Compression in a 2 stroke outboard?

[Trundlebug]

I bought an old Johnson 4 seahorse (1974) back in February, as a non runner from a boat jumble. Luckily, it was cheap..

A bit of a gamble as to whether I could get it running, but I've always been good with engines (up till now) although I have no previous experience of 2 strokes.

After going through all the key items and checks on everything I decided to check the compression.
I got nearly 7bar / 100psi on one, and about 5.5bar on the other.

I know the second is a bit low, but is 7 bar OK?

My compression tester is for a 4 stroke and it's just below the green zone on its dial. But I read somewhere that 2 strokes can be around 130psi but not to expect it to be as high on a hand started engine like an o/b. Not sure if that's right?

After finally putting a fresh petrol mix in it on Saturday I managed to get it to fire for nearly 10 seconds on one cylinder! Then it died and won't play any more. Despite me using my electric drill on the flywheel nut to turn it over...!

What should the compression be on a small 2 stroke outboard?

 

[Matata]

135 psi but pref 140.

 

[Fire99]

The compression is less important than the variation between the cylinders.
If the variation is more than 15psi between the two then you need to investigate.

It may just be a lazy piston ring due to being left standing but you never know.

Have you tried a 'wet test', spraying some fogging oil into the weak cylinder to see if it brings the compression up a bit?

If compression is too low it wont have enough gumption to suck the fuel mixture into the cylinder.

 

[oldharry]

The compression is less important than the variation between the cylinders.
If the variation is more than 15psi between the two then you need to investigate.

It may just be a lazy piston ring due to being left standing but you never know.

Have you tried a 'wet test', spraying some fogging oil into the weak cylinder to see if it brings the compression up a bit?

If compression is too low it wont have enough gumption to suck the fuel mixture into the cylinder.

Well its not quite as simple as that:

Compression testing a two stroke tells you very little about the state of the engine. They dont 'suck' the mixture (at least not much) like a 4 stroke: the suction part takes place in the crankcase as the piston rises; as it comes down again it compresses it and forces it in to the cylinder when the ports open. So although cylinder compression is important, more so is the crankcase compression. Worn rings will upset the crankcase compression and suction, but far more commonly the crankcase seals leak, reducing the crankcase suction / compression cycle. If the crankcase compression is low, it will not blow through the combustion chamber enough to overcome the pressure of the exhaust gasses, so they will not purge properly, again preventing the engine starting or running properly. It is very difficult to measure crankcase compression, but very often it is the main cause of poor starting/running

Many two strokes, and nearly all outboard engines have reed valves on the induction system, and any problems or leaks here can cause starting and running problems.

2 Strokes are incredibly simple, but everything has to be spot on for them to work - which is why they are so much less reliable than the 4 stroke counterpart. 4 strokes run in a bath of oil, but the 2 stroke always works on the point of oil starvation so is more prone to wear and component failure

 

[VicS]

7 bar is fine but the two should be within 10% of each other.

Even if they were both as low as 5.5 bar it should be a goer.

In the USA they would give it a decoke with Seafoam. (Its not available in the UK, I suppose Redex might be worth a try) but you have got to get it running first.

Check plugs ... currently recommended type are Champion J6C gapped at 0.030"
check for good sparks
check points gap = 0.020"
inspect coils if sparks poor.

try spraying fuel mix into the carb throat. It should start and run briefly even if the carb needs cleaning

(fuel = 50:1 but a bit extra oil would be no bad thing for that model as it does not have needle bearings throughout)

See http://forums.iboats.com/showthread.php?t=299680 for many helpful ideas including the Seafoam treatment and how to do the compression test.

.

 

[Fire99]

Well its not quite as simple as that:

Compression testing a two stroke tells you very little about the state of the engine. They dont 'suck' the mixture (at least not much) like a 4 stroke: the suction part takes place in the crankcase as the piston rises; as it comes down again it compresses it and forces it in to the cylinder when the ports open. So although cylinder compression is important, more so is the crankcase compression. Worn rings will upset the crankcase compression and suction, but far more commonly the crankcase seals leak, reducing the crankcase suction / compression cycle. If the crankcase compression is low, it will not blow through the combustion chamber enough to overcome the pressure of the exhaust gasses, so they will not purge properly, again preventing the engine starting or running properly. It is very difficult to measure crankcase compression, but very often it is the main cause of poor starting/running

Many two strokes, and nearly all outboard engines have reed valves on the induction system, and any problems or leaks here can cause starting and running problems.

