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Author Topic: why is a 1969/70 faster than the remaining years ?  (Read 1346 times)

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Offline rokhedjo

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why is a 1969/70 faster than the remaining years ?
« on: June 25, 2019, 06:31:31 am »
My Dad had a real nice K0 stocker, it would run off and leave any other CB750 I have ever ridden. I know the carbs are different, but what would make it so much faster?
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Offline the-chauffeur

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #1 on: June 25, 2019, 09:31:28 am »

From what I've read - and I'm far from an expert on this - there's no single reason.  There are a number of developments between the K0's and later models that on their own, probably wouldn't make a difference that was particularly significant but their effect is considerably more noticeable when put together.  It's pretty straightforward stuff - like spark advance design changes and modifications to the exhausts to name two - trouble is, there's a lot of them.

Your best bet is to read the thoughts of HondaMan thread; it's a good starting point.  From there, you can click on his profile and read some of his other posts.  Hopefully someone better qualified than me will be along to give you a list, but Mark also documents many of the changes and how they affected performance of the first model and those that followed through the K series in his book.
« Last Edit: June 25, 2019, 09:48:56 am by the-chauffeur »

Offline seanbarney41

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #2 on: June 25, 2019, 09:32:39 am »
It is a combination of several subtle tuning and engine changes...good news is it can all be done to most 750s just gotta find the parts
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Offline seanbarney41

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #3 on: June 25, 2019, 09:33:48 am »
Single biggest factor is sprocket size, in my opinion
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Offline pjlogue

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #4 on: June 25, 2019, 10:18:25 am »
The biggest changes Honda made were to the valve timing/lift/duration and cylinder head and intake/exhaust had work.  The early ones received a lot of hand finishing to the heads.  When the energy crunch came in and the EPA was formed the engineers started downgrading the engine (valve timing/duration/lift and leaned out the jets) to reduce emissions and increase fuel mileage and this was about the same time Honda stopped hand finishing the heads.  (lack of head finishing was to reduce production cost) All together the engine output dropped quite a bit.  The early engines could put out ~67 Hp.  The later ones were in the low 50's if I remember correctly.

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Offline seanbarney41

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #5 on: June 25, 2019, 02:24:12 pm »
I have yet to see anyone list cam specs different enough to make more than 1 or 2 hp difference...but that could just be that there are few who take the time and have the required tools to measure the values that are comparable.  We know that cam grinds changed at least 4 times between 69 and 78.  Some of these changes may be too subtle to make any difference on a dyno.

The cylinder head cleanup issue should be obvious to anyone who has peeked down the intake port of both early and later heads.

In my parts swapping experience...that lazy k5 wakes up the most with a countershaft sprocket tooth count change from 18 to 17.  Next up, get an early advancer because it is easy.
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Offline MauiK3

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #6 on: June 25, 2019, 06:47:03 pm »
Also, the red ones were the fastest in the quarter mile, candy blue green always took all out top end.
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Offline PeWe

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #7 on: June 25, 2019, 09:31:35 pm »
K0 was more normal, the later were detuned with less breathing carb filter box, exhaust, leaner jetting and ignition? Tuning that should be easy to apply on the later bikes.
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Offline ekpent

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #8 on: June 26, 2019, 06:49:52 am »
  Might be an old urban myth but I remember reading that on top of the major gas shortage we experienced Honda also took notice of all the 750's that were being outfitted with Vetter fairings/bags etc. and being turned into touring rigs before the official birth of the Goldwing. I think they were trying to re-tune the engine for a little better and lower rpm torque curve for these applications along with making them more eco-friendly.
  Early bikes had better free flowing pipes,bigger jets etc.. My 1971's feel faster then the later models on the butt dyno  :D
« Last Edit: June 26, 2019, 06:52:00 am by ekpent »

Offline PeWe

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #9 on: June 26, 2019, 08:52:02 am »
17:48 gearing feels better than 18:48 too
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Offline HondaMan

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #10 on: June 26, 2019, 07:30:14 pm »
Yep, the hand-worked heads, the cam changes, the carb recalibrations, the exhaust pipes, the airbox(es) and the spark advancers.
The K0, prior to about 3/1970, was a raucous, lighter, hand-made engine. That was about the time Honda discovered they were caught completely off-guard by the popularity of this ground-changing bike (which became a history-changing bike), and they had to get serious about making thousands of them instead of hundreds.

