Hi - Performance Cam for XJ Bikes

Because for the 750 engines, all of the air-cooled engines* used the same camshaft on the intake and the same camshaft on the exhaust (they are different shafts, but the same between engines......there is no "750 Seca specific camshaft")...........all of them EXCEPT for the 1984 XJ750RL Seca exhaust camshaft, which was different (but it was shared with the XJ700 air-cooled engines). Here are the goods:


HCP3841
OEM Intake CAMSHAFT 4K0-12171-00-00
Lift: .346" or 8.80mm
Duration:
Intake opens 38-degrees BTDC
Intake closes 58-degrees ABDC
Overlap: 74-degrees
Applications:
XJ700 air-cooled
XJ750 all air-cooled including XJ750RL
XJ750 Police (37H)



HCP3842
OEM Exhaust CAMSHAFT 4K0-12181-00-00
Lift: .307" or 7.80mm
Duration: not given
Overlap: not given
Applications:
XJ750 all air-cooled (except XJ750RL)
XJ750P (37H)



HCP3843
OEM Exhaust CAMSHAFT 22N-12181-00-00
Lift: .327" or 8.30mm
Duration:
Exhaust opens 56-degrees BBDC
Exhaust closes 26-degrees ATDC
Overlap: 74-degrees
Applications:
XJ700 air-cooled
XJ750RL



MODEL NAME: XJ750RL
STREET NAME: 1984 XJ750 Seca
MODEL ID CODE: 44X (Canada)
FRAME ID: 44X (Canada)
MODEL ID CODE: 45T (Australia)
FRAME ID: 45T (Australia)
SERIAL NUMBER STARTS AT: 000101
NOTE: All XJ750RL models feature an XJ750 engine in an XJ900 frame. Only 601 made for the Canadian market and just 286 made for the Australian market. Engine actually displaces 749cc (1cc larger than standard XJ750 models, which were only 748cc's) and uses a hotter exhaust cam, larger intake valves (= 34mm, or +1.00mm vs. standard XJ750 valves), different crankcase, cylinder jugs, and pistons/rings, the XJ900 type clutch assembly and middle gears set, as well as the strengthened 2nd gear set as used in the XJ650 Turbo, XJ750-X, and XJ900 models. It has been noted that all the Red/Silver XJ750RL models (Canada) seem to have black motors while the Blue/White models (Australia) have silver motors.

 

The first cam is a 38 plus 180 plus 58, equals 276 deg, 346 thou lift - quite a tame cam by any standards.
The second you can discount.
The last one is a 56 plus 180 plus 26, equals 272 deg, 327 thou of lift. The first, the 4k0 would be the best bet, but it’s marginal.
The next question would be, what are the thickest valve shims available? This dictates the maximum allowed to be ground from the base circle of the best cam - my guess would be you need 80 thou to make it worth the bother, but cams are complex, so would need to speak to a cam grinder to evaluate. Longer valves could help, if they exist off the shelf, expensive to buy as race specials, but then you only need 4...
If I was in a fictional one make xj 700 race series I would be looking for larger, longer valves, having the cams reground with a target of 290 to 300 degrees duration and 400 thou of lift.
Then fitting or cutting new seats, blending and lapping, stiffer springs.
Then cutting the piston crowns to allow clearance, with something like 50 thou off the head face to increase compression to 12 to 1.
The engine would need bigger carbs, maybe some opening up of inlet and exhaust ports, K&N air filters, bigger exhaust headers, and a 4 into 1 pipe.
An extra 20 bhp? Maybe...

 

The 750 models all got the same cams.

 

Thanks for that little bit of non fake news. I nearly bought the cam.
Cheers, 50gary

 

So I made a discovery that led Me to abandon this Project. Yes the Intake Cam has more Lift, but on further inspection I found that the Exhaust Cam actually has a little more Duration. I'm guessing this was done to improve Scavenging, and They went with less Lift to Improve Velocity Lower in the Rev range where a 2 Valve Engine Shines. I've concluded that an Intake Cam on the Hot Side will Reduce Overlap and Kill Velocity, and I will See a Significant Loss in Power if the Bike even Runs..

 

Of course it would run. Maybe a little hotter, but it would be small differences. Velocity is built in with the exhaust ports. Ie it goes out with a bang. You have come up with theories to reason why the makers did something forty years ago - fair enough, but it's guesswork really.

 

