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CB750 F2

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K2-K6:
You're right Roy,  in 14 pages of official F2 manual they don't even mention breathers as far as I can see.

Pictures/ schematic show nothing more than external drainage from float bowl vents,  but no specific data about maintenance of the intricacy of anything else.

I worked much more on them when contemporary,  so no real need to strip and replace components just setup.

I can see that opinions of these PD series are based on things like this in justification for an awkward reputation,  but they are a very good carb design and certainly address their predecessors (which are just essentially run rich to cover design limitations) shortcomings in accurate fuel metering with increased delivery for acceleration phase. They are in reality how an engine should be run efficiently and with power when needed.

Just need very diligent cleaning and setting to fully optimise their metering accuracy.

blackeagles:

--- Quote from: royhall on July 15, 2020, 07:36:37 AM ---

There's a small air vent in the side of the carb bodies, if one or more is blocked it's impossible to get even running. The vent is hard to see and cannot be cleaned with carbs assembled. Also, I have never seen that vent mentioned in any manual.

My F2 runs spot on after the second carb rebuild to clean the vents, where previously it would not balance up at all. And don't forget, the air screws work in the opposite direction to earlier 750's.

--- End quote ---

ok, something i didn't clean when they are off.  can you indicate where the 'vent(s) are ?

Bryanj:
What we found in dealer terms was that from the F2 onwards the carbs were set for california emmissions rules and if stood on showroom floor for more than 3-4 weeks without being run they were a swine to start and pilots blocked for a passtime.

royhall:
Depends on your type of PD carbs, there appears to be several different types with no information on them at all. The main bowl vents are between the outside pairs of carbs and are vented through a tee piece and tube to behind the swingarm. Not all 750 PD carbs have these vent tubes. Some carbs also have tiny vents on the sides that need to be clear. Could only find this grainy image as my bike isn't here at the moment for a photo.

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This one belongs to someone on the forum but I cant remember who.

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It's also absolutely vital that the idle jets are 100% clean, 95% is not good enough as it will prevent correct setup. These carbs are a pain because they are a very accurate metering device and any dirt or blockages of any kind upset the delicate balance. The older K model carbs are much more forgiving on setup.

blackeagles:
ok thanks.  mine are different - pd42b - but i have found what  believe is them and if i get a pic of the location i will put it up.

this is a pd42a but it shows the bit i'm talking about:

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left side of carb 1 has one but its a blank, the right side of 4 has none, the two facing each other on the right/left of 1/2 and the right/left for 3/4 have what looks like a blank in the hole but a bit of probing with a calor gas pricker found a hole in the centre of which one was blocked and the other 3 clear.

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