Help with Yamaha engine rebuild
Sean Bacon
I have taken up the project of rebuilding the engine (new rings, valve
grinding, etc) on my 1978 Yamaha SR500 for a winter project. I have gotten the
upper assembly apart and started to put it back together when I started
running into problems. I bought a Clymers manual for 1977-1980 SR500's, but
it is a bit confusing. It says that when installing the Cam, to get the
timing right, bring the piston to TDC with the points begining to break.
Earlier in the book, it states that the 1977-1978's had electronic ignition
(ie. without points). From what I can tell, the plug fires on every rotation
of the cylinder (no harm firing when the exhaust valve is open and no fuel is
being put in). So my questions are:
Is this the case?
Does it matter which rotation the cylinder is on with this year?
If so, how do I tell which rotation it is on when at TDC?
If anyone has some suggestions for me, or can look in a shop manual, I would
greatly appreciate it.
Thanks
**************************************************************
* Sean Bacon -'91 Beretta GT *
* Milwaukee Design Center -'78 Yamaha 500 SR "Thumper" *
* Test Systems Design *
* "Some people without brains do an awful lot of talking" *
* - Scarecrow from 'The Wizard of Oz' *
* -c32...@mwbcsy01.mke.delcoelect.com *
**************************************************************
Alex Kantor
<del>
ý Earlier in the book, it states that the 1977-1978's had electronic ignition
ý (ie. without points). From what I can tell, the plug fires on every rotation
ý of the cylinder (no harm firing when the exhaust valve is open and no fuel is
ý being put in). So my questions are:
ý Is this the case?
ý Does it matter which rotation the cylinder is on with this year?
ý If so, how do I tell which rotation it is on when at TDC?
I'm not exactly sure; I have an 86 650 Savage Single w/electronic ignition, and
it gets the _fire_ signal from a little stub on the alternator: the book just
skips the possibility you could have the crankshaft 360deg off.
I assumed it fires twice /cycle, and it worked (or I just got the lucky 50%).
I don't know how many cyls your SR500 has, but a distributor could indicate if
a plug is about to fire or if it's just moving from one to another (No distrib
on single :-()
Goodluck
--
dHHHHHP" dP '"THHHHHHb
HHHHH dP "THHHH Ales Kantor
HHHP dHHb dHHb aHHH Hewlett-Packard Company 206 212 4632
HHH dP dP dP dP HHH Vancouver (USA) Division aka...@vcd.hp.com
HHH. dP dP dHHP" aKHH
HHHHi dP .aHHHH company opinions not included
"HHHHHb. dP .adHHHHHP
mr...@ibm.net
>
> Does it matter which rotation the cylinder is on with this year?
> If so, how do I tell which rotation it is on when at TDC?
the piston reaches the top of its travel (on compression and exhaust). You can
tell whether it's compression or exhaust by jiggling the valve rocker arms. If
they both jiggle then they're both closed and the piston is on the compression
stroke.
When you're reassembling the engine all you have to do is make sure the crank-
shaft is in the TDC position, and make sure the cam (I assume overhead) is
rotated to the proper position when you string the chain around the cam and
crank. One tooth off will denigrate engine performance, so try the two or three
adjacent tooth positions when positioning the chain, to find the one that puts the
crank and cam into the closest alignment.
-------------------------
-- The truth according to
-- Michael R. Clements
-- mr...@ibm.net
-------------------------
John Buckham
c32...@mwbcsy01.mke.delcoelect.com (Sean Bacon) writes:
>
>
>I have taken up the project of rebuilding the engine (new rings, valve
>grinding, etc) on my 1978 Yamaha SR500 for a winter project. I have
gotten the
>upper assembly apart and started to put it back together when I started
>running into problems.
>It says that when installing the Cam, to get the
>timing right, bring the piston to TDC with the points begining to
>break.
Easy: You are right about the points vs. SSI, Your SR has solid state
ignition, just turn the crank to TDC, then install the cam with the
timing mark at 12:00 up, on some it's even with the case, your book
tells you. At this point both valves are closed, after you install the
rockers it's a good time to set the clearance.
Put a couple of the head bolts in, leave the plug out and turn the
engine by hand to make sure you get compression showing the cam was
installed properly before you finish the assembly.
BTW- This is the best time to install a new cam chain and chain
tentioner.
Happy Thumping.
Christopher Lawson
:<del>
:
: Earlier in the book, it states that the 1977-1978's had electronic ignition
: (ie. without points). From what I can tell, the plug fires on every rotation
: of the cylinder (no harm firing when the exhaust valve is open and no fuel is
: being put in). So my questions are:
:
: Is this the case?
: Does it matter which rotation the cylinder is on with this year?
: If so, how do I tell which rotation it is on when at TDC?
Look at your coils. If you have a single coil feeding two plugs,
then no it shouldn't matter what rotation the crank is on.
If you're 360 degrees off, you've exchanged the intake spark with the
exhaust spark, which makes no difference.
This single coils feeding two plugs is the reason for having the extra
spark on the exhaust side of the cycle. One of the cylinders is gettingc
its spark for the ignition and the other gets an extra (somewhat useless)
spark. Go arround one crank and they switch roles.
-Chris
Jeff Fitzsimons
: running into problems. I bought a Clymers manual for 1977-1980 SR500's, but
: it is a bit confusing. It says that when installing the Cam, to get the
Not that I have anything productive to add, but I'd like to take this
opportunity to bash Clymer's and suggest that you buy the factory manual.
My CX650 (actually CX500) manual from Clymer's is VERY innacurate in
certain crucial areas.
Since the Clymers manuals have to cover so many different models, they often
have certain aspects left out concerning certain models, or even have
conflicting information. The factory manuals (at least from Honda) are
hardly more expensive, and have MUCH greater detail and accuracy!
Jeff Fitzsimons
(sorry about any formatting/spelling errors, problems with xterm!)
--
Jeff Fitzsimons
[Proof that I am capable of using vi to edit a .signature file! :-]