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Compression

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Replied by Gr8uncleal on topic Compression

davidscott95 wrote: Hello all. I am new here. could not figure out how to post a new question!!! so I am posting here.
I just got a 1976 (i think) IT400. does not run. no spark. my first assumption is the kill switch. it is really rusty. the bike was sitting outside and has a lot of rust on it, so my next step will be looking at the magneto. But I also want to ask about compression? what range is acceptable? and on this bike, it has a auto decompression box on the front of the cylinder? how does that work? and how do i check compression with that thing in it? Also, the head has TWO spark plug holes! I seem to recall back in the day, we had the second one used as a decompressor. so why would they have one on this if it has the automatic one?
I am sure I will have lots of questions as I get into this bike and tear it down. will probably rebuild the entire bike. so hope to get help from you experts out there!
thanks.


Welcome! David.

I've attached a link below to the IT section. To start a new thread, press "New Topic" on the left.

yamaha-enduros.com/index.php/forums/all-about-the-it-models
22 Jul 2020 12:26 #11

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  • RT325
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Replied by RT325 on topic Compression

Welcome.
If the decomp isn't working but is shut & sealing pretty good then you should have compression you could just about stand on for a second or two. But so long as you can feel a reasonable bump then it should run. if the decomp is leaking then it should still run as it's designed to start with it open & just bleeds off some off the compression from a hole above the exhaust port. With the rusty kill switch the problem probably isn't in there as just shorts out to stop so chances are it won't short out--so in other words it shouldn't stop it running. Can simply unplug it to eliminate anything going on there. Be nice if you can get it running before dismantling it. I had an IT400 motor the same in a DT250 frame years ago. Was a fun bike. Just be careful with no kill switch if something goes wrong like the throttle sticking. Be sure throttle is operating & shutting right down before trying to start it. I'm forgetting it doesn't spark. CDI spark never looks very strong.
Last edit: 22 Jul 2020 15:57 by RT325.
22 Jul 2020 15:56 #12

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Replied by davidscott95 on topic Compression

thanks for the info. when I did a slow kick over I could feel some compression. supposedly it ran a couple of years ago. I determined the kill switch is defective, but not the problem for no spark. it was sitting out in the rain for a couple of years so I am guessing corrosion. next I will check the magneto. Is there a way to verify CDI units? i heard there was a test harness back in the day. Before I try any starting of the engine, I need to drain the tank, and clean the carb.
23 Jul 2020 12:57 #13

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Replied by Blukas on topic Compression

Rings might be stuck from sitting. Try putting some penetrant or marvel mystery oil in the cylinder.
04 Oct 2020 12:21 #14

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