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Old 25 October 2012, 04:06   #1
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Country: UK - Wales
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Old mercury 80 - how easy to work on, how thirsty?

I am looking at buying Steve66's old Glastron (too poor for a rib). This boat comes with an old Mercury 80. The engine is probably done relatively little work, but needs a service. How difficult is it to change the impeller on one of these engines? It was pretty trivial on my baby newish 4hp Yamaha, but everything came to bits exactly as it should then. The Merc has almost certainly been used in salt water.

How thirsty is this engine likely to be? I have seen comments that it will be bad, but how bad is bad both in terms of actual fuel usage and in comparison to say an engine only 10 years old...

Are there any known problems with these engines?

All the best

David
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Old 25 October 2012, 06:36   #2
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Great engines, easy to service,

Get a manual, get it going, before you buy see it running, check the compression out, should all be within 10% of each other and around 100psi+

If you buy, a kit of service parts is inexpensive from KEYParts in Kings Langley or maybe better use Blackdog on here. A complete set of service bits is about 70/80.

Regarding fuel use then it will use more than a modern O/B but the cost wouldn't in my view justify the significant expenditure.

My old merc 60 2S can use about 1,1ltr/nm or more but its a very good weeks holiday when i use 200lt +.

Do the servicing yourself, be guided by the manual and the forum and use the american iboats forum, which is also very good.

I really like old mercs for being reliable and simple, One top tip is to drop the LU off asap as the longer they get left after use in salt the more difficult they are to drop the impeller is then easy to replace.

If you buy post some pics and how you get on.

Good luck Dave
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Old 25 October 2012, 06:37   #3
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David,

I had a quick look at the for sale section - couldnlt see it - any chance of either a linky / pic or tell us exactly what flavour of 80 it is?

What I can say with reasonable confidence regardless of the age is that an impreller change on an 80 will be no differnet (bar the space below the engine you need to get the shaft out the leg) than a 4. half a dozen bolts fto hold the gearbox on, maybe one of the gearshift (unless it's got a rotating gearshift & it's simply another spline) - drop. One bolt will inevitably be huidden under the anode!

As for fuel economy, I;m probably going to start a big debate here where people wil ltell you a 4- stroke is soooo much more efficient, but unless it's a really old one from the 1960s, chances are you won't notice a lot of difference at WOT regardless of the age. Fuel consumptrion drops only really kick in if you do a lot of idling / low RPM. 1L/ nautical mile seems to be a good guess whether you have a 25 on the back of an SR4 or a 250 on something a lot bigger. I am getting about 0.83L/NM with a 1970s 60 on a 5m boat. Do a search in both the engines & ribs & ribbing sections for "fuel consumptrion" or "fuel economy" - plenty of posts to rwead about it.
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Old 25 October 2012, 07:04   #4
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mercury 80

Hello, I dont want to comment too much as its my engine and boat that im selling as my comments may be construed as being biased, but here is a short clip of the engine running a couple of months ago.
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Old 25 October 2012, 07:15   #5
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The engine is old - probably early 70s at a guess and I am pretty sure that it has not been maintained for at least 10 years... so if things tend to seize they probably have on this engine! It does apparently run though. I'll post the picture Steve sent me, I hope he doesn't mind.

Thanks again

David
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Old 25 October 2012, 08:34   #6
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Hi, I dont mind at all.
Steve
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Old 26 October 2012, 06:32   #7
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Ok, so it's an 80Hp version of mine!

Impeller change is easy. 5 bolts round the bottom of the leg, one hidden under the trim tab. (remove the trim tab by the bolt accessted through the grommet on the top above, if it's still there! Only thing to remember is engage forward gear before you drop it - the gearchange is a rotating rod rather than the more conventional "up down" motion, and the gearbox will spring into forwards if the change rod isn't holding it elsewhere by engaging forward at least the shift mechanism is in the right place when you reassemble. When reassembling, just make sure the gearchange splines are engaged before you tighten the bolts. the drive shaft will have a spring loaded pin on the top end, so no faffing about with packing shims etc.


The only thing that the 50/60 clamshells were reknown for was destroing their top piston. Hopefully Uncle Al or or TurboD will be along soon to confirm or rubbish this, but IF the 80 is a similar design, on mine the top carb has a slightly bigger (+0.002") jet to prevent the top cyl running too lean & hot. THere was also a small reduction in spark advance & max RPM to prevent those failures, but this may not apply to the 80, and if Steve's engine is running then it's either not a problem on the 80 or his has already been modified -there is no "halfway" with that failure - it either runs or it's f*cked".

