Honda CB750 Sandcast
General Category => General Discussion => Topic started by: 736cc on February 18, 2010, 08:28:22 pm
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Is it the cams that make the sandcast motors faster than later CB750's? I've ridden several sandcasts, K0's, and K1's,-the sandcasts are without a doubt always MUCH livelier; even the exhaust note has more snarl to it. Its also rumoured the early sandcasts were factory blueprinted and assembled w/ extra special handling. Maybe the unstamped pipes flow better. Smaller front sprocket may be a contributing factor, also. Anybody ever draw the same conclusion in comparison?
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Posted by Steve "Guest"
no it is a myth the k1s are the same as k0s, the f1s had bigger valves that made them the faster engine, and by then they where better made,the only advantage if any,is that the k0s had was bigger main jets
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The earliest exhaust had a louder sound, probably due to insufficient baffling... later corrected in the "300" series of exhaust
Since only the earliest sandcast had these exhaust, perhaps a few extra ponies could be scoured from the engine, but I would want to see a speed test to state this factually
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Early unstamped pipes may be less restrictive than later HM300's, etc. Bikes fitted with a 16 tooth front sprocket definately feel more lively than those with 17 or even 18's fitted. Camshaft / carburettor specs are the same for all sandcasts, K0's and K1's, so I would put any difference down to exhaust / sprockets.
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I don't think that stock Sandy's were faster than other 69-70 CB750's. I can attest to front sprocket changes having a big affect on how the motor feels.
I think all of the previously mentioned items are valid for a comparison to late K1's and up. I have read recently on SOHC4 forum that the spark advancer unit had different specs than later bikes that helped preformance on the 69-70's.
I also think that while the cam specs were the same, the cam degreeing was different - maybe even on the Sandy's as opposed to later 69-70 diecasts.
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Posted by Steve "Guest"
k0s had was bigger main jets
Dear Steve;
What main jet and low jet sizes used are consensus in America and in Europe ?
Marcello
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sea level main is 120 and slow is 40.
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A sandcast cam was recently measured over on sohc4 and it had 370 lift. I think the 4 cable carbs are better too. Later engines K2-5, had poor ports. They were hurrying the build to meet production and stopped hand porting them.
Not scientific fact, I haven't looked inside my sandcast. I have seen the port difference though, in k3 and k4 engines.
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as i wrote elsewhere, a long time ago, back in the day, for what it's worth, Norm Mathis had a new red k1 and i had my 4779. we did a number of rolling starts, 1/2 mile+, ran them through redline, traded bikes, 4779 would beat Norm's K1 every time by a half to nearly a full bike length. i don't know how many miles were on Norm's K1, maybe 4779 beat him because his K1 was not broken in or maybe because 4779 was just that much quicker... :) ???
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All the cams I own have a number cast into the shaft. So far I've noted my sandcast cams have numbers that range from R1 to R4. The R1 cam I have I just received, which was won on eBay, came from a very early engine. I can tell you that the lobes on this cam are hot as you can see the lobe angles clearly without the need to dial the numbers. Obviously there would be a need to dial the numbers to find out the actual profiles of each of these cams but I have neither the gear or expertise to do that
Steve, Mark and Keith, can you shed any light on these "R" castings and what are your cam casting numbers. My 958 engine has an R2 cam as does 1771
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The number on the cam from E100 is R1.
(http://cb750sandcastonly.com/coppermine/albums/userpics/10002/DSC05603.JPG)
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i don't know what the "r" means. perhaps Vic know.
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Why do I remember someone to have put into this forum measures to prove differences on cam lopes for the early CB750s?
In those days at least there were talks of a cam a little fiercer in my original cb750 E1638. It did pull 9000rpm in 5th.
Youers Erling Danmark.
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My understanding is the R# is simply a casting/forging number and has nothing to do with the grind on the cam. My first hand experience indicates the SC's were definitely faster than subsequent bikes for most of the reasons previously mentioned. The intake ports in the heads were hand finished (major difference). No seals on the exhaust valve guides. Full floating rocker arm shafts. The cam was hotter. The spark advancer had a different curve. The exhaust had very little restriction, especially with the baffles out. The carbs behaved differently even versus K0's. The smaller front sprocket made a big difference. These early bikes probably made more than the claimed 67bhp and like already mentioned, would exceed redline in 5th gear.
I think I have already told this story but here goes again. Eight of us rode to Daytona in February 1970 for the BIG race. I was allowed to ride the 1st SC we ever received (don't know the VIN) as it was in the shop having been traded in on a K0. There were 4 other CB750's, all K0's on this ride. The SC I was riding was clearly faster than the other bikes, from a standing start, to midrange roll on, to top speed. We were riding across the South on I-10 which was pretty wide open for long distances back in the day and we could run WOT for miles at a time and did. The K0's all topped out right about 118-120MPH. I could run right along side them rolling the throttle on and off. You know the wah ba, wah ba, wah ba thing. Screw the throttle open and leave them in my dust, running well over 125MPH on the speedo.
Having said all that, I did own an early K0 that was HOT. Maybe close to the SC. Also owned a K1 that was HOT. Maybe close to the SC. Like any bike or car for that matter, some are just faster than their twins. Break your new bike in like Honda says and you will have a slower bike, typically with glazed cylinder walls. Run it hard from day one to produce a good piston ring seal. The precision of the tune can make a huge difference - valves, ignition, and especially carb sync. I always use a mercury manometer to sync the carbs precisely. Let 5psi out of your tires and see what happens to your top speed. It ALL adds up.
Keith
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When i rode back from California, May 1970, on the Great Salt Flats, i held 4779's throttle full open for over 5 miles. it did exactly what Honda advertised, speedometer needle passed over the 125 mph mark, slightly above 8500 rpm.... i remember thinking, "So, it will do 126." it would not come close to 130. As a total package, the CB750 WAS like a space ship compared to the bikes of the era. On the way out, i remember meeting up right before dusk with a guy by the name of Vance Reed, just west of Salt Lake City, he had just taken delivery of a brand new Trident and he lost a cylinder shortly after we met up. I stayed at his house in Elko that night.
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Reviving an old thread here, but I thought Id confirm the R1-R4 range on the camshafts.
I recently purchased some parts, all from a low use sandcast engine. The rocker cover was dated 30/4/69, the clutch basket late may 69. It has the early rocker gear and the camshaft which came with it was an R4. Im unsure of the engine number but assuming it was at least a June 69 assembled bike. Pics below (although the R4 is not visible it is confirmed. Ill post a pic when it arrives).
A question for those who have been able to compare R1-R4 cams - Did you notice a lobe difference in the cam profiles between these cams? Im thinking it is possible given the exhaust changed from the lotus root to no number c. 4148, the cams may have been changed accordingly?