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Motorcycle Forum / General / Yamaha / August 2007Hot vs Cold Start
Why is it harder to start a cold engine vs starting an already warmed- up engine, special on older bikes. Based on the three primary factors: Spark , Fuel/Air mix and Compression. What improve so much? >Why is it harder to start a cold engine vs starting an already warmed- >up engine, special on older bikes. Based on the three primary >factors: Spark , Fuel/Air mix and Compression. What improve so much? The distribution of burnable fuel/air mixture in the combustion chamber is rather random. In order to ignite, the cold engine needs a richer mixture to improve the odds that there will be burnable molecules in the spark gap as current jumps to the ground electrode. Once the engine is running, a leaner mixture can be used. Older Japanese motorcycles with ignition points and low voltage coils were harder to start than modern bikes with electronic igniton. They either needed more voltage so the spark plugs could be gapped wider, or they needed a rather rich mixture. This resulted in spark plug fouling and the need to clean the plugs every three or four thousand miles. >>Why is it harder to start a cold engine vs starting an already warmed- >>up engine, special on older bikes. Based on the three primary [quoted text clipped - 12 lines] >rather rich mixture. This resulted in spark plug fouling and the need to >clean the plugs every three or four thousand miles. maybe on 2 srokes but none of my 4's, among whic are xs-654'd-750's-1100's and many sr 500's. the plugs on those stayed fine. i do cheat, though, i keep them tuned, adjusted and started at least once every 2 weeks. many yamays have weak sparks, but hotter plugs help a lot. >The distribution of burnable fuel/air mixture in the combustion chamber is >rather random. In order to ignite, the cold engine needs a richer mixture to >improve the odds that there will be burnable molecules in the spark gap as >current jumps to the ground electrode. When the engine is warm, the gasoline atomizes better, so the engine starts on a leaner mixture. On Aug 10, 1:56 am, "Albrecht via MotorcycleKB.com" <u33665@uwe> wrote: > >The distribution of burnable fuel/air mixture in the combustion chamber is > >rather random. In order to ignite, the cold engine needs a richer mixture to [quoted text clipped - 6 lines] > -- > Message posted via MotorcycleKB.comhttp://www.motorcyclekb.com/Uwe/Forums.aspx/yamaha/200708/1 Does the Compression play a roll? The manual (xt550 '83) say it should be 156psi, I only get about 130psi (wet/dry). What is the tolerance on that. My tester can't be 100%, because it works with a rubber end on the sparkplug hole (at a place where you can't see nothing!) How much compression do I loose with the automatic compression release, will it help to disconnect that? How do I know if I have a low voltage coil? Where do you kick from? TDC, pre-, post? Thanks for all the help! >On Aug 10, 1:56 am, "Albrecht via MotorcycleKB.com" <u33665@uwe> >wrote: [quoted text clipped - 22 lines] > >Thanks for all the help! http://www.thumperpage.com/ go the the message section and ask there. real experts.... On Aug 10, 7:51 pm, some...@some.domain wrote: > In article <1186783216.805428.135...@57g2000hsv.googlegroups.com>, Gerrit <ingerr...@gmail.com> wrote: > >On Aug 10, 1:56 am, "Albrecht via MotorcycleKB.com" <u33665@uwe> [quoted text clipped - 26 lines] > http://www.thumperpage.com/ > go the the message section and ask there. real experts.... That does not exist anymore! (due to hackers) >On Aug 10, 7:51 pm, some...@some.domain wrote: >> In article <1186783216.805428.135...@57g2000hsv.googlegroups.com>, Gerrit [quoted text clipped - 34 lines] > >That does not exist anymore! (due to hackers) son of a bitch. go ahead and use the old forum, it's safe. we checked the back. ask big ed, if he's around, if not, you'll stii get help from reid and the boys. it really is ok... >Does the Compression play a roll? It's about 10% more efficient to burn the fuel at 10:1 than it is to burn fuel at 5:1 compression. As long as you have at least 90 psi cranking compression, the engine should start. >The manual (xt550 '83) say it should >be 156psi, I only get about 130psi (wet/dry). Your valves are burned or not seating well for some other reason, such as a build up of carbon. >What is the tolerance on that. 130 psi is about the minimum. >My tester can't be 100%, because it works with a rubber end >on the sparkplug hole (at a place where you can't see nothing!) OK, there is some extra volume inside the compression tester and the tube connecting the tester to the spark plug hole. That's why you cannot get a full pressure reading the first time the piston comes up on compression, you have to kick the engine through about 20 to 25 turns so you get 5 compression strokes. There is a tire valve inside the rubber end of the tester to capture a fraction of the pressure from each compression stroke. The pressure should rise evenly, the same amount for each compression stroke. But using one of those testers that doesn't screw in makes compression testing difficult. Also, the test is supposed to be done with the engine warmed up and the throttle open so the engine can get some air to compress. >How >much compression do I loose with the automatic compression release, I dunno, do you hear the compression leaking out the exhaust when you kick the engine over? >will it help to disconnect that? It would make sense to disconnect the cable. >How do I know if I have a low voltage coil? The spark from low voltage coils would only jump about 1/4 of an inch. Higher voltage coils make long sparks, but you risk having the spark jump from coil to coil *inside* the coil, ruining it. >Where do you kick from? TDC, pre-, post? It doesn't matter what position you start from. You have to kick and kick and kick continuously while holding the compression tester in one hand and the throttle open with the other hand. You need to kick the engine through about 20 to 25 turns. When the compression stops rising, you've trapped all the pressure inside the tester that the engine can produce in 4 or 5 compression strokes. >Why is it harder to start a cold engine vs starting an already warmed- >up engine, special on older bikes. Based on the three primary >factors: Spark , Fuel/Air mix and Compression. What improve so much? it's all lubed, the carb is sucking right and it's warm and all expanded loose. >it's all lubed, the carb is sucking right and it's warm and all expanded >loose. If you had just left the word "carb" out, that statement would have been very sugestive. ;-) |
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