Lowering compression for rideablilty

I have a '80 XL500S that had been bored out and a pop-up piston installed befor e I got it. The bike has decent power but is a tad tempermental. It gets hot very easily and will die on me for a few minutes until it cools off a little. I'd like to sacrifice some power for reliablity. I'm thinking about tearing the motor out this winter and cleaning some things up a bit, so I was thinking while I was in there I can stack a few base gaskets in the cylinder. Richening the fuel mixture has helped some, but not quite enough...

if your going for reliability i wouldnt just stack base gaskets up. Maybe start by trying a colder spark plug.

Probably be easier to run 2 head gaskets. The only real problem with stacking gaskets is you screw up the squish in the head and mess up the flame front in the cylinder head. But it doesnt sound like your racing the thing anyway so I'd doubt you notice. It'll definitely run cooler. One extra head gasket will probably lower the compression ratio by close to a full point. You may want to do some math there so you know what you'll end up with. You may want to check the timing too. If its been retarded too far it will make it run hot also...

You could file a tiny (don't go nuts with the file!!!) bit off the dome of the piston. That would likely help a little.....

I guess stacking head gaskets would inded lower the compression, but I'm not certain it's the greatest way in the world to go... Wouldn't 2 head gaskets effectively create another potential source of a compression or oil leak? :bonk:

I know my cam chain ain't long for dis whirlled, & has stretched way beyond what it should be runnin' at.... Maybe the 2 head gaskets deal is what I need to do to my bike to take up some cam chain slack.... :cry::cry::cry::applause:

Stacked base gaskets scare me. Double-up the head gaskets; make sure your timing is spot-on; and run a cooler plug.

I quite often use two base gaskets to set my squish band, but I would never, ever run two head gaskets. You are asking for trouble to even consider it. There is no compression on the base gasket, but the head gasket needs a solid material on both sides of it. What do you think will happen in between the two?? You can get a thicker, copper gasket from several sources, but as mentioned above, it will compromise the squishband clearance and hamper performance. If too high of a CR is a problem, the correct way to deal w/it is to change the piston to a flat top. It is very difficult to get a low compression ratio, w/a 500cc engine and keep the squish band under .040. Happy New Year! Clint

Since I will mostly be using this bike to scoot around town a ride and to work, I think I'm getting a slightly larger sprocket to get my cruising RPMs down and try a colder plug. I was a bit concerned with leaking compression if I added head gaskets instead of base gaskets. Can you add some sort of sealant to the gasket surfaces to stop this? Is it really OK to take a file to the piston? I'd be afraid to make it too thin. I'd love to be able to find a flat piston for it, but all I've been seeing is are the domed Wisecos.

When I was younger, I had a Honda S-90 that I was able to stuff a piston out of a CL175 twin, if I recall correctly. It opened the bore up enough to give a calculated 100cc, but had a much higher crown than the stocker. I had to file a little off that piston to bring down the CR, and make life easier on the kickstarter's shaft.

Now, that was a Honda piston that I filed on, and I doubt that I filed more than .02 off of it. But I did it over the entire high point of the piston's crown, and a little of the "sloped portion" of the crown. I can say honestly that I never burned a hole in a poston, or had any type of failure due to lowering my compression ratio that way.

If anything, it made the bike more reliable..... I wasn't having to replace the kikstarter shaft all the time after that... :bonk:

If I couldn't afford to put in a piston with a lower dome, & if there was enough meat (that's a BIG prerequisite we're talkin' about here.... :cry: ) on the dome of a high compression piston, that's what I personally would do. Myself, I wouldn't stack gaskets.... But again, that's my own personal philosophy.... :applause:

Ive had to stack head gaskets in the past. Never had a problem. I was scrambling to get my CR500 together after they ported and milled the head. After I put it together and kicked it the first time, I thought something was wrong, way too hard to kick. So I pulled the plug and did a compression check and it hit 245 psi. Too damn much. I was trying to get it ready for 5 day trip in the dunes and was leaving the next day. I just popped a spare head gasket in and was able to drop the compression down to 195 psi which turned out to be perfect for the port job and pipe Im running. Its been together that way for years...literally.

Filing the piston off is an option I guess. Before I did that, I think Id make a clay mold and see what the piston dome volume is and figure out what your actual compression ratio is. Then figure out how much you'd need to remove (volume wise) to achieve your desired C/R. You may be surprised how little you need to remove.

An extra head gasket is a hell of a lot easier. And you can measure the one you take off to find out how thick it is after its been squished to give you an idea of what you'll do to the final C/R.

If you do end up running two gaskets be sure to check your timing. raising and lower your head will change your timing

edit, i meant cam timing, sorry.

As far as I can tell there's no way i can adjust the ignition timing on my bike. The pickup is mounted in a fixed position, unless there's some other way to change it on a bike with no points.

You won't need to worry w/the static timing. I build a certain comp engine several times a year that uses a much taller head and a longer cam chain. It has no effect or change on the cam timing. The cam can be 3 feet above the crank and it won't change the cam timing. The crank sproket turns once and the cam turns the same with it regardless of how far apart they are. I have hashed this out before and like I said, I use the longer chains all the time. Clint

bianese. The distance apart isnt what i meant could cause a problem. The cam and the crank will still turn the same degree relative to eachother no matter the distance because they are both sprockets. The problem I meant lies when you raise or lower the head relative to the bottom end you will rotate the cam while the crank is still. If your cam sprocket is adjustable this isnt a problem but if its fixed then you could potentially see a problem. I doubt it would make enough of a difference but you never know.

My suggestion is to take your cylinder head to a local machinist and have the "squish band" clearance enlarged to about .100". That would be about .100" clearance between the cylinder head gasket area and the nearest part of the combustion chamber, and I think it'll have to be cut on a roughly 17 degree angle. Not much material is removed and it's a fairly simple job and then you'll easily be able to run pump gas without detonation and overheating.


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