Topic: Anyone know of a decent set of carbs that will fit a 650z?

[speedy gonzalais]

Ive come to the conclusion that the standard Keihin mechanical carbs are useless, any one know of other carbs that fit the 650, or has any one got a set of cv's they wanna sell.

cheers

[heffay]

i've been told they don't swap due to mounting differences... have not checked into it.

i'm having trouble w/ the cv version if that helps at all.    

[speedy gonzalais]

the problem im having is with the mixture and the accel pump, the carbs where designed to have a weak mixture due to u.s emission standards or sank. I cant seem to get a nice mix well i can but it misses when i open the throttle quick, i think this is cause of the accel pump making the mix too rich if you know what i mean, I just think the bike can be alot faster if it wernt for these carbs.

[mlinder]

even with accel pumps, carbed bikes do not like to be whacked open, especally from lower rpm, or from less than 3/4 throttle open.
It'sjust the nature of old non-cv carbed bikes. You need to learn the balance of rpm, speed, and throttle position, and you can get these mechanical-carbed SOHC's to really scoot.
CV carbs use vacuum to lift the carb piston, so it 'feel's ok to whack the throttle open, because the carb-piston doesn't lift except by engine vacuum.
Learn your mechanical carbs, because at the end of the day, if you learn them, you get better control and performance (in my opinion) over the CV carbs.

[Soos]

Yaha....


Yes, there are replacement carbs.

I would REALLY suggest a search.
I KNOW of 2 types of carbs that are replacements for the cb650 head that uses the mechanical(PD50a/PD50b) carbs.

CB750
GPZ600 CV carbs work(off a '85 GPZ600, AKA kawasaki ninja) but the carb isolators are needed as well..

cb650 mechanical carbs -  26mm venturi.
cb750 stocker carbs      -  28mm venturi.
GPZ600 carbs               -  32.5mm venturi, and CV to boot.
CB650 CV carbs using GPZ500('85) carb isolators work as well.
Not too sure on the venturi size offhand of the stock cb650 CV carbs though.
If interested I can measure them though.


Heffay, just to let you know(in case you didn't read my GPZ600 carb thread) those GPZ600 carbs are a DROP in for the cb650 head that uses the CV carbs.
Nearly perfect alignment.
Check out my thread on this, it gives more specific measurements than I can remember offhand.


http://forums.sohc4.net/index.php?topic=40287.0


l8r

[speedy gonzalais]

I know what your saying about sudden throttle with carbed engines its best to gradualy open the throttle but its still not right even if i shift down theres still a slight miss, There cant be a blockage any where they are mint inside, had them stripped last night
is there any way to disable this pump some how

cheers

nice to see you back online mlinder

[speedy gonzalais]

cheers soos il take a look at the breakers now i know what bikes im looking for.

[Soos]

I'll agree on the control of throttle position vs RPM's.

If you don't get the right feel for them, your bike will feel as if it running like a dog.


And as for performance... I dunno.
That whack it open ability of CV's is nice though.
The ones i'm running definately have a flow advantage over the 750 carbs i've run on my 650, but the only major difference is top end. say 7500+rpm.

The 750 mechanical carbs are nicer starting (once properly sorted out for the 650) and have a WAY cool side choke lever.


The search function is your friend, spend some time with it!
 

l8r

[speedy gonzalais]

You say the cv isolators is that the same thing as the boots on the mechanical ones?

[Soos]

And you saying these bikes can be faster with bigger carbs?

Not really.
Now it may take a bit longer to get there.

But the top speed i hit with the 650 stockers on my 650, About 110-115mph.
With the 750 carbs, about the same speed, mabey 5-10mph more.

Thats the last top speed comparison I can give.
Shortly after adding my 750 carbs I bored my engine out 1.2mm.
So top speed now compared to before the overbore is skewed now.
Although i would like to feel these CV carbs on a stocker just to see the difference.

However since the overbore i hit well over 125mph.
Not too sure, as my 650 speedo ends at 120mph, but there are a few unmarked hash marks past that.


However, with the 750 carbs acceleration definitely benefits compared to the stocker 650 carbs.
With the GPZ carbs, there are no hesitations in acceleration. But that is due to the CV design.
I like 'em and plan on removing them this winter to store for my 65mm overbore cb650 motor.

