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Message |
    
Martin Vincent
New member
Username: wheelnut
Post Number: 1
Registered: 07-2010
| | Posted on Thursday, July 01, 2010 - 12:35 pm: | 
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I have a Stuart P55 twin in my 21ft clinker-built sailing cruiser that has a tendency to cut out on the rear cylinder after a few hours of use - usually when I'm about to enter Portsmouth harbour and when I really need the power!
The rear plug oils up and I find that the rear crankcase has a build-up of oil that needs draining. This used to happen very occasionally but it has got worse of late. It may be due to the type of oil I'm using (Quicksilver - a synthetic 2-stroke oil) so I'll try changing to a mineral oil.
Also, I have been using a cooler plug in the rear as recommended by Fairways (NGK B5HS for front cyl, B6HS for the rear), a system which has been fine until recently. I'll try using B5HS for both cylinders to make the rear run hotter and therefore less likely to oil up.
With the engine apart for decarbonising, I notice that the head gasket has five coolant holes for the front cylinder but only one for the rear. Is this the correct way round, and why is it that the front benefits from better water circulation? Could this be the cause of the front plug running fine and the rear oiling up?
Also, I wonder if fitting a thermostat is a good idea. I do have one but never fitted it before as it seemed to me to be one more thing to go wrong. But perhaps the engine is running too cool without the thermostat, hence the oiling-up. Doesn't seem likely bearing in mind most Stuart's seem to run without one.
Any advice would be welcomed. |
john.kenneway
New member
Username: jbz2079
Post Number: 1
Registered: 09-2009
| | Posted on Thursday, July 01, 2010 - 02:08 pm: |
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I wonder if it's possible that the rear cylinder is pulling oil through it's rear seal from the gearbox? as both the cylinders are fed from the same carburretor this would make you think if it was a fuel mix or carb problem effect both cylinders. as for the thermostat, any Stuart turner I've ever had anything to do with has never had a stat fitted. |
David Ball
Member
Username: daveincaol
Post Number: 4
Registered: 06-2010
| | Posted on Thursday, July 01, 2010 - 05:20 pm: |
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Hello
I have no experience of the engine directly but do have a copy of the parts list that has small drawings including the cylinder head gasket. It appears from the drawing that apart from the orientation there is no difference in the number of holes surrounding each cylinder, and that five on each provide for the cooling water circulation.
Regarding a thermostat. The S-T catalogue offers a thermostat as an extra and states that it is available for those who idle the engines for a long period ie fishermen. I suggest that 1 way to check your own installation would be to clip a thermometer to the outlet pipe. S-T suggest a running temp of 80deg C. |
Matthew Balkwell
Senior Member
Username: matt
Post Number: 115
Registered: 04-2006
| | Posted on Thursday, July 01, 2010 - 05:50 pm: |
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We now use champion L90C plugs in our p55 and have found them to be the best for low speed runs as they are a hotter plug and less prone to oil up.
You have excess oil in the rear crankcase because the plug has oiled up and ceased firing, therefore the oil is not being expelled as exhaust.
After every run we do there is always about a teaspoon in both crankcases.
If we have done some slow running for a while , we give the Stuart a few more hundred revs for a minute and it seems to clear the plugs.
As for cooling water temp, after about twenty minutes running some steam is expelled with the exhaust and cooling water.
We have always been hesitant about the engine running hot but three years running in the Australian heat have not done it any harm.
Matt and Ron. |
Martin Vincent
New member
Username: wheelnut
Post Number: 2
Registered: 07-2010
| | Posted on Friday, July 02, 2010 - 01:20 pm: |
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Thanks for the suggestions.
The root cause seems to be that the rear cylinder operates at a different temp than the front one due to the inherent design of the Stuart Turner cooling system. That's why Fairways Marine recommend different grade spark plugs for front and rear cylinders.
The head gasket does indeed have five cooling ports around one cylinder and only one for the other one. I have three gaskets as such and it is shown thus in a drawing in my original P66 parts and instruction manual. It may have been different for earlier years. But why the change?
The oil in the rear crankcase is not coming from the gearbox as the level has not dropped over the past three years of use. Also, the oil build up is not a result of the rear cylinder failing to run - it is quite the oppposite - the rear cylinder fails to run as a result of the build up of heavy fuel.
My engine does seem to run quite cool though, which may be part of the problem. The cooling water is never really hot to the touch.
Still unsure as to which way round the head gasket goes though. Should it have more coolant holes at the front or at the rear? |
David Ball
Member
Username: daveincaol
Post Number: 7
Registered: 06-2010
| | Posted on Sunday, July 04, 2010 - 12:47 pm: |
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Hello Martin
Quite a puzzler this one. Certainly the gasket on the P66 isn't symetrical and it would be interesting to know why. But your engine is a P55 and so far I haven't come across a diagram showing the head gasket to have anything other than five holes surrounding each cylinder for cooling water.
On your engine water to one engine would be resticted compared to the other and the unrestricted cylinder would have a lot of water flowing around it.
Another point is that the parts list for the P66 seems to indicate that a thermostat was standard, (there isn't an annotation that states 'where fitted').
Have you confirmed the engine type and perhaps had its number dated?
Dave B. |
David Ball
Member
Username: daveincaol
Post Number: 8
Registered: 06-2010
| | Posted on Sunday, July 04, 2010 - 12:57 pm: |
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Martin
Check this thread "P66D No2 Cylinder gives up" perhaps this relates to your own problem.
Dave B. |
Martin Vincent
New member
Username: wheelnut
Post Number: 3
Registered: 07-2010
| | Posted on Monday, July 05, 2010 - 01:43 pm: |
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Just read the thread that David mentions above.
I don't think it's the rear seal as there has been no loss of oil from the gearbox, although there is certainly a lot of unburnt oil escaping down the exhaust.
My engine is actually a bit of a hybrid P55/P66. It started off as a P66 but when my boat was out of the water for restoration, I drained the cooling system so as to avoid frost damage. It was a bad move as the cylinder block, the exhaust elbow and the expansion chamber all cracked and I had a heck of a job getting them off, as the pistons were almost seized due to the distortion. So much for draining the system to avoid damage...
Anyhow, I bought another engine - this time a P55, and removed the cylinder block, head, pistons and expansion chamber cover from the P55 engine and fitted them to the P66. Everthing seemed to be identical except for the expansion chamber design. This is a hollow casting for the P66 but, with the P55, the casting is a much simpler design and the exhaust gases are separated from the coolant by means of a solid copper gasket, rather than the casting itself.
Also, there are differences in the way the coolant is routed - my P66 had a pipe leading to (or from) the expansion chamber cover whereas the P55 has the pipe to (or from) the cylinder jacket instead.
When taken apart, both engines had an identical head gaskets (with five coolant holes for one cylinder and only one for the second cylinder). |
