Cams

[tarzan]

I have the head and cylinders off my XS.The chambers on the head were crusted with carbon and I couldn,t decide if the valves were good or not.I have not disassembled the head.The reason for disassembly was I couldnt get rid of the popping back through carb 1.Then I figured while I have the head off I should check the rings.
The bike has 40,000 miles on it and the cylinders are glazed with no ridge at all.
The bike did burn a little oil a little sometimes.Although I have on had this one running a total of 4 or 5 hours while trying to get it to run right.
Heres my plan,I bought a 81 head on ebay.Supposed to have 4000 miles on it.
All the exhaust valves are tan colored and look like it ran good.
Can I just put the cams from my '78 in these heads.I know I can,but is it plug and play?
I am going to hone the cylinders and clean all the ring grooves and pistons and reassemble.Will my old rings seat in do you think?I know I cant get stock rings,right?

[dbeardslee]

The cams in the 78 have more lift and duration than the 81 cams. It's supposed to be a hot mod - hotter cams, along with the bigger valves in the 81 head - but I haven't actually done it myself. I plan to though. Might not completely answer your question, but at least it will bump the thread to the top of the list.

[Geezer]

I wouldn't mess with the honing the cylinders or cleaning the ring grooves unless you are installing new rings. You could easily make things worse.

At 40k miles the smoking was from probably from a leaking valve guide seal.

Geezer

[Ivan]

Is it close enough in the cylinder to mash valves with the old cams on a new scoot?

[tarzan]

That was my concern too Ivan.
I may just go with the 81 cams anyway,they do have way less miles on 'em.
On the other hand,every ounce of power is worth its weight in gold.

[trbig]

I ran 78 cams on a late head with the bigger valves for quite a while. It ran alright, but with the timing the way it was, we could get almost no vacuum to do a carb synch.. like maybe 1-2hg. It would run good at high RPM, but I didn't realize what I was missing on the bottom until I put the right cams back in it a little over a year ago. I'd say at LEAST double the low end grunt versus the 78 cams. There's plenty of piston to valve clearance.. it just isn't the hot mod that people think.

And before you say anything, the valves were good and shimmed correctly and the cam lobes were all still in spec. So.. short answer to my rambling... use the cams that were designed for the head.

What I will say though for your plan, is the later heads had a higher valve relief area versus the early heads..(Space above the piston in the head while at top dead center) and by going to the later head on your early motor, you're going to lose some compression. The early 78-79 model pistons were actually LOWER domed than the later 80-82 pistons. They domed the pistons higher in the 80-82 models to try to make up for some of the compression lost in the larger valve relief area.. but didn't get it all made up. By putting this late head on the early motor, you will drop your compression numbers even further still than the stock later motors.

Clean your 78 head up, put in some new valve guides, lap your valves, shim them.. and it'll be like a new motor if it's only got 40k miles. The 78 was the fastest of these bikes... stick with the winning combination.

Now.. someone with a LATER model engine with the higher domed pistons (Later stock pistons are even higher domed than the Wiseco replacement pistons) that can find a 78 head and cams to put on...

Tod

[Ivan]

I read somewhere about using an 80-82 intake cam and an early exhaust cam. Either the author didn't report about performance or I just don't remember. I can see the early cams creating more more torque, usually lower lift works well for that.

Maybe we need a variable cam timing setup on an Xs!!!

[TopCatGr58]

Hey Tarzan and Ivan,

Dan Hodges is the one that has posted about the pros and cons of mixing and matching early and late cams/heads, so a search for posts by him would reveal them. But Ivan, I believe you're right, in that he suggested the late model intake with the early model exhaust with the late heads, but finding his post would be best to be sure.

I remember Tod's posts and his problems with low idle vacuum with the later head and early cams, think it was due to the higher lift and valve timing overlap. The hotter cams provide more power at the higher rpm range where the larger valves can help feed the engine more air/fuel mixture, but apparently can cause a loss of low rpm grunt.

So it might also depend on where you plan to do most of your engine running, low or high rpm as to which cams or combination you decide to put in.
T.C.

[trbig]

I could be wrong, but I think it's the other way around. It was suggested to use a 78/79 intake cam and leave the later stock exhaust cam... wasn't it?

