79 F head on an 80 LG...

[tcoop]

Will the 79F Head fit and work on the 80LG engine? It is my understanding the crown of the 80LG is different from the 79F.

The reason I am asking is the 80 head is leaking oil from the exhaust port (bad valve stem seals) and has a spark plug stuck. The 79 head is in good condition and I would like to swap them while I get the 80 head refurbished.

[BA80]

It might work if you stay with the 80 cams. If your going to use the older cams it won't run right with the timing curve the newer engine has.

If you change the centrifugal advance unit you might have a real runner though. I've been thinking about doing that exact thing for a while now. With the higher dome on the later pistons and the smaller combustion chamber in the older head it should raise compression considerably.

[tcoop]

Would I need to worry about bent valves because of less clearance?

If I changed the centrifugal advance unit would I need to change the TCI (or is that what you are talking about)?

[BA80]

I'm kinda curious about that myself.

The way to check, if your willing, is to bolt the head (with the old gasket, no need to torque or even put all the nuts on) on with the cams in time and slowly turn the engine by hand. Back in the day we would lay some modeling clay on top of the piston when we did that to see how close the valve comes to the piston. Rule of thumb is you should have no less than .100.

I personally don't think it will be an issue because the intake valve is the only one I would worry about and the intakes on the older head are smaller diameter so it should make up the difference in the dome.

However, the older cams have a longer duration so there is a possibility that since the piston will be higher in the bore before the valve starts to close that it might come pretty close to the crown.

Clear as mud?

[tcoop]

Clear as Mud!

First check would be with out the cams making sure the pistons do not touch anything then adding the cams (with everything lined up) to check clearances (turning by hand). I would imagine the amount of molding clay would be very little.

I have a 78 engine (hole in the #3 piston and crank case) that I can use parts from. This should be fun. I will have to take lots of pictures as I do this (I tend to forget that part).

[tcoop]

Well I have successfully put the 79 head onto the 80 engine. I did use the cams that came with the head. Got all the timing marks to line up and I very carefully turned the crank...it turned. I rotated it about 10 times and rechecked the timing marks and all was good. put everything back on cranked it and it started right up! So far I have only needed to adjust the idle and the bike purrs. Took it for a 10 mile ride and she is running better than ever. No longer have a low end stumble and she pulls strong from any RPM range. I do need to sync the carbs tonight but other than that all is well!

I may even check the compression just for giggles.

[skids]

Well I have successfully put the 79 head onto the 80 engine. I did use the cams that came with the head. I may even check the compression just for giggles.

The clearance between the open valves and the piston crowns must be pretty close. I would just recommend that you do more frequent valve/shim checks than the intervals that are specified in the manual.

[BA80]

Well I have successfully put the 79 head onto the 80 engine. I did use the cams that came with the head. Got all the timing marks to line up and I very carefully turned the crank...it turned. I rotated it about 10 times and rechecked the timing marks and all was good. put everything back on cranked it and it started right up! So far I have only needed to adjust the idle and the bike purrs. Took it for a 10 mile ride and she is running better than ever. No longer have a low end stumble and she pulls strong from any RPM range. I do need to sync the carbs tonight but other than that all is well!

I may even check the compression just for giggles.

You should try changing the centrifugal advance unit with a 78 or 79 unit. It will have the timing curve to work with those cams.
You may get MUCH better performance with it.

This is interesting, I've been thinking of doing this for years.

[BA80]

The clearance between the open valves and the piston crowns must be pretty close. I would just recommend that you do more frequent valve/shim checks than the intervals that are specified in the manual.

I've always thought that the difference in the valve size would make up for the difference in the piston crown.

[BA80]

Well I have successfully put the 79 head onto the 80 engine. I did use the cams that came with the head. Got all the timing marks to line up and I very carefully turned the crank...it turned. I rotated it about 10 times and rechecked the timing marks and all was good. put everything back on cranked it and it started right up! So far I have only needed to adjust the idle and the bike purrs. Took it for a 10 mile ride and she is running better than ever. No longer have a low end stumble and she pulls strong from any RPM range. I do need to sync the carbs tonight but other than that all is well!

