They allow more gas/fuel mixture to flow in and out of the cylinders. More air/gas = more power.
75 XS650B with 79 forks and dual disc, braided lines, disc brake on back, Pamco ignition, Pamcopete's reg/rec mods, kill switch relay, home made harness, all LED turn, brake and tail lights, tapered head bearings, bronze bushes in swing arm, soon to get shock and fork upgrades
An inexpensive way to get a set of oversize valves is to buy a couple of heads from a XV750 Virago. They spec out right and are larger. You will still have to have a machine shop cut and install new larger seats. I have been using a set for a couple of seasons in my flattrack bike and have no complaints. Use heavier/aftermarket springs too.
I had originally bought the standard size stainless steel valves from MikesXS:
05-0067 XS Performance Polished Stainless steel Exhaust Valve with collets. Fits: All 650's 1970-84 Replaces OEM #341-12121-01
05-0065 XS Performance Polished Stainless steel Intaket Valve with collets. Fits: All 650's 1970-84 Replaces OEM #341-12111-02
But I decided getting these oversized ones too after I got a answer to my question. I'm very curious how much this might help performance.
This is a full set of performace valves for the Yamaha XS650. There are two 2mm oversize intake valves and two 2mm oversize exhaust valves. These are custom made in small quantities to exacting tolerances, not mass produced. Exhausts use an inconel alloy which has a high temperature threshold. This alloy is normally used in turbocharged applications which makes it a good material for air cooled engines which can run very hot. Custom valves can be made in any size for many applications and more will be listed as they become available.
Inconel alloys are oxidation and corrosion resistant materials well suited for service in extreme environments. When heated, Inconel forms a thick, stable, passivating oxide layer protecting the surface from further attack. Inconel retains strength over a wide temperature range, attractive for high temperature applications where aluminum and steel would succumb to creep as a result of thermally-induced crystal vacancies (see Arrhenius equation). Inconel's high temperature strength is developed by solid solution strengthening or precipitation strengthening, depending on the alloy. In age hardening or precipitation strengthening varieties, small amounts of niobium combine with nickel to form the intermetallic compound Ni3Nb or gamma prime (ã'). Gamma prime forms small cubic crystals that inhibit slip and creep effectively at elevated temperatures.
How difficult is it to cut the larger valve seats into the head? Can you use the normal valve seat cutting tools, as in those available from Mikes XS? I would think all you have to do is measure where you need to to be and cut away until you get there. Sounds too easy, what am I missing? It's been 30 years since I cut a valve seat and my guess is it's been that long since your local Yamaha shop has done it on an XS head.
you have to open up the throat too,you want it some where between 80% to 86% of valve diameter,some even go higher then radius the bottom cut in and unshroud the valves or you'll loose flow at all lifts.
You can cut head to accept oversize intake but it is quite a lot of cutting. You have to have correct installed height (about 0.020" less than stock is OK and same as shimming springs) Cutting seats to 43mm pockets the valves so you then have to open up combustion chamber to restore flow. I don't do oversize exhausts, doesn't really need them even with 800cc motor. I have no experience of Mikes XS seat cutters but it looks like they would work. PJ
There are three types of people in the world, those who can count and those who can't I dabble in rocket science, when I'm not picking my nose
I'm only going to 700cc, but have the Lillie porting on the head and am running 36mm flatslide carbs, so this would probably help, but I'm thinking the affect would be marginal, and this is way too much trouble for my lazy arse. I also want to keep peak power down near the stock redline, so don't want her breathing too hard.
I think I'll probably stick with stock valves, MMM recommended the new stainless steel version, but I've heard they may be a little soft for the radical cam I'm planning to run. Anyone have an opinion on this?
Yes..........stay away from undercut valve stems, once the uncut stem is exposed to the mixture or gases you create greater resistance by forcing a change of opposing air speeds at the uncut and undercut stem sections and the flow #s will fluctuate up and down rather than stabilize thru the curve. you can hear this change when flowing,you'd be better off with a straight cut stem and thicker valve margins over all to assist in straightening out the flow past the margin and reducing resistance,I know I'm biased when it comes to margins but they flat out work to your advantage in performance.
Hey Dennis, I'd recommend you stick with the stock sizes if you are going to run a radical cam, especially with 700 cc. The smaller valves will help the low rpm/midrange operation. You can always consider going to the larger valves if it dynos out too soft on top. Hope your project is going well. Craig
Slimbob: Did you try Mike's XS or 650 Central? They usually have them.
Dec 6, 2013 14:29:26 GMT -5
joshelrich: Hey guys. new guy here. Where is the map and where does one introduce himself to the group? I've got a pretty cool project in the works. Trying to relive my youth. Ha ha. 80 frame, 71 engine case with 650 central long rod, big pin 750 kit. Shell 1 cam.
Dec 19, 2013 21:02:48 GMT -5
mwb122: Another newbie here. I appreciate the page and being accepted. Just brought home a 78 xs650 special. She is running good after a little TLC. So far I have cleaned the carbs, battery, fixed leaking clutch push rod oil seal. Now what to do with her.
Jan 3, 2014 8:50:55 GMT -5
mwb122: Bought a 79 special in 79. Was a great bike then and it is great to see they have such a large following!!!
Jan 3, 2014 10:30:04 GMT -5
oldcodgerhenry: Gooday ,all .new Pamco basic ign replacement arrived yesterday ,But the coil wires are different color ,to the trigger wires ,don't want to fry new one.
Jan 29, 2014 15:41:34 GMT -5
floris: at this moment, my XS is with Jerry v.d. Heiden, a well known XS tuner in the Netherlands.
Feb 4, 2014 16:28:08 GMT -5
floris: The engine become's a rephased 270 crank.
Feb 4, 2014 16:28:37 GMT -5
floris: Also a bigbore cilinder set of 890 cc.
Feb 4, 2014 16:29:10 GMT -5
floris: Also a long stroke is gona be applied, what makes the total about approx. 1000cc.
Feb 4, 2014 16:30:12 GMT -5
floris: 965 cc to be exact.
Feb 4, 2014 16:30:35 GMT -5
mahanlee: need help identifying a gas tank i bought on Ebay for my 79 xs. it has a tall neck and the new mikesxs cap i ordered will not work on it. any suggestions?
Apr 10, 2014 13:43:49 GMT -5
mahanlee: Need help identifying this part. It came in a box from the previous owner. I have searched Google and Ebay and haven't found something similar. It has a short bolt with a flat side. The only place I figure it goes is underneath the battery box.
Apr 23, 2014 8:14:07 GMT -5
djonesxs650: Batteryless setup for '78 XS650 with points ignition....I need a wiring schematic, simple as possible...does anyone know of any posts that can help?
Jun 28, 2014 16:02:09 GMT -5
gwc1147: I snapped the outer push rod for the clutch while removing front sprocket. I want to change to the longer rod. 77 650d I would also like to replace the seal and front bushing. How do I remove the broken rod? Any help please.
Aug 10, 2014 20:29:00 GMT -5
boxertwin1: I have a 76' XS and need petcocks for the tank. I don't want to rebuild, I want new ones. Weird, but you can only seem to buy right-sided ones. Any source for both sides?
Aug 21, 2014 14:56:18 GMT -5