Where to start, motor rebuild

[Dead Roman]

Just picked up a motor from a guy in ft worth. Im about to open it up to see what kinda shape its in. Where should I start? What needs to be replaced right off the bat? How can I make it a strong, bomb proof runner? What is max hp you can wrangle out of an xs before turbo or nitrous is nessecary?

Also, can you do a 750 on stock cylinders?.

 

[JP50515]

To start, I would go from the top down. Check to see if you can turn it over by hand before anything....figure out if its seized or not.


Pull the valve cover, check the cam for wear/damage. If you can run your thumb over the lobes and feel any damage your F****D! (in the words of the local harley shop guy)

ha

if you're planning on rebuilding either way I'd cut the cam chain, and move downwards. Check that valves and seals. Then get to the pistons.

If you pull the pistons, hone/bore the cylinder walls, replace the rings and all gaskets in the top end you should be set up for a solid runner.

I believe that a stock engine produces right around 50HP and a bit more than that in torque. However it is very possible to punch these out to a 700cc with some upgrades, however I am 99% positive you need new sleeves to do that. I've actually heard of 800cc engines on the euro versions....but never actually seen one

 

[Dead Roman]

It turns by hand. Imma do a compression test today bit the guy i got it fr said he tested and it was good. I will pull the cover and check the cam after.

 

[retiredgentleman]

The only way to know what needs replacing or machining, is to open up the top end and measure the parts. Compare what you find with the specs that Yamaha has in their Service Manual.

A compression test is fine to do, buy good numbers do not mean that all is well inside the engine. When I bought my 78SE, compression was in the 145 psi area. However, upon taking the top end apart, the piston tops and the heads had a very thick layer of carbon from a passing head gasket. The thick carbon would actually increase the compression.

 

[sseres]

I wasn't in a hurry to need a running engine and so was able to enjoy the frack out of the process. If it's old, it'll appreciate a rebuild. Even if it's just to replace oil seals and the thrust washer on the clutch, it'll have been worth it. If you're in a hurry, tho (eg, no other engine in a running bike) it might be a little frustrating to cut it all apart.

 

[Liftedzuki]

I'm rebuilding mine because I found plastic in the oil filter, small piece. looks like the lower end has been touched and its a 1980 motor in a 75 frame.

I'm thinking of leaving the lower end together and just redoing whatever I have to (I usually go way too deep into a project, trying to not do this now)

I'd do a full rebuild if i was going for a big HP/or big money project.

 

[cobrasneverdie]

since we are on the topic, why would you want to hone/bore the cylinder walls.. hmm what would make you want to do it?

like let's say I wan to replace the piston rings, or clean it up a bit, or replace valves

 

[Dead Roman]

So i literally have to cut the timing chain with bolt cutters?

 

[XSLeo]

That's one way. I carefully grind the peened ends of the pins on one link then drive the pins out.
If the cam chain isn't worn to bad You can just remove the tensioner, lift the cam a bit, slide the bearings off the cam then slip the chain off the sprocket and with a bit of wiggling you can slip the cam out from under the chain.
If your going to replace the chain, just go ahead and cut it.
Leo

 

[I am Carbon]

you can cut it with a cut off wheel. you can allso lift the cam up a bit and slide the bearings off the end of the cam and slide it off that way.

 

[Dead Roman]

After lookin through the exhaust ports/spark plug ports the inside of the cylinders appear to be black and dry with a little debris from storage, so I am gonna go ahead and re-build the thing. I will most likely go ahead and do the re-phase while I have got it apart.

 

[peanut]

T

I believe that a stock engine produces right around 50HP and a bit more than that in torque. However it is very possible to punch these out to a 700cc with some upgrades, however I am 99% positive you need new sleeves to do that. I've actually heard of 800cc engines on the euro versions....but never actually seen one

I read the other day of an XS900cc

 

[JP50515]

wow....gimme!

 

[Dead Roman]

Read about it but his e-mail is dead now

 

[Dead Roman]

Manged to break the oil lines that come up over the top of the valve cover not paying attention.

Ugh

 

[peanut]

dayum are they repairable ? difficult part to find used I should have thought.
Can't find mine at the moment to look at it but I assume it can be brazed repaired ? any chance of a pic ?

 

[Dead Roman]

It may be repairable, but ive already resgigned myself to sweating new lines out of soft copper. I wanted to use a little copper here and there on the bike anyway just to add some accents.

 

[peanut]

thats a great idea ! I was going to suggest that but wasn't sure if you had brazing experience
look forward to seeing the result

 

[Dead Roman]

The only thing I am iffy about is the splitter at th top of the downpipe, its like a weird 45 degree job. Maybe I can use the existing one but I am not sure yet.

 

[peanut]

not sure about the standard internal dia but if it is around 8- 10mm there might be some copper or brass 'y' connectors in central heating brazed fittings .You can certainly get 'T' connectors which you could cut down and modify.
take a look in this lot
http://www.pegleryorkshire.co.uk/MEDIA/Downloads/CC_001/48435435_Endex_Data_Book.pdf