.

I set the pressure at the water hose so as to be sufficient to keep the engine cooling stream running, but not excessively high. I am afraid that excessive water pressure might cause a seal to leak--I don't know if that is common or not, but it seems possible if you crank the hose faucet fully open and have high water pressure.

Legobusier posted 04-23-2004 08:20 AM ET (US)     Profile for Legobusier  Send Email to Legobusier     
Good point on the pressure jim.

I do the same thing. Especially after the "pressure tesing debacle" I just went through with my lower unit seals. If you pressure test with air at a max of 14.5 psi, why on earth would you want water pressure of 30 or 40 psi (gussing here) pushing the seals the other way.


islander posted 04-24-2004 07:51 PM ET (US)     Profile for islander  Send Email to islander     
When you run the engine on muffs, remember it will sound a lot louder. It also may run a bit rough. This is okay.

While the vents are rectangular(on my '87 90 hp, round muffs work fine.

Legobusier posted 04-25-2004 10:54 AM ET (US)     Profile for Legobusier  Send Email to Legobusier     
Why would the engine run rougher on the muffs than in the water. Louder for sure, but why more rough? This doesn't make sense to me. As long as the engine is getting cooling as it would in the "real" water, what difference to the engine is there?
islander posted 04-25-2004 11:06 AM ET (US)     Profile for islander  Send Email to islander     
Hey Chris -

When I said a bit rough, I meant just a tiny bit.

My mechanically savy nephew said it may be due to lack of water pressure on the exhaust. Then again, it could just be my engine.

gil carpenter posted 04-26-2004 07:55 AM ET (US)     Profile for gil carpenter  Send Email to gil carpenter     
thanks to all for the advice. My vents are rectangular so I used rectangular muffs and they worked great. I see what you mean by the high water pressure from the hose, although the seal of the muffs was not real tight so water did expel from around the muffs while flushing, not a problem evidently. Maybe I had the water pressure too high. Still had the flow needed at the confindence stream nozzle, thankfully no problems. Definitely louder and MAYBE a bit rougher while flushing. It was a great weekend on the water.
Legobusier posted 04-26-2004 08:18 AM ET (US)     Profile for Legobusier  Send Email to Legobusier     
Lack of pressure on the exhaust? Huh. I don't know. I guess it's possible. There's not very much pressure in 18" of water, but ceratinly more than there is on land :). I've not noticed any difference on my engine with muffs or in the water.

Bigshot posted 04-30-2004 02:20 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I have not had great luck with rectangular muffs, those are made for Force/hrysler I believe. The round give a better coverage on the vents. The ONLY thing water pressure will do is knock the muffs off the lower unit. SOme use a bungee cord to hold muffs on. This is a GREAT idea if you run the boat inside and can't see if they have fallen off, etc. Do NOT run more than 1500 rpms or so. It is NEVER a good idea to rev an engine in neutral but especially bad with no backpressure which in itself causes a lean condition. This is also why they run a bit "rougher" on muffs and have a tendency to not want to idle down or idle on the high side. This si also why they will pop or go ding....ding...ding when you rev them and release the throttle...hence do NOT rev the throttle. If your engine is relatively warm, you only need to flush for less than a minute. Once the salt is washed away....all you are doing is pissing off the neighbors and fouling your plugs more.
Bigshot posted 04-30-2004 02:25 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Actually water pressure at the surface is twice that of atmospheric pressure. Go down 30' and it becomes 4x.
gil carpenter posted 05-10-2004 08:20 AM ET (US)     Profile for gil carpenter  Send Email to gil carpenter     
thanks bigshot and all others. A relatively experienced neighbor boater and fellow 23' Whaler owner suggested I flush for 10-15 mins. I thought that seemed excessive but that's what I've been doing. I've been flushing withing 30 mins of getting her out of the water so the engine is still a bit warm. Next time I'll reduce that time considerably. Maybe my neighbors will like me better. thanks.
jimh posted 05-16-2004 12:25 AM ET (US)     Profile for jimh  Send Email to jimh     
The engine may run differently when operating its exhaust into air rather than when exhausting under water. The engine tuning may have accounted for some restriction or back pressure of the exhaust created by the underwater exit that is normal when the engine is in the water. When running the engine with "ear muff" cooling, the exhaust is going directly into the air, and there is probably much less restriction than normal.

On some engines I believe there is an exhaust relief valve which helps some of the exhaust exit at idle or slow speeds without using the through-hub exhaust under water. As boat speed increases the through-hub exhaust becomes more efficient due to the flow of water around the hub which probably tends to suck the exhaust out. At idle the exhaust has to displace a volume of water. Water weighs more than air, so that is going to create more back pressure on the exhaust system.

Chuck Tribolet posted 08-18-2004 11:29 PM ET (US)     Profile for Chuck Tribolet  Send Email to Chuck Tribolet     
Bigshot, that's wrong.

Water pressure at the surface is 1 atmosphere, all from the
miles of air above.

Water pressure at 33' is twice sea level air pressure, not
4x. One atmosphere of pressure from the air, one more from
the 33' of water. At 66' it's 3x, at 99' 4x.


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