I am a champion of the PD series Keihin carb as fitted as original equipment on some of Honda's SOHC four models: the late CB550, the early CB650, the 750 Automatic and the late CB750. I do not agree with those on forums who say this carburetor is troublesome. I think it is a much better carburetor than folks assume. Easy to work on (the easiest of all the SOHC four carbs), uncomplicated, and a good performing carb. I have a carburetor rebuild how-to book specifically on the PD carb that covers all of these models.

Since the PD carb is found on several 70s Honda models, this brings a host of potential issues into the mix when considering engine running symptoms. While the carburetor may indeed (and I think inevitably) require attention, it is false to assume the carburetor should necessarily be the focus of performance issues. Ignition service, especially up through 1978 models, is virtually assured to be much more in need of proper maintenance than will the carburetor, solely on the basis of correct Kettering ignition service unfortunately being a lost art. And not only a lost art but a forgotten and unaccepted ethos entirely. Owners of these bikes just can't be convinced that the ignition system is as crucial to good engine running as it is. I have tried for decades. Nobody will believe it. Nor do people understand how cylinder compression affects carburetion. Low compression leans your carbs. And all of these old Hondas suffer low compression. Take care of the other stuff before focusing on the carburetors.

A few more things to consider. The idle jet really must be removed for cleaning when rebuilding these carburetors. It is necessary to get the idle passage under the jet clean, and the jet itself can't adequately be cleared without it in hand. You must clear the air bleeds also. If you're not sure what those are, get help. The float bowl venting has to be vetted also. The PD carb has two different kinds of bowl venting. The 77 750 models have a more crude system, the 78 750 is more sophisticated. Both have to be attended to. The pilot screw o-ring must be replaced and the original one dug out of the carb body or the engine idle will not be consistent. And set the pilot screw to 3 turns out. All PD carbs on all stock bikes need this. The 77 750 carb has an adjustable jet needle and the 78 750 not, though the 78 is easily adjusted in a non-conventional way. However, needle adjustment is very rarely a good idea--basically only when engine modifications are in view. This carburetor is not, as many insist, wrongly jetted at the factory. Folks who say this have bikes with either modifications or with neglected maintenance, with the latter being a given. Get the circuits clean, use the factory specifications--except for the pilot screws--and sync them carefully and these carbs will work very well for you. But this assumes you have good intake sealing, over 150 psi cylinder compression, fuel that is less than a month old, the stock intake system (air filter element, box, ducting, cover, etc.), stock exhaust, good ignition coils, good plug caps, clean genuine NGK plugs, and the timing set properly (both statically and dynamically) using factory points. On properly modified and set up 750s, these carbs really shine. The older carbs don't come close.

Many seem to angst about the float level. To begin with, set the float level with the carburetor right-side-up. Use the factory spec--the 77 and 78 750 carbs have different specs--and don't view the float hanger's angle as anything important. The forums' insistence that the hanger must be perfectly horizontal is wrong. It has no bearing on correct adjustment. Many carbs' floats when properly adjusted do not have their hangers precisely horizontal. Some models do, but many more do not.

More don'ts. Take these to heart, they're important.

1. Don't use carb rebuild kits.
2. Don't use Pine-Sol to clean your carbs, especially in an ultrasonic cleaning machine. If Pine-Sol will get your carbs clean, so will paint thinner. Meaning, it won't. Do the cleaning correctly.
3. Don't reuse the choke plate screws if you have removed them (usually to unrack the carbs). The screws will be heavily burred and this will ruin the choke shaft threads. Those shafts are very pricey to replace.
4. Don't unbolt the throttle bellcrank. There is no reason to, and it will just add complications to reassembly.
5. Don't test your PD carbs' floats by dunking them in water. They are solid, not hollow.
6. Don't reuse the o-rings in your carburetors if you are unracking them. Replace with new.
7. Don't clean your jets with one of those gas welding torch type "carb wire" sets. They're actually metal files and have no business anywhere near your carbs.
8. Don't bang on the float pivot posts.
9. Don't install your idle jets with thread locker.
10. Don't abrasive-clean your float valve seat.
11. Don't neglect to clean and vet your carbs' air bleeds.
12. Don't forget to use an aerosol cleaner on all of the many circuits in each carburetor body (casting). The idle circuit by itself has more than five openings.
13. Don't regard the factory-specified float level as merely a suggestion.
14. Don't overlook the adjustment for the fast idle mechanism.
15. Don't lose the lockwashers on the special shouldered slide lift arm screws, and don't lose those special screws.
16. Don't use a drill bit or anything else under the carburetor slides to bench sync them. That wasn't even a good method when it was done on Brit bike carbs.
17. Don't use a mechanical vacuum gauge style sync tool unless each gauge has a calibration screw.
18. Don't worry about ethanolized fuel--it's a non-issue.
19. Don't use gasoline to leak test your carbs.
20. Don't forget to run Sta-Bil all the time, full-time.