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Dennis Witt
Visitor
| | Posted on Friday, October 13, 2006 - 04:09 am: | 
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I have a Stuart P6D that I purchased new in Tasmaia, Australia in 1975, as an auxiliary in a small yacht. It has been used for occasional short periods of running, usually at low throttle settings. Apart from a few minor problems with the water pump along the way, it has run faultlessly until recently.
It still starts well, idles well, and runs well at low speeds(up to 1/4 or 1/3 throttle). There does not seem to be any loss of compression, and the engine is not losing power. However, if the throttle is increased, it very soon starts to misfire in the most alarming way. The sound is very much one of an explosion of a fuel/air mixture, somewhere inside the engine. I can't work out if this "firing" is occurring in the crankcase or the expansion chamber, but it doesn't seem to be coming from the silencer or further back in the exhaust system. I have checked the timing and it is spot on.
I am afraid I am stumped. I would be very grateful if anyone could give me some advice on what may be going wrong and what the remedy might be. |
Peter Ogborne
Senior Member
Username: peterogborne
Post Number: 171
Registered: 09-2002
| | Posted on Friday, October 13, 2006 - 05:47 am: |
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Hi Dennis, a P6D .Can you describe the engine ,ie single or twin cylinder .To be honest I am not familier with a P6D. Can I assume it is a single cylinder like the P5? Anyway your problem.........is the mixture setting on the carburetor ok ? There are a variety of carburetors fitted to S/T engines .You say that the ignition timing is correct are you sure it has not slipped and is retarded? If it is retarded it would not impede starting but would ignite any fuel in the expansion chamber at increased rpm.If the fuel /air mixture is not correct and is over rich you will have backfireing .If it is too lean ,ie too huch air then there will be spitting back through the carb.As you would know there is not much in the way of adjustments to make in these engines. Maybe the expansion chamber is coked up as well as the silencer.
I guess it is a case of just checking the basics, to be honest there is not much to go wrong except that they wear out!
Try a new spark plug .Sorry i can not give any specific diagnosis..keep us informed as to how you go. |
Malcolm Russell Ward
Member
Username: russell
Post Number: 12
Registered: 08-2003
| | Posted on Friday, October 13, 2006 - 03:32 pm: |
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I would check the fuel supply, Dennis. They will backfire though the carb if there is starvation. The fact that all the other basics are there doesn't leave much else. Are you still using the same sort of sparking plug -not changed it recently for a different heat range? Check the fuel lines I reckon. Then lets look for the difficult stuff. It is a natural human tendency to asssume it is a disasterous problem when it frequently is simple. Your engine has had a closseted existance and not been wrecked by indifferent sailors with large hammers.... |
Dennis Witt
Visitor
| | Posted on Friday, October 13, 2006 - 06:53 pm: |
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Thanks Peter & Malcolm,
The engine is a single cylinder (5HP) with coil ignition and a SIBA dynastart. The carburetter is an AMAL R622 with a fixed .105 pilot jet and an 80 main jet.
Both the fuel supply and timing have been double checked and the plug has not recently been changed.
There is no sign of spitting back through the carburetter - does this eliminate crankcase ignition?
The carburetter seems to be well adjusted for idle and low revs, but perhaps the mixture is too rich at more open throttle settings when the main jet comes into play. Can the main jet and needle wear over time resulting in an over-rich mixture at higher throttle settings?
Can hot coke in the expansion chamber cause backfiring even with proper mixture settings or does the mixture need to be over-rich to start with?
Sorry for all the questions, I really appreciated your quick and considered responses. |
Peter Ogborne
Senior Member
Username: peterogborne
Post Number: 172
Registered: 09-2002
| | Posted on Saturday, October 14, 2006 - 12:27 am: |
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Dennis it is a long shot but has your engine ever been dismantled? If so I wonder if the piston has been put back incorrectly.The piston crown gas a deflector to divert the incoming fuel charge away fro the exhaust port.Not a hard job to see if this is so .Take off the end cover of the expansion chamber and look through the port.You will see the piston top and the deflector should be furthest from you.
In answer to your other questions.Yes the jets do wear but only after years of service. It is very unlikely that there is an explosion occuring in the crankcase.Excessive coke in the exhaust could cause a problem but it is usualy ''strangling'' of the engine.Not sure if you have tried a new spark plug .Plugs can be absolute bastards of things dont persevere with the old one get a new one.
Hoping between malcolm and myself we can sort your problem out. |
Dennis Witt
Visitor
| | Posted on Monday, October 16, 2006 - 04:51 pm: |
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Thanks Peter. No, the engine has never been dismantled, and the deflector on the piston is in the right place (as viewed through the spark plug hole). But you were right about the spark plug, they can be "absolute bastards". I put in a new one and it runs perfectly, no sign of backfiring at any throttle setting!
I guess the plug occasionally failed to spark, and the charge of mixture was ignited in the expansion chamber by a hot piece of coke or the like. Maybe I should schedule a de-coke in the not too distant future anyway.
Many thanks for your help. |
Peter Ogborne
Senior Member
Username: peterogborne
Post Number: 173
Registered: 09-2002
| | Posted on Monday, October 16, 2006 - 06:48 pm: |
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Glad to hear that Dennis .Malcolm Ward and I were confiring on this and scratching our heads as to what the problem could be...something as simple as the plug. I will take heed of my own advice ,I have a small J.A.P. four cycle engine that should go....maybe a new plug !
Have you noticed the amount of Spam that is getting into the List? |
Andrew Menkart
Moderator
Username: andrew
Post Number: 769
Registered: 11-2001
| | Posted on Monday, October 16, 2006 - 07:00 pm: |
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Peter,
We had a batch of spam this afternoon. About a hundred unwanted messages got posted here. I deleted them almost right away, but if you receive email confirmation of postings, you would have recieve them by email.
Sorry about it. I have various safeguards in place and I am going to try to tighten it up a bit, but until I require everyone who posts to register (no posting as a "visitor"), then it is very difficult to prevent. |
Andrew Menkart
Moderator
Username: andrew
Post Number: 770
Registered: 11-2001
| | Posted on Monday, October 16, 2006 - 07:25 pm: |
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Peter,
I tried to reply to the email you just sent me but it was bounced back to me as "undeliverable". |
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