Patches
XS650 Junkie
This article was sent to me by the person who wrote it, Mike. It was put on another site apparently and I wanted to share the info with you all. I have talked with Mike over the phone several times about this motor and have been looking into build one myself. I know when Hugh posted about a ling rod motor there was a lot of interest.
I left-out Mike's info and signature at the bottom, just in case he didn't want people trying to contact him, but I have been sitting on this info for a while, and apparently he is OK with sharing it now.
Enjoy.
Here ya go:
The quest for the ultimate XS 750 dirt track engine
This project started with the intent on giving our rider an advantage as compared to the competition (XR750) in vintage dirt track racing. As the reader may or may not know this is a real challenge. It is so much of a challenge that the owner (CD Martin) considered the purchase of a XR. CD spent several years in the 80's as an owner/mechanic attending grand nationals, so he had experience with the XR 750. However, he already had two Champion framed XS's one a 700 the other a 750. After much discussion CD decided to stay with the Yamaha's.
We began by purchasing the Engine Modification Guide from Craig Weeks (a must have in my opinion) and searching the web, we began to develop our build plan. The plan included the following.
The Lilly CNC ported head including guides and valves
Megacycle #30 cam with adjustable sprocket
Venolia pistons/perfect circle rings (gapless)
RD valve springs
Honda CR500 rods
Weighted crank, balanced
Barnett clutch and springs
ARD ignition
36mm round slide Mikuni carberators
JEMCO exhaust
So we started by sending the head to Craig's vendor for the porting and guide installation along with early XR 750 valves. Then came the custon Venolia pistons that have the Lilly domes (11.25-1 compression) but with the wrist pin moved upward 9mm compared to the early 256 Yamaha rods and using the late 20mm pins. This was done to accept the Honda CR 500 rods. Why the longer rod you ask, check out http://www.stahlheaders.com/Lit_Rod Length.htm, this will explain the why. Our intent was to smooth out the power as the XS's are known for it's 2 stroke type of power delivery on the dirt. That is the one of the advantages of the XR 750 a smooth power delivery and I believe the long rods are one of the contributing factors. The XR has a rod to stroke ratio of 2.115-1, I believe they, H-D, would have perferred a 2-1 ratio. The following is what I have researched from avaliable off the shelf rods that I am aware of, although a custom rod may also be purchased.
Yamaha early -256 135mm length rod to stroke ratio of 1.824-1
Yamaha -447 130mm length rod to stroke ratio of 1.756-1
Yamaha -533 Europe 140mm length rod to stroke ratio of 1.89-1
Honda CR-500 144mm length rod to stroke ratio of 1.945-1
We used Wiseco Hot Rods for our project, I believe you have to buy a complete crank from Honda as a rod kit is not offered. Also of note due to piston pin location it requires a teflon button because the pin encroches on the oil ring location, these are supplied from Venolia. For those who might want to use these pistons and long rods, order from Venolia job number 135554, these are 80mm bore which is standard 750. The rings I sourced thru Craig Weeks.
As for the crankshaft we installed .9 lb rings each on the two inboard flywheels, they are avaliable from Hoos Racing call Roger Johnson his number is on their web site. The assembly of the crank was done by Chet Wilson Engine service in Wichita Ks. Jerry Wilson runs the shop and is an avid motorcyclest, his dad Chet was the first one to put a V-8 Chevy in a sprint car. As far as the rod instl the small end was bushed to accomodate the 20mm pin and the crank pin was shortened 3mm (compare to the 650 pins to be sure) Jerry also balanced the crank to a factor of 64% (I think that's right, consult Craigs paper). As a final step weld the crank pins to the flywheels, this is a must as they will twist, even on a dirt tracker, I know of thru first hand experience.
Engine assembly is next. The upper and lower cases will need to be relieved to accomodate the flywheel rings, just set the crank in one case at a time to determine where to and how much to relieve, then join both cases with crank installed and check for proper rotation. The rest of the asembly is pretty straight forward just follow the tolerances described in Craig's paper. To get the proper squwish we had to remove .035 thou from the top of the cylinder with no base gasket and a .026 copper head gasket. So it is critical that this is checked. We degreed the cam per Mecacycle specs, only took 9 attempts but that's how it goes.
So, how did it work? As all projects we were behind the 8 ball for time as we wanted to go to the Kansas fair races and it was a last minute thrash. We finally got the whole thing assembled and went to the dyno. Now, we had a Maxxis dirt track tires and they will slip on the dyno, we were lucky enough that they had a rear wheel with a Dunlop CD 8 and they don't slip. This next part is really strange, we could not get the air fuel ratio right on the L-H carb, changed needels, needel jet, pilot jet and main jet to no avail. So we switched carbs left to right and the problem followed the carb. What was happening was a blubber up to about 6,000 rpm or so. It made 68.6 hp with the blubber. We never could find the problem with it, replaced it and no more blubber.
As I said we were short on time so we went to the first race untested, I will never do that again as we fought gremlins for the whole series of races but that is another story. I have to say the thing will really turn the RPM's and seems to be smooth, we geared it too low as we were turning it 9,000 or so. The good thing is our rider liked it as the power was smoother then the short rod he was used to riding, as he said it took the 2 stroke hit out of it. We still have things to do and I believe more power is there. Might try flat slide Mikuni's and a # 40 cam, I would think the the power would be somewhere in the 70's.