My '79cb650 is getting my ported cb650 head and I plan on re-installing my cb750 carbs.
A ported head (IMO) gives more than any carb upgrade you will ever throw on these SOHC's though. Even when using the stocker carbs.

http://members.tripod.com/cb750k2/Technical/porting/Porting_the_Honda_Head.htm

I have brought this article up and discussed it with the old man that helped me port my head, and he agreed 100% with it's stated results of a ported head with stock carbs.
However bigger carbs and a ported head...
He did say that going over 120-130% of the diameter of your intake valve is not only pointless, but inefficient due to the lack of vacuum created in the carbs for drawing fuel out.
I have been told (by several motorcycle freaks locally) that 80-90% of the intake diameter is about perfect for any motor when naturally aspirated.

Not sure I remember right if he mentioned the valve diameter, or the diameter of contact the valve makes with the seat.


The cb650 intake valve diameter is about 31mm.
The seat diameter(where the intake valve contact the valve seat) on my ported cb650 head is roughly about 30mm.
Not sure what a stocker runs at, but i'm guessing about 29mm or so.

So cb750 carbs are about perfect for a cb650 head IMO.
The 28mm carbs would be 90% roughly based on comparing to valve diameter.



l8r

[Soos]

You say the cv isolators is that the same thing as the boots on the mechanical ones?

no.

CB650 CV and nonCV isolators are differend diameters and lengths.

The CV carb isolators for the GPZ600 carbs have a jog in them allowing for the carb spacing of the CV carbs to be used on the non CV head.

But with new isolators you can fit things that a old set would not allow... they are stretchier.
One example...
cb750 carbs on the motor side are a bigger diameter by 1 or 2 mm than mechanical cb650 carbs, but they fit in a mechanical carb stock isolators ok. Just a tighter fit.
New carb isolators makes the fit a ***LOT*** easier though.
I have a post somewhere on cb750 carb measurement...

the search function really is your friend 


For example:

CV cb650 carbs with GPZ carb isolators fit (but I have not run them) on a mechanical carb cb650 head.

GPZ carbs fit a mechanical carb cb650 head with GPZ600 carb isolators.

GPZ600 carbs fit on a CV cb650 head with stocker CV cb650 isolators.

CB750 carbs fit on a mechanical carb cb650 head with stock mechanical carb isolators.


I have run both the cb750 carbs,  and the GPZ600 carbs and isolators on my '79cb650 with a mechanical carb cb650 head.

I have only fitted up the GPZ carbs to a CV cb650 head, and with stocker CV cb650 carb isolators they fit at least, would imagine it would be fine.
I have only fitted CV cb650 carbs on a mechanical carb cb650 head as well.
it does fit, but have not run them.
I would imagine they would run fine as well.


l8r

[Soos]

nevermind... double post.

[Soos]

You say the cv isolators is that the same thing as the boots on the mechanical ones?

The CV carb isolators are slightly bigger ID on the carb side, and shorter.


l8r

[speedy gonzalais]

Oh i see so i must get the isolators off the gp if i find one, also you say 115 tops? ive had mine of the clock 135 is what a friend was doing behind me on hes CBR, It takes a bloody long time to get there though.

[Soos]

That is my problem.
No long enough, good condition, remote enough run areas to take her up that fast.
Unless I pony up the $$ for a run out in the salt flats.(a dream of mine actually!)

And yes it takes time to get to top speed on my 650.
It's quicker to top end with these GPZ600 carbs though.


Now if I lived in kansas, texas, mabey california... some flat lowland state.
I would have no problem finding a straight flat LONG run.

But living at the foothills of a mountain range limits the stretches of road that are a flat run, or straight enough in remote enough areas to do a real top speed run.

As well the 4500+ ft elevation I ride at drops the power output(theoretically) 3% per 1000 foot.

I would guess a bit more than 10% loss in power would be why I am loosing the top speed.
I would love to run at 0ft elevation on a straight stretch just to see one day.



l8r

[tonycb650]

Make sure your acc. pump is ajusted correctly. I have the pd50b carbs on mine and i got them running perfectly. I was getting a rich bog only at higher rpm in top gear (@ 6000) but fixed that by running my float bowl vents to the air filter box.

[mlinder]

125+ with 63ish HP on an unfared bike sounds not quite right to me.
Not trying to be a naysayer, but as far as I know, recorded top speed on them, by professionals, was in the 110mph range.

