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i have seen some talk about the 501 rotary valve i have an mxz 670 ho and want to get more out of it. is there a noticeable difference in performance, or any suggestions would be helpful. right now i have a goodwin clutch kit, 192 studs, roller secondary, and a b&b eliminator on it the last two things i'm not to sure if they were worth the cost but you live and learn
 

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It depends what you like about your sled. If its torque you want leave it alone.At a minimumyou should  verify that the closing edge is set to spec or slightly past. If its peak power than the 501 is what you want. You lose a little throtle response but you can get that back by adding the short boots when you put in the RV.
Dont buy a new one just get a degree wheel and modify your existing RV.
 

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</span><table border="0" align="center" width="95%" cellpadding="3" cellspacing="1"><tr><td>Quote (H.O.RIDER @ Aug. 04, 2002, 9:32pm)</td></tr><tr><td id="QUOTE">Dont buy a new one just get a degree wheel and modify your existing RV.[/b][/quote]
The only problem with that suggestion is if you don't like the result you have to buy a new stock disc.

Just my 2 cents CDN
 

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If he wants to try a 501 he has to buy one anyway, so he might as well modify his first and if it works the way he likes than its money in the bank. If he modifies his own he can get the exact timing he wants, if you try to get a "501" to work he could be out by 4 deg or more no matter which way the valve goes in.
 

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Hey H.O.RIDER,
Can you recall the exact degree setting you cut yours too,  on your H.O?

I put a 501 in mine and didn't notice a bit of difference.  This year i am going to do what H.O.RIDER suggests and custom cut my old one....
 

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I think anything you do to an HO is a compromise. The motor was tweaked to get as much power out of it without the meticulous matching and massaging of a hand ported motor.
I found the best mod on my two HO's has been short carb boots, dne can, and ufo's. I ran the ufo's for a season and hated the mileage so i took them off. I loved the response and without them my machines could never properly pull the stock 53 deg start angle of the helix. I am trying them this year with a different setup that according to Lon at Thunder Products WORKS. I played with the RV but to be honest the stock setting works pretty well. Without the UFO's a straight 50 helix works better.
 

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HO there was an article on this website about the UFOs. They said they were very difficult to calibrate. For the average rider not worth the effort. I don't know if the article is still up.
 

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Yes I did read that article and many other re:UFO's. Lon is very certain that this setup will work as promised. I had great performance with them last time with poor gas mileage. I am still hoping for the performance with some better mileage this time. You really have to hear the throtlle response with a DNE can and the UFO to believe it.
 

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I ran the 501RV, short boots, and UFOs on my HO last year and it worked well.  My mileage was still quite acceptable.  While these three things together did not make a huge difference I do think they helped a little.  I used the recommended set up for the UFOs and it worked well immediately.  The only place that I think it is off is in the first 1/8 to 1/4 throttle.  It is a bit rich in this area.  I am going to make some small changes this year to correct this.  670s still rule!
 

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I have a 99HO, where do you get the short intake boots, what did they come on? will i have to modify the air box ? and will i have to change jetting? Thanks
 

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FAST used to sell them. You might want to try the Crank Shop in Essex Jct. VT.
 

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In another topic,the guy said he got the short boots from his 'doo  dealer. Ask your parts guy if he can come up with the #s.
 

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I am thinking the shory intake boots came on one of the early 670 mach 1's.
 

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HO Rider,

I was afraid someone would ask that question!  I kept all of my notes on this stuff on my computer and then had a hard drive failure and lost it.  I will probably have to tear into my carbs to get this info back.  I did not drill out the center of the UFOs.  I drilled small holes in the hood of the needle jets (tubes).  I ground my slides to 3.5 cut-aways but they ended up slightly more than 3.5.  I will have to dig around for the size of the holes in the tubes, pilot jet size, and clip positions.
 

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hey guys..shorty boots suc* carbs hit the oil pump and there not level(some of the time)to keep a constantfeul level....take your long boots turn them inside out ..an sand them on abelt sander to match your rotery cover and your carb...this works great if you tunnel port and bore the carbs!!!
PS...lonn was great help with my UFOs   make shure you have AA3 OR richer needle jets...
 

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Im not sure which boots you are talking about but with mine the carbs certainly dont touch the oil pump and they sit as level as they do with the stock boots. The only issue some people talk about with the short boots is foaming of the fuel in the float bowl due to more vibration.
 
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