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1978 sp 370

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Hi guys I just recently acquired a 1978 SP 370 I got it from a friend and it had some sort of problem and it wouldn't start. I took it apart and the cylinder walls were all scratched and the piston was shot. I have replaced the piston and honed the cylinder, changed valves and springs an have it back together now. When I push it down the road and drop the clutch it starts and runs really ruff, it seems to have no power at all and you cant rev it, it stays at an ideling pace. It only runs for a couple seconds then shuts off. I have taken the engine out of it twice to check the timing and it still doesnt resolve anything, im starting to think it might be something in the crancase. Any help is appreciated

Thanks,

Matt V

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Check cylinder pressure.

Check Timing.

Check Spark.

Check the Carb and throttle cables.

That's about all I can think of for now. Good luck!

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post this in the DR section--did you take the carb apart and clean it? a must do if it has been setting.

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Make sure the cam timing is set correctly (mechanically) the piston has to be at top dead enter (TDC) and the cam lobes down and the cam shaft sprocket should have some marks on it that ensure you are correctly aligning the cam paosition with the position of the piston (TDC) Honda uses horizontal lines that align flush with the surface of the cylinder head for this purpose. At the same time there should be a mark on the flywheel that lines up to indicate TDC - maybe a "T" (such as honda uses)

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Does anyone know the correct cylinder pressure for this engine I have measured 87 psi and I think that that is way to low.What would cause this, a leaking head gasket or valves are not closing properly possibly the piston is not sealing?

Thanks for all the help so far guys,

Matt V

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When I reassembled the engine I reused the head gasket because it looked ok still, is it possible that it is leaking?

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When I reassembled the engine I reused the head gasket because it looked ok still, is it possible that it is leaking?

YES and 87 psi is way way low.

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Thanks Mike,

What range of psi should I be looking for? from what I have heard no less than 105 psi?

Your help is greatly appreciated 👍

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You originally said that you replaced the piston and had the cylinder honed.

Did you re-bore the cylinder to the next size over before installing a new (correct for the new bore) piston?

If you had a worn cylinder liner and simply honed and replaced the piston with a stock size...that might very well be your compression problem.

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yes I re-bored the cylinder and put the correct size of piston in it, I'm pretty sure that's not my problem. Today I took the head and cylinder off and found that the intake valve was hitting the piston, it looks like this is most likely my problem. I could not turn it over by hand when the spark plug was out, thus I think that the valve was hitting the piston and the piston could not finish the cycle out. Will the valve be bent or need replacing?

Thanks Everyone for the information so far 👍

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Today I took the head and cylinder off and found that the intake valve was hitting the piston...

Sounds like the position of your timing chain on the cam is incorrect.

With the head off turn it upside down and pour a little bit of solvent in the combustion dome of the head. Leave it like that and see if it leaks out that valve which should be sealed. If it's leaking it is most likely bent a little bit from the impact with the piston and will need to be replaced.

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Good news the valves are not bent, I re-seeded them with some valve grinding compound and have put them back into the head. I was wondering what you guys would do for a head gasket, mine looks Ok but I'm not sure if I can trust it, the problem is I cant find a new one since the bike is so old and parts are not easily found for it. Also the piston and cylinder walls have some vertical scratch's on them not sure if I sould leave it alone or hone it before I reassemble every thing, you can barely feel the scratch's but they are there.

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these guys in Kalifornia should have it

http://www.bikebandit.com/

go to suzuki, year, model, list for US $39 , a bit steep.

otherwise make sure that you have a good seal, you can use some loctite sealer a little bit to seal the little nicks, otherwise a little polishing of the head and cylinder head on a flat glass surface could help.

But as said what is your compression now, specs should be in above 140, I do not have the specs at the moment. I have the 400 off road version and I can almost stand on the kick starter, I really have to kick.

you can check ebay and put an add in the wanted section, only base gasket are listed

Michel

80DR400

83XR350

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Yes my compression is good now, but to my great disappointment i cant figure out why i don't have any spark. I have spend a long time trying to figure out the wiring but the previous owner I think did some creative wiring a couple times too fix his problems 👍 Anyways does anyone know where I can find a wiring diagram for this model or the sp400's,

Help will be greatly appreciated because im getting ready to take this thing to the dump if im not able to fix it soon🤣

Thanks,

Matt V

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I can send you a copy of the diagram(s), pm me for a private email

M

Yes my compression is good now, but to my great disappointment i cant figure out why i don't have any spark. I have spend a long time trying to figure out the wiring but the previous owner I think did some creative wiring a couple times too fix his problems 🙂 Anyways does anyone know where I can find a wiring diagram for this model or the sp400's,

Help will be greatly appreciated because im getting ready to take this thing to the dump if im not able to fix it soon :banana:

Thanks,

Matt V

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