Why my dinghy won't plane

[Trevethan]

Why my dinghy won\'t plane

I now know my my recently acquired tender won't plane with its 9.5 hp engine -- only going on one cyclinder.

Outboard is older evinrude. lower clylinder doesn't fire.
So-- checked the spark plugs . seem OK. Swapped them, but still only top cylinder fires. HT cable is fine.

At one point managed to get both firing, but that was short lived -- seems it tries sometimes, but fails. {;enty of fuel coming out so no not carb.

A friend said there might be a little water getting in as it sort of coughs from time to time.

Took of head, and gasket appears fine -- but will replace. Failing that what else? the crank case seal? If so how much and can anyone suggest somewhere around london?

Cheers,

Nick

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[aitchw]

Re: Why my dinghy won\'t plane

What's compression like on faulty cylinder? The fact that there is plenty of fuel getting through is only partly reassuring. If there is too much it could be choking to death.

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[Trevethan]

Re: Why my dinghy won\'t plane

Haven't had compression tested. I want to change the head gasket before anything else, but the cylinder looks OK, just a bit oily -- which is par for the course.

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[aitchw]

Re: Why my dinghy won\'t plane

Have you tried heating the plugs up before you try starting? This can sometimes overcome the cause at least briefly. When it stops running cleanly take the plug out and have a look to see what state it's in. The rings on the bad cylinder may also have become really caked in crap with not firing.

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[Colin_S]

Re: Why my dinghy won\'t plane

I don't know much about outbaords but have rebuilt a couple of 2 strokes so base my suggestions on that.

If it's a single carb you can pretty well rule out that as the problem. Is the induction via reed valves? If so, have a look at them.
Only other areas I can think of, if spark is OK, is crankshaft and case seals and worn piston / rings.
Before replacing the head gasket, have a look at the old one and the engine surfaces for any signs of blow by.
It's always a good idea to do a compression test on a 2 stroke before stripping. Before fitting a new one you could refit the head with the old one and some instant gasket to seal it up temporarily to get a compression test done.

Have you checked the spark on the non running cylinder? I would try an old plug in the lead to see if it sparks strong and regularly when running on the single cylinder.

Perhaps someone can advise if both cylinders get a spark at the same time. If that's the case you could try swapping the leads between cylinders to see if misfire follows. I know some, but not all, 2 strokes twins get a spark twice each revolution so this does work as a check for ignition.

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[William_H]

Re: Why my dinghy won\'t plane

I have had 2 cases (mine and a friend)on Johnson Evinrude 6 and 8HP lately where the ignition module has failed for one cylinder. I am pretty sure the plugs fire in sequence so it won't run if you swap plugs. I assume yours has the CD ignition about 1980 on. The ignition system has a pick up coil under the flywheel which generates several hundred volts AC this is rectified to DC in the ignition unti. There are also 2 coils one for each cylinder which cause an elctronic switch (SCR) to discharge the high voltage when the magnet in the flywheel passes the coil and the induced AC voltage rises sufficiently. The current from the SCR switches feeds to the primary of the ignition coil one for each cylinder.
An easy check is to see if there is a spark on the plugs pull both out and provide an earth wire to the plug body. Assuming you see a spark on the good cylinder plug and none on the bad cylinder plug then check with a multimeter (ohms) for resistance from the plug connector to earth through the coil. This shouild be no more than a few thousand ohms probably far less. You will find a plug and socket in the harness to the under flywheel with I think 3 wires each should have no more a hundred ohms to ground.( looking at the under flywheel coils) If you can identify the 2 sensing coil wires and can swap these over by cutting then you can swap the plug leads and if it is a faulty ignition unit it should run on the other cylinder If these checks fail to show a problem then look for a new or secondhand ignition pack. it doesn't take much to change or wire in temporarily if you can borrow a pack to test.. If the sparks are similar on both plugs disregard all above Good luck regards will

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[ex-Gladys]

Re: Why my dinghy won\'t plane

Two stroke twins do not have 360 degree cranks, they have 180 cranks since they fire every compression, so whilst you can have a 360 4 stroke which fires both plugs together, you wouldn't normally find it on a 2 stroke since you'd possibly ignite the fresh charge at the botom of the stroke.

