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What should the compression be on a 1975 DT400?

14 Aug 2019 10:23
Isaac
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What should the compression be on a 1975 DT400? #1
Hey everyone!

After finally getting my stator issue sorted out ( I ended up buying a Rex's Speed shop kit) I was having fun riding my bike.

It started to make some bad piston slap noise and then stopped running altogether. I am considering buying another engine to drop in and skip the process of rebuilding. My bike is not a show bike and I would like to ride it vs build it.

Looking at some used engines on ebay I found this one

It says it has 100 PSI compression. I searched everywhere but have not found spec for compression on that engine. Does anyone know what it is supposed to be?

Should I buy this engine or do a rebuild?

Thanks!

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14 Aug 2019 14:52 14 Aug 2019 14:58 by RT325.
RT325
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What should the compression be on a 1975 DT400? #2
Welcome, or Hi if you're a regular. If the price of freight doesn't kill the deal then 'i'd do it'. Ok it's dirty, but under all that she could be a good runner. Compression may get better with a run so just 'do it'. Bonus is that it's complete with cdi magneto by the looks. I'm guessing broken kickstart means just the lever. Hope it works out. Cheers.
Lower comp could even mean the decomp valve isn't fully seating. Looks like a very complete motor & a good deal. Think i'd premix the fuel initial;y but bleed the pump out the screw on the side so you know the line from the tank & the pump is full. Then when running hold the pump on full stroke for a minute to get the line to the cylinder full--crack the line at the cylinder just to see it pump a few strokes for peace of mind. if the line from tank to pump wasn't bunged i hope not dirt has jumped down there.

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14 Aug 2019 19:39
bluntz465
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What should the compression be on a 1975 DT400? #3
I would by it for parts and fix what you have not depending on the new one to be a runner.
I have never found my luck to hold but maybe yours is fine.

1957 Cushman Eagle 1969 Honda 90 ATC 1969 Honda CB250
1973 Yamaha AT3 1974 Kawasaki KH440 1974 Honda XR75
1975 Suzuki TS75 1975 Kawasaki GreenStreak 90 1975 Suzuki GT550
1975 Suzuki TS250 1976 Suzuki GT550 1976 Indian Chief
1976 Yamaha YZ80 1978 Yamaha IT175 1980 Suzuki GS550
1982 Yamaha IT465J

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14 Aug 2019 20:13 14 Aug 2019 20:18 by Don Dieago.
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What should the compression be on a 1975 DT400? #4
IDK thats almost 350 bucks get a new piston kit and gaskets for 175ish bore cylinder for 50.00-80.00 takes few hrs to do and then you know what you have and it will be fresh. Buy that used engine and its also over 40 years old and god knows how long sense last rebuild So either gamble or have a sure thing . If you dont want to do it then send it to someone who will on here both ways your changing a engine and both ways your paying freight. Take any two stroke with 4000 miles and running good tear it down and chances are you will see some piston scuffing and wear I bet you! a two stroke is a fun engine but not a efficient long life engine in my opinion .

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15 Aug 2019 06:48
MarkT
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What should the compression be on a 1975 DT400? #5
DD, you're certainly welcome to your opinion... I just keep seeing your posts about how bad two strokes are in terms of "long life" and "efficiency".

The Yamaha two stroke power for the displacement was incredibly high for the day... especially considering they are air cooled and were cheap to buy. A four stroke motorcycle with that kind of power per liter (or cubic inch if you prefer) was "race only" and not very reliable at all.

From the time these bikes came from, they are very good... maybe not compared to a modern car that will go a couple hundred thousand miles. But even compared to a Chevy small block from around 1975 that was extremely lucky to make 100,000 miles without major expensive repairs and had maybe 100 HP, these bikes are great in my opinion.

So comparing apples to apples... more modern to more modern... I have a Honda CRM250 from the 1990's. Street legal. Two stroke. Very efficient. (runs as a "diesel" when cruising) About 40 HP from only 250cc (that's about 160HP/liter... not fair to compare directly but roughly equivalent to a 900 HP Chevy 350) And yes, has the Honda reputation for running a long, long time without a rebuild. (Mine had over 10,000 miles on it when I got it and piston/cylinder/rings looked virtually brand new.)

I have to wonder where we'd be if two stroke motorcycle development had continued full force all these decades...

:Buds

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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15 Aug 2019 07:02 15 Aug 2019 07:04 by Don Dieago.
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What should the compression be on a 1975 DT400? #6
Hey I love 2 strokes I have 2 stroke lawn mower 2 stroke outboard 2 stroke pwc and 4 bikes. And I think you know exactly what Im saying. Im still partially anti 4stk myself but you know what I mean. Im just saying for few hundred bucks he can have a fresh new top end or spend few hundred and bolt in a unknown used 40+ year old engine. Facts in my mind is he knows his clutch works he knows his transmission is fine etc etc Hes already prepared to remove engine so with head nuts and 4 cylinder nuts and 2 circlips he has a brand new fresh top end and very very little labor

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15 Aug 2019 07:09 15 Aug 2019 07:14 by Don Dieago.
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What should the compression be on a 1975 DT400? #7
But with all that being said I also have a 1972 honda xl250 I wouldn't be afraid to head of to Colorado tomorrow 700 miles and maintain 60mph all the way. I wouldn't on my 2stk 400 other than on a trailer behind my truck. But Honda is dog slow and the fun factor award goes to my Yamaha and suzuki 2stks

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15 Aug 2019 08:55
Isaac
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What should the compression be on a 1975 DT400? #8
Thanks for the reply DD.

Do you have links for the piston kit? Will any machine shop know how to bore it out or do I need a place that knows these bikes?

One of my concerns is that I had a lean condition that I think was due to an air leak. I know the new/used engine could have the same issue, but I am certain the one in the bike will need it sorted.

Don Dieago wrote: IDK thats almost 350 bucks get a new piston kit and gaskets for 175ish bore cylinder for 50.00-80.00 takes few hrs to do and then you know what you have and it will be fresh. Buy that used engine and its also over 40 years old and god knows how long sense last rebuild So either gamble or have a sure thing . If you dont want to do it then send it to someone who will on here both ways your changing a engine and both ways your paying freight. Take any two stroke with 4000 miles and running good tear it down and chances are you will see some piston scuffing and wear I bet you! a two stroke is a fun engine but not a efficient long life engine in my opinion .

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15 Aug 2019 08:56
Isaac
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What should the compression be on a 1975 DT400? #9
Thanks for the push.

RT325 wrote: Welcome, or Hi if you're a regular. If the price of freight doesn't kill the deal then 'i'd do it'. Ok it's dirty, but under all that she could be a good runner. Compression may get better with a run so just 'do it'. Bonus is that it's complete with cdi magneto by the looks. I'm guessing broken kickstart means just the lever. Hope it works out. Cheers.
Lower comp could even mean the decomp valve isn't fully seating. Looks like a very complete motor & a good deal. Think i'd premix the fuel initial;y but bleed the pump out the screw on the side so you know the line from the tank & the pump is full. Then when running hold the pump on full stroke for a minute to get the line to the cylinder full--crack the line at the cylinder just to see it pump a few strokes for peace of mind. if the line from tank to pump wasn't bunged i hope not dirt has jumped down there.

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15 Aug 2019 08:58
Isaac
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What should the compression be on a 1975 DT400? #10
I don't know about y luck with used engines lol. The last time I bought one it was for my FJ40 Landcruiser and it smoked so bad I swapped to a diesel. Didn't fix the smoking but at least I had a diesel :silly:

bluntz465 wrote: I would by it for parts and fix what you have not depending on the new one to be a runner.
I have never found my luck to hold but maybe yours is fine.

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