2 Strokes are incredibly simple, but everything has to be spot on for them to work - which is why they are so much less reliable than the 4 stroke counterpart. 4 strokes run in a bath of oil, but the 2 stroke always works on the point of oil starvation so is more prone to wear and component failure

Well i'm quoting some very basic basics from a Mariner Workshop manual to get the guy started. So it's simple enough.

 

[Bosun Higgs]

A much more likely cause of your problems is the fuelling and the likelyhood of gummed up jets. I wouldnt worry too much about the compression at this stage - first get the engine going and the difference in compression wont stop that happening. Indeed getting the engine going might well cure the compression issue anyway.

Assuming a reliable spark and fresh fuel, then remove the carb, strip it and soak it in carb cleaner. Better still your local lawnmower specialist could well have an ultrasonic carb cleaner.

 

[maxi77]

A much more likely cause of your problems is the fuelling and the likelyhood of gummed up jets. I wouldnt worry too much about the compression at this stage - first get the engine going and the difference in compression wont stop that happening. Indeed getting the engine going might well cure the compression issue anyway.

Assuming a reliable spark and fresh fuel, then remove the carb, strip it and soak it in carb cleaner. Better still your local lawnmower specialist could well have an ultrasonic carb cleaner.

Certainly my experience with older outboards has been that the big problem is getting the fuel supply right, check the state of flexible hoses in the engine from the tank, they rot from the inside and little bits of rubber block the jets, with extrnal tanks what is the state of the connections, are they sucking air, then once you are sure you are getting a supply of clean fuel into the carb, clean the carb and the jets and set them up from first principles. You should now be able to get the engine running and ready for getting the carb adjustment right.

 

[Trundlebug]

Thanks for all the replies. The compression test was just about the final thing I tested, after the following:-

Spark plugs - new, gapped at 0.030" (NGK)
Points - were oily, cleaned up and checked gap at 0.020"
Spark - healthy blue on both. Sparking at about the right time, plug leads checked for correct connection (right cylinder)
Fuel - fresh mix at 50:1
Fuel pump - checked, working fine


Carb - stripped, cleaned although it was clean before, needle valve checked OK. I didn't blow it all out though, I may do that. But there is definitely petrol getting through to the engine when I turn it over. Low speed jet set 1.5 turns out, high speed at 1 turn out, as initial settings to get it going.

Reed valves - removed, and the baseplate polished on my polishing mop to achieve a better seal. A previous owner seems to have cleaned it with sandpaper, leaving it scored. There were slight gaps in the reed valves which I've reduced by careful assembly, although there is still some daylight visible as the reeds lift off very slightly at rest. Could this be the root of the problem? It did seem to have better compression after I'd worked on this.

I'm now running out of ideas as to just why it won't go, hence the question about compression!

At least I'm saving arm ache when trying to start it. I'm using an 11/16 socket in my power drill on the flyweel nut!

So why won't it go? From your answers above I think the things I need to check further are:-

Crankcase seal
Reed vales (again)
Carb jets - blow out with airline.

Anything else?

 

[maxi77]

Even if the fuel is fresh is it clean by the time it gets into the carb, I had a problem with a dodgy fuel hose that looked OK on the outside and was depositing small bits of dirt into the carb every time fuel flowed. Replaced the hose and fitted a filter and that solved it, only after I had learnt how to strip and clean the carb in minutes every time the thing stopped.

As for the reed valves I have had no problems with them on an engine but a set did b*gger up a compressor. It does sound as if there could be a sealing problem though.

It is amazing the impact dirty jets can have though.

 

[VicS]

Try the trick I mentioned earlier. Spray some fuel mix directly into the carb throat using a trigger spray bottle. It should fire and run briefly if its only the cab that's at fault. If it really wont then it's time to look at some of the other suggestions.

 

[macd]

7 bar is fine but the two should be within 10% of each other.

Agree with that. Cylinder compression doesn't actually start in a two-stroke until the exhaust port has totally closed, so even with crankcase compression the true ratio is somewhat less than the nominal one, especially at the revs attained at starting. Equally, at these revs, the reed valves are less efficient at sealing. Even in racing two-strokes, cylinder compression is rarely much above 7:1.

