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Auto Decompressor Not Working

  • Bsimpson
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Auto Decompressor Not Working was created by Bsimpson

I decided to work on the decompression system on my 400 yesterday. I didn't have the cable connected because it never really seemed to work anyways. But after watching multiple videos and reading the manual I still cannot get it to work. When I take the unit out of the cylinder the lever moves as it should and the pin moves as well. I'm sure they are called something different but the lever that the cable pulls moves up and the pin that goes in the cylinder moves out at the same time. The first issue is when I install the unit back into the cylinder the lever has limited movement. It no longer moves as much as it did when it was removed. 

Second issue is the cable isn't moving that much when I depress the kickstarter. To try and solve this I adjusted it at the adjustment point near the kickstarter. When I adjust the cable to pull more, the weirdest thing happens. I cannot explain it. The kickstarter no longer engages the engine. It has no resistance and when pushed down it will not pop back up by itself. This prevents me from using the decompression system. I can kick the bike without it but I don't like it. Any ideas?
1977 DT400D Brick Red
1978 DT125E Leaves Green
22 Jun 2022 14:52 #1

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Replied by RT325 on topic Auto Decompressor Not Working

Without looking i'm thinking loading the cable so much is bottoming out the decomp valve & that's back loading on the kick gear & overcoming the tension clip that makes the gear move out to mesh. Actual not sure those have a tension clip but same result in the end--won't move to engage. So next thing is valve not decompressing--well, 'is' lifting but not decompressing. Sounds like the hole into the cylinder is blocked with carbon--Or--hole down to the exhaust outlet at the port is blocked verticaly--maybe both blocked. Think those decomps have a washer too which might give more room for lift, but won't need much lift to get the result. I guess the head is off? other wise kick it over or start it won't hurt & see if you have a New exhaust hole putt putting away out the empty decomp hole. Should be a hole straight into the cylinder if you poked it with the piston at bdc. Then with exhaust off you should also be able to poke from bottom to top of the other hole if you look--stand on your head--& look up from in the exhaust outlet. Your pokey thing will appear up top. So the valve links those two holes. Both holes are only about 5mm--or zero if blocked of course.
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Last edit: 22 Jun 2022 16:37 by RT325.
22 Jun 2022 16:33 #2

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Replied by Bsimpson on topic Auto Decompressor Not Working

Head is on the bike, I just removed the unit from the cylinder briefly to verify it was functional. When I rebuilt the top end a few months back the hole going from the decompressor to the cylinder wall was clogged up with carbon. I cleaned that out - but I did not know there was a second hole. It goes from the decompressor to the exhaust? I will have to removed the decompressor again and look. I have a hard time understanding exactly how the system works. At first I thought the valve pulls away from the hole allowing decompression, but when the bike is kicked over the valve moves forward (towards the piston), and for some reason my brain cannot piece it all together. I will look for that hole tomorrow. 
1977 DT400D Brick Red
1978 DT125E Leaves Green
22 Jun 2022 19:58 #3

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Replied by MarkT on topic Auto Decompressor Not Working

I'm trying to remember...  I think when head of valve moves towards piston it unseats allowing pressure past and around stem into a little chamber in the threaded-in part.  Hole from that chamber goes down into exhaust port below.  So pressure goes in at valve head and exits down into top of exhaust passage.. 

 
1963 YG1-T, 1965 MG1-T, Allstate 250, 1970 CT1b, 1971 R5, 1973 AT3MX, 1974 TS400L, 1975 RD350, 1976 DT175C, 1976 Husqvarna 250CR, 1981 DT175G, 1988 DT50, 1990 "Super" DT50, 1991 RT180, 2017 XT250
Last edit: 23 Jun 2022 02:03 by MarkT.
22 Jun 2022 20:36 #4

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Replied by RT325 on topic Auto Decompressor Not Working

Yep, what Mark said in a sentence--which takes me an essay to write haha.
The down hole could be close enough to the outer end of the exhaust port outlet to get something--extended drill--& get up there. Without looking i think the port exits at an angle so give more of a chance of getting at it. Be a bugger if you need to go from the top as will have a small bung like a glued in ball bearing in the top cylinder fin from where it was originally drilled. So might get lucky with a bit of bronze welding wire, or a knitting needle made jagged on the end to spin in a drill & flail the carbon away in the down hole. The valve could get carboned up but would be obvious. Has a ring of holes to let it get to the down hole--i'll call it. Might need the cylinder off to get at it but hope not--frame in the way.
22 Jun 2022 23:42 #5

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