Thoughts about the 1977-78 pd carbs.

[Lucky 1]

By LUCKY 1

After having spent the last 6-7 months involved with my CB750 project
I have a new outlook on syncing carbs.

The vacuum operated carbs starting in 1979 left no option as far as how you were going to get the 4 slides sync'ed when there was no mechanical or cable connection to the actual slide.
The slide is opeated on vacuum completely up or down and the "down "is helped with a spring.
By drilling the small hole in the slide larger you can get it to drop quicker, but other than that it is all about the friction of the slide and vacuum or possible air leaks.


But the 1977-78 Honda CB750 has a mechanical linkage connected to the slide. It is pulled upwards by a cable and goes down with a cable and a spring. But rather than measuring the vacuum to get the 4 slides in the same position in relation to each other I have discovered that by setting carb slides height #1.#3,and #4 with a measurement (vernier calipers) in relation to the top edge of the carb, that I got much better results than by using a sync gauge! I was getting the 4 slides within .001 thousandths of one another.


Also all the talk about float height and then after the 2nd or 3rd rejetting of the carbs I noticed that the float levels could also be set in a unconventional way with good results and no special tools were needed. Only a steel ruler. But I just saw it by accident. Float is .570 thousandths above carb body and carb body is upside down. No pressure on the float at all.
I will post a photo.


Also the accelerator pump was a new learning experience for me too. The importance of getting that whole system of check balls to work right was so important.

These are just some of the things I learned after working on this 1978 CB750.

[Re-run]

I love the pd carbs. I like the range of adjustment offered by the use of a fuel screw as opposed to the earlier air screw. Many people have been able to 836 their engines and just adjust the fuel screw to accommodate the change. I also believe these carbs are partly responsible for the power the bike gained back in 77 with the idle mix running leaner but giving that squirt of fuel from the pump.

I will have to try the float measurement you showed and see how that works.
The other setting I will have to test. I don't think my carbs will be an issue but worn carbs can affect the vacuum pulled so periodic checking with gauges may still be in order. This is only a theory though.
Still an excellent idea though.

[Lucky 1]

Thanks for checking out my post re-run.

At first I did not like the PD carbs compared to the early carbs
but I am learning more about them.

The float level thing was just something I noticed after taking
float bowls off several times.
Makes me wonder if it might work on other carbs as well.


I was surprised at how good the results were when measuring slide height
as opposed to using vacuum to get a measurement of a mechanical controlled slide.
The vacuum method makes more sense to me on vacuum operated carbs.

[Re-run]

Yeah it does. But again, if you have a carb a little more worn, it MAY end up being off. Again, only a theory but since I have no set of bad carbs, I can't test it. It makes sense since a worn carb will not control air the same and I know of no sources for the slides if they wear out. Carb wear should be minimal though if they are kept clean so maybe it will not matter.

Its a trick worth trying but I will need a gauge better than the crappy motion pro mercury sticks I currently have. A morgan might be in the works sometime.

[Lucky 1]

A Morgan?I will check that out.

[Re-run]

Yup, morgan carbtune. A little expensive but supposedly one of the best you can get.