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I worked on a CB750 K5 all winter and now it is on the road. I didn't touch the engine and I don't know the history. The bike had an oil cooler but I don't know if it had any engine work done to it.

I have found different specs for the bike that say it should top out between 110mph and 125mph.

I am trying to gauge how well mine is running.

Is it normal to be riding 90mph at 5krmp's in 4th? (I am at 7000ft above sea level)
 

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Hey Mut;

First off being at 7K feet - don't expect the cb750 to perform like it's at sea level - I'm sure you already understand this. My cb's in stock form will do about 110mph - although I will deny this in any court of law. I changed my sprocket gearing on one of my cb's to give it more performance from 0 to 100. I have not got my 836 engine on the road yet so it tough for me to make any comparison between the two. In top stock form a cb750 turns about 56hp while I've read dyno reports that an 836 will turn close to 70hp at the rear wheel. If you have an RC cobra motor - that might give you a 100hp at the wheel.

I like to tell other folks - it's not how fast you go but how fast you get fast...

Let us know how you make out...
 

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quote:Originally posted by mut


Is it normal to be riding 90mph at 5krmp's in 4th? (I am at 7000ft above sea level)
at 60mph, i'm around 5000rpm in 5th. so no, thats not normal. have someone pace you in a car for the speed, and make sure you have a cb750 tach. The next time i ride, i'll get a 90mph rpm for you. then i'll come back to here and post it. this may be a while, it hasn't stopped raining in 4 days.
 

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I've never seen a road test from any credible motorcycle magazine that said a stock CB750 would even break 115

and I've never owned one that would

I guess I've owned at least one hundred of them
 

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down here at near sea level I have had my K5 to an indicated 125mph. However with speedo error and just the regular nuances of it being a 30+ year old bike I figure it was probably closer to 105. It is really hard to get a 500lbs, unfaired, bike making 58hp at the wheel much further past the ton than 110mph total.

honestly, if you want to figure otu how well the bike is running - top speed is not the way to do it. Measured distances over time (like drag strips) and using conversion tables to adjust for the atmospheric condition are the way to go.

Also dynomometers are an excellent way to test your state of tune. Most bike shows have a traveling dyno where you can pay like $60-70 for 2 or 3 runs.

want to know how well your bike runs?

1) do a compression test. all the cylinders should be above 90psi and with in 5% of each other.

2) read your plugs. A plug chop test tell you about immediate problems. however there is plenty to learn from your regular plugs.

3) check the dwell on the points.

4) run it on a dyno that can test your air/fuel ratio. Not onyl will you be able to tell where your flat spots are, you will also learn what changes you need to make to the carb.
 

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depending on the current amount of friction with local LEO

I may get one of them to laser me

or I run the superslab timing mile markers

or, strap on the GPS which is almost as accurate as timing mile markers

I agree that most off of the vintage bikes seemed to have built in errors


Geeto....... how many stockers have you actually seen put 58 to the back tire on a good dyno?

I've never actually put a CB750 on a dyno or seen one on any dyno.
 

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quote:Originally posted by HackAsaw

depending on the current amount of friction with local LEO

I may get one of them to laser me

or I run the superslab timing mile markers

or, strap on the GPS which is almost as accurate as timing mile markers

I agree that most off of the vintage bikes seemed to have built in errors


Geeto....... how many stockers have you actually seen put 58 to the back tire on a good dyno?

I've never actually put a CB750 on a dyno or seen one on any dyno.
The problem with dyno's is that you can manipulate them to ready whatever you want, even by accident, so the final output is really irrelevent. Really what you want is the power curves and the Air/fuel ratio readings to see where you are lean, where you are rich, and where you can fine tune it.

My own K5 made 58hp on a dyno at the old Marcus Dairy supersunday bikes shows. However I think the roller resistance wasn't optimal (set up for a heavier bike) and that was only after makeing several adjustments to the chain and one to the needle height. At the time it did this I was still running points, but I had given the bike a massive tune up right before including new plugs, points, set the dwell, new k&n in the stock airbox, fresh exhaust gaskets, set the timing, fresh oil (synthetic) and filter, and blueprinted the carbs - you name it. I think the proper number should have been around 55 hp. I have seen other mid year k-bike pull mid 50s running the later carbs or gs750 carbs and more with any kind of porting and headwork.

