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Old 29 July 2016, 16:48   #21
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Yeah that's not a bad shout, I did something similar with my last motorbike only using blowtorch to warm the plugs gently...she ebough it fits up - turned out to be a weak spark and a dodgy HT lead. I will try again when back in Bristol. I've just been round I circles as originally told it had no spark.

Luckily I'm in Cornwall for the weekend with the 6hp motor as a backup! Off out around st Michaels mount for a spot of fishing tomorrow so the project engine will have to go on hold...
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Old 29 July 2016, 17:04   #22
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Thumb over hole means nothing, you'll blow your thumb off at 30psi...

Put it on a compression tester. Sub 100psi or a 10% imbalance means it's shot.
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Old 29 July 2016, 17:05   #23
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>>>What would cause the spark to be weak under compression

The voltage needed for a spark to jump the plug gap depends on the amount air is compressed above atmospheric. So the standard basic test of looking for a spark when the plug is out of the cyl is quite basic and doesn't represent the condition of an engine running or trying to start. So old plugs, iffy leads or weak coils could all result in a weakened voltage that can't jump the gap under compression.

Also on the "path of least resistance" theory if under compression the spark is reluctant to jump the gap for any of the reasons mentioned above it will look for an easier path that might be to earth through carbon/oil/fuel contamination.... but you wouldn't see that earth leakage testing plugs out of the cyl.

Hence my thought brand new plugs wouldn't go amiss as they're cheap for that engine.
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Old 29 July 2016, 17:52   #24
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>>>I've tried a little bit of everything, choke no choke, primer and easy start. I'm not exactly sure what you mean

Well on these engines the "choke" knob does three things... by turning clockwise it advances the ign a little to speed up the idle and so set the timing right for starting... then as you pull the knob out and push back in it works a plunger/diaphragm on the side of the carb that injects neat fuel each time you do this... and finally the knob left pulled out is the start "choke" position.

So I wondered which combination of these actions you were using?

Also I've noticed with these engines they often don't need anything more than the idle increased to start in the summer... using priming/choke too can mean they will not start unless you take the choke off... don't operate the plunger primer and then pull over loads with the throttle wide open.

Also as these engines have the gearchange/throttle combined it isn't possible to open the throttle wide on the twistgrip to clear or rev in neutral a possibly flooded engine. You have to take the hood off and operate the throttle lever on the side of the carb.
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Old 30 July 2016, 05:21   #25
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Richard are you Richard2B... who has posted about this engine on Fribs?

Assuming so I note you believe this OB is a 2006 10hp leg fitted with a 15hp engine of uncertain but likely greater age. Also that it has the tiller removed which will mean the stop wiring will have been fiddled with... and it has no fuel filter.

This is all extra helpful info for those on here trying to give advice.

For a start you pose the question on the other site about the fuel filter... yes it should have one just above and to the right of the carb as you face the engine from the front.

Here are two images of my unmolested Mercury 10hp from last year... you can see the fuel filter bowl in the left image. The top of it mounts by being pushed into the black moulding adj the pull start.
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Old 30 July 2016, 05:34   #26
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Also something else has come to mind...

There was an upgrade to the whole ign system around 1999/2000. The parts are not interchangeable. The whole lot must match which includes the flywheel, ign stator(pickup) and the CDI box (switch box).

So if your powerhead has been swapped and is older than the original one then you need to ensure each of the components I mention above is either pre 1999 or post 2000 for them to match.

In the image below when I had both a 1997 (on the right) and 2006 at the same time you can see the different looking CDI units.
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Old 30 July 2016, 09:13   #27
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but a 10 compression tester, Nos4r2 has hit the nail on the head, if compression is ok then invest in a manual you could be wasteing money just buying spark plugs
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Old 30 July 2016, 12:25   #28
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Hi Fenlander, yeah that's me with the molested engine; I need to establish what's original and what's not.

I've actually reinstated the fuel filter (from my 7.5hp donor mariner), I also intent on putting the tiller on and using the gearing mechanism manually on the side like the earlier models, on the basis it runs.

From what people have said a compression test is a good place to start.

Having had another look this morning, before an awesome day out from Penzance and around St Michaels Mount on the 6hp, that actually fuel migh not be getting through. I tried warming the plugs to no avail. And actually whilst I was messing with the plugs they did seem bone dry, even after they'd cooled and I had another go.

I did strip and rebuild the carb and it all looked pretty good, however My air tools are back in Bristol and there's a chance something could still be blocked.

Perhaps a slightly unorthodox approach, but if I was to swap the carbs over for one which I know runs (6hp), assuming everything else is good then might this fire her up? I know it won't run great, but just by way of process of elimination as well as a compression test.

Thanks
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Old 30 July 2016, 12:39   #29
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Thanks for the pics as well Fenlander, pretty much what I was trying to swap over - the old style CDI with the new. If they're not cross compatible then a new CDI could be pricey (assuming the engines not shot and I can't resolve it with a more thorough carb clean).
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Old 30 July 2016, 13:14   #30
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So if you have put fuel directly into the pots with hot/ warm plugs and it doesn't fire it takes out of the equation fuel right from the tank to the reed block? You have a spark but no idea how strong under compression assuming correct plugs/ gap so as the guys say 10 for a compression tester and what ever the plugs cost you will no know the engine is in good or bad Nick and that as fenlander has said parts not compatible for a spark at that point I think it's fair to say this might not be a viable project to sort in terms of dosh & time but might be worth looking for a runner and keep this for parts or sell parts.
Easy for me to say but this could easily drain a lot of money and you still have a ringer at the end of the day IMO

Cheers
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