Is the installation of the long rods worth the effort? In my opinion, yes. I am so convinced I think I will put one in my TT-500 based dirt tracker.
I left-out Mike's info and signature at the bottom, just in case he didn't want people trying to contact him, but I have been sitting on this info for a while, and apparently he is OK with sharing it now.
Enjoy.
Here ya go:
The quest for the ultimate XS 750 dirt track engine
This project started with the intent on giving our rider an advantage as compared to the competition (XR750) in vintage dirt track racing. As the reader may or may not know this is a real challenge. It is so much of a challenge that the owner (CD Martin) considered the purchase of a XR. CD spent several years in the 80's as an owner/mechanic attending grand nationals, so he had experience with the XR 750. However, he already had two Champion framed XS's one a 700 the other a 750. After much discussion CD decided to stay with the Yamaha's.
We began by purchasing the Engine Modification Guide from Craig Weeks (a must have in my opinion) and searching the web, we began to develop our build plan. The plan included the following.
The Lilly CNC ported head including guides and valves
Megacycle #30 cam with adjustable sprocket
Venolia pistons/perfect circle rings (gapless)
RD valve springs
Honda CR500 rods
Weighted crank, balanced
Barnett clutch and springs
ARD ignition
36mm round slide Mikuni carberators
JEMCO exhaust
So we started by sending the head to Craig's vendor for the porting and guide installation along with early XR 750 valves. Then came the custon Venolia pistons that have the Lilly domes (11.25-1 compression) but with the wrist pin moved upward 9mm compared to the early 256 Yamaha rods and using the late 20mm pins. This was done to accept the Honda CR 500 rods. Why the longer rod you ask, check out http://www.stahlheaders.com/Lit_Rod Length.htm, this will explain the why. Our intent was to smooth out the power as the XS's are known for it's 2 stroke type of power delivery on the dirt. That is the one of the advantages of the XR 750 a smooth power delivery and I believe the long rods are one of the contributing factors. The XR has a rod to stroke ratio of 2.115-1, I believe they, H-D, would have perferred a 2-1 ratio. The following is what I have researched from avaliable off the shelf rods that I am aware of, although a custom rod may also be purchased.
Yamaha early -256 135mm length rod to stroke ratio of 1.824-1
Yamaha -447 130mm length rod to stroke ratio of 1.756-1
Yamaha -533 Europe 140mm length rod to stroke ratio of 1.89-1
Honda CR-500 144mm length rod to stroke ratio of 1.945-1
We used Wiseco Hot Rods for our project, I believe you have to buy a complete crank from Honda as a rod kit is not offered. Also of note due to piston pin location it requires a teflon button because the pin encroches on the oil ring location, these are supplied from Venolia. For those who might want to use these pistons and long rods, order from Venolia job number 135554, these are 80mm bore which is standard 750. The rings I sourced thru Craig Weeks.
As for the crankshaft we installed .9 lb rings each on the two inboard flywheels, they are avaliable from Hoos Racing call Roger Johnson his number is on their web site. The assembly of the crank was done by Chet Wilson Engine service in Wichita Ks. Jerry Wilson runs the shop and is an avid motorcyclest, his dad Chet was the first one to put a V-8 Chevy in a sprint car. As far as the rod instl the small end was bushed to accomodate the 20mm pin and the crank pin was shortened 3mm (compare to the 650 pins to be sure) Jerry also balanced the crank to a factor of 64% (I think that's right, consult Craigs paper). As a final step weld the crank pins to the flywheels, this is a must as they will twist, even on a dirt tracker, I know of thru first hand experience.
Engine assembly is next. The upper and lower cases will need to be relieved to accomodate the flywheel rings, just set the crank in one case at a time to determine where to and how much to relieve, then join both cases with crank installed and check for proper rotation. The rest of the asembly is pretty straight forward just follow the tolerances described in Craig's paper. To get the proper squwish we had to remove .035 thou from the top of the cylinder with no base gasket and a .026 copper head gasket. So it is critical that this is checked. We degreed the cam per Mecacycle specs, only took 9 attempts but that's how it goes.
So, how did it work? As all projects we were behind the 8 ball for time as we wanted to go to the Kansas fair races and it was a last minute thrash. We finally got the whole thing assembled and went to the dyno. Now, we had a Maxxis dirt track tires and they will slip on the dyno, we were lucky enough that they had a rear wheel with a Dunlop CD 8 and they don't slip. This next part is really strange, we could not get the air fuel ratio right on the L-H carb, changed needels, needel jet, pilot jet and main jet to no avail. So we switched carbs left to right and the problem followed the carb. What was happening was a blubber up to about 6,000 rpm or so. It made 68.6 hp with the blubber. We never could find the problem with it, replaced it and no more blubber.
As I said we were short on time so we went to the first race untested, I will never do that again as we fought gremlins for the whole series of races but that is another story. I have to say the thing will really turn the RPM's and seems to be smooth, we geared it too low as we were turning it 9,000 or so. The good thing is our rider liked it as the power was smoother then the short rod he was used to riding, as he said it took the 2 stroke hit out of it. We still have things to do and I believe more power is there. Might try flat slide Mikuni's and a # 40 cam, I would think the the power would be somewhere in the 70's.
Is the installation of the long rods worth the effort? In my opinion, yes. I am so convinced I think I will put one in my TT-500 based dirt tracker.