I may even check the compression just for giggles.

You should try changing the centrifugal advance unit with a 78 or 79 unit. It will have the timing curve to work with those cams.
You may get MUCH better performance with it.

This is interesting, I've been thinking of doing this for years.

Look her to see what I'm talking about....http://www.xs11.com/forum/showthread.php?t=37189

[trbig]

He should have bumped up the compression a decent bit with doing this. I'm curious what kind of compression numbers he'll get. This has been talked about several times to try, but never heard of anyone actually doing it. I wish he would have checked the piston to valve clearance to know exactly. Until it's a sure thing knowledge-wise, I'd sure set those valve measurements to the loose side.

Congrats to guinea pig tcoop!

[tcoop]

Well I did the compression 150 across the board. this was with WOT and engine cold. That didn't seem to be any big increase. IIRC the last time I checked compression on this bike it was at 140 to 145.


As far as the centrifugal advance unit I'm going to hold off on that. The bike is running better than ever right now. I may look into it this winter.

I did set all the valves clearances to the loose side. she sounds like a well oiled sowing machine.

[Dan Hodges]

VP clearence

Well I did the compression 150 across the board. this was with WOT and engine cold. That didn't seem to be any big increase. IIRC the last time I checked compression on this bike it was at 140 to 145.


As far as the centrifugal advance unit I'm going to hold off on that. The bike is running better than ever right now. I may look into it this winter.

I did set all the valves clearances to the loose side. she sounds like a well oiled sowing machine.

There are no clearence issues when using the early cams with the late pistons or when putting the early heads & cams on a 80-82 motor. You can safely set the valve lash on the early cams using the late pistons per stock specs but do keep an eye on the cam chain adjustment.

You can determine your running compression ratio by dividing the pumping compression by 14.7 (atmospheric pressure)-----hot battery, good starter with carbs fully open required. This number is not the same as the mechanical static compression ratio which is based on swept volumn.

Pumping compression has everything to do with the intake valve closing in that the quicker the intake valve closes the more pumping compression you will have with a given amount of static compression.

A smaller cam will pump more while a more radical cam will pump less all other factors being equal.

The late pistons have a dome volumn of 6 cc's and the early pistons are about half that. The Wiseco 74 mm piston for the XS measures 6.6 cc's.
Using the late head with the early pistons is a no, no for two reasons. One, you would have very little compression due to the loss in piston dome volumn and two, the valve reliefs in the early pistons will not provide sufficient clearence for the big valve late heads.

Mention is made that a point of static compression is worth about 4% more power untill you reach about 11 to 1 and then the gains begin to diminish.

The late head is worth about 5% more power than the early head all other factors being equal.

A set of late pistons with the early heads will fetch about 3% more power.

The ignition advance can be easily changed on the 78-79 and the 81 doesn't need to be changed because it's already at 40 degrees at WOT.
One can study the advance curves in the Yamaha shop manual and see for him/herself.

Gas isn't what it once was however 93 octane BP gas is pretty good stuff and works well even in motors with 200 pounds of pumping compression.

Some pump gas is better than others but it all goes bad in a hurry and it's all very corrosive because of the 10% added ethanol and of course ethanol leans out the motor slightly however E-85 gas should be avoided, period.

Horsepower is where you find it.

[trbig]

Hey, Dan. Here's a pic of the domes of the early vs late piston on the same pin.





The 74mm Wiseco piston is domed somewhere between those two. I couldn't find my picture of it. Not as high as the late OEM, but higher than the earlier OEM. So, effectively, Wiseco will bump up compression over OEM on an early model motor, but decrease compression a touch on the late models.

The late head is worth about 5% more power than the early head all other factors being equal.

I don't get it? Larger valves, but larger valve relief area and less compression than earlier models, even with the higher domed pistons. Was this a typo, or were you meaning that if compression was the same, the bigger valves win?

[XS11UFO]

I love mine

I put an '82 motor in my '79F with the old cams, head, and ignition. She runs like a scalded dog. Seems like more than 3% to me but it's great.