Testing outboard compression?

[prv]

You may remember a while back I asked about cylinder head bolts for a 4hp 2-stroke Johnson twin. Well, I put it back together without any trouble, and it seemed to run ok. However, it turned out to only work in neutral - as soon as I put it in gear, it would die. Or rather, five seconds or so after putting it in gear, it didn't die instantly.

Google suggests there are various possible causes, but having had the head off, my natural worry is that I damaged the seal while removing the old gasket. So I decided to splash out fourteen quid on a compression tester: http://www.ebay.co.uk/itm/290949832664 to find out. On Sunday I finally got round to trying it. I had the set assembled according to the instructions, and the adaptor firmly screwed into the spark plug hole. But when I pulled the cord, the gauge showed nothing whatsoever. Not a flicker. On either cylinder.

I'm puzzled; the only explanation I can think of is that the gauge is faulty. There can't be zero compression in the engine, otherwise it surely wouldn't have run at all. But I am of the generation for which even crappy old cars were fairly reliable, so my experience and skill with engines is limited, and I turn it over to the forum for thoughts.

Pete

 

[macd]

What's the range on the tester? Two-strokes don't run much above 7:1 compression, and I doubt an elderly but otherwise healthy put-put would even manage that.

 

[prv]

It goes up to 20 bar. Presumably 7:1 would equate to 6 bar, so nearly a third of the way round the gauge. I'm getting not a wiggle.

It's a 1999 engine, so not new but not ancient either.

Pete

 

[Seajet]

That was my first thought; engine don't run, guage says zero; " there ain't no such things as coincidences "...

However-failure as soon as load is applied - is it up and running to temperature, happy, when you put it in gear ?

If it is, start looking at the coil.

 

[Matata]

I've got a spare compression gauge set I used to rebuild mine. You are welcome to try it.No charge. , you can drop it in to my lad as he lives in Southampton when your finished. You should be looking for 135psi approx with no more than 10% difference between cylinders. hope this helps Nik

 

[William_H]

The failure of the engine under load sounds more like a fuel problem. Does it recover with application of choke? It might also be bad spark plugs or coils. Hardly likely to be bad compression which would give hard starting. Surely you can feel on pulling it over if there is normal compression. No compression feels very strange you would know it imediately. ie try with no plugs. good luck olewill

 

[VicS]

You may remember a while back I asked about cylinder head bolts for a 4hp 2-stroke Johnson twin. Well, I put it back together without any trouble, and it seemed to run ok. However, it turned out to only work in neutral - as soon as I put it in gear, it would die. Or rather, five seconds or so after putting it in gear, it didn't die instantly.

Google suggests there are various possible causes, but having had the head off, my natural worry is that I damaged the seal while removing the old gasket. So I decided to splash out fourteen quid on a compression tester: http://www.ebay.co.uk/itm/290949832664 to find out. On Sunday I finally got round to trying it. I had the set assembled according to the instructions, and the adaptor firmly screwed into the spark plug hole. But when I pulled the cord, the gauge showed nothing whatsoever. Not a flicker. On either cylinder.

I'm puzzled; the only explanation I can think of is that the gauge is faulty. There can't be zero compression in the engine, otherwise it surely wouldn't have run at all. But I am of the generation for which even crappy old cars were fairly reliable, so my experience and skill with engines is limited, and I turn it over to the forum for thoughts.

Pete

Take both plugs out to start with and pull the kill cord off to kill the ignition.

Fit tester in place of one plug.


give it at least 5 good full pulls on the starter cord. It should then build up to a reading. Which the tester should hold.

note the reading. reset the gauge and repeat on the other cylinder

best done warm if possible

You wont find a spec figure but its not likely to run on a compression below 60psi ( 4 bar)

Should run on 80psi

be absolutely fine on about 100psi

120 psi or above = optimistic gauge

There should be no more than about 10% difference between the cylinders.

Sorry about the units!


If it starts and idles the compression cannot be too bad.

Check you have two good sparks...... they should jump best part of a cm


Maybe only firing on one cylinder ???? Do cylinder drop test to check.

 

[essexboy]

Did you have the throttle wide open ? If not you're wasting your time !

 

[VicS]

Did you have the throttle wide open ? If not you're wasting your time !

Not necessary with a two stroke

 

[30boat]

I'd look at the carb.Do you feel resistance when you pull the cord slowly?If so then you have compression,at least some.

 

[prv]

Take both plugs out to start with and pull the kill cord off to kill the ignition.

Fit tester in place of one plug.

give it at least 5 good full pulls on the starter cord. It should then build up to a reading. Which the tester should hold.

Nope, it doesn't. That's the point.

Pete

 

[prv]

Surely you can feel on pulling it over if there is normal compression.

It feels different to my single-cylinder Suzuki, but I put that down to having two eeny-weeny cylinders rather than one larger one.

Pete

 

[prv]

Did you have the throttle wide open ? If not you're wasting your time !

Yes, the instructions on the tester said to do that.

Pete

 

[prv]

If it is, start looking at the coil.

As stated, my knowledge of petrol engines is limited, but I don't think it has one. The coil is what we had before electronic ignition packs, right? This engine has the HT leads attached to a sealed, potted block which I assume is an electronic ignition (the magazine cutting I have says that's what the engine uses). Also, given it runs OK in neutral, I think it must be sparking ok. I can't see why the additional load of turning the prop would affect, or be affected by, the spark.

Pete

 

[prv]

Maybe only firing on one cylinder ???? Do cylinder drop test to check.

It does sound a bit funny, almost flatulent, so I could believe it's only running on one. The trouble is I don't really remember what it sounded like before, as I've never used it in anger. Of course the vendor did show me it running, but that was a few minutes a couple of months ago.

What is a cylinder drop test, and how do I do it?

Pete

 

[macd]

As stated, my knowledge of petrol engines is limited, but I don't think it has one. The coil is what we had before electronic ignition packs, right? This engine has the HT leads attached to a sealed, potted block which I assume is an electronic ignition (the magazine cutting I have says that's what the engine uses). Also, given it runs OK in neutral, I think it must be sparking ok. I can't see why the additional load of turning the prop would affect, or be affected by, the spark.

It's probably CDI: Capacitor Discharge Ignition. Instead of a coil it has a substantial capacitor which fulfils more-or-less the same function. It will be potted with a few other bits you don't really need to know about, probably under the pull-cord housing. It's not unknown for an engine to show a spark under test, and even to run without load, and then die when load is applied, usually because the spark is weak. But this doesn't explain the lack of compression.
Have you tried turning it over with a finger over a spark plug hole?: you should definitely feel the pressure (but it won't hurt).

The only thing I can think of that might account for low compression and refusal to run under load is knackered crankcase seals. I don't know the details of your engine, but most two-stroke parallel twins will have a conventional seal on the outboard* side of the crankshaft for each cylinder, plus a seal between the cylinder...probably a labyrinth seal. I'm assuming, of course, that the pistons have rings

Another thought: is it a reed-valve engine? Are the reeds OK? (Reeds are composite petals, usually translucent, which live in a housing just downstream of the carb. They act as a one-way valve. It will be obvious if any reeds are broken or frayed.)

* outboard as in direction, not as in type of engine :ambivalence:

 

[30boat]

It's probably CDI: Capacitor Discharge Ignition. Instead of a coil it has a substantial capacitor which fulfils more-or-less the same function. It will be potted with a few other bits you don't really need to know about, probably under the pull-cord housing. It's not unknown for an engine to show a spark under test, and even to run without load, and then die when load is applied, usually because the spark is weak. But this doesn't explain the lack of compression.
Have you tried turning it over with a finger over a spark plug hole?: you should definitely feel the pressure (but it won't hurt).

The only thing I can think of that might account for low compression and refusal to run under load is knackered crankcase seals. I don't know the details of your engine, but most two-stroke parallel twins will have a conventional seal on the outboard* side of the crankshaft for each cylinder, plus a seal between the cylinder...probably a labyrinth seal. I'm assuming, of course, that the pistons have rings

Another thought: is it a reed-valve engine? Are the reeds OK? (Reeds are composite petals, usually translucent, which live in a housing just downstream of the carb. They act as a one-way valve. It will be obvious if any reeds are broken or frayed.)

* outboard as in direction, not as in type of engine :ambivalence:

CDI ignition system still have coils,I'm afarid.

 

[Birdseye]

you may well have damaged the gasket putting things back together but even that would not cause a no compression result. The only thing I can imagine causing that is if the pull string wasnt rotating the engine or it you had forgotten to install the pistons. Even missing out the reed valve wouldnt cause total compression loss because once the pistons have got past the cylinder ports the reed valve is irrelevant.

IMO the compression tester has to be faulty. Try testing it somehow - say with an air compressor or even blowing into it. As someone has said, when you put your thumb over the spark plug hole and pull the cord you should feel some compression.

As for the problem, I once had a Mariner O?B that did just what you describe. It turned out to be an impellor issue. The impellor ran in a stainless cup which was a press fit into an ally housing. Corrosion between cup and housing squashed the cup and trapped the impellor which was too much resistance for the engine to run.

 

[VicS]

It does sound a bit funny, almost flatulent, so I could believe it's only running on one. The trouble is I don't really remember what it sounded like before, as I've never used it in anger. Of course the vendor did show me it running, but that was a few minutes a couple of months ago.

What is a cylinder drop test, and how do I do it?

Pete

It sounds as though your compression gauge is faulty. Can you try it in the car? or lawn mower

Drop test =
Short out or disconnect one plug at a time. If one is not firing disconnecting it wont make any difference. You should get an equal effect from both. Take care Use insulated pliers or you'll get one hell of a shock!

You have not given the model number but you should find useful exploded parts diagarms at http://www.boats.net/parts/search/BRP/JOHNSON/1999/parts.html
Or on the BRP parts website at http://epc.brp.com/Index.aspx?lang=E&s1=5ba95294-65d2-40a4-8851-3f49b994181d

Non firing on one plug could be due to bad plug, bad lead , HT coil assembly ( £50 or more), CDI module ( probably over £100). Difficult to test without at least a DVA adapter for your multimeter. (easy enough to make if you area up to it) I assume youve tried new plugs already.

what plugs are fitted should be Champion QL86C. No direct NGK equivalent but be sure you are not using resistor suppressed ones if NGK are fitted. ( ie NOT with an R in the number)

If ignition is all OK turn your attention to the carb and clean thoroughly. It's possible that the high speed jet is blocked.

Instructions for adjusting the carb at http://forums.iboats.com/forum/engi...nts-for-asorted-carb-variations-by-joe-reeves
(Note basic settings, if you've already disturbed them, are low speed needle out 1½ turns, high speed out 1 turn)

 

[VicS]

As for the problem, I once had a Mariner O?B that did just what you describe. It turned out to be an impellor issue. The impellor ran in a stainless cup which was a press fit into an ally housing. Corrosion between cup and housing squashed the cup and trapped the impellor which was too much resistance for the engine to run.

Not likely to be Pete's problem because on this engine the water pump is turned by the drive shaft so putting in gear make no diffrenece to that.

Howver worth ensuring that the prop turns freely when in neutral.