cr500 decompression valve, great idea!

i just received my head, which has been modified with the installation of a decompression valve, and i went ahead and had it milled down as well. now it kicks out more power from the extra compression, and it kicks over with ease! i can rest my foot on the kick starter when the valve is open and it will turn over.

"oldtimer" from this forum did the machine work for me at his shop and it beats every price on the internet. if you want your cr500 to turn over easier without tearing up your boots and leg, hit him up!

sorry if this sounds like a commercial but i just wanted to let the forum know that i had him do this work for me and it turned out great!:moon:

thanks oldtimer!

They save starters on the big bore, high compression V-Twins as well. I bet it is nice having one on a 500.

yeah it kicks over like it's a 125 now

wow I gotta see this, post up some pix

i dont know how to post pics on this site, i dont have photobucket account, but i'll link up another thread where oldtimer posted some pics of the work he does, hang on




here it is

cool man;)

Hello Matt,

I just happen to stumble on your thread.

Glad to hear that you got the bike running already.

So the milled head turned out more power????....just as we thought it would.

Sounds crazy, but just milling the head offers great benefits.

Did the idle quality improve also????

I remember taking pictures of your head, but I never downloaded them.

I'll try to post some pics of a spare head that I started on. I lost your pics at some point.

The pics you posted give an idea, except that's a "chainsaw" valve. The ones I use are stainless steel pieces intended for big v-twins.....really high quality valves.

I also cut down down the outer welded area for a flat/true surface for the valve to seal to. Drilling, tapping, and clean up is done in a mill. Then I polished his chamber.....the one in the pic is sandblasted.

I'm happy to hear that you're satisfied w/my work.

Still waiting for the t-shirts to come'll get one asap.

Thank you Matt,


i just put a new 39.5mm PWK carb on it, so im still getting the jets sorted out but this 500 is going to rock between the new carb and the milled head


The only one I can show installed is the one on my bike.

This one was the easiest I've ever modded. The waterjacketing was so thin (thick castings), that I didn't have to do any welding:


All the other heads so far have needed plug welding, to seal out the water.

This is before any machine work took place:


Hope the carb works out....I know tuning can be a chore.

Not to get off subject or anything, I thought some extra info would be helpful.

At part throttle acceleration, does it blubber at any point?

Perfect jetting should feel 4-stroke'ish.

I'll use this as an example.....

I have my 480 jetted pretty close for cooler days. It's not as crisp on a hot day.

Noitce the take-off from low RPM is smoothe and torquey, but on the 2nd return there's brief points of low throttle burbles. It could be tuned out by just by leaning down the idle mixture screw. More severe blubbering would need a smaller pilot jet.

Hope this helps:

turning the air screw in richens the mixture, right?

i'll have time to take the bike out to some open space this weekend where i can really ride it and try to get it dialed in. im having a hard time doing it in my parking lot

im having a hard time doing it in my parking lot

I know how that feels.

You need wide-open spaces for one of these.

The screw meters is leaner.

Good question, no doubt.

What's the scoop on your carb?

You have a link to it?????

The screw meters is leaner.

Good question, no doubt.

This is incorrect for PJ and PWK Keihin carbs. It isn't a "fuel screw", it's an "Air screw" so turning the screw "in" (clockwise) is RICHER, not leaner...

If you need help jetting that 39.5 PWK let me know and I can help... Make sure you let me know what's in there before you ask! (Main Jet, Needle jet and clip position, pilot jet and the slide #)

thanks, ill definately take you up on that.

right now i have a 178 main, 58 pilot, needle clip on the very bottom. it was too rich today, but it is about 15 degrees warmer today than earlier this week.

i ran it through the gears today at 1/4 throttle to get a plug reading, i havent checked the plug yet though, i was in a hurry. ill let yall know tomorrow.

What needle is it??? Where's the air screw adjusted to??? What's the slide #???

I think you're waaay too rich. I would try a 55 pilot, bottom the air screw (lightly) all the way in clockwise then turn it out 1.5 turns out, and then put the needle's locating clip in the "middle" slot on the needle.

Your Main might be rich (175 is probably best) but that's ok for part-throttle tuning and it is getting colder out. Just moving the needle clip to the middle should be a big help.

ok ill give that a shot. ill have to look at the carb and tell you what needle and slide it is, it's whatever one came on it (i just bought the carb last week)

btw, im at 400-500 ft, 40-55 degrees (today was 62) and humidity goes from 50%-85% typically i believe.

btw, yes it was running verrry rich. it felt good and strong off the bottom, but after 1/4 throttle or so it felt like the choke was on

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