2 stroke troubles!

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surekandoo

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My boat is equipped with a Vire 12 HP 2 stroke petrol engine. The engine has a magneto ignition system and whilst basic normally runs well & has been pretty reliable.

This season the engine has had new plug & points and started 1st time when the boat was put back in the water.

During my first sailing trip this weekend, the engine after running OK all of a sudden started running roughly, misfiring, & lacking power when prop is engaged and occasionally backfiring.

I have:

Checked & reset points gap
Changed the spark plug
Checked fuel flow to lift pump, - OK
Checked fuel lift pump operation - OK
Removed carb, stripped jets and blown out as per manual
Checked float chamber - OK

Problem still remains.

Any suggestions please as to what else may be causing this problem?

Thanks
 
I had problems with a Vira once which were eventually put down to the plug being the wrong 'heat' grade. It would start OK and run for a while and then inexplicably fail. There was a problem with the cooling (no blockage; it was a brand new engine!) and a change of plug to a different grade sorted it out.

I am not suggesting its your problem, but I offer the experience for what its worth.

Do you have one of those 'Dyna start things'. Usually called a Dyna fail to start on the boat we sailed in those days.

PS I am talking about 40 years ago... :(
 
surekandoo said:
Yes, I have tried 2 diff heat grades of plug. No difference.

Dynastart OK so far. I have a spare too, just in case. £28 from boat jumble.

This is the Vire's last season, provided I can persuade SWMBO to let me spend the money on a diesel replacement.
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Points properly resurfaced before adjusting? and cleaner than clean?

Condition of magneto coil.? If it looks manky the insulation maybe deteriorating.

Misfiring, esp if back firing through the carb, can be a sign of a week mixture. Clean the carb again, poking a soft copper wire through all the jets.

Not using stale or contaminated fuel ?

My money would be on carb not properly clean!
 
VicS said:
Points properly resurfaced before adjusting? and cleaner than clean?

Condition of magneto coil.? If it looks manky the insulation maybe deteriorating.

Misfiring, esp if back firing through the carb, can be a sign of a week mixture. Clean the carb again, poking a soft copper wire through all the jets.

Not using stale or contaminated fuel ?

My money would be on carb not properly clean!
Click to expand...

Is there not something funny with Vire's and the exhaust choking causing random stops?
 
VicS said:
Points properly resurfaced before adjusting? and cleaner than clean?

Condition of magneto coil.? If it looks manky the insulation maybe deteriorating.

Misfiring, esp if back firing through the carb, can be a sign of a week mixture. Clean the carb again, poking a soft copper wire through all the jets.

Not using stale or contaminated fuel ?

My money would be on carb not properly clean!
Click to expand...

Confess fuel stale but been topped up with 5 litres fresh and had stabiliser in at end last season

Will have another go at jets as suggested.

Re. Weak mixture. Must admit when I removed spark plug to change it, it did look a lighter colour than normal.

I also think fuel supply to blame, as fault first occurred after crossing some rough tidal over falls on way into R. Ore. Maybe stirred some crud up although there's a glass bowl type filter in the fuel line and fuel appears to flow freely to carb.

Thanks
 
I was going to suggest water in the fuel but that should become evident in the glass filter bowl. We used to have similar problems because the fuel take off wasn't quite at the bottom of the tank and water built up and then in rougher conditions got sucked in with the fuel.

Yoda
 
Check the igntion timing. If the key on the flywheel shears, and the flywheel slips on the crank, it can produce the symptoms you describe. Or at least, mine did.

The exhaust problem happened when the cooling water flow reduced overheating the inside of the rubber exhaust. This would break down and bits of the rubber liner could act as an exhaust choke valve, causing sudden power loss, or even complete stoppage. Pull the exhaust hose off the manifold, and you will see straight away the blistered inside of the rubber hose. If it looks OK then it is!