The K0 early engines had hand-ported heads, partly because their hand-shaped sand-filled socks that formed the ports were all over the place in size and shape. Looking over these heads, you will see one with the valve guides protruding as much as 14mm inside the port(s) while the next one has about 6mm of them sticking out. Their ports will look very different, too. The most common feature is: they are larger in diameter and have more headroom over the intake valves than any of the later heads, especially the late K2 and the K3 heads. But size wasn't everything: there were 'doggy' K0 bikes, too, and mostly it came from poorly-shaped intake ports, probably from the hands of the "new guy" in the production line...

The cam changed 4 [documented] times between the K0 and the K5/6, but I have (had) notes of there being at least 7 identifiable differences in both the intake and exhaust lobe timing. Generally, the intake duration that started out opening at 5-6 degrees BTDC and closing at 35 degrees ABDC in the K0 lost 7 degrees overall by the time of the K5, and the opening time got progressively later during those years. The lift dropped almost 1mm along the way (intake side), too. By the K4, which excelled in smoothness over all of the others, the 2-3 intake cam lobes have less lift by 0.25mm, with the same duration, as the 1-4 intake lobes. I believe (still) this change was aimed at increasing fuel mileage (up to 50 MPG then) for the US gasoline crisis that OPEC caused in the mid-1970s. Honda knew its largest market (USA) was limited to 55 MPH freeway speeds for bikes pushing [mostly] Vetter full fairing, saddlebags, and 2-up riding with camping gear aboard, so many of the changes were directed toward increasing midrange torque at the sacrifice of sheer top-end speed and performance, so that drove much of the change.

All of the carbs before the emission-controlled PD series of the F2/3 and K7/8 bikes were essentially the same parts, but with differing jetting to match their cams (mostly) and their pipes. The K2 pipes (HM341) were at first overestimated by Honda, and the K2 had overly-rich carbs before about 2/72 production, causing LOTS of plug-fouling troubles. This was fixed by increasing the airbox vacuum (read: smaller intake vents) and shortening the total spark advance, along with gearing the final drive up (again) over the K1 gearing. this was great for touring, not so much for street fighting and racetrack work. My original 'claim to fame' came from reverting the K2/3/4 bikes closer to K0 tuning, which added much top end and acceleration joy to the typical college kid's 750.

These changes were:
1. Increased the spring tension of the spark advancer, in those days usually by cutting 1/2 turn off one of the [then new] springs. Today this requires 1/2 turn from both springs, as the springs are annealed from engine heat much more.
2. Lenghtening the carb bowl vent tubes to reach up under the seat, or alternately to reside in 2 small holes drilled into the face of the airbox. The K0 bikes had 4 such tubes, while the K1 and later bikes had 2, or (in PD carbs) only 1, which limits fuel flow above 7000 RPM quite a bit.
3. In the [in]famous 7-chamber HM341 pipes, I drilled 4 holes 1/2" each into the last baffle plate of the pipe, then reinstalled the "plugs" on the ends and made sure the rubber couplers were snug. These matter...if the owner had installed some aftermarket pipe, this upper-midrange power was simply lost, no help there.
4. Adjusted the spark angle to reach further. This depended on the spark advancer in the bike: in most cases this can be viewed as being as far past the longest timing advance mark plus the gap between it and the earlier one, or about 3 degrees more advance. This advance must not arrive before 4000 RPM, though.
5. Switched from paper air filter in the airbox to K&N's filter, oiled lightly.
6. Changed the final-drive gearing. The K0 first started with 16/45 teeth, which broke chains and cases because the 16T front whipped most of the lube out of the chain at hiway speeds. Honda's fix was an even better 17/48 combination, which, when combined with a slightly larger rear wheel, upped the top end speed to 135 (indicated) MPH on many bikes in those days. It also increased the gear ratio, making quicker takeoffs. This engine is made to rev, don't ever lug it...
7. In the end, riding it some (hard) and then jetting the individual bike to get a light brown plug burn on premium fuel after 10 miles of riding at 70 MPH speed, after the other changes were done. This sometimes took altered float bowl levels in the carbs, maybe jetting changes, maybe new needle settings (groan...), or, very commonly, altering the size of the emulsifier's holes in the mainjet emulsifier and the pilot jet. I experimented with polishing the air screws enough to know it was a waste of time: they just need to be clean...