[Soos]

It takes me about 2 miles to hit those speeds.
The last 10-15mph takes most of that run length.

Thank god I never got pulled over trying to do that.
Thats actually my #1 worry...
A 919 will do that speed in less than 2-3 blocks i would guess...
it takes me a while, and WOT most of the ride.


You ever want to confirm, you're more than welcome to visit!



l8r

[mlinder]

I'm a bit laid up at the moment... but thanks for the offer

I've had the 750 up to 120ish, which makes a bit more power than the 650's. I didnt hold it wot for 2 miles though. 135 is, if I feel like doing the math, impossible for an unfaired 63 crank HP motorcycle, though. I can tell you that most modern sportbikes (and sporty-ish bikes) have EXTREMELY 'optimistic' speedos. Usually off 5 to 7 mph at 100 mph, sometimes more.

[kayaker43]

I agree, the HP and speeds being thrown around don't always match. Since I've been riding with GPS I realize how far off most speedo's are. I'm finding 10mph or more (high) is typical of a lot of bikes at 100+, and a 5-7 error is typical at 60mph. Also a barely perceptable grade or wind will add or subtract 5-10 mph. That's why speed runs are done in both directions and averaged. Go stand outside in a 5 mph wind and you'll think its calm.

The only fair comparisons are carb changes done to the same bike and runs done on the same day and place. Even at that I've read that changes under 10% can't reliably be felt by seat of the pants on different days. Having said that,.. very small changes can be felt if done instantly while riding,.. one example is switching off the charging system under a heavy charging load.

[speedy gonzalais]

One day il strap on a helmet cam and use a digital speedo, Trust me i wasnt going that fast for long @1100rpm the engine wont last verry long but it does get there.
I must admit i was in a cars slip stream and going down a slight slope on the dual carrige way and she didnt feel safe at all.
For a bike of its age i cant belive what she goes like, over 1/4 mile she will pull a ton, Ive raced various too fast to furious boys and won plenty of times, As for bikes ive come across a few modern riders that were suprised how i could keep up on such an old bike.
Ands thats almost stock, I think a better set of carbs will make a difference and i wonder if you could play about with the gear ratios a bit and maybe even add another gear, would that be possible does any one know?

cheers

[mlinder]

adding a gear would cost more than a new bike. An expensive bike.
Im betting in a slipstream and downhill, you were still no faster than 125mph, due to errors from the speedos. Dont try for 135 on your bike on my account, please. They probably arent safe at that speed, especially if you are still running stock suspension, etc.
11k is prolly ok for short periods. Did it all the time on the cb750, but for very short period of time.
redline on your bike is 124 mph in 5th.. sounds well balanced to me.
If you add a tooth up front, youll hurt acceleration, and may actually lose top speed, with as little bhp the 650 puts to the rear wheel. Need to keep in the powerband to continue to accelerate. Remember than air resistance increase exponentially.So must your HP to increase top speed. Not going to happen in the case of an old SOHC4.

good luck and have fun, though, and be careful!

[Pinhead]

I think most of you 750 guys greatly underestimate the lowly 650. Also, they were greatly smog choked from the factory; simply changing characteristics of the carbs, exhaust, and/or airbox make quite a big difference in horsepower (read the posts that I've made about the mods that I've done to my bike).

[Hush]

Hey MLINDER, pleased to see you in better condition.
DIDS I know a secret only total idiots with CB650Z,s know, don't tell everyone but if you put the bloody diaphram spring on the wrong side of the diaphram (hey it looked right!) you will most certainly disable the accelerator pump.
However my bike wouldn't start until I found and reversed this error.
As for top speed I'd be happy to get more than 55mph out of my bike, apparently I don't thrash it hard enough!

[speedy gonzalais]

Hush you only need to thrash it hard to reach the sort of speeds talked about in this thread, My 650 gets to 55 cruising im talking no more than 1/4 throttle, If i remember right you was sayin how sooty your plugs are i think you are def facing a mixture problem, If your inlet valve guide was leaking it would smoke like hell no matter what rpm range, and if an exhaust guide was leaking the oil would go strait down the pipe and not into the combustion chamber, the best way to check is look at your silencer ends they will be covered in oil if you have guide issues, And from experiance here a weak spark wont affect performance.
And thanks for the info on the gears mlinder


cheers