I have seen 2 77-78 F's pull 58hp at the rear wheel but those are big valve motors with accelerator pump carbs and are supposed to make 67hp. I have had one other of my personal bikes on the dyno and it was running like crap and could only muster 44hp. It had a power curve that spiked throughout the rev range and ran from lean to rich to lean in several spots. Afterwards I cleaned the carbs and got it to run much better but I never did get the chance to run it on the dyno again. All in all I have probably seen about 6 or 7 cb750s on a dyno, most at shows in the 1990s. For a while (1997-2002) I wasn't seeing any cb750s on the road as opposed to the late 1980s, early 1990s when they were still kinda common.

Right now I want to run my 78 dunstall on a dyno but the local dealer I use has decided he is only going to allow bikes he sells on the dyno (the old dyno guy used to run anything for a fee). As soon as I find another I will run my '78 and give a full report. I have not had the motor open on this bike and I think she is a little hotter than stock.

A properly tuned cb750 with a healthy stock motor should be able to pull at least 50hp at the rear wheel.
 

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huh, ive my 550 to 110 indicated. it had more, but i was a little scared at that point. i do it aout once a month. at one point even looked over to see a kid on a cbr600 right on my ass. literally just 6' away.



jc
 

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Once I add a warp drive unit to my cb750, these pesky cbr's should be a problem....
 

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I just finished buttoning up my 900cc kitted CB750. There still is a lot to do, but it should at least be running and driven around the block by the end of the week. I have had about 12 SOHC CB750s, never had any on a dyno, and this is my first "hot rod" CB750 to build. I will be sure to post some speed numbers.

As far as checking top speed, a GPS is easy and accurate, but I have never had one on a CB750. I will certainly keep one on my CB as I am breaking it in.

Peace and grease
-steve
 

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I just finished buttoning up my 900cc kitted CB750. There still is a lot to do, but it should at least be running and driven around the block by the end of the week. I have had about 12 SOHC CB750s, never had any on a dyno, and this is my first "hot rod" CB750 to build. I will be sure to post some speed numbers.

As far as checking top speed, a GPS is easy and accurate, but I have never had one on a CB750. I will certainly keep one on my CB as I am breaking it in.

Peace and grease
-steve
 

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thanks Geeto and the rest of you

I just cut a very fine original Paul Dunstall Decibel Muffler and put it on a nice 4 into 1 header a local was running around town with just an open megaphone

think it is a nearly bone stock 74 CB750

the cops didn't like it and kinda told him about it

The collector size on the header was larger than some found on SBC's

stupid huge at 2.5" and much larger than the very fine Yoshimira used on my big CB had

pegged my db meter at 110 from more than 25 steps and not on full song

so I cut the dunstall and welded it to the collector and also to the support brace

much quieter and he says it's much more powerful now

go figure, pick up all that lost midrange torque and unleash a monster

I will have to get a picture of this bike, it has some of the scariest tires I've ever seen anyone use on the road


some of the kids around town gave me a chuckle suggesting that particular 45 inch CB should get a shot at lining up against my Seca Turbo

I asked him and he seemed to indicate he knew better
 

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thanks Geeto and the rest of you

I just cut a very fine original Paul Dunstall Decibel Muffler and put it on a nice 4 into 1 header a local was running around town with just an open megaphone

think it is a nearly bone stock 74 CB750

the cops didn't like it and kinda told him about it

The collector size on the header was larger than some found on SBC's

stupid huge at 2.5" and much larger than the very fine Yoshimira used on my big CB had

pegged my db meter at 110 from more than 25 steps and not on full song

so I cut the dunstall and welded it to the collector and also to the support brace

much quieter and he says it's much more powerful now

go figure, pick up all that lost midrange torque and unleash a monster

I will have to get a picture of this bike, it has some of the scariest tires I've ever seen anyone use on the road


some of the kids around town gave me a chuckle suggesting that particular 45 inch CB should get a shot at lining up against my Seca Turbo

I asked him and he seemed to indicate he knew better
 

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the majority of any race is in the rider. I have taken street squids with cbr600s (mostly f2s and f3s) on the street with my cb750 because they were afraid to lean the bike over or didn't know how to properly launch it. But you have to be real good to make vintage iron run with the new stuff since hp goes a long way in compensating for lack of rider skill.