In all of this, I never had a set of vacuum gages for carb synch work, but then, these were stock engines with typically less than 5k miles since Honda set them up. On those bikes that got 'built' with Action Fours parts (most popular then) like cams and bigger pistons, those engines only got bench-sync jobs from my shop. Gages would have let me smooth some of those out more...

In the end, the most bang-for-the-buck in performance parts came from cam changes and [sometimes] port jobs done to the post-1972 heads, if the rest of the engine was in top condition - as most were then. The 2 cams that stood out from all the rest were one from Action Fours (no longer made, and was too soft for touring use, it wore quickly) and the still-made Megacycle 125-00 series (which is the only non-stock cam I'll install today). This cam is a copy of the so-called '90-HP' Daytona cam of the 1970 race debut of the Big Four, although in street trim with standard carbs and pipes it more closely approaches 80 HP levels with full tuning. It lasts like an OEM cam, though, and makes a good touring power curve, too.

Other things that 'happened' to the engine were: in the K4-5 early engines the cylinders were 1mm taller than the others, to lower compression slightly to allow US riders to run on Regular gas (when they could find it) during the gas shortage. It dropped the compression just a bit, but power and crispness with it. Today, when I get one of these, I fit that...today's head gaskets are all 0.2mm thicker, which acts much the same way, so you can actually over-mill the cylinders a tad to recover that, too, if you really want it. The pistons will peek over the tops of the bores when you do, so don't get too crazy with it, and measure carefully. The K4/5/6  heads also have slightly more volume (i.e., lower compression ratio) with the original swirl-directing ridges gone: this happened with their new injection-molding equipment of the K4 that removed the vagaries of the hand-finished (by then, sometimes) ports and chambers. To those who recognize "the smell" of a well-tuned SOHC4 CB750 exhaust, this changed that particular signature of this bike, making its exhaust smell just like any other Honda exhaust (or Suzy 4-stroker, or Kawi...) of the day, because it changed the engine from its original stratified-charge burn system to a more conventional combustion system, in order to increase midrange torque and improve fuel mileage at low-to mid-throttle settings, where this engine ligs along somewhat unfulfilled.

This engine is made to rev: it does not come to life until 5000 RPM when it suddenly burns the fuel more fully and the exhaust is no longer pushing back into the chamber during overlap (no, the pipes don't control this...the cam and spark timing do it). This is the "surge point" any K0/1/2 rider is well acquainted with, that simply isn't there in the later engines. ;)
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Online Erny

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #11 on: June 26, 2019, 11:06:12 pm »
Reading all this, I really would like to have one early 750... If somoeone would like to sell one, let me know
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Offline DickL

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #12 on: June 27, 2019, 04:04:12 am »
If you buy Hondaman's book you can modify your bike to bring it to the 69/70 configuration.

Mark,

When was the recall for the sprocket change? I  took delivery of my 70 K0 around April 1, 1970. My memory was that it was a few months later. I opted out and did not change the delivered config. One of the guys I rode with was a very heavy man, about 350, his failed under warantee and engine was,replaced. That was probably JUNE JULY OR AUGUST of 1970.

