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Low Compression

  • Shauno
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Low Compression was created by Shauno

Hi everyone; been rebuilding my 1978 DT 400; come to get it started and its a no go? checked spark that's good; carbs been rebuilt and should be in the ball park; timing is set at the marks left by the factory so should be OK. The only thing I noticed is the compression is low at 85psi? Its had a new Wisco piston and fits snug, new head gasket; and decompressor has been overhauled. Has anyone got any thoughts, I've read one article which stated you need to bump and run the bike in gear to get a decent compression test? Lastly if the reed valves are passing on the down stroke will this prevent the bike form starting? If the compression is Ok by you guys I think the timing may need a re-visit? Any ideas welcome.  
05 Jul 2021 08:50 #1

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Replied by DT400FRANK on topic Low Compression

I also have a dt400e.  I just rebuilt my decompression valve also. Make sure the valve is working properly and make sure you disconnect it to do a compression check.  Only other thing that would cause low compression would posibly be a wrong piston but that's doubtful.  I'm not even sure what the compression should be on the engine but I will be going through my top end soon with new piston so I'll need to know :)
05 Jul 2021 10:40 #2

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Replied by Pedalcrazy on topic Low Compression

I'll be starting a complete rebuild of my 78 DT400E motor in the next week or so. Been through a few other bikes already. Just my opinion as I'm no expert like some of the guys here who worked for dealers back in the day (what a cool job)...but 85 is a no-go. Should be maybe 130 to 140 with a correct fresh top end. Also, get a gauge for measuring TDC down the spark plug hole. Never assume you are interpreting some sort of previous timing mark. Should be 2.9mm BTDC and align factory flywheel mark and the pulser coil locating mark. Make sure you are using the mark on the flywheel cast into it and not someone's chisel mark.
1978 DT400E
1976 DT400C
1973 RT3
1971 RT1B
Last edit: 06 Jul 2021 16:11 by Pedalcrazy.
06 Jul 2021 15:48 #3

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Replied by RT325 on topic Low Compression

Red flywheel, cool, never thought of painting mine but covers the rust i guess. 85psi i'd think would run & only get better. I have a 'Trojan Mini-motor pushbike motor from the "50's or late "40's with almost zero compression after breaking a stuck ring & saving the other so put it in the top groove. Have to spin it hard to get it running, but keeps running. So i think 85 would work on yours & only get better with running. So "is it flooded". Or is the crank key in place as could have all the lines aligning on flywheel & stator but if the piston isn't at whatever--2.9btdc--then you're out of luck. Sorry if all this is covered previously.
06 Jul 2021 16:02 #4

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Replied by Shauno on topic Low Compression

Thanks for the comments guys, think I'll go back to the start and check the timing again, lost my flywheel puller at the moment so will have to get a new one!
07 Jul 2021 00:43 #5

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Replied by Schu on topic Low Compression

"lost my flywheel puller at the moment so will have to get a new one!"

Don't worry. You will soon have two flywheel pullers. Just like I have doubles on many specialty tools.
Schu

CT175B, CT175C, JT1' JT2' DT360A, GT80B, DT100B, DT125B,
DT175B, DT175C, DT250B, DT400B, Z50, SCR950

Someday, you'll own some Yamahas
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07 Jul 2021 09:18 #6

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Replied by mdscott on topic Low Compression

Check compression with the de-compression valve disconnect, or plugged off if you think it leaking. 
Thanks Mark
Roseville, Ca.

'06 GL1800
'98 KDX220
'75 Norton
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'01 Honda CT110
'87 PW50
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07 Jul 2021 11:14 #7

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Replied by shyted on topic Low Compression

Are you testing wet or dry?
You should see a marked increase with half a dozen drops of oil in there if you're not.
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07 Jul 2021 23:56 #8

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Replied by Shauno on topic Low Compression

Good call; I'll give it a go, Cheers
08 Jul 2021 00:34 #9

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Replied by Shauno on topic Low Compression

Already tried that, cheers
08 Jul 2021 00:34 #10

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