If you a 2 stroke twin does have a 360 crank, it's really a split single....

<hr width=100% size=1>Larry Botheras

Anderson 26 "Amber"

 

[Trevethan]

Re: Why my dinghy won\'t plane

Thanks!, but I get good sparks from both plugs via both cables!

I tried running it very very briefly out of water, and it still only ran on one clylinder, but after a second it coughed and ejected aerosol liquid - fuel presumeably - from the exhaust.



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[aitchw]

Re: Why my dinghy won\'t plane

Given everything you've checked appears OK you are getting down to some long shot possibilities. I would discount crank seal because that should affect both cylinders equally, shouldn't it? If everything going in is OK then what about when it is coming out, ie exhaust restriction. Is it easy to check for blockages to outlet on that cylinder. Do you know if the previous owner stripped and reassembled the engine, particularly the pistons? I know on some 2 stroke engine designs there is shaping/step in the piston to aid exhaust ejection. If the cylinder is the wrong way round (turned through 180 degrees) the cylinder won't clear and will choke.

Finally, can you get hold of some engineers blue dye (crack detection) fluid? There could just be a hairline crack in the head.

Sorry, really clutching at straws with those comments.

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[Trevethan]

Re: Why my dinghy won\'t plane

The one thing that makes me still think seals and water is that the cylinder is arrangement is over and under and its the bottom cyl -- perhaps small dribble in is enough to retard detonation. -- the engine sounds like it it trying to go on two, but never quite gets there.
cheers,

Nick

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[Colin_S]

Re: Why my dinghy won\'t plane

"I would discount crank seal because that should affect both cylinders equally, shouldn't it?"

I would assume there are seals on the ends of the crank where it goes up to the flywheel and down to the prop. One 2 stroke engine I had to rebuild had lost power due to a bearing breaking up and debris from it damaging the crank end seal causing a low speed misfire. At high revs the offending cylinder seemed to come back to life. A compression test did show this up.

When I've rebuilt 2 strokes I carry out a leakage test before final assembly. This consists of sealing up all the openings to the engine which I did with rubber sheet cut to shape and fitted over the exhaust and intakes. Using a homemade billy bodge pump I pressurised the engine through a plug hole and checked that it held the pressure. About 5psi for a few minutes from memory. This will check most of the important seals, eg Crank, head, case, etc.

 

[paddywackcocker]

Re: Why my dinghy won\'t plane

How old is the motor, is it old enogh to have twin coil packs. (may be twin points)
Lift fly wheel and check.
When looking for a spark across a plug cap, it does not take as much power for the spark to jump as it does under cylinder compresion, so you think you have a good spark but it might not be present when in the the cylinder.


<hr width=100% size=1>Sorry for the spilling

 

[JimMcMillan]

Re: Why my dinghy won\'t plane

Agree with Paddy. Older Evinrudes had twin coils etc. so lift the flywheel and see if this is the case and check points,condensers and especially the coils. It could be damaged wiring producing an intermittant fault or as one old Evinrude of mine which suffered badly from damp and the coils needed cooked in the oven (low.Mark 1) for a short period.

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[Trevethan]

Re: Why my dinghy won\'t plane

Engine is old enough to be steam driven -- early 1970's I guess.

OK will give that a try -- I will also see if I can get it running on just the lower cylinder.

I did disconnect the working cylinder spark lead last night and gave it a pull, and it almost started -- but subsequent attempts failed.

Cheers,

Nick

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[JimMcMillan]

Re: Why my dinghy won\'t plane

Another thing. When you remove the plug thats not firing is it wet with petrol or are there beads of water on it. If electrics prove ok then it looks like piston/rings problems but check compression first because stripping an old aluminium engine is fraught with danger and usually ends with lots of broken aluminium studs most of which are impossible to remove.

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