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Offline Don R

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #13 on: June 27, 2019, 11:08:50 am »
  Having lived through it, I remember gas being terrible quality and expensive. On Sunday you couldn't buy any. The insurance companies were taking no prisoners, pricing insurance on any bike considered fast. A lot of new riders were getting on 4 cyl bikes and crashing.  There was a lot of bad publicity from wrecks and Honda was seeing competition, so cost cutting and the resultant reduced performance was allowed by Honda, in the later years the performance came back around a little. I always wondered why there was no 900 single cam.
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Offline HondaMan

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #14 on: June 28, 2019, 07:07:47 pm »
If you buy Hondaman's book you can modify your bike to bring it to the 69/70 configuration.

Mark,

When was the recall for the sprocket change? I  took delivery of my 70 K0 around April 1, 1970. My memory was that it was a few months later. I opted out and did not change the delivered config. One of the guys I rode with was a very heavy man, about 350, his failed under warantee and engine was,replaced. That was probably JUNE JULY OR AUGUST of 1970.

DICK
The Service Bulletin I had for the sprocket change was dated (in my handwriting, Honda didn't date them then) April 1970, so you must have picked up yours just as they were changing the setup. The kits did not come out until mid-summer, though, and they contained a curved guard to put on via the bolt in front of the final-drive bearing: they called it a 'guide'. The first ones were real thick, later ones were like sheet metal and sometimes got crumpled up if the chain was let too loose. I still see them sometimes on the K0/1 engines I get to rebuild. You can't fit more than 17T inside that guide, so to keep it you must stay with that 17/48 gearing. It's not all bad, though: the 17/48 is about 1% lower than the original 16/45 setup and actually let these bikes run FASTER by about 5 MPH top end (126 for K0, 131 for K1), like my K1 did.
I remember one July night when I raced with a Chevelle sporting a 454 CID engine who thought he would outrun me: after he passed me I followed him to 105 MPH, then downshifted and pulled up next to the driver, waved at him, hit high gear and pegged it. When I ran out of road it was indicating 132 MPH that night, and the Chevy had given up. Boy, do I miss those empty country roads! I don't think I have the ability to do that today, though...  ::)
The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
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Offline 754

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #15 on: June 28, 2019, 11:01:59 pm »
What rpm at 105 when you went into 4th ?

 I had a 900 Kawi beside me, came up to a straight, left it in 4 th to 115 mph indicated...9000 rpm, then I shifted.. !
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Offline Jerry Rxman Griffin aka MuthaF'er

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #16 on: June 29, 2019, 10:52:19 am »
Mine is about 10,000 in 3rd at 105. 17/48
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Offline DickL

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #17 on: June 29, 2019, 12:31:33 pm »
When my first K0 was new. It would pop out of fifth gear about the time I was just about 100 yards or so short of the finish of the 1/4 mile at 102 to 105 mph. The bike was on warranty and my dealer didnt want to fix it on warranty. I argued that it was under warranty, outside the break in period and under red line. I won the argument.

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Offline Mark M

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #18 on: June 29, 2019, 12:37:25 pm »
I blame Nixon and the oil crisis, heralding speed limits and gas station queues
In the UK anything over 40 years old only needs insurance and Fuel.

Offline HondaMan

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #19 on: June 29, 2019, 07:32:36 pm »
What rpm at 105 when you went into 4th ?

 I had a 900 Kawi beside me, came up to a straight, left it in 4 th to 115 mph indicated...9000 rpm, then I shifted.. !
At 105 when I dropped into 4th, it was pulling a little under 8k on the 8500 redline tach of the K0/1 bikes. I ran it up to a little over 8500 to come up next to the driver so I could shift right then, probably putting it about 112 or so. That K1 ignored the redline like my ex-wife ignored the gas gauge...