A cb750 is a pretty quick bike for an old bike. A flyweight rider who knows what he is doing should be able to get a stocker to run low 13 second 1/4s. most people have never driven cars that accelerate like that. as far as vintage bikes go it is not hard to get other vintage iron to run similar numbers, but to do it reliably and consistently as a cb750 can is really where the marvel in it lies. It isn't that it makes 53 or 55 or 58 hp it is that they make it 365 days out of the year. My norton commando may have been faster some days but the number of days it was running in peak form were less than half that make up a year.
 

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I must agree

this guy rides this bike like the hounds of hell were hot on his heels

frankly, he's nuts









pull that baffle plate out and it looks like a gatlin gun



crappy welds but he was in a hurry and didn't want to take it off

so far it's holding up just fine



I wouldn't ride this one around the block with those tires
 

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That is so bad I hardly know where to start. I wouldn't push that thing around the block on those tires, a worn out Dunlop F-11 on the front and a slick Cheng Shin on the back? Not to mention the paint job and the stock shocks. That is just scarey. Do you take pictures of every POS that you work on or only the ones that are worthy of such documentation?

Ken
 

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I seldom do any work for the public, but when I do, there seems to be no rhyme to the reason.

He bought the mirrors and one of the guys told me. So he was parked close to my shop and I walked over to tell him thank you.

Looked at his bike and thought, this guy may even be crazier than me.

So I asked him how things were going with the huge open megaphone and he asked if I could help him.

Don't guess it was about money but he really does ride....... umm the tread off that bike and has even embarrassed a couple of the tentative younger kids on their brand new 600cc whatever.

Hated I didn't have a clutch cable for him yesterday and that he was too uncertain to ask if I'd trade out something to teach him how to make his own. I told him I had everything on hand to make a new one, and if I just happened to be lacking some barrel end or whatever, the metal eating machines can make it happen.

The one before this was a 25K+??? Busa that was absolutely insane and inhumanly fast.



normally I'd pass but the guy that owns it is really down to earth even though he has 4 or 5 similar machines

and I think he may also be as crazy as I am

small farm town and I really got tired of training the next group of kids/adults

about how they don't even need to dare roll their ATV or dirt bike onto my property covered in mud or cow dung

it was so bad I had to put up a sign addressing the subject and instructing them to take it to the car wash before I'd even look at it

so now I just help out a few friends here and there but don't really have much time to be able to do that
 

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something about that bike gives me wood...

maybe it is the fact that he painted is safety orange with the bodywork still mounted on the bike, or that it is a 1977-78 SOHC with a 1979-1982 cb750K DOHC tank on it. Or maybe it is just the fact that it is a junkyard dog of a motorcycle and I like things that when you are done beating the absolute ever living piss out of it, if there is anything left you can set it on fire and walk away.

I have ridden on worse but those tires are toast. A new set of metzlers and I gaurantee the guy will have brown moments on that bike he never thought possible (until I got my lazertechs I didn't think it was possible to scrape the left side dyno cover in a turn without dropping the bike - how wrong I was.
 

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what would be a good non-DOT race tire for that bike that would almost pass for DOT? rain tire I'd think best

I think he's about ready to consider it

he gave a hundred for the thing, or something like it and told me he appreciated that I never tried to talk him into selling it to me.

it really does run good and sounds very healthy.

he knows I'd probably pop three or four on him if he got weak
 

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why do you want non DOT? That bike will run out of bike (clearance)before it runs out of tire on modern rubber so handeling is not an issue. Besides most racing "rain tires" are DOT. As low as that pipe hangs he will never even get into the meat of a modern tire's connering grip.

You are going to want a ribbed front tire (like an avon speedmaster or a newer version of that dunlop that is on there), otherwise cb750s get head shake on deceleration. If you can't find ribbed the metzler lazertechs probably have the least shake.

The two tires I recommend for cb750s that are ridden hard by competient people are metzler lazertechs and Avon venoms. Bridgestone bt45s are also really good but I have never used them.

The lazertechs have a classic tread up front, more sporty in the rear. The Avons are all business sport bike tires.
 
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