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Old 03 August 2016, 02:50   #41
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Country: UK - England
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>>>It flows great at first and occasionally stops completely - I didn't run it for long in fear of over hearing.

To expand on Stig73's replies to that...

If your previous experience or expectation is of a cooling tell tale that always pumps like a water jet from idle speed upwards (as many OBs do) the normal behaviour of the Mercury/Mariner tell tale looks likes something's wrong.

There is almost no stream when started and then at idle there is an erratic and intermittant stream. I always advise popping the thermostat out on any "new to you" motor of this type so you can test without to make sure you're happy with the true stream. Also gives you the chance to clear any salt deposits from the stat and test it in a saucepan. The amount of salt deposits in the stat housing will give you a good idea how well it's been flushed post outing in the past.

If held at a very fast idle in the test tank... and once under load on the water... the tell tale should pump all the time.

If yours pumps well immediately when started and then stops firstly I'd guess the stat has been removed... and a bit of old broken impellor rubber, old gasket debris etc could be moving through some part of the cooling passages until it blocks... then falling back when stopped to go through the same cycle next time.

You are right to be wary of leaving it running like this... I'd have the stat out temp and see if that changes anything.

Some folks are stressed by the way these motors behave at idle and just take the stat out. I never do being happy once I know there is a good flow with stat temp out and the stat is either new or tested in a pan of water.

BTW just for interest which gasket had fallen apart? Some part of the carb/inlet or crankcase related?
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Old 03 August 2016, 04:20   #42
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I'll whip the thermostat out tonight and take a look. I also intend on servicing the impeller so that should identify it's integrity; good for piece of mind also!

I attach a couple of pictures, not the neatest of jobs but at the time I was thinking hmmm I wonder if this is could be the cause of the fault! On the parts diagram the plate is part No. / called "826383F 1 - Intake Cover". It doesn't actually appear to show a gasket which is slightly strange.

I don't understand why it's an intake cover - since it's meant to be sealed. It only really served as an inspection chamber showing the condition of the pistons (to which No.1 piston shows a little scoring.. I've seen worse!).
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Old 03 August 2016, 05:13   #43
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That's interesting and not a plate that would often be taken off in the life of an engine.

Offering advice online is difficult and we all advise slightly different preferences for a first thing/things to check.... all valid.

The only reason why I don't go for an initial compression test is I've found most engines will at least start... even if they take a few pulls or run badly... with low compressions. Also on a 2-stroke as important as the cyl compressions is the crankcase sealing which ideally needs a pressure/vac test at the same time as you do a compression test to give a good base idea of the engine condition.

The crankcase pressure/vac test would have found your problem immediately but I really never expected issues in that area... but perhaps should have done knowing the engine had an uncertain history.

The slight piston scoring could be due to this air leak if it had been present to a lesser degree for a while as it would be "running lean".

You've got a Honda monkey bike then?
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Old 03 August 2016, 06:53   #44
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All of the comments, suggestions and advice have been spot on and actually really helpful for future diagnostics of cars, bikes and boat engines! I should have arriving today in the post a compression and spark test, which is sure to make future diagnostics a doddle; I'd always just battled on though without...

I see you have an eye for quality Fenlander, indeed an original 1970 Honda Dax (ST70). My pride and joy, recently registered back on the road. Not rib.net related but a picture for your pleasure; St Ives where I hope to play with the SIB this summer!
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Old 03 August 2016, 07:10   #45
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Country: UK - England
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Posts: 3,241
That's a great picture and a fun bike in fantastic condition.

In the mid 70s whe I was a teen my aunt (still going at 83 but carving up the pavements on a mobility scooter these days) was the coolest woman and rode one of those in the same blue as well as a Honda CB400F-4cyl of that era. Her husband who is 89 now only gave up riding his 1000cc BMW at 87!

There's hope for us all.
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