Another weak point is the flange at the base of the cylinder block. Even slight leakage here will be difficult to see, but will thoroughly upset both carburation and the crankcase compression essential to proper 2 stroke functioning. Ditto the joint between the carb and the inlet manofld on the block. Crank case seals can cause the same problems, but very rarely suddenly, as wear gradually increases the leaking.

Otherwise as suggested, poor fuel supply, blockage etc.
 
oldharry said:
Check the igntion timing. If the key on the flywheel shears, and the flywheel slips on the crank, it can produce the symptoms you describe. Or at least, mine did.

The exhaust problem happened when the cooling water flow reduced overheating the inside of the rubber exhaust. This would break down and bits of the rubber liner could act as an exhaust choke valve, causing sudden power loss, or even complete stoppage. Pull the exhaust hose off the manifold, and you will see straight away the blistered inside of the rubber hose. If it looks OK then it is!

Another weak point is the flange at the base of the cylinder block. Even slight leakage here will be difficult to see, but will thoroughly upset both carburation and the crankcase compression essential to proper 2 stroke functioning. Ditto the joint between the carb and the inlet manofld on the block. Crank case seals can cause the same problems, but very rarely suddenly, as wear gradually increases the leaking.

Otherwise as suggested, poor fuel supply, blockage etc.
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Thanks for that.

Engine has been out this year and there was no evidence of exhaust problems you describe. Cooling water ok .Engine sounds the same as normal when running, doesn't sound muffled or anything like that.

I've had the flywheel of to check points, condenser, coil wiring etc. Key fine.

Engine starts fine, problems only appear when load is applied.
 
Definitive test. When it stops, check to see if there is a spark immediately. My money is, in part, on a weak coil that 'breaks' rather than 'makes' when warm..
Then I would get a bit of rubber pipe and blow back through the fuel supply line and verify that it is not resisting with crud
 
surekandoo said:
Thanks for that.

Engine has been out this year and there was no evidence of exhaust problems you describe. Cooling water ok .Engine sounds the same as normal when running, doesn't sound muffled or anything like that.

I've had the flywheel of to check points, condenser, coil wiring etc. Key fine.

Engine starts fine, problems only appear when load is applied.
Click to expand...

Fuel starvation?

or engine misalignment? but do not see how that would cause backfiring... Just guessing...

Any problems before it was taken out? Anything changed whilst it was out?
 
Alignment checked with feeler guage in coupling. But may need re-visiting after "bedding in". No problems before removal.
1st day of sailing trip it performed faultlessly. It only developed problems part way through day 2

Fuel starvation unlikely in it's basic form as fuel pump delivering strong flow, also petrol in float chamber.

Engine doesn't appear to be working hard, just runs rough. Bloody thing!
 
low tension fault has my money
mag's don't just suddenly stop then work then stop
There have been cases of cheap copies re points
You could fit electronic ignition, the same type as fitted on model planes, just engines sell the kits.
cheers
mick
 
mtb said:
low tension fault has my money
mag's don't just suddenly stop then work then stop
There have been cases of cheap copies re points
You could fit electronic ignition, the same type as fitted on model planes, just engines sell the kits.
cheers
mick
Click to expand...

You say low tension fault. Presumably this is the wire from the coil to the condenser & from the condenser to the points. How can I check this please, electrically that is?

Thanks.
 
Also check that the Mag auto advance/retard is free to operate. that the is on the drive end, you should be able to twist the mag by hand,let go and it should fly back, small movement. This also will give you the symptoms as described.
 
pampas said:
Also check that the Mag auto advance/retard is free to operate. that the is on the drive end, you should be able to twist the mag by hand,let go and it should fly back, small movement. This also will give you the symptoms as described.
Click to expand...

Not sure this engine is that advanced. No mention of advance/retard in manual, but I'll have a look.
 
Inbuilt in the mag. its true purpose is to Flick the rotor during starting, thus giving a slightly more voltage during start up. If its got dirty it may not be in the run position and CAN affect the timing.
 
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