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I have a Derbi Senda SM 50 2005 plate. It has a good spark, however, it's not reading any compression. It's had a new top end and gaskets!

What could be wrong?

Sometimes it starts when you bump start it, but then it will just die!

Any suggestions?

3

This is a two cycle engine

This particular motor (and most Derbi's) is a 2-stroke engine and as such has multiple places where an air leak from the reed valve (intake track after the manifold) all the way through to the head gasket, base gasket, spark plug AND crank seals can give you a low-compression issue.

Things to check

  1. Reed Valve* - The reed valve is just after the carburetor and component of the manifold. It's marked as 21 in the screen grab. Reed valves are a one way valve and prevent pressure from the piston's down-stroke from escaping out of the intake through the carb. I believe this is the most likely cause of the issue. enter image description here

  2. Engine Seals - If you didn't split the cases then you should not have to worry about this. It is worth noting that in two - stroke engines the crankcase is a component of the carburation system, any air leaks in the crankcase can cause a compression issue.

  3. Base Gasket - Same as above regarding air leaks. Make sure you have good seal here. The crankcase is a component of the fuel delivery system in a two stroke engine. When the piston comes down it pushes the fuel charge into the combustion chamber. Any air leaks in the crankcase gaskets, crankcase metal to metal seal or crankshaft seals can lead to an air leak.

  4. Spark Plug - Ensure you have the aluminum or copper crush washer that comes with the spark plug. This is essentially to sealing the head up.

Tricks

  1. Soapy Water - Use soapy water and a brush as you are troubleshooting, you can find bubbles where positive pressure is escaping.

  2. If you get it running poorly - Use a carb cleaner or some other very 'thin' hydrocarbon to spray around seal points. If it is 'sucking' air into the system you can hear the idle change when you spray that point (possibly) as well as smell it coming out of the exhaust.

Reference

Here's a picture of a two stroke engine cycle so you understand the points that require a good seal.

enter image description here

*Pulled this from oemmotorparts.com

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Any internal combustion engine needs three things to run: fuel; air; ignition source (spark). Almost always, air can be assumed as being present. You should check the other two things, those being the spark and the fuel.

  • Remove the spark plug wire from the spark plug and check to see if you are getting spark through the wire. You should see a spark coming from the wire if it is held close to the head of the engine, then spin the engine over (two person operation). Be careful because you can get shocked doing this if not careful (though if you are shocked, it won't harm you, but you will definitely know there is spark).
  • If you get spark there, check the spark plug as well by pulling the plug, connect it back into the spark plug wire, ground the plug at the head and rotate the engine. You should see a spark over the gap. If you don't, it's probably the spark plug.
  • Remove the spark plug and smell it to see if you can smell gas on it. If it's there, you know you are getting fuel. It could be that the carb isn't doing it's thing correctly. Too much or too little fuel will cause you issues. Running ethanol laced fuels does not help this, either. The ethanol tends to gum up the carb.
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just had this problem on a friends bike after a late night recovery call I couldn't fix roadside so threw it in my garage - 99 derbi gpr50r symptoms and quick checks... the spark was there, carb was cleaned out, felt like compression when finger over plug hole but nowhere near enough, wouldn't even fire with carb cleaner poured into combustion chamber, but occasionally would start bump starting on a hill (but ran rough and sounded wrong ooh so wrong)

you might be able to do this in bike but I dropped the engine for fun. pull off the head then the barrel from the piston push out the gudgeon pin and remove piston for best view - there was scoring on the barrel and the ring had 'notched' causing a stuck ring on one side and was flush with the piston (see the problem yet) a new piston (£13) and a little wet and dry paper would work but no guarantees on how long for (I don't advise this)(+£13 for gaskets = £26) a new barrel piston gaskets and all the little extra bits start at £50 on fleabay but we spent £65 to get a slightly better quality one you are open to decide if you want to go big bore kit as this is the best time to make the decision - up too you.

search google for carl salter to get manuals for torque numbers etc - I advise to buy beg borrow but not steal a torque wrench and doing it right - and add some copper slip on your nuts (as in nuts and bolts) doing it right now saves doing it wrong later and MOST IMPORTANT COVER NEW PISTON AND BARREL IN NICE NEW 2T OIL WHILE ASEMBLING TO EASE WEARING IN AND MAKE SURE YOUR OIL PUMP IS WORKING AND HOSE ON CORRECTLY AS THIS IS OFTEN THE CAUSE OF THE PROBLEM SHE WILL SMOKE A LOT TO START WITH BUT DONT WORRY ABOUT IT TOO MUCH IT IS A STROKER AFTER ALL. -HAVE FUN, OZZY

  • Great answer and welcome to the site. Thanks for contributing! – MooseLucifer Aug 16 '16 at 18:49
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Well, if you're not getting any compression, that explains why it's not running - you just need to work out what's causing the lack of compression.

If you've already replaced the top end and head gasket, the next suspect would be the piston rings letting the gases into the crankcase - though this usually results in poor compression rather than no compression at all.

One trick I have heard to test this (although I've not tried it...) is to pour a small amount of diesel into the cylinder, and watch to see how quickly it drains out - if it is quick, the rings have failed. Don't try and start it while the liquid fuel remains in the cylinder though - if it doesn't all drain, siphon it out!

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