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Blackhawk Paramotor blows a piston


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If you read my last post you will know that yesterday I had an engine out and had to make an emergency landing between crashing waves and a river (10-15 meters apart).

Today I found out why, after about 5 hours of total use time over about a week, my Blackhawk Paramotor blew a hole in the piston. My instructor/mechanic says it could be that a bit of sand got in the motor from my turtling 20 minutes prior, either way it sucks as I have about 10 flights and was just starting to get a bit of confidence up.

I read a lot about pistons blowing, especially in the pollini and a few about the Blackhawks, what the hell! Why don't they put some kind of filter over the muffler if it is so easy to bugger the engine with a few grains of sand?

So now at least a week while I wait for the pieces to be sent to Guatemala, sucks a fat one, and potential issue with the Velocity Elektra wing.


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I do not claim to be a mechanic by any means, but I am finding it hard to work out how a bit of sand would cause a blown piston?

I flew a Parajet in the Sahara for 50 days with no air filter, by the end it was running like crap but after a good carb clean out she was back to her chirpy self. :-)


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Tyrhone, where is the hole in the piston?

If it's in the top face, it would not have been directly caused by sand. It may be possible that sand in the carb could cause it to run lean, but if it were running lean enough to burn the piston you'd feel a serious lack of power. A hole in the top face of a 2 stroke piston is most often the result of over advanced ignition.

If it's in the area of the rings, it could be the direct result of sand ingress, although it's difficult to see how it would have got all the way back through the exhaust system to the cylinder.

Any engine may be buggered by a few grains of sand in the cylinder, but under normal circumstances, everything that goes into the engine (air, fuel & oil) is filtered.

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Thanks for the replies guys, the hole is in the top, I have attached an image but it doesn't seem to be working.

That is interesting and good to know about the sand not being a likely culprit, being completely ignorant of mechanical things myself, can I ask what you mean by "over advanced ignition"?

We had a lot of trouble starting the motor (despite it being an easy start) and so I know my instructor had to play around with the carb and throttle setting a lot to get it firing.

Any ideas what I should be setting it to or watching out for to prevent this from happening again?

Thanks again guys, really appreciate the help.



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Ah thanks Hann,

Well the mix is at 3 percent with Motul 710 as the engine is still under the 10 hour mark.

I wish I knew more about motors so I knew how to check for those things you guy mention, as it stands I have to have faith in the skills of others, something I have never been particularly fond of doing.

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Isn't the affect of upping the oil ratio, in fact to weaken the mix? Less fuel for given air intake. So it maybe more lube but it's a weaker mix. Weaker mix hotter running. Maybe not enough of its own but it's pushing it the wrong way.

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I run mine at 2% with Motul 710 and I make sure that it's running just rich. You have just got to avoid a lean mixture and that is either too little fuel or too much air, hence the post further up that says check for air leaks.ie too much air being inducted.

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tyrhone best advice you could have at the moment is try to find someone

who understands 2stroke engines ' and the type of carb" your motor has

what you should aviod is turning the needle jets in and out . thats a quick way to hole a piston and another engine out .

looking at the picture it looks like as the engine bedded in' the compresion

would of increased in your cylinder ( as the piston got to the top postion)

that would cause a lean run and detination to start ' thats when your fuel ignites before it should ignite. also in the pic it looks like the head gasket

wasnt long for this mortal world theres carbon on the outside of the sealing ring ( in the pic under your finger) was the cylinder studs re checked after a

short run to working temp' and once it cooled down ' when you fist got the motor . i know this cant help now but it will help you when you get the motor back together . good luck

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Thanks for the help everyone. I am trying to make up a list of info from your comments and from the HE pdf for this engine.

Unfortunately some of what is here is greek to me, and I don't speak Greek!

cas_whitmore what do you mean "avoid turning the needle jets in and out"?

Also I don't know what cylinder studs are or what to do with them, and what should I do about the carbon around the sealing ring?

I am going to suggest to Yankell that we try the 2% mix, that we turn the carburetor screw down to 1.5 turns. Anything else I should suggest for during or after he installs the new piston thingy?

I wish I knew more about this stuff, and I am definitely going to learn, but for now I appreciate all the help I can get to prevent this happening again.

Thanks all

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A great place to start learning is the Carburettor. I think its safe to say most 2 stroke engine problems are caused by it not being set up correctly.

If it sucks in to much air, it will run hot.

if it sucks in to much fuel, it will struggle to pick up revs and kill your spark plug faster.

I would advise as above. Maybe take it to a motorcycle shop to set the carb up?


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Engines 101:

When we refer to 'weak' or 'lean' mixture, this means not enough fuel in the fuel / air mix going into the cylinder. This may cause overheating, loss of power and heat related damage (like siezed or holed piston)

'Rich' mixture means too much fuel in the fuel / air mix. This may cause loss of power, smoking, spark plug fouling and carbon deposits which impair performance.

'Over advanced ignition' means the spark is happening too early before the piston reaches th top of the cylinder. This can cause overheating and heat related damage, but is less likely to cause a noticeable loss of power.

The opposite is 'retarded ignition', which means the spark is happening too late. This will cause a lack of power, possibly smoking and fouling due to incomplete combustion, but is not likely to cause permanent damage.

Far too much of my youth was wasted on 2 stroke engines. They're powerful, but noisy, dirty, unsophisticated and fickle.

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Thanks Simon and ptwizz, very helpful.

Well I am going to make sure that when the engine is repaired I will use maximum 2% Motul oil, and I will set the carburettor to 1.5 turns instead of 3 (as it sounds like the mixture was too lean), and the rev screw to whatever is required, I guess about 1 turn.

