Topic: Performance of a k1 versus k8 CB750, much difference?

[Texwing]

HI Guys, I have not been around for a while, and needed a little help: I have running nicely a 1971 750 K1, stock except for a 4 into 1 header.  Runs smooth, tuned, valves adjusted, carbs synched.  I am looking at a 1978 cb750 K8, nice and stock, still with the original pipes, and man, that thing seems to have a lot more beef than my 71-I mean it really ran strong!  I cant find any performance specs, but would have thought the 71 faster....Or about the same.  Makes me suspicious that my 71 has low compression, or the 78 has a kit (I doubt it with stock pipes) or something....Like I said my 71 K1 has clean carbs, idles great, timed and runs really well, just not as strong as the 78.  Your comments appreciated.

[Hockers Choppers]

does the  71 still have stock jets? may need larger ones with the 4-1.

[feliz]

Well, I have around 100 road tests of these bikes from the 70s and I've noticed the later bikes are usually 1-2 seconds slower in the quarter mile. It didn't happen in any one year just seemed to take place gradually. I know my 71 runs a lot better than my 73 and 74.

To be sure I wasn't exaggerating I just took a look and there's about 150 road tests in the Gold Portfolio book alone plus another hundred or so on the "Honda going strong" CD I got and the bikes get consistently slower.

[cafe750]

Well, I have around 100 road tests of these bikes from the 70s and I've noticed the later bikes are usually 1-2 seconds slower in the quarter mile. It didn't happen in any one year just seemed to take place gradually. I know my 71 runs a lot better than my 73 and 74.

Don't let Eldar hear you say that....

[seaweb11]

Well, I have around 100 road tests of these bikes from the 70s and I've noticed the later bikes are usually 1-2 seconds slower in the quarter mile.

Yeah, well I have "Ass stats" all day that say different

 
78K8

[feliz]

Well, I have around 100 road tests of these bikes from the 70s and I've noticed the later bikes are usually 1-2 seconds slower in the quarter mile.

Yeah, well I have "Ass stats" all day that say different

 
78K8

I don't care one way or the other but the bikes also lost 10-15 hp during that time, hard to see how they could go faster.

[Old Scrambler]

The K8 pistons appear to be somewhat domed for higher compression......I'm not sure, just from observing pictures. Also some comments that the head was somewhat reworked but not to be confused with the F2 or F3 heads.  I agree about the later models being slower. I was very disappointed with my K3 over the K0 until I changed the jets. 

[feliz]

I was very disappointed with my K3 over the K0 until I changed the jets. 

Sure, I was thinking stock bikes. Reverse the exhaust, intake, gearing, and jetting changes that took place over the years and it's a different story. I've never ridden a K0 but I understand the difference between them and the K1 alone is significant. By 73 they were already complaining the Ks had been casterated.

[eldar]

Truth be told, the k7/8 is about on par with the 69. The f3 is actually faster than the 69. This comes from cycle mags whos articles can actually be found here. the k8 is probably the most powerful of the K engines. Remember the F/f1? That engine with upgrades is in the k7/8. So your butt was probably telling the truth.

The K-model is all this plus another 3 percent. It has an edge because it's built and sold as a touring bike, so factors such as ground clearance and weight aren't as significant as they are to a bike with "Super Sport" in its name. Ideally the K model would also have ComStar wheels, dual horns, a rear disc and the F's power, but even without them, it's a fine touring machine tainted only by less than satisfactory suspension compliance. Its 1/4-mile time of 13.33 doesn't seem impressive compared to the F's hot-blooded 12.7, until you realize it's still three tenths quicker than the original CB750, three tenths quicker than the current Yamaha 750, and quicker than the BMW 750. In short it's no slouch, and there's a simple explanation why: the K engine is last year's Super Sport engine with the new accelerator-pump carburetion system.

From this article:
http://www.sohc4.net/index.php?title=1977_Cycle_Feature:_Honda_CB750F2_%26_CB750K

[Hockers Choppers]

might have something to do with why the early bikes had 120 main jets and then dropped to 105's EPA was cracking down on emissions. had to chock em down to pass?

[eldar]

It is possible. The K8 has 110 mains though. I think the f3 has 105 mains. Both use the accel pump which I think really helps. Many do not like the later carbs, they might be more finicky but they work very nice.

[Jerry Rxman Griffin aka MuthaF'er]

The 75 750F was the beginning of the rebirth of the detuned for emmissions and reliability 750. This engine carried over to the 77/78 K with only slight differences in cams and carbs. While many may think the 75/76 F and the 77/78 K Fugly it signalled the SOHC4's exit and they tried to improve over the 76 750K which could out run shoe leather. Honda always used to make the early models hotter for sales before detuning for reliability.

[GammaFlat]

Look what you did... now I'm gonna cry

My K6 is supposed to be "among the" (if not THE) slowest of the 69-78 750s (ok, 750 automatics are prolly slower).  I just love riding it though.  I am cheating some - ignition, 4-1, K&N air filter, jetted up a bit and 17 teeth in the front (thanks HondaMan - excellent upgrade).  And trust me, I plan on cheating more but more horsepower just isn't important (until I have it  )

I have a K7 and K8 - (cause I wanna be just like Eldar ) that I'll get running well this spring.  I'll report back my findings. 

