High performance XS650 engine with CR500 rods

650performance

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Here's some great info for anyone contemplating building a long rod engine. This one uses CR500 rods, but Michael Morse suggests that the KX500 rods may also be suitable (with appropriate modifications, of course). Check it out.
Craig


The quest for the ultimate XS 750 dirt track engine
By Mike Mendenhall Derby, KS mmendenhall1@cox.net


This project started with the intent of giving our rider an advantage when compared to the competition (Harley 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 XS650/750 Engine Modification Guide from Craig Weeks (see www.650performance.com - a must have in my opinion) and searching the web for all the additional relevant data we could find. After doing our research, we began to develop the build plan. When completed, it included the following:

The Lilley CNC ported head including guides and valves
Megacycle #30 cam with adjustable sprocket
Venolia 750cc pistons with Total Seal gapless rings
R/D valve springs
Honda CR500 rods
Weighted and rebalanced XS650 crank
Barnett clutch and springs
ARD ignition
36mm round slide Mikuni carburetors
JEMCO exhaust built to specs in the Engine Modification Guide

We began the project by sending the head to Craig's vendor for the CNC porting and guide installation along with replica early XR 750 valves. Then came the custom Venolia pistons that have the Lilly domes (11.25-1 compression) but with the wrist pin moved upward 9mm when compared to the early 256 Yamaha rods.

The pistons were drilled to accept the late 20mm XS650 pins. This was done to match the Honda CR 500 rods which were bushed to share the 20mm wrist pin sizing.

Why the longer rods? See 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 notorious for the sudden onset of power as the throttle is opened. On the dirt this characteristic overloads the available traction and causes the rear tire to break loose prematurely.

A smooth power delivery is one of the advantages of the XR 750, and I believe its long rods are a key contributing factor (the other is the firing order, but that’s another story). The XR has a rod to stroke ratio of 2.115-1. Research suggests H-D would have preferred a 2-1 ratio, but no one will argue the present set up doesn’t work well.

We wanted a similar ratio for the new XS engine, so I began doing some research. The following is what I have found from available off the shelf rods that I am aware of, although a custom rod set may also be purchased from Carrillo or other sources.


Yamaha (later) -447 130mm length rod to stroke ratio of 1.756-1
Yamaha (early) -256 135mm length rod to stroke ratio of 1.824-1
Yamaha (Euro) - 533 140mm length rod to stroke ratio of 1.89-1
Honda CR-500 144mm length rod to stroke ratio of 1.945-1

Adapting the Honda rods appeared to be technically feasible, so we plunged forward. We used aftermarket Wiseco Hot Rods for our project. The OEM crank bearings supplied with the Honda rods were used as-is. I believe you have to buy a complete crank from Honda as a rod kit is not offered (of course, an alternative parts source is a good, low time, non-abused used engine, or you might get lucky on eBay Motors).

Also of note is that due to its location, the piston pin requires a Teflon button because the pin encroaches on the oil ring location and if the buttons are not fitted the oil ring will not be supported properly. These buttons are supplied on a special order from Venolia, and require hand shaping by Venolia personnel. For those who want to use these pistons with the CR500 rods, specify job number 135554 from Venolia. We ordered our pistons with an 80.0mm bore, which is the “standard†750 diameter and leaves room for later overbores, if desired. The Total Seal rings I sourced thru Craig Weeks.

As for the crankshaft, we installed .9 lb. rings on each of the two inboard flywheels. They are available from Hoos Racing. Call Roger Johnson - his number is on their web site. Assembly, welding and truing of the crank was done by Chet Wilson Engine Service in Wichita, Kansas. Jerry Wilson runs the shop and is an avid motorcyclist. His dad Chet was the first one to put a V-8 Chevy in a sprint car.

Regarding preparing the rods for installation, the decision was to use 20mm pins. Brass bushings were swedged in place (expanded to the rod) and then honed. The pin to bush tolerance is .001†or slightly less. Oil feed holes were drilled through the bushings to match up with the stock oiling holes in the rods and then carefully chamfered to ensure the pin wouldn’t be abraded by the bushing.

The crank pin was shortened 3mm (compare to the 650 pins to be sure). Piston pin clearances were set up per Yamaha OEM specs. Jerry also balanced the crank to a factor of 67%. As a final step, be sure to weld the crank pins to the flywheels. This is a must as they will twist, even on a dirt tracker. Unfortunately, I know of through first-hand experience.

Here’s a piece of trivia: when I was at Wilson Engine Service we weighed the pistons, rings, pins, keepers and teflon buttons as each separate cylinder. They were the exact same weight to the gram. I asked Jerry how often that occurred and his response was "Never." Based on that, Venolia and their personnel put out really accurate products.
.

Engine assembly was next. The upper and lower cases will need to be relieved to accommodate the larger diameter weighted flywheel rings. Simply set the crank in one case at a time to determine where to and how much to relieve metal, and then join both cases with the crank installed and check that the crank rotates with absolutely no interference.

The rest of the assembly was pretty straight forward. Just follow the tolerances described in the Engine Modification Guide. To get the proper squish we had to remove .035 thousandths from the top of the barrel. We then assembled the barrel and head with no base gasket and a .026†copper head gasket. [Note that due to production and machining tolerances your engine may be different … you must carefully measure everything!] Squish is critical, so it must be checked very carefully with clay or similar material. We then degreed the cam using the adjustable sprocket per Megacycle specifications. It took 9 attempts to get it really dialed in, but that's how it goes.