Once I experimented with air pressures in the (OEM Bridgestone) rear tire to see if I could "tweak" the gearing some more. The stock air pressure was supposed to be 30 PSI, but I took it (in the end) up to 40 PSI. I found I could pick up a few more [indicated] MPH for a given RPM, made perfect sense. But then I also discovered that at 40 PSI with just me on the bike (all 132 lbs then) that a puff of side wind could make the bike change lanes. When I was loaded with another rider it was more stable, but I never could get more than 121 MPH with 2 aboard that K1. Still later, I discovered the rear wheel was offset from the front by more than 1/2" because the frame was welded crooked: from that day until I traded it for the K2 I didn't ride over 70 MPH!
The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
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Offline MRieck

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #20 on: June 29, 2019, 08:19:46 pm »
 I think most of this is all speculation and fond memories. Does anybody have some Industry Standard RWHP #'s like off a Dynojet? There have to be some "original" 69's and 70's out there to test. Do it and put this thing to rest. Personally I get tired of the 2 degrees more duration, .003 more squish, 2 degrees more advance @ 5,000RPM etc etc etc etc etc.
 Get some of these "ponies" on a dyno and lets see. They are out there!
 Other than that I think this Unicorn chasing. No disrespect to Dick.....I've spoken with him and he is a nice fella.....wish I had the time to help him out.
BTW....I think my CB is faster and quicker than any 1969. ??? ;D
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Offline PeWe

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #21 on: June 29, 2019, 11:17:45 pm »
Good idea about Dyno results, Mike!
That will show what's possible when everything is correctly tuned!
Hp, torque
and optional fuel/air ratio that can be trickier to get.

The shop I have visited use a tube to be stucked into the exhaust as long as possible, no baffle possible. The tube connected to the  sensor at outside where the gases are sucked into it.
My stock K2 is not as fast as my K6 was with stock engine back in 1979. But bike has to carry more weight today 8)

My K6 today is scary with modifications. I can pass many cars very quick on tight roads. Next time my stock K2 will be scary when I try to pass cars, not much happen when twisting and getting close moments... 8)
« Last Edit: June 30, 2019, 12:36:23 am by PeWe »
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Offline seanbarney41

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #22 on: June 30, 2019, 12:07:09 am »
I think most of this is all speculation and fond memories. Does anybody have some Industry Standard RWHP #'s like off a Dynojet? There have to be some "original" 69's and 70's out there to test. Do it and put this thing to rest. Personally I get tired of the 2 degrees more duration, .003 more squish, 2 degrees more advance @ 5,000RPM etc etc etc etc etc.
 Get some of these "ponies" on a dyno and lets see. They are out there!
 Other than that I think this Unicorn chasing. No disrespect to Dick.....I've spoken with him and he is a nice fella.....wish I had the time to help him out.
BTW....I think my CB is faster and quicker than any 1969. ??? ;D
yup... stock k1 is certainly faster than stock k5, but it is mostly set up stuff.  Most of these Honda only guys have not had their minds blown by a Z1 or a Suzi gs...or even a V4 Honda for that matter.  Hell, they think an f2/3 is only good for bad valve guides...I have witnessed a kz650 spank the pants off a K0 before.
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Offline DickL

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #23 on: June 30, 2019, 08:14:58 am »
Mike,

I think that's a good idea but the dyno tests should be with bikes that are exactly pure stockers. Not with Hondamans improvements or your port and polishing. There aren't  many of the pure stockers out there. So, did I ever tell you about the time...

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Offline HondaMan

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Re: why is a 1969/70 faster than the remaining years ?
« Reply #24 on: June 30, 2019, 08:30:44 am »
There's no doubt that the HP of other bikes made after 1969 is higher, the bikes are torquey-er, etc.

...but the 750 was the FIRST...and it is still on the roads, by the hundred thousands, while the others are rarely to be seen.
 ;)
« Last Edit: June 30, 2019, 02:16:54 pm by HondaMan »
The demons are repulsed when a man does good. Use that.
Blood is thicker than water, but motor oil is thicker yet...so, don't mess with my SOHC4, or I might have to hurt you.
Hondaman's creed: "Bikers are family. Treat them accordingly."

Link to Hondaman Ignition: http://forums.sohc4.net/index.php?topic=67543.0

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Link to website: www.SOHC4shop.com

 

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