Ricky Posted April 14, 2013 Share Posted April 14, 2013 Probably a stupid question but is it possible to do a compression test with the engine out of the bike?Reason for it is because I've still not got my frame back and my compression tool has arrived so thought I could give it a go?Like use a drill to spin the fly wheel or something or will that seize the engine? Thanks for your help Quote Link to comment Share on other sites More sharing options...
Chrissb6 Posted April 14, 2013 Share Posted April 14, 2013 Hi its is possible but not with a drill l would say. Days gone by you could of turned it over by the kick start. Compression on modern bike are quite high "if alls good" so it might take some turning any chance of getting a temp supply to the starter motor? just make sure the coils are disconnectedWhat bike is it and why the test? Quote Link to comment Share on other sites More sharing options...
mattycoops43 Posted April 14, 2013 Share Posted April 14, 2013 Get 12v across the starter motor it should spin up ok. no reason why it needs to be in a bike to do it, but it wants to be securely propped/wedged so it can't move when the starter jerks. Quote Link to comment Share on other sites More sharing options...
Ricky Posted April 14, 2013 Author Share Posted April 14, 2013 Thanks for the help guys I attached my 12v battery to it and it spins normally but the air gets pushed out of the exhaust hole now so there's no reading on my compression tool.It's a Peugeot XR6 and the reason for testing it is because I've just rebuilt the engine and need to check I'm getting good compression before I put it back in the frame because if I've missed something out I'll to have to take it out again Quote Link to comment Share on other sites More sharing options...
mattycoops43 Posted April 14, 2013 Share Posted April 14, 2013 Can someone else answer this? I didn't think it needed the exhaust blocked up to do a compression test? you are testing the sealing of the valves on a 4 stroke to see if the compression ratio is ok. Not sure how a compression test works on a 2 stroke but if it's like a nitro engine it will get compression whether exhaust is on or not, it should be measuring the compression in the cylinder head as the piston reaches the top, exhaust port is not open at this point.If you have tried it and got no compression that would be worrying me, but like I said, someone with 2 stroke experience needs to answer. Quote Link to comment Share on other sites More sharing options...
Ricky Posted April 14, 2013 Author Share Posted April 14, 2013 I would of thought if I plugged the exhaust surely under pressure it'd pop straight out before I got a proper reading :/Thanks a lot for your help Quote Link to comment Share on other sites More sharing options...
Bogof Posted April 14, 2013 Share Posted April 14, 2013 I would of thought if I plugged the exhaust surely under pressure it'd pop straight out before I got a proper reading :/Thanks a lot for your help Dan, courtesy of Google, has the answer...http://www.dansmc.com/vacuum_testing.htm Quote Link to comment Share on other sites More sharing options...
mattycoops43 Posted April 14, 2013 Share Posted April 14, 2013 I would of thought if I plugged the exhaust surely under pressure it'd pop straight out before I got a proper reading :/Thanks a lot for your help Yep, but you said all the air was escaping from the exhaust, so I thought you were saying you were going to plug it up for a reading. Quote Link to comment Share on other sites More sharing options...
acting_strange Posted April 14, 2013 Share Posted April 14, 2013 Here you go..http://www.your-adrenaline-fix.com/comp ... -test.htmlIf you are not getting a reading then you have a problem somewhere... Quote Link to comment Share on other sites More sharing options...
eastanglianbiker Posted April 14, 2013 Share Posted April 14, 2013 is your compression gauge one that you just push into spark plug hole using a rubber bung on it to seal or does it screw into the spark plug hole using a washer to seal? Quote Link to comment Share on other sites More sharing options...
Ricky Posted April 14, 2013 Author Share Posted April 14, 2013 Sorry for wasting your time guys I'm an idiot...There's a little buttony pin than you push in and it must open a valve in the compression testers pipe...So yeah call me what you want but moral of the story is to read your manual first Sorry guys thanks for your help much appreciated Quote Link to comment Share on other sites More sharing options...
fredc Posted April 14, 2013 Share Posted April 14, 2013 the little button will be the release buttonit lets the pressure out and clears the reading when you're done Quote Link to comment Share on other sites More sharing options...
Ricky Posted April 14, 2013 Author Share Posted April 14, 2013 I'm so confused I followed that tutorial and to no avail I just keep getting no reading But the air that gets pushed out of the exhaust hole is massive I cant keep my hand over it...The piston has an arrow on it which is meant to be pointing forward, towards the front wheel, right? I don't get why the compression tester isn't picking anything up :@ Quote Link to comment Share on other sites More sharing options...
Chrissb6 Posted April 15, 2013 Share Posted April 15, 2013 If as l think your motor is a 2 stroke engine there's a lot more to it than just checking the piston ring seal. This below l grabbed off the net it explains it well.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, 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.If it an't broke don't go fixing it! Quote Link to comment Share on other sites More sharing options...
acting_strange Posted April 15, 2013 Share Posted April 15, 2013 Have you tried putting two squirts of oil into the cylinder?The barrel/rings etc need to be wet to work, in a two stroke this is done by the mixing of oil with the petrol in a four stroke it is done by oil thrown from the crank... Quote Link to comment Share on other sites More sharing options...
Ricky Posted April 15, 2013 Author Share Posted April 15, 2013 If as l think your motor is a 2 stroke engine there's a lot more to it than just checking the piston ring seal. This below l grabbed off the net it explains it well.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, 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.If it an't broke don't go fixing it! That explained a crap load to me thanks A few people I've spoked to and some people on the web don't even check it after a rebuild but because its my first time one or two things could of gone wrong...It's all sealed with liquid gasket stuff, spread evenly so it's all put together properly so it should be okay and is had a new barrel and piston put on...Crank seals were a pain in the arse but eventually put on...So it should be good right?Only thing I can think could go wrong is the timing on it but I put it on exactly how it came off and I can't test timing until its back together Quote Link to comment Share on other sites More sharing options...
Chrissb6 Posted April 15, 2013 Share Posted April 15, 2013 Hi, if as you say have just completed a complete rebuild of the engine, fare play and well done. Looks like by all accounts it’s going to be a bit of a suck it and see exercise to see if all gone back together properly. One thing l would strongly suggest you do before you start the engine unless you gave everything a good lub up with 2 stroke oil on the rebuild is to put a small squirt of 2 stroke oil into the spark plug hole put the plug back in and give it a good spin over without starting the engine - (take the plug cap off and turn the engine over) The first seconds of life after a rebuild is the most critical part more so on a 2 stroke engine. Make sure that engine got extra oil in there. It will smoke like hell when you first start up but soon clear, but with this you can be sure that oil is in there doing its job. Fingers crossed for you mate. PS This op might just fowl the spark plugs on the first start up, but it will soon clear up Quote Link to comment Share on other sites More sharing options...
Ricky Posted April 15, 2013 Author Share Posted April 15, 2013 Hi, yeah it's costed me a fair bit to rebuild it but everything seemed okay in the engine anyway but while it was apart it got rebuilt i lubed all cogs etc with 2 stroke oil might be to much :/ but better safe than sorry...I'll be sure to add oil in the spark plug hole I've bought a pack of three NGK spark plugs so if I foul one I've got some spare.I hope it starts up okay Thanks for your help guys Quote Link to comment Share on other sites More sharing options...
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