So, after all this work, how did the engine perform? As with all projects, we were behind the 8 ball for time. We wanted to go to the Kansas State Fair races and it was a last minute thrash. We finally got the engine assembled and installed. It started up easily and sounded pretty good, so we loaded the trailer and headed for Salina, Kansas to get some tuning time on the dyno.

Unfortunately, our dyno session had its own set of problems. We had installed a used Maxxis dirt track tire on the bike and it slipped on the dyno drum. We were lucky that there was a handy rear wheel fitted with a Dunlop CD 8. We did a thrash wheel change and discovered the Dunlop didn’t slip. So far, so good.

Then things got really strange. We could not get the air fuel ratio right on the left carb, and in attempting to cure it we changed needles, needle jets, pilot jets and main jets, all to no avail. With hours wasted and rapidly running out of time, 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 an obviously malfunctioning carb with less than optimum jetting. We never could find the problem with the carb and after the dyno session replaced it with a used 36mm Mikuni from another race bike and the engine immediately ran great with no more blubber. We haven’t run it on the dyno again, so the true power potential remains untapped.

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 chassis and brake 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 and were turning it 9,000 or so at the end of the straight. The good thing is our rider liked the “feel,†reporting the power was more linear than the short rod XS engine he was used to riding. He said the long rod engine no longer had the sudden 2 stroke hit when exiting corners.

We still have things to do and I believe more power is there. We might try flat slide Mikuni's and a Megacycle #40 cam. With those upgrades, I think the rear wheel power will be somewhere in the low to mid 70's.

Is the installation of the long rods worth the effort? In my opinion, yes, it is. I am so convinced I think I will put one in my TT-500 based dirt tracker.

If you are putting one of these long rod engines together you can email me if you have a specific question. If you have general questions about high performance XS650 engines please refer to the Engine Modification Manual.

Good luck,

Mike
 

270YAM

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Nice work!
I wonder if it was the long rods that made the power come on much smoother or the weighted flywheels? or both?
Thanks for sharing:)
 

650performance

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Probably a combination of both. The premier AMA flat track engines in the '70s all had weighted flywheels (as did the OU) but most stuck with the 256 length rods. I believe the XS engines previously raced/ridden by this rider had later, shorter, 447 length rods, so his experience going from one to the other isn't exactly apples to apples.
Craig
 

Jack

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Nice work!
I wonder if it was the long rods that made the power come on much smoother or the weighted flywheels? or both?
Thanks for sharing:)
Craig,
Can't thank you enough for chiming in and for Mike supplying that all so important feed back on the long rod conversions,which in my opinion offers the best of both worlds,smoother operations when combined with light weight pistons(which is my goal) to reduce the vibes some what and providing wider power band over the shorter rod ratio. Gotta Thank my good friend PJ at the Garage for initiating the idea of using the CR500 rods in the XS. One thing Mike left out was that Brandon's XS is the one to beat,gotta love it "the old XS still kicking ass in the dirt"

Matt,
Mike is the guy I was referring to:thumbsup: hope to hear that Beast of yours fired up some day :)
 
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Highside

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I thought I had already de-bunked the myth of long rod engines. :shrug:

Spare yourself the effort of building a long rod engine to see the difference in piston movement. There's this branch of math called "trigonometry". You can use it to see the effect of of a long rod vs a short rod without having to spend a dime.

You want to know the difference in piston movement between a stock and long rod at 10 deg ATDC?? .0004" :laugh:
 

Punkskalar

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Thank Mike, I am planning a Rephased Long Rod engine in the very near future. Those 2 combinations should really bring out the "Doubters" :laugh:
 

Tnainc1

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Mike, Thank you for sharing this. Upon pondering over your tasty words, a thought came to mind in reference to the pin location in the piston. Would it be possible to move the pin further away (down) from the oil wiper ring and make up the difference by shimming up the cylinder base accordingly. Upon first thought I considered the fact of the timing chain / timing the cam, but quickly remembering your adjustable sprocket. If I am missing something, please set me straight. Thank you in advance for your reply.
 

650performance

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Shimming the base would open up the squish to an unacceptable extent. I think a more promising approach is to confer with your piston manufacturer of choice and ask about the latest ring and ring spacing options. Some new four stroke engines are only using two rings, so perhaps that's something to look at. You might also check out 650Central.com's use of KX500 long rods because they've sourced a piston that has the rings situated above the pin and doesn't need the teflon button. I don't see any reason why that piston wouldn't work with the CR500 rods with the right pin size, etc.
 

TwoManyXS1Bs

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Craig, belated thanks for posting (3 years ago?) Mike's write-up. I spent some time with Chet Wilson in the mid '80s building my 327. Impressive shop. Small world.

I'm wondering if anyone has ever investigated or pursued 'offset' boring (like that used in the MT-07) on these XS650 engines. It's a way to artificially mimic the effects of the long rod...
 

650performance

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TMX, I've never heard of anyone trying an offset approach. Interesting.
Since Mike's writeup they've raced the bike throughout the Kansas and surrounding flat track scene with good success. Still issues with handling and the frame, but no engine problems. I'm told they are going to ramp up their effort in 2015 with an eye to being able to stick with and occasionally beat the local XRs.
 
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