Try removing one spark plug at a time, then turning the engine over. You should be able to feel any large disparity in compression between the two cylinders.

If there's a spark and fuel, it should run -- or, at least, show some signs of trying. You seem confident about the spark and its timing, which probably points to a fuel problem as others have suggested. Be prepared to keep changing the plugs regularly as you work on getting it going: two-stroke plugs easily oil up = no spark. Sometimes you can dry the plug without taking it out by turning off the fuel and turning the engine over a few times with the throttle wide open.

 

[Trundlebug]

Since the motor ran for a few exciting seconds on one cylinder before dying out, I have repeatedly removed the plugs, put them over a flame of the gas stove to dry them out and heat them up. Having replaced them, I've tried to start it a few times but it just won't play any more.

The plugs do seem to be getting wet though, indicating there is fuel getting to them ( I may have flooded it) and therefore that there's no problem with the carb.

When I saw the plugs were getting wet I even turned it over for a while on the drill with the throttle wide open and no choke, to try and dry it / clear it out.

I may have another try tomorrow night. If no go again, I'll definitely have to strip the carb down again, and blow through with an airline. I'll have to dig out my trusty compressor!

Is it very easy to flood a 2-stroke?

I must say, it seems remarkably temperamental. If it won't start at 2,000rpm with a drill, how on earth am I ever going to start it by hand?

 

[VicS]

You sure you still have good sparks ?

 

[macd]

Often easier than a 4-stroke, yes.

You seem to know what you're doing. Maybe the problem is that it's far too rich rather than not enough fuel. Blocked pilot air bleeds? Float height too low? When it did start, was that on choke with throttle closed or almost closed? Maybe try no choke and more throttle.

If, on other hand, it's too lean whatever you do, could be failed crank seals (which bleed fresh air into the system).

If it starts but dies when throttle opened (assuming you get that far), symptom same as on 4-stroke -- usually blocked main jet.

 

[Trundlebug]

Maybe the problem is that it's far too rich rather than not enough fuel. Blocked pilot air bleeds? Float height too low? When it did start, was that on choke with throttle closed or almost closed? Maybe try no choke and more throttle.

If, on other hand, it's too lean whatever you do, could be failed crank seals (which bleed fresh air into the system).

If it starts but dies when throttle opened (assuming you get that far), symptom same as on 4-stroke -- usually blocked main jet.

Gold dust! Thank you. Some great info there.

When it fired, it was on 1/2 choke and throttle in start position. As it started to go I was conscious of not flooding it and pushed the choke in whilst opening the throttle. It started to die, so I pulled out the choke again and that killed it altogether! Pah!

 

[macd]

Hope that helps. I might add that I know nothing about that particular engine or, more pertinently, its carbs, so I'm in no position to say precisely how the choke mechanism works. My guess is that it's a straightforward mechanical choke, but I imagine you'll be able to work that out.

 

[Bosun Higgs]

When it fired, it was on 1/2 choke and throttle in start position. As it started to go I was conscious of not flooding it and pushed the choke in whilst opening the throttle.

Need to give it time to warm up a bit before moving the choke in. But it does sound as if the mixture is the problem. I have a Johnson single 3.5hp and that will not run at all without a little choke because the mixture is weak.

Suggest you try opening out both main and idle jet settings - you are unlikely to flood things when the engine is running anyway. And I do wonder about an air leak too.

 

[maxi77]

Gold dust! Thank you. Some great info there.

When it fired, it was on 1/2 choke and throttle in start position. As it started to go I was conscious of not flooding it and pushed the choke in whilst opening the throttle. It started to die, so I pulled out the choke again and that killed it altogether! Pah!

That sounds like problems with the slow running jet

 

[VicS]

Suggest you try opening out both main and idle jet settings

That sounds like problems with the slow running jet

One of the items in the link I gave earlier gives the procedure for adjusting the mixture needles
You do have to get running first of course but it should run with the needles in their basic settings.

If the carb is in need of further cleaning I would expect it to run by fiddling with the choke ... It is an ordinary strangler type BTW.

Compressed air may not get the jets clean. It may be necessary to poke something through which will not damage them. I use soft copper wire but I have seen nylon monofilament fishing line suggested.