And Ivan.. ol' machinist buddy ol' pal. When you get back to operating speed, I sure wouldn't mind having a couple sets of cam sprockets degreed if that would be anything you could do very easily.. Probably could have a small market on here doing it if you could get set up for it. I think Weber has a degreed cam for our bikes, but they aren't cheap... like me!


Tod

[Ivan]

I could be wrong, but I think it's the other way around. It was suggested to use a 78/79 intake cam and leave the later stock exhaust cam... wasn't it?

And Ivan.. ol' machinist buddy ol' pal. When you get back to operating speed, I sure wouldn't mind having a couple sets of cam sprockets degreed if that would be anything you could do very easily.. Probably could have a small market on here doing it if you could get set up for it. I think Weber has a degreed cam for our bikes, but they aren't cheap... like me!


Tod

I have thought about this several times. I had figured on just elongating the mounting holes and making some thick washers to fit. The problem is I don't have a rotory table on the conventional mill at work, which would make that a snap. I would have to make a fixture, and a lever to manually pull the sprocket against the endmill. It wouldn't be very precise or accurate. If I had the use of a rotory table, I would have several sets already.

[trbig]

What about mounting an old head to the table,(Minus all the springs/shims/valves) mounting the sprockets onto some cams and bolt the cam caps down. Find the proper size cutting bit, and put it through the existing bolt hole. You could use the slots on the cams to use a wrench and turn the cams... leaving the cutting tool stationary. Some kind of gauge on the cams so you got a max of @ 5 degrees? That way it would be cutting true.

Or maybe not... lol.


Tod

[Ivan]

What about mounting an old head to the table,(Minus all the springs/shims/valves) mounting the sprockets onto some cams and bolt the cam caps down. Find the proper size cutting bit, and put it through the existing bolt hole. You could use the slots on the cams to use a wrench and turn the cams... leaving the cutting tool stationary. Some kind of gauge on the cams so you got a max of @ 5 degrees? That way it would be cutting true.

Or maybe not... lol.


Tod

Well, using a good jig like that could work. The problem is the cutter would have to parallel the cam. I was thinking about just turning a spud on a block of aluminum, that the sprocket would center on, and figuring out the amount of rotation needed with a couple dowel pins to bump against the bog holes in the sprocket. it would be very repeatable, but take lots of time and material. Usually, for me any way, things like this rarely work in the alpha stage of engineering, sometimes in beta stage but usually in the range of foxtrot to zulu...

With a rotory table, it just mounts to the mill table, and you count the number of turns on the handle to figure out the degrees. easy, but the cheap ones are $500+

[tarzan]

Hey Ivan,
If you just need two dots for indicators,you could mount the sprocket on the table and indicate the mark so it is at a spot you can verify.Then trig out the dot locations and drill or use an endmill and make a couple dots at those locations.
I could do it,but I just changed jobs and we cant use the machines for personal use.
Am I misunderstanding what needs to be done?

[petejw]

Hi Tarzan,
Its easy to check the valves on a head when its off the engine,
tilt the head on its side and fill the ports with petrol, then check for leaks
or seapage, if it doesnt leak all good to go if it leaks, just lap the valves,
then while the head is apart clean off all the carbon build up in the ports,
replace the valve guide seals and your good to go.

As Geezer said, I wouldnt mess with the cylinders unless you replace the rings.

In relation to the cams, the intake cams were the same through out the series, only the exhaust cam is different, the early model cam had the longer exhaust duration.

[Ivan]

Hey Ivan,
If you just need two dots for indicators,you could mount the sprocket on the table and indicate the mark so it is at a spot you can verify.Then trig out the dot locations and drill or use an endmill and make a couple dots at those locations.
I could do it,but I just changed jobs and we cant use the machines for personal use.
Am I misunderstanding what needs to be done?

The point being quick machining, with high repeatability. Dialing in the sprocket center each time and then relying on a 35 year old knee mill with out a DRO would be a pain. If I were to make a fixture that would allow for one setup that I could slap sprockets on and make them quick, it would almost be something I could do after work for a nominal fee for anyone that wants to send me their sprockets.