Honda outboard descaling advice needed

[Longliner]

I have a 10-year old 8hp Honda outboard which has probably done less than 50 hours from new, all in salt water. Having had problems with blocked cooling channels on previous engines I've done what I can to try to reduce the buildup of muck in the Honda. I always let it cool by running at idle for a few minutes before shutting down, and then run it in a barrel of fresh water for about 10 minutes after every use. I have not had any problems with overheating but I pulled the thermostat out today and there seems to be a fair amount of scale in the water channels that I can see (see photo). Some interesting colours in there - I assume that the thermostat is made from an alloy containing copper.
Given that I have a good flow of cooling water at the moment I'd like to flush the engine with some sort of chemical descaler to clean it out before it gets so bad that I need to dismantle and clean it manually. I'll probably run it in a barrel, possibly with the thermostat removed to make sure I flush all the channels and galleries. Can anyone recommend something that is powerful enough to descale the engine without dissolving the whole motor? I've heard of different chemicals being used but I don't know which would work best for my needs, for example:

• Rydlyme (hydrochloric acid)
• Acetic adic / vinegar
• Fernox DS-3 boiler descaler (sulphamic acid)
• Star Brite Descaling Engine Flush (appears to be hydrochloric and oxalic acid)

Thanks! Once I have a plan I'll post photos to show how well it worked...

[rossymtb]

Any joy with giving it a good clean up?

[paddlers]

I got some cheap vinegar on eBay and tipped it into the tub when I flushed my Tohatsu 20. The flow from the tell tale increased straight away. It didn't need much and when I had a look the thermostat was very clean.

[Lee argyle]

Salt-Away Home Page always get good reviews, I brought some on amazon

[HDAV]

Originally Posted by Lee argyle

Salt-Away Home Page always get good reviews, I brought some on amazon

Supposed to be really good but tricky to find and expensive (not compared to a new motor i guess)

Seen this and wondered about trying it https://www.boatworld.co.uk/blu-thru...r?search=flush

Also been told commercial dishwasher cleaner works and is very cheap (if you have access to it)

[Poly]

In reality any of those potions will work to remove scale (calcium carbonate) and any salt that is 'stuck' to the scale. With the more aggressive ones the issue will be making sure you stop before you start damaging any vulnerable materials (copper, zinc anodes etc would be more of a concern than the alloy engine block) . I'd make sure you are flushing it very thoroughly afterwards as that's when nasty stuff could happen with any remaining treatment. Also how are you going to dispose of your waste?

If you are worried about the damage to the engine the proper Descalers should be designed to minimise/avoid that.

[Longliner]

The first problem ( I now realise) is that 10 minutes at a fast idle in a barrel probably doesn't get the engine hot enough to open the thermostat so all the fresh water does is circulate through the bypass without going near the passages where the deposits actually accumulate.
So, step 1 was a 30 minute fresh water flush using a garden hose connected to the flush port, as per the Honda instructions, with the engine off. The port does not need the thermostat to be open to flush the passages in the head and the water seemed to shift quite a lot of the crystalline deposits. After a few minutes of flushing a lot more water started draining through the leg and less through the tell-tale which I think means that the flow through the head was improving.
I decided that step 2 needed to be an acid flush to dissolve the lime scale and was offered a breathtakingly expensive bottle of diluted hydrochloride acid and 'flushing kit' at the chandlers. I substituted toilet lime scale remover, which turns out to have exactly the same HCl concentration as the 'marine' product, and a £7.99 submersible 12V fuel transfer pump from ebay + plastic box for the flushing kit. I connected the pump to the flush port and recirculated the lime scale remover using the plastic box as a reservoir under the engine at 1:2 dilution for 10 minutes, followed by 30 minutes of fresh water from the hose, followed by rinse with Salt Terminator which apparently contains a corrosion inhibitor (Sodium Nitrite) as well as detergent. At the end of all that I had an impressive amount of sediment in the acid and initial water rinse and cooling passages that look a lot clearer (see photo).
After the flushing I dismantled the lower leg and changed the impeller. Everything looked good and there were no signs of the descaler eating the engine.....yet.
In future, I think I'll give up on the barrel and use the flushing port after each use and possibly the limescale remover every few seasons, but only if it's needed. My boat has a single engine, and the only other source of propulsion is me and a pair of oars so I'd rather keep the engine running if I can!

[Phil M]

I did something very similar with dilute vinegar and a sump pump on my old Mariner 75 clamshell. Took the leg off, stuffed a hose up the water channel, stood the whole thing over a large bucket and circulated the vinegar mix for about 6 hours. Worked a treat.

Phil M

[rossymtb]

That was a certain improvement Mr longliner.
I may give mine a bit more attention from now on instead of just flushing for 5 mins with a hose.