Could this be of any help? From Megacycle Cams, San Rafael Ca.
CAM VALVE DURATION LOBE INTAKE OPEN/CLOSE RUNNING
NUMBER LIFT AT .040" CENTERS DESCRIPTION AND APPLICATION EXHAUST OPEN/CLOSE CLEARANCE
256-00 .354" 254° 108° OK with stock pistons and springs. Good 19 btc/55 abc .005"
.354" 254° 108° mid-range. Some top-end. Best in stock motor. 55 bbc/19 atc .007"
256-30 .375" 257° 108.5° Should clear stock pistons, but must check. 20 btc/57 abc .005"
.375" 257° 108.5° Best all around road race. Must use #106-Y. 57 bbc/20 atc .007"
256-40 .410" 250° 105° Drag race. Mid-range torque. Great throttle 20 btc/50 abc .005"
.410" 250° 105° response. Use racing pistons and springs. 50 bbc/20 atc .007"
256-20 .417" 268° Road race. Best for top-end. Use #106-Y + 12:1 or more compression.
256-60 .438" 258° Shim under! Drag race. Use #106-Y + 12:1 compression.
Stock .335" 236° Stock XJ-650 for your reference.
.305" 232°
YAMAHA XJ-900 SECA 4-cylinder (1983—1984)
Hardfaced cams, per pair, on customer cores.
CAM VALVE DURATION LOBE INTAKE OPEN/CLOSE RUNNING
NUMBER LIFT AT .040" CENTERS DESCRIPTION AND APPLICATION EXHAUST OPEN/CLOSE CLEARANCE
262-00 .375" 257° 107.5° OK with stock pistons and springs. Street and 21 btc/56 abc .006"
.375" 257° 107.5° road cam for improved mid-range and top-end. 56 bbc/21 atc .007"
262-10 .410" 250° 108° Must use modified pistons and springs. Race 17 btc/53 abc .007"
.410" 250° 108° only. Racing pistons and springs needed. 52 bbc/17 atc .008"
Stock .343" 240° 108° Stock cams for your reference. 12 btc/48 abc
.326" 236° 108° 46 bbc/10 atc

Cheers, 50gary

 

"Hardfaced cams" - I wonder if he built them up with welding rod? That would mean they are steel of sorts. Perhaps I'll try one, I think I have s knackered one here somewhere. Then of course I'd need to get it ground.....

 

No Sir, it has a significant impact. Because the Exhaust Port is much Smaller than the Bore the Gas's have to be Condensed/Compressed to Pass through. Compressing the Gas causes it to Speed up. And because the Valve determines the Size of the Hole the Gas is forced through, it determines exactly how much it needs to be compressed initially which has a huge impact on the Gas's Velocity exiting the Engine... The Exhaust Valve Creates the Velocity that then Builds as it Pass's through the Port.

 

Now for the Cams, as I said the Exhaust Cam has a little less lift, but a little more Duration than the Intake Cam, this means its going to stay open longer. It does this so it's still slightly open when the Intake Valve Opens, this is called overlap. It causes a Scavenging effect where the exiting Exhaust Gas's actually help draw the A/F Mixture into the Combustion Chamber. The Intake Cam will cause it to Open more but close Faster, and if the Exhaust Valve has already Closed when the Intake Opens You're not going to get that Scavenging effect causing a major loss of Power, and it's probably gonna Run like Sh*t...

 

No, they are a finished product, buy and install. Megacycle Cams has been around for years. This is not a backyard business.
Cheers, 50gary

 

Finished product or no, they are based on customers cores - ie reground after hardfacing - this from your post -
"Hardfaced cams, per pair, on customer cores."

@Jerrod Robinson1086 - your theory on thermodynamics and combustion and cylinder heads needs some work. Your definition and explanation of scavenging effect is correct though, but with the difference in numbers on the standard cams the effect will be tiny, hence my assertion that it would run, albeit slightly hotter, maybe.

 

I uploaded the pic from megacycles online brochure - sure they are welded up standard cams, but they are not a backyard outfit as @50gary says. Hopefully the pic makes their options clear. Note they don't have a spec for the standard 700 exh. Cam.
One other thing to note is they use 40 thou as the measuring clearance, which is why their numbers are less than quoted by the factory. Oh, and also in my earlier post commenting on @chacal s listing of cams my mental arithmetic let me down - the 700 exhaust cam has 262, not 272 degrees duration. Ie it is substantially less than the intake cam. So, want more power, fit an intake cam, or have your cores reground, or hardfaced and reground for more still. But if you go to these lengths you want to be gas flowing your head.
Oh, lastly, the frame above the xj cams is for the xs - that had more lift and duration if the info is correct, with the exhaust having more of both - do these fit in an xj head??

 

XJ700 air-cooled and xj750RL (1984) used the same exhaust cam, specs here:

OEM Exhaust CAMSHAFT
22N-12181-00-00
Lift: .327" or 8.30mm
Duration:
Exhaust opens 56-degrees BBDC
Exhaust closes 26-degrees ATDC
Overlap: 74-degrees
Applications:
XJ700 non-X
XJ750RL


They also used a 1mm larger (34 vs 33mm) intake valve than the XJ650/XJ750 engines; exhaust valve was the same as the 650/750 models.

So, to summarize for the air-cooled engines (700 engines vs. the 650/750 engines):

- have the same intake cams
- 700 has a larger intake valve
- have the same size exhaust valves
- 700 has an exhaust cam with significantly greater lift (8.30 vs 7.80mm) than the 650/750 engines


And as long as we are comparing YICS cylinder heads (1982-up 650, all air-cooled 700, all air-cooled 750), they are all the same casting number, so basically all the same head but the intake valve pocket opened up on the 700's ----- and I don't know whether any other changes were done to the intake or exhaust port shape, etc.

P.S. Megacycle has been in business for a long time, so I'll have to assume they know what they're doing.

 

Thanks for that @chacal , confirms its a lesser timing cam than the intake. How available are the 700 intake valves?
No one's questioning Megacycle cams heritage or ability here...