Topic: Solved: K7 PD carburetor on K6 Engine?

[Kaze]

My K6 engine has stock 76' KeiHin 086A carbs. I also have a set of K7 PD carbs.
Would there be any performance benefit to putting a set of PD carbs on an engine like mine, or are they really only benefitial when paired with the whole K7 (F "1") engine package?

Answer: Not really.

[scottly]

Same same, except PD has an accelerator pump.

[Kaze]

Thanks Scottly.

[bryanj]

Personaly i would say no benefit at all and nothing but problems

[BenelliSEI]

I have a K7 and a K8, both on their stock carbs. I don’t have any problems with them but it does seem that everyone else thinks they are a pain.

[bryanj]

But i guess from looking at your pic they get run relatively regularly

[Kaze]

BenelliSEI: That's a clean machine. 

Thanks guys. I was just wondering because the K7 (first year SS) are clearly quicker than the previous couple years. I know there's a different cam, sprockets, and F had that nifty "bazooka" exhaust etc. to add performance but I assumed that at least some of that power must be attributed to the carburetor design. So PD just dealt with boggy acceleration, huh? Learnin' all the time.

[bryanj]

The PD was built because of the emission rules in various state that meant the engines had to run a lot leaner.
We found that if a bike was on the showroom floor for up to 2 weeks you had to drain the carbs, much over and they had to come off to be cleaned BUTwe went from K2 direct to K7 in uk so pd was mostly F2 750 and K3 550

[newday777]

BenelliSEI: That's a clean machine. 

Thanks guys. I was just wondering because the K7 (first year SS) are clearly quicker than the previous couple years. I know there's a different cam, sprockets, and F had that nifty "bazooka" exhaust etc. to add performance but I assumed that at least some of that power must be attributed to the carburetor design. So PD just dealt with boggy acceleration, huh? Learnin' all the time.

The FII has bigger intake valves, different cam to help get more out of the motor, not just the carbs being PD.

[HondaMan]

The K7/8 has higher compression (9.2 or 9.5, depending on which literature you're looking at) and a lower primary-drive internal ratio (as do the F2/3 engines) so they rev higher at the same speed/gearing as the K6 and earlier ones. The F2/3 and K7/8 cams are made with later intake valve opening (0 degrees) and less valve overlap duration, plus more scavenge time, to lower emissions: the PD carbs became necessary then to prevent a nasty "flat spot" in the performance just off-idle when accelerating. This "flat spot" shows up clearly when the accelerator pump's diaphragm gets old and stiff, causing the engine to stumble whenever the throttle is opened sharply, and especially in the lower gears. The tradeoff for all these changes was a handful more HP but with a handful less low-end torque below 4400 RPM as compared with the earlier ones. The increased HP at 80+ MPH is welcome, although some SOHC4 riders from the "old days" complained of these later ones feeling "buzzy" at hiway speeds when they traded up to a newer one.

Heck, compared to modern smallbore engines, that's nothing! I hear 14k often in my neighborhood.

[Kaze]

Thank you all for taking the time to lay that out. And thanks Hondaman for the homework. Your replies are often like a lesson.

Guys who remember the "old days" are important. I never rode any of these when they were new, and a lot of you did...or raced them, fixed or sold them. Many of us appreciate your sharing these things.

Until recently I just assumed that the CB750 would have gotten better and faster as the years went by. Now I hear that 69' and 70' were actually the performance years, it seems rather backward. Honda must have detuned the following years for a reason... ?

UK went from K2 right to K7? Was there just no market? UK had enough cool bikes without Honda I suppose. Those bikes sound like sort of a handful on the showroom floor. I just thought they sat there peacefully waiting to be bought.

[newday777]

Thank you all for taking the time to lay that out. And thanks Hondaman for the homework. Your replies are often like a lesson.

Guys who remember the "old days" are important. I never rode any of these when they were new, and a lot of you did...or raced them, fixed or sold them. Many of us appreciate your sharing these things.

Until recently I just assumed that the CB750 would have gotten better and faster as the years went by. Now I hear that 69' and 70' were actually the performance years, it seems rather backward. Honda must have detuned the following years for a reason... ?

UK went from K2 right to K7? Was there just no market? UK had enough cool bikes without Honda I suppose. Those bikes sound like sort of a handful on the showroom floor. I just thought they sat there peacefully waiting to be bought.

Detuned because of EPA involvement

A bit of the history by year changes in the FAC section of the forum

http://www.sohc4.com/cb750/cb750k/

[bryanj]

Nope in uk we just liked the K2 better, same as 500 we only ever had 500K1 until the 550F and K3.
Supose uk is smaller market than us

[Kaze]

BryanJ: I completely glazed over the significant population gap between the UK and USA too, which obviously affects the market. I'm that "Can't see the forest, because it makes me sneeze." type of guy.

NewDay777: That's a good link. Is there a full list of changes somewhere? I typed a list of K7 changes for my own reference while working on this current bike, and it was very long. I'll bet the list of changes between 1970 and 71 must be huge.

I always thought the sandcasters had a "finless" oil filter. I could swear it was in John Wyatt's book, but I don't have it anymore so can't be sure. I've read recently that it was a sham to get people to buy fake parts on Ebay... what do you think?

[bryanj]

First 750's had unfinnef oil filter casing but people broke them by overtightening so it was modified

[Kaze]

Okay, so it WAS real. Thanks.

[Don R]

  My brother rode and worked at a Honda shop when these bikes were new. The way I remember it, Honda had a problem with guys that didn't bother to learn how to ride bought a new 750 and promptly wrecked it. The insurance companies were complaining to congress to do something about these dangerous machines. The fast fours got a reputation much like the GSXR's did later.
  That, as well as exhaust emissions had to do with the tuning changes, Honda in some cases allowed the tolerances like compression and cam timing to slip, some things like rougher ports helped production, some helped emissions, but the result was lowered performance.
 When Kawasaki was stealing the sales, Honda did restore some horsepower. My opinion is it was too little, too late and there should have been a SOHC CB900. My brother went on to work at the Kawasaki shop and for a couple years the 900 LTD was king around here.

[Kaze]

I like the K7 a whole lot, but it does feel a bit like something Honda came up with for waning sales and changing times. Style wise it has a very late 70's vibe, especially that gold cursive "750Four".

CB900K would have been cool. The C and F are fun (I loved riding the 900C) but totally different market I reckon.

Not actually Honda parts, but have you come across the retro kit for the CB1100? It's cool, but I'm in the wrong income bracket for $6,000 worth of eye candy. https://www.samurider.com/?p=24221/#itemImage