Author Topic: CB750 K7 - PD carbs replacement by K1-K6 one  (Read 274 times)

0 Members and 1 Guest are viewing this topic.

Offline Erny

  • Hot Shot
  • ***
  • Posts: 426
CB750 K7 - PD carbs replacement by K1-K6 one
« on: February 07, 2019, 01:59:59 pm »
I start to think whether it is worth both for reliability and power point of view to replace PD carbs (41A) by older non-PD carbs (K1-K6) type.

Questions comes to my mind:
- is there benefit in terms of power, torque or engine flexibility? Do I see some improvement on my K7 that is otherwise 100% stock?
- can it be fitted? If yes, then what else to be changed? Rubbers to head? Rubbers to airbox? Airbox itself? Trottle cables?

I suppose somebody tried it already or investigated...
CB750K K7 USA model (1977)
CB550K1 USA model (1975)

Offline Bankerdanny

  • Eventually I will be old enough in reality to be
  • Really Old Timer ...
  • *******
  • Posts: 6,682
  • Endeavor to persevere
Re: CB750 K7 - PD carbs replacement by K1-K6 one
« Reply #1 on: February 07, 2019, 02:58:20 pm »
I start to think whether it is worth both for reliability and power point of view to replace PD carbs (41A) by older non-PD carbs (K1-K6) type.

Questions comes to my mind:
- is there benefit in terms of power, torque or engine flexibility? Do I see some improvement on my K7 that is otherwise 100% stock?
- can it be fitted? If yes, then what else to be changed? Rubbers to head? Rubbers to airbox? Airbox itself? Trottle cables?

I suppose somebody tried it already or investigated...

I don't think that the PD carbs are any less reliable and any power improvements would likely be from a richer mixture, which you could do with your stock carbs.

You would need the correct carb/head boots and the matching air box. I wouldn't think it would be a very hard conversion, but I don't think there would be sufficient improvements to make it worth the cost and effort.
"The problem with quotes on the Internet is that you never know if they're true" - Abraham Lincoln

Current: '76 CB750F. Previous:  '75 CB550F, 2007 Yamaha Vino 125 Scooter, '75 Harley FXE Superglide, '77 GL1000, '77 CB550k, '68 Suzuki 80 (K11 probably, it's been 30 years), '68 Yamaha YR2, '69 BMW R69S, '71 Honda SL175, '02 Royal Enfield Bullet 500, '89 Yamaha FJ1200

Offline Erny

  • Hot Shot
  • ***
  • Posts: 426
Re: CB750 K7 - PD carbs replacement by K1-K6 one
« Reply #2 on: February 07, 2019, 03:05:28 pm »
...I wouldn't think it would be a very hard conversion, but I don't think there would be sufficient improvements to make it worth the cost and effort.

This is what I wanted to hear, thanks, so I keep it and rather focus on other ways for improment - richer mixture, change chain from 630 to 530 (weight saving), mod airbox (additional vents)...

Honestly still fed up by topspeed only 100mph and highly likely accelaration too (but I can only compare top speed as I never rode any other earlier pre-K7 CB750)
CB750K K7 USA model (1977)
CB550K1 USA model (1975)

Offline seanbarney41

  • not really that much younger than an
  • Really Old Timer ...
  • *******
  • Posts: 8,311
Re: CB750 K7 - PD carbs replacement by K1-K6 one
« Reply #3 on: February 07, 2019, 05:06:03 pm »
If you seriously want to ride faster than 100mph regularly, than no cb750, of any year, is a good choice.  Actually, going that fast on any bike is a bad choice imo, unless you are racing on safe and sanctioned track.  Although any cb750 should be able to do it, they really are not very good at it compared to more powerful, stable, aerodynamic, and well braked machinery.  Just not enough excess power and brakes to make these speeds comfortable and pleasant...

I have gone 100-110mph for about 45 continuous miles before on the freeway.  It was a harrowing experience of perpetual fear of explosion and catastrophe.  It was a k5 bike with k7 engine.  It used roundtop(k1-k6) carbs.  If your PD type carbs are working correctly I do not think it would be any advantage to change to roundtops.  However, I do think it is more difficult and expensive to get the PD carbs to work correctly. 

To use roundtop carbs on your K7 you will need the correct boots, airbox boots, clamps, and throttle cables for a k1-k6 bike.  I can't remember if you need to change air boxes or not.
« Last Edit: February 07, 2019, 05:09:56 pm by seanbarney41 »
If it works good, it looks good...

Offline Erny

  • Hot Shot
  • ***
  • Posts: 426
Re: CB750 K7 - PD carbs replacement by K1-K6 one
« Reply #4 on: February 07, 2019, 10:56:01 pm »
Don't get me wrong, I don't want to ride faster than 100...
I'm looking for power that K7 lost compare to previous Kx.
Top speed is at the moment the only eay how I can compare with other guys here on the forum. I know it's not the most proper way to compare but still indicates I'm lacking some HP.
CB750K K7 USA model (1977)
CB550K1 USA model (1975)

Offline flybox1

  • My wife thinks I'm a
  • Really Old Timer ...
  • *******
  • Posts: 14,297
Re: CB750 K7 - PD carbs replacement by K1-K6 one
« Reply #5 on: February 08, 2019, 07:20:14 am »
Tune. Tune. Tune.
timing/advance, jetting, vacuum sync....
Get your accel pump working perfectly.
Get your bike on a gas analyzer to see if there is a jetting issue you cant feel with your butt-dyno.

