CB650 carbs

[Raymond_Y]

Hi, I have a 1980 CB650 engine that I intend to use for a future care racer project and have a question on the carbs.  I have a couple of sets of carbs (069A and PD46) which I hope I can use for the project.  My question is around the carb size.  All the specs I have found suggest a 1980 CB650 had PD51 carbs that were 26mm.  However, if you measure the internal diameter of the inlet stubs they are 30mm, which seems strange.  Could I use the PD46A’s or should I consider perhaps taking the opportunity to fit bigger carbs?

Raymond

[andy120t]

I use carbs from a GPz600 on mine and it runs OK with those and four separate air filters.   I seem to remember there were a couple of different heads / inlet ports spacing when I was investigating.  As it was built up from a box of parts I didn't ever have the orginals so I'm not sure of specs for those.

I don't have a link to the info I used at the time, but it's somewhere on  the US SOHC website - they have a lot more cb650 stuff / interest on there.

..and I just searched 'gpz carbs on cb650' in google and it brought up a few threads and youtube videos that could help. (eg) http://www.dotheton.com/forum/index.php?topic=68816.210

Hope that helps! 

[Raymond_Y]

Thanks for the input.  Fortunately, I have managed to source a set of PD51A carbs that I can refurbish.  Did not expect there to be any available as the CB650 wasn’t very popular.

Raymond

[hairygit]

Thanks for the input.  Fortunately, I have managed to source a set of PD51A carbs that I can refurbish.  Did not expect there to be any available as the CB650 wasn’t very popular.

Raymond

Truth is PD carbs are even less popular than the CB650, most owners quickly look for different carbs, especially on 550K3's and 750F2's.

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[Raymond_Y]

What is it specifically people don’t like about PD carbs.  Is it just that they are a bit more difficult to set up accurately, because they are a “lean” carb to meet emissions regs?

Raymond

[Nurse Julie]

What is it specifically people don’t like about PD carbs.  Is it just that they are a bit more difficult to set up accurately, because they are a “lean” carb to meet emissions regs?

Raymond

They are a pain in the arse if the bike is not ridden regularly, ie, daily. They need constant fiddling with. Best place for them is the USA or in the bin.

[matthewmosse]

I loved my cb550 carbs. Mpg especially on motorways was pretty good, Fort William across the A9 then down to nearly Whitchurch, Shropshire on a tankfull one time - could be s frugal beat

[K2-K6]

As above,  not liked because they have very fine jets and air passages to get clean if not running. Some have pushed in jets too, making routine dismantling and cleaning harder than straight screw in types.

However, the jetting strategy is a better way to run an engine in pure, pedantic accuracy terms. They are referred to as lean burn,  but really they are more towards optimum fueling for constant running with accelerator pump facilitated extra fuel to accelerate when needed. 

"Conventional" carbs have to be run richer than ideal constantly to make sure they don't go too lean on snap throttle opening,  in effect the inverse of PD carbs.  Effectively running too rich under most light load conditions and resulting in more unburnt fuel passing through the engine.

It seems odd to say it,  but most advancement in engine oil condition has come from running jetting leaner so that less unburnt fuel is washed past the rings to dilute the oil.  It also impacts exhaust life too, with the liquid deposits containing significant acid concentration the more fuel is left unburnt. That should be of concern to those running new sets of four pipes on these bikes given the cost they are, along with limited mileage. None of the pipes being in any way protected from corrosion internally.