I have this from the engine manufacturer (it is an HE engine):

You can set the carburetion at the desired revs. By the long black downward screw C. By turning it anticlockwise you rich the mixture, while the opposite you lean it. The basic setting is between 1,5 and 1,7 turns from the shut position. The C screw must be turned very gently, always verifying the cleanness of the "passage" (the point where the nozzle of the idle enters the nozzle of the full, at about half revs.) in revving up. If the engine pops, you tighten it; if it abates you loosen it.

The D screw sets the revs at idle, by opening the butterfly valve (for additional information please contact your local distributor). If the idle is high, turn the screw in an anti clockwise direction. If the idle is low, turn the screw in a clockwise direction.

ptwizz, how would I go about changing an over or retarded ignition? I have seen many a post about it around Google so won't complain too much as I know the reasons, but it does still amaze me how unreliable these things seem to be. In the future maybe I will try and go for a 4 stroke, the only viable option of which seems to be the bailey.

But for now I just have to learn how to read these engines better. Any where you guys could point me for basic 2 stroke or even paramotor maintenance courses would be great.

As for a motorbike shop mechanic, well my trainer is a mechanic and has been flying and working on paramotors for a decade, of course more opinions is always better in my mind, but I am in Guatemala and wouldn't know where to even begin looking for a second opinion. So if it is OK, you guys are kinda taking that place :)

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That's classic detonation. I's not caused by sand, unless the sand got into your carb fuel line. Its's caused by:

1) too lean a mixture, or

2) ignition timing not set correctly,

3) really crap fuel

1) can be caused by many things, badly adjusted carb, an air leak in the crankcase (were your head bolts loose?), crap in the carb filters, bad carb membranes, a fuel restriction in the supply line - e.g. primer bulb, or forgetting to open the tank vent valve, etc.

2) can happen if it was wrong from the factory, or the timing flywheel has slipped on the crankshaft (have that happen).

Make sure you have gotten all the metal from the bottom end of the crankcase out before you think about putting it back together again.

Edited by Guest
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To add to the above.

I believe my engine (bh125) has a WG8 walbro carb. Everything I have seen says close then open about 1.5 turns. It seems to me like having it open 3 turns is going to make the mix very lean and therefore very hot, could this be a likely cause of the blown piston?

Also once rebuilt I am going to tune the carb, and bear with me here bacause I know this might be a ridiculous question to most, but...

Do I turn the long screw all the way clockwise to close, and then 1.5 turns anti-clockwise to open, and that should do it? I am googling it, but no one has that basic answer because no one thinks it is an askable question I would guess.

Thanks for bearing with my ignorance :)

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Yes mate as above.

All the way in (clockwise) and then 1.5 turns out is standard. (this does not mean it will be correct for your engine) it just an average setting from the factory so normally a good place to start.


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Thanks Simon,

Really hoping not to have too many more problems as I have a long way to go (all through Central and South America) and can't imagine repairs being an easy thing to get done in some of these countries.

As such you guys are my life line, lucky you huh ;)

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The WG8 carb has adjustable low needle only, therefore if you needed the jet open to 3 turns for normal running it means your high jet is Blocked !

Hence the lean mixture and the holed piston.

Strip the carb down and clean before using again.

Sent from my iPhone using PMC Forum mobile app

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what about if we get tyrhone to do the plug test .

when your motors ready to run tyrhone and youve set the carb neddle to 1.5

turns and if you can run it at moderate rpm , run it up for 3 minites stop it and let the motor cool down ( repeat this 6 times ) after its cooled down

after the run ' start it up and give as much throttle as you can for thirty seconds . then hit the kill button and throttle closed , remove spark plug

take a pic and post on here so we can have a look .

if its wet black and looks like you held it over a candle its way to rich on fuel.

if its dry and a light brown you are in the ball park and safe for now .

if its looks similar to your holed piston in colour its way to lean .

remember those haynes mauals evry one

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Cool, that sounds great, I am waiting for my instructor to order the parts from HE and then will likely have to wait about a week to get them and have the engine running again, but as soon as I do I will do that test.

Spigot I am looking back on everything now and thinking that we probably didn't need it open to 3 turns, that the engine flooding likey caused the hard start, and that we compensated with 3 turns of the carb, which is obviously a no go (damn that hindsight).

Also I should mention that I have 2 practice areas, one is at 1500m and the other is at sea level, so I believe the carb needs to be adjusted for both.

Such temperamental little bastards these engines, but I guess you learn how to finesse them with time.

Once we get the engine back together I will do that test and take photos of the spark plug, thanks for the suggestion.

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Unscrewing the L screw makes the carb richer not leaner, i.e.

3T out = rich

1.5T out = normal

1 turn out = lean

My engine won't run with 3 turns out, so you either have a massive air leak somewhere, or a carb with a load of crap in it.

Do you have an in-line fuel filter? Was the petrol from a "good" source?

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Re. your question about setting ignition timing:

I am not familiar with your engine.

Most 2 strokes have a flywheel magneto with either a contact set (also called 'points') or a sensor coil.

Either the contact set, the whole coil assembly or just a sensor coil may be mounted on a plate with slotted attachment holes which allow adjustment around the axis of the crankshaft.

The first thing to do is check that the screws are tight and there is no evidence that the plate has moved. Next check that the flywheel is tight and hasn't moved on the shaft.

If all looks good, leave it alone. If not, get your instructor to look at it.

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I never thought I would need to be a mechanic as well, I should have, I just didn't think outside of the box enough and got carried away by the awesomeness of flying. But I guess I can try and look at this as just another nifty aspect of the sport to get involved in.

I am writing down everything you guys tell me and when we clean and rebuild the engine in a couple days will check on all of it and try and remember how the thing goes together. Hopefully it will make more sense to me once I have seen its guts!

Thanks everyone

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