As for fuel economy, they are lightweight engines for their HP, which seems to counter the inefficiencies of an old design. As I have said above, I get 0.83L/NM out of mine on a 1/2 ton deep V hull. IMHO at WOT 80Hp needs a certain amount of fuel to generate the gee gees, and so the economy at Vmax is going to be on a par with most 80s. If you spend all day pottering at idle, be parpared to cry.... I did a canal trip in mine to the Falkirk wheel 10th anniversary. Spent 7 hrs between idle & 1500 rpm doing pretty much walking pace & got 2L/N Mile out of it!
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Old 26 October 2012, 06:43   #8
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We did up one of those old beasts...


http://www.rib.net/forum/f36/1978-me...0-a-47513.html

It stripped the crankshaft splines on its maiden voyage!

I have plenty of parts should you need any.
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Old 26 October 2012, 07:15   #9
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You can date the engine from these side decals

http://www.nymarine.ca/SIDE%20DECAL%...ES%2072-83.gif

http://www.nymarine.ca/SIDE%20DECAL%...ES%2084-93.gif

Editing: another photo added



my Merc 500 50hp ( 1976 ) uses 5 gals per 55 mins/60mins at WOT ( 5450rpm ) giving 42mph using a 17" alum prop.
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Old 26 October 2012, 07:49   #10
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mercury 80

Hi All, Its a mid 80s engine, top piston is fine and doesnt need any parts (at the moment lol) apart from usual service items and it absolutely flies!!
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Old 26 October 2012, 08:38   #11
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Thank you for all the replies.

With regards to efficiency, it sounds like the engine is not going to be enough worse than a younger one to be worth worrying about, but I am glad I had the opportunity to brace myself for the running costs! It sounds like it is going to use at least 1 gph at tickover rising to perhaps 6 - 7 gph WOT. I think I am going to need more than one 25lt tank! The Glastron hull is less steeply Vd than most ribs so there should be an efficiency gain there, but it will ride more harshly in a chop. One of the jobs I have for the boat is teaching scouts, which will be lots of low speed work. I'll just have to charge extra for those sessions

A1an, it is a shame to hear that your engine broke. I found that thread earlier and was impressed at you dedication. I hope that the splines failing is not a common fault! On the other hand I may well want spare parts when I try and get the thing to bits.

It is good to hear from Steve that the engine is slightly less ancient than I thought, also confirmed by matching with the decals - thanks T15.

I'm also glad to hear that changing the impeller is generally easy, thanks 9D280, and that normal people would consider using an engine this old.

The biggest problem left seems to be transporting the boat. It is tempting to just launch it and go, but I am definitely not going to try that. There is scary water on the way (Bardsey sound, Caernarfon Bar etc), I haven't done the impeller and I really don't have the experience. It would also cost a bob or two in petrol! Asking to borrow a trailer is a bit much as it would have to be adjusted to fit the boat. Buying a trailer is going to make this too expensive...

Thanks again everyone

David
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Old 26 October 2012, 08:49   #12
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trailer

Hi David,
There is a galvenised trailer in usable condition for sale in the same boat park that i could find out how much the guy wants if its any help?
I dont think he wants a kings ransome for it
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Old 26 October 2012, 09:00   #13
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If you would see about the trailer Steve, that would be great. I have booked Tuesday off next week to go and look at the boat. I'll probably look at the boat in the morning and then take the kids for a walk or something later on if the weather is kind.

Is there a fuel tank with the boat?

All the best

David
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Old 26 October 2012, 10:19   #14
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If you need a tank I have the original tank that came with the engine. Free if you want to pick it up/arrange collection. It could do with a new hose.
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Old 26 October 2012, 10:23   #15
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Hi There is a tank with it but i doubt it is much good now. We used the tank off our other boat when we ran it. Will need a battery too
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Old 29 October 2012, 06:09   #16
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Quote:
Originally Posted by steve66 View Post
Hi All, Its a mid 80s engine, top piston is fine and doesnt need any parts (at the moment lol) apart from usual service items and it absolutely flies!!
Steve,

Sorry, I have re- read my post and realised I maybe didn't emphasise the tenuous nature of the link between the two engines (they have different cyl head / blocks) - I wasn't trying to put him off - I'm a fan of the Clamshell!

IF (and that is an if!) the 80 suffered the same problems as the 50/60, a mid 1980s machine should have all the modified bits in already, as proven by yours still running!
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Old 29 October 2012, 06:19   #17
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No Problem, no offence taken
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