Once these are done....conclude this is as good as it will get (on a 40 year old bike) without some serious engine mods.

Dont mod the airbox unless youve created the need with an exhaust upgrade(which might also save you some weight)
If you gear for acceleration, you lose top speed.
If you gear for top speed, you lose acceleration.
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline macvit7906

  • Enthusiast
  • **
  • Posts: 114

Offline flybox1

  • My wife thinks I'm a
  • Really Old Timer ...
  • *******
  • Posts: 14,297
Re: CB750 K7 - PD carbs replacement by K1-K6 one
« Reply #7 on: February 08, 2019, 11:00:46 am »
The stock bike is only capable of so much.  No matter what you bolt on to it  :-\
'78 750K (F3 engine) PD42b's, Modified airbox w/K&N  filter, 40/110 jets, 1 needle shim, IMS@ 1 turn out. Kerker + Cone 18" QuietCore

Past Bikes
1974 550K0 (stock), 1973 CB350F (stock), 1983 Yamaha XS400K (POS)
77/78 cool 2 member #3
"Knowledge without mileage equals bullsh!t" - Henry Rollins

"This is my CB. There are many like it, but this one is mine…"

Offline HondaMan

  • Someone took this pic of me before I became a
  • Really Old Timer ...
  • *******
  • Posts: 9,488
  • ...not my choice, I was nicknamed...
    • Getting 'em Back on the Road
Re: CB750 K7 - PD carbs replacement by K1-K6 one
« Reply #8 on: February 08, 2019, 07:21:31 pm »
The K7/8 speed limitation is largely a function of that one's cam. Honda was faced with meeting strict (California) emissions standards, using an existing engine design, starting in 1976. They used lean-burn carbs and extra exhaust scavenging to meet the standards.

If you do nothing else but drop in the Megacycle 125-00 (or 04 hardweld) cam with the existing PD carbs and jetting, you will be surprised...you might wish to then raise the mainjets in the PD to about #115 and drop the needles a notch (if yours have the adjustable needles, many do not), or else just check to make sure the carb float bowls are at the right level. If they happen to be the later PD42b version of the carbs (mostly found on F3/K8), the float level is set at a lean 12.5mm, so altering that to 14mm like the early K7 will also help.

The PD carbs have an upper fuel-flow rate restriction, which was accomplished by using a single hose on one side of the rack to feed the whole rack AND a single bowl vent hose for all 4 carbs. This makes it VITAL that the vent hose breathes from still air. In the past I have altered the bowl vent hoses on the PD carbs to have them connected 2x2 like the earlier K1-K6 setup (keep in mind, the K0 had all 4 hoses), then run the 2 breather hoses up under the seat, and tied it down next to the tools in the tool tray. This is one of the quietest air spots on the bike (unless it has a full fairing like Vetters with lowers, then there are better places), and this will help ensure the fuel is pushed up the jets to a higher RPM rate. This was common with hi-speed roadracing back in the day: some riders made "breather boxes" to receive these hoses, which were small vented boxes placed in any site where the fast-moving air would not hit them, and the breather hoses were glued into the box. This worked well, too.

On my own 750 (Vetter equipped, and lowers when on long tours) I have the 2 hoses poked into 2 holes in the lower front of the airbox (see my book) to pick up the quietest air possible with stock parts. Don't open up the vents in the airbox: this will only disturb the air as it heads into the carb bells, and this, too, will lower the emulsifier pressures, which is exactly what you DO NOT want to do for higher speeds.
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

Link to My CB750 Book: http://forums.sohc4.net/index.php?topic=65293.0

Link to website: www.SOHC4shop.com

Offline PeWe

  • Really Old Timer ...
  • *******
  • Posts: 8,201
  • Bike almost back to the 70's 2015
Re: CB750 K7 - PD carbs replacement by K1-K6 one
« Reply #9 on: February 08, 2019, 10:50:33 pm »
About cam...
I have measured my cams a with a vernier caliper. Maybe not that accurate, same tool of a good brand on all cams.
I use that 78 (K7) cam in my K2 build since the K6 cam was in bad shape, rusted, pitted. I took what I had. The K7 cam was not much used. The earlier cams are said to be even better.
I have not measured the duration which can be even worse too.

[mm]
K6 -76 original   (sat in my blue K6 since new, replaced by hotter cams)
IN:Base    Lobe+base     Lobe           lift            EX:Lobe+base           Lobe       Lift
14.05        35.95          21.9          7.85                35.3                   21.25     7.2
 
K7 1978 original (eBay USA)
IN:Base    Lobe+base     Lobe       lift             EX:Lobe+base          Lobe       Lift
14.05        35.7               21.65     7.6              35.25                 21.2      7.15
« Last Edit: February 08, 2019, 11:10:51 pm by PeWe »
CB750 K6-76 1005cc JMR Billet block.
CB750 K2-75 Parts assembled to stock K2

 

;