Engine with points swapped with engine with cdi. Advice please!

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Jimlovelady

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Hey everyone,

I have a 79 special that needs a new engine because the right piston has an ugly hole in it caused by the spark plug. (Sad day since I'm poor and love my bike!)

Anyway, I am looking at two engines. One is a 78 with unknown mileage and the other is an 80 with 10k miles (supposedly) but it has CDI rather than points. My bike has points and the guy selling the one with cdi is telling me that it is easy to just put the points from my old, dead engine onto the engine that had cdi. Is this true?

I'm by no means a mechanic so this has been an adventure!

What advice can you give me? Would you get the 78 motor, which is about $100 cheaper and seems to be a straight up swap but of uncertain quality? Or would you get the 80 motor which has cdi, comes with a 30 day guarantee and is about $100 more? If you got the 80, would you convert back to points or make the bike compatible with cdi? And how would you go about doing these conversions?

Thanks!
Jim Lovelady
 
Welcome to the site Jim.
Don't take my simple answer as a definitive "how to" sometimes my brain gets a skip in it.

The CDI system wasn't used on these. The mechanical advance was replaced with a
(T)ransistor (C)ontrolled (I)gnition or TCI.
To use the '80 motor as is you would need to source a TCI box to go with it.

To convert the '80 you would need to move the mechanical advance to it. Including the cam because it has the bushings for the advance rod to ride on. Or source the bushings for the '80. A word of caution here. Some folks are experiencing bad results with installing bushing into the later cams. Is it because the inside wasn't finished to accept the bushings or the bushings are out of spec and oversize? I don't think that has been determined yet. The fact is some cams are splitting when the bushings are installed.

Another thought. Mileage on these motors doesn't mean a thing. Both motors in my opinion should be considered as unknown. May be one was better taken care of but is higher mileage. May be both were thrashed? May be both are pristine?
So maybe a rebuild of your old motor would let you know what it is what?
 
And fyi, holes in xs650 pistons are almost always from over advanced timing.
Unless you broke and electrode off....then that may do it.
Posted via Mobile
 
74-76 Pistons. These will fit any XS650 447 engine from 74-83

There is a mob on ebay, (link above), who sell a set, (2), of Pistons/rings/pins and clips for a good price and a lot of people have used them with good results.

Cheap way if the rest of the internals are good as others have said it is a pig and a poke on buying second hand engines
 
I agree with the other that say getting another used engine should be considered as an unknown. The seller will tell you whatever he thinks you want to hear.
You may get a good one., or you may not. Rebuilding what you have is a good idea. This will give a very good idea of what you have. It will also teach you a lot about your engine.
Cost wise a new engine may be cheaper but if it is a bad engine, it will cost you more.
If your engine is half way ok a fresh top end isn't expensive.
Leo
 
Thanks everyone.

So just to clarify, it I got the '80 engine I would take the points parts and cam from my old engine and it would work? This would be the easiest solution but also an unknown solution because I don't know this engine, right?

The idea of overhauling an engine seems pretty daunting to me but deep down it's what I'd rather do. I figured I'd just replace the pistons but then I thought about that little blown piece of old piston floating around who knows where. Maybe it blew out the exhaust. Maybe it fell down but won't I have to take the whole thing apart just to make sure? Kind of a scary thought.

I'll start looking at threads that talk about supplies needed for an overhaul. I wish it was December!

Thanks!
 
I'll add my .02 the 80 eng. would be the beter choice but as weekendrider said
there are some things you need to do 1st check the cam in it some 1980 eng. had the bushing allready in the cam ( left over 79's ) no big deal to install the bushings, but you'll also have to change the voltage reg/rec the 80 has an Solid State Rectifier/Regulator or you'd have change your stator over to it, Installing new pistons take a little work, but no matter what you do you;ll have to fix what caused the problem in the firest place a holed piston is most likely from running lean dirty carb,vacuum leak
but if you don't you'll burn the new one too
 
By using 87 octane your engine started to detonate while on the highway. The detonation scoured the insulating layer of gas from the top of the piston allowing the intense heat of combustion to reach the aluminum piston. Holes blown through the softened piston top are the result. ALWAYS use the highest pump octane if your old bike is going to hit the highway.

Tom
 
Tom,

What are you, a yamaha prophet or something? That's exactly what happened!!! I was on the highway when it broke down. I didn't know about the octane issue until it was too late.

So what do I need to fix this myself? Pistons. Gaskets. Seals. What else?

Jim
 
Guarantee it was because of advance timing swinging wide causing detonation. 93 may have helped. Everyone sets it at idle and never checks advance. Atu units wear. Holes in xs650 pistons are pretty common.

Posted via Mobile
 
Jim
If you get into trouble I am in NJ in the CAPE MAY AREA and I rebuild motors all the time. Your best bet is to rebuild your motor with new pistons and rings. The cost is going to be close to what it would be buying a unknown motor. If you want to learn how to do it contact me or if you just want it done and no headack I can do that too.
You can get me at DADDYGCYCLES@YAHOO.COM You are about 2 hours from me where I live and you can drop off motor or complete bike if need be. My shop is about 3+ hours from you and I have all the tools and the machine shop to bore.
If you are just looking for advise BUY the 80 Motor and buy a Pamco and go ride.
 
These pistons listed on ebay say "oversized" so does that mean that they are not the stock pistons and that I would have to bore out the cylinders?

Also, the I took the pins out of each side of the piston ring but the ring is not coming loose. Am I supposed to remove anything else before that comes loose? I don't want to start hammering and bend the piston rod or something.

And one more thing, it looks like there is a plastic guide on each side of the cam chain and on the side opposite of the cam chain tensioner, the black plastic (rubber?) piece is broken on one side. It still fits in the metal groove but it's missing the lip on one side. It looks like it will still work but do I need to replace that? And where is that piece that seems to have broken off? Is it floating in the trany?

How do I post pics on this site?
 

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