Topic: franken-carbs? Well, it's an idea anyway (I didn't say it was a GOOD one... )

[alacrity]

Before I get too deep - can anyone easily shoot me a link that shows all the measurement specs for the internal parts and passageways for these two carb types?

PD46a and PD50b

Onward:  So here's the idea.

I am doing a cb550 performance-oriented cafe (unlike the focus-on-pretty cb500 I did).

I have a rack of done/perfect CB550 carbs - pd46a. Several hours work getting these all gorgeous. Yes I could just bolt these on and be done.
Too easy.

I want an accelerator pump.

I also have a set of PD50b carbs from a 1980 cb650.

I'm thinking of doing a bastardized mix-match to get the best functional four from this gang of seven SOLELY because I have these pieces in my garage right now - a perfect set of pd46a carbs and three good pd50b carbs I bought recently and had shipped to me (#4 had a completely seized pilot mix screw - the PO overtightened and cracked housing around that as he (she?) seized it in place.  And then to "finish" the job, the usps cracked the head of the carb body when they smashed it in shipping. Yeah... it wasn't packed well.  That body is garbage now). I guess I should feel grateful all the damage was on that one carb, right?

So options could be
1 -- most obvious:  BUY a loose pd50b #4 carb.  Been trying that for two days. Cant find one. I don't wanna buy a whole new rack for over $100.

2 -Idea here is to borrow whatever linkage bits and tubing etc I would need to make 1,3 and 4 from the original (and done/prepped) pd46a set work with the bastard pd50b #2 carb to get the accel pump function.

3 - reverse idea: use the three "good" but needing restoration pd50b carbs and sandwich on the #4 pd46a carb.

Downside of the 50b carbs is the epa crapola... including non adjustable needles.  But good news grasshopper, I measured and they are same as previous needle - seem to be same taper under magnification too. I can just chuck the fixed ones and insert 4 adjustable ones, or shim the fixed ones.


SO  - questions:
Has anyone tried this?
Is it just beyond stupid? The pd46a have those magnesi-aluminum alloy slides.  The pd50b have chromed slides.
The pd50b have a bunch of extra air lines and tubes flowing into the venturi (smog stuff i believe). Not sure how much that would affect running.  the jet positions, bowls, needles, seats, etc otherwise seem identical.

Please don't just tell me I'm an idiot for even considering this.  I am already well aware of that.  Please DO tell me if you KNOW why it could or could not work. That's the kind of thing I'd find very helpful.
Thanks.
:-)

[TwoTired]

The PD 46 carbs have 22mm venturis.  Aren't the PD 50 carbs 26mm?

[alacrity]

Oh... and if it wasn't obvious in all the previous prattling I did, IF anyone HAS a good #4 pd50b from a 1980 cb650 they want to sell, I think they just found a customer.

:-)

[alacrity]

Two-Tired:
Actually they are both 26mm on the engine side.

The Pd50b is a few mm larger than the pd46a on the intake (choke) side. I could go measure again but it's something like a 5mm diff.   I figured MAYBE I could tune around this since the business end was identical...
And if the oddball carb is the one "fatter" intake side and the accel pump, maybe that makes the most sense to do it that way.
I don't have the benefit of a flow bench to see how the different intake bell size affects that comparatively, but I could perhaps throw ballpark idea at it and jet the pd50b carb down a step (main and pilot)...   Maybe?

[TwoTired]

Two-Tired:
