Testing outboard compression?
- Thread starter prv
- Start date
prv
Well-Known Member
Yep, that was my initial conclusion, but I thought I'd ask in case there was something obvious I was missing. Diesel car and electric lawnmower, but I do have a 4-stroke petrol generator I could try it on.
Thanks - I may give that a go. Leads can shock even through the rubber, then?
At first I thought I had, because I remember buying them (I'm working on this engine in dribs and drabs when I have time). But when I took them out to do the compression test, they were the old ones - I realise now that the replacements I bought were for spares!
In general I haven't done much in the way of troubleshooting - testing the compression was to be my first approach because if it turned out that I'd knackered the head with excessive use of wet&dry to remove the remains of the old gasket, there'd be no point trying other things. The compression test seemed a nice positive yes-or-no answer.
Already given the carb a good clean when I had the whole engine in bits. I didn't deliberately disturb or adjust anything.
So, I think my next step is to try the compression tester on the generator and confirm that it's knackered. Then get my hands on one that works. Then if that shows ok, start looking at other possible causes.
Pete
macd
Well-Known Member
Of course they do :ambivalence:, although they work in a quite different way to inductive ignition systems.
VicS
Well-Known Member
Oh yes !
Always worth changing the plugs.
Always advised on the American outboard forums
You might have to clean it again more thoroughly. ( BTW I now realise that there is a only a slow running adjustment on that engine) The diagram shows a funny little bladder thing. That is a very basic fuel pump Is that OK I'm abit puzzled by the fact that it does not die for 5 seconds or so........ makes me wonder about fuel flow although a problem there would not cause it to die even that soon
prv
Well-Known Member
I can't quite remember, but I think the fuel pump is only involved when using a remote tank.
Pete
CreakyDecks
Well-Known Member
I hesitate to ask this and apologize for doing so, but your compression tester hasn't got a pressure release valve that you have forgotten to close has it?
prv
Well-Known Member
No, it has a button which you press to release the pressure. I've wiggled it and I don't think it's stuck in the "release" position.
Pete
VicS
Well-Known Member
You have the option to use the integral tank or a remote tank then. They vary.
To lift fuel from a remote tank there is normal type of diaphragm pump driven directly by vacuum pulses from the crankcase. It has two hoses, one from the fuel line connector and one to the carb.
Also there is a sort of minipump consisting of a "bladder" and a couple of one way valves built into the carb ( #31 and #32 in the parts diagram) which assists the flow from the integral tank. It is also driven by vacuum pulses from the crankcase via a hose.
The fuel tap/ choke control switches between integral tank and off/remote tank.
prv
Well-Known Member
Ah, right, yes. I remember that hose. Without thinking too hard, I assumed it was some kind of crankcase breather, but of course that makes no sense on a 2-stroke.
Pete
Matata
Well-Known Member
I'd convince myself of the compression first, easy to do and hugely diagnostic........
lw395
Well-Known Member
TBH, with all the dodgy two strokes I've owned, if it makes a convincing bit of pressure when you put your thumb over the plug'ole and crank it, compression is not what's preventing it from running.
The air that comes out should smell convincingly of petrol.
And not have too much oil in it....
The air that comes out should smell convincingly of petrol.
And not have too much oil in it....
lw395
Well-Known Member
For completeness, it might be worth ruling out the 'odd' possibilities,
As the motor has been rebuilt, it's conceivable something is out of place.
If the air filter/intake can't flow the air, or the exhaust is closed off by the baffles out of place or something, that can have these symptoms.
I would guess carb though.
Because it usually is.
As the motor has been rebuilt, it's conceivable something is out of place.
If the air filter/intake can't flow the air, or the exhaust is closed off by the baffles out of place or something, that can have these symptoms.
I would guess carb though.
Because it usually is.
Birdseye
Well-Known Member
leaving aside the compression issue which I reckon is a faulkty guage, the first place to lookl with the syptoms you give is fuel starvation . Could be that the main jey is blocked , with the engine starting and idling on the starter jet. If engines are left with fuel in them and not used for a few years, the fuel evaporates and leaves befind a sort of varnish. This blocks jets and passiageways.
charles_reed
Well-Known Member
At a guess, crankcase reed valve - it happened to me on Johnson 9.9 twin, in 1982. Had to spend the night anchored just by #16 channel buoy, just out of the channel near Brownsea.
Exactly the same symptoms!!
Exactly the same symptoms!!
lw395
Well-Known Member
Good thought.
I'm not into Johnsons, didn't realise they were reed valve.
VicS
Well-Known Member
Not usually something that's troublesome as far as i can make out but here they are #4 ( only one of two shown. ) between the inlet manifold #1 and the crank case
lw395
Well-Known Member
They have been known to stick shut, get broken etc.
Usually on moped belonging to hoons though!