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SP370/400 compression

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Hi all:

 

I told the story (somewhat) of the electronic to battery points ignition on my 1980 SP400. Dead CDI box and used ones are hard to find and very expensive. So, I mated a SP370 head and rocker box to my 400 so I could use simple and cheap points. The SP370 head looked almost identical to that of that of the 400, with the expection of the area in which the points and mechanical advance go. Also, the rocker box didn't have the compression release of my 400. Valves had same part numbers.

 

I never used the compression release when my bike was all SP400. Never needed it. Now that the SP370 head is in place, the compression seems to have jumped sky high. I can stand on the kicker and it won't budge(remember, compression release is gone). The engine will eventually turn over, but my leg and 190 lbs. are having a hard time spinning the engine fast enough to start it. It does run, however. Did the SP370 have a higher compression ratio? I wonder if someone had previously milled some material off the mating surface of my 370 head? Other than taking the head off and adding a second (or third) head gasket, does anyone have any ideas of what's going on and how to correct it?

 

Thanks for any suggestions and or sympathy.

 

Scott

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I was sure both models had a manual compression release.....I've never had a bigger single that would just  kick through tdc if the valves and the compression where good.....and that's why they had a compression release to ease it by.   It sounds to me like maybe the old 400 head had slightly leaky valves allowing the bike to spin and start easy...and the newly installed head doesn't.   Also, the 370 head might have a smaller combustion chamber (even without milling)  than the 400 which would compound the compression ratio....did you cc them when the engine was apart?

 

If there is an SP370 compression release available I would install it.

 

patuca

Edited by patuca

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Patuca:

 

No, I never cc'd the 370 head, and the 370 rocker box definitely definitely has no provisions for a compression release. Funny thing is that when the bike was entirely SP400, I could kick it fairly easily without using the release (before CDI death). I'm not sure the 400 rocker box (which has the release) will work but that's a very good idea. Someone told me that the rocker box and head are machined to match. Does anyone have any thoughts on issues that may arise if I mix the two? I have to go and check to be sure the 400 rocker box will accommodate the points assembly. I just want to go for a ride and can complicate anything.

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Checked everywhere and couldn't find an SP370 rocker box with decompression setup. Looking around the web tells me the 370s didn't have compression releases, which a lot of people complain of. Can't make the SP400 rocker box (with compression release) on the SP370 head and retain points. Guess it's back to the gym for some work on my right leg and time to get a good starting technique.

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