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Re:76 YZ400 compression PSI

  • Judohead
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76 YZ400 compression PSI was created by Judohead

I'm rebuilding a 86 YZ400. I've got fire, gas, and air. It just feels low on compression for a 2-stroke 400. Does anyone know the stock compression PSI. I can't seem to find it anywhere.

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27 Oct 2023 11:12 #1

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Replied by Gr8uncleal on topic 76 YZ400 compression PSI

I'm not familiar with that model. However, I would observe that it is fitted with a decompression valve linked to the kick start, so that will have an effect on what you're feeling when you kick it over. Unless, of course, it's been disconnected. 

I'm not sure that compression figures were published by Yamaha.
Last edit: 27 Oct 2023 12:02 by Gr8uncleal.
27 Oct 2023 12:01 #2

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Replied by akara1 on topic 76 YZ400 compression PSI

with the decompressor cable removed and the valve not leaking, it should be 140-160psi on a healthy engine, tested with a small engine compression tester. 120psi rings tired, 100psi or less top end refresh time.
27 Oct 2023 14:03 #3

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Replied by Sneezles61 on topic 76 YZ400 compression PSI

Is compression to be tested on a cold engine?
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27 Oct 2023 14:32 #4

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Replied by MarkT on topic 76 YZ400 compression PSI

There's way too many variables (the gauge used is a biggie, factory two stroke gauges are very expensive but probably most accurate on these engines) to what readings you get vs what someone else will get with a different technique, etc. 

Probably why Yamaha never published specs?
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
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27 Oct 2023 14:41 #5

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Replied by Judohead on topic Re:76 YZ400 compression PSI

Well, I didn't have but 30 PSI. Decompression is unhooked. I got the top end off, thankfully the bore looks good, and bottom rod bearing is still good. Head is a little rough, going to get it polished, and a new top end.

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27 Oct 2023 15:44 #6

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Replied by MarkT on topic Re:76 YZ400 compression PSI

Sounds odd if bore looks good?  Hope you still don't have 30 psi after rebuilding top end due a very inaccurate gauge reading or something..  (Been there, done that)

Do you have access to another similar two stroke you could try the gauge on?
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
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27 Oct 2023 16:48 #7

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Replied by Judohead on topic Re:Re:76 YZ400 compression PSI

Yeah, tried the Guage on my Husky 125, it was reading close. I'm not sure how long this bike has sat up. Hopefully I'll get it cranked after a top end.

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27 Oct 2023 17:03 #8

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Replied by MarkT on topic 76 YZ400 compression PSI

125 was reading close to what?
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
27 Oct 2023 17:19 #9

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Replied by Judohead on topic Re:76 YZ400 compression PSI

It was close to factory spec. A little low, but it's been awhile since a top end on it.

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27 Oct 2023 17:37 #10

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