One thing I feel certain about.. if you have one of the pokey middle years, throw away your 18 sprocket and replace with a 17 - especially if you do a decent amount of city driving.  You will definitely grin. 

[void909]

ok im not sure if this is a thread jack but here goes...

I have a 74k a 75k and a 78k. To me it seems as though the k8 frame isnt as well built as the earlier frames. Why would this happen. An example would be the rear peg mounting points. Another would be the area under the neck on the frame. It just doesn't look as well made. Any thoughts?

[eldar]

32 years and no issues on my frame. I doubt the pegs are different by much and the remember, the 77/78 have longer wheelbases with a bit more rake and that could be what is contributing to what you think may be a weak looking.

[goon 1492]

Look what you did... now I'm gonna cry

My K6 is supposed to be "among the" (if not THE) slowest of the 69-78 750s (ok, 750 automatics are prolly slower).  I just love riding it though.  I am cheating some - ignition, 4-1, K&N air filter, jetted up a bit and 17 teeth in the front (thanks HondaMan - excellent upgrade).  And trust me, I plan on cheating more but more horsepower just isn't important (until I have it  )

I have a K7 and K8 - (cause I wanna be just like Eldar ) that I'll get running well this spring.  I'll report back my findings. 

One thing I feel certain about.. if you have one of the pokey middle years, throw away your 18 sprocket and replace with a 17 - especially if you do a decent amount of city driving.  You will definitely grin. 

I'm on the fence with this one too cause mine will scoot, hell last summer I went along side a 07' HD softail deluxe(whale of a bike) and was right behind him.
I have heard goods and bads about the k6.....

[coyotecowboy]

You have to be very careful comparing preformance numbers of different bikes based on what they ran at the dragstrip.  Waaaaay too many variables to take into account.  Hell, you can put a different rider on the same bike and see several tenths difference.

All things being the same (clutch, gearing ect...) if it makes less horsepower on a dyno and weighs more, it aint gonna go as fast as something that makes more power and weighs less.  Simple physics.

I'm not trying to step on anyone's toes, but don't always take 30 year-old mag articles for gospel truth.

[eldar]

Yeah but you also cant take hondas word either on their HP ratings. If you did that, the f3 should be slower and it is not.
I can tell you that if I decide to twist the gas, I get to 60 in a VERY short time. We have no drag strip within 100 miles so I cant speed trap it but it goes very well.

[coyotecowboy]

This is all vapor talk then

[eldar]

Maybe but then the thought that the k1 is better might just be vapor too. My ride goes, it goes good. I can easily bust the speed limit on the interstate here in 3rd, that is 75mph. It only takes a few seconds to get there. As I am not the only one to report that, there might be something to the old articles. Like I said, if you went by honda hp ratings, the f3 should be slower and it sure isnt.

Maybe it does not matter but honda was playing catch up with kawi and suzi and put some extra fire in the later years.

[moham]

Maybe but then the thought that the k1 is better might just be vapor too. My ride goes, it goes good. I can easily bust the speed limit on the interstate here in 3rd, that is 75mph. It only takes a few seconds to get there. As I am not the only one to report that, there might be something to the old articles. Like I said, if you went by honda hp ratings, the f3 should be slower and it sure isnt.

Maybe it does not matter but honda was playing catch up with kawi and suzi and put some extra fire in the later years.

You could also make the argument that the success of the K0 and K1 spawned many offspring, whereas the K8 spawned the yawning chasm of nothingness. My logic here isn't foolproof but neither are the rest of the arguments offered thus far (with the possible exception of the dyno testing theory)...

[Don R]

Put a Pee Wee Gleason on your bike and see how fast it is. He rode most of the road tests. If he couldn't make it fast it just wasn't fast.

[moham]

Put a Pee Wee Gleason on your bike and see how fast it is. He rode most of the road tests. If he couldn't make it fast it just wasn't fast.

I think I saw one on eBay the other day...(a peewee Gleason, that is...)e

[BobbyR]

The K8 head is the choice for building the tricked out motor. No one runs away from me other than crotch rockets. I run 120 mains with a dyna, no problems and gives me a better range.

The early CBs are very pretty and they were quaint looking. Time and style moved on and Honda responded. The K7/8 had more fork rake and began to less quaint and more like modern motorcycles. All of the model years are outstanding, and they are still running.     

[Jerry Rxman Griffin aka MuthaF'er]

While cleaning out all the used, NOS, and new parts in my basement, office and garage and getting them organized by page of the parts manual over the last 3 days, I found my K8 cam and my F0 cam. Now while the K8 engine is the offspring of the F0 engine it has a different cam according to the Honda shop manual and appearance. I'd say the K8 cam IS milder than the F0. The lobes appear to be approx the same height but the F0 has a wider lobe. Appears to open earlier and stay open longer. Duration. Perhaps this is one reason that the bike will go slightly above 100 in 3rd. Now this has nothing to do with the K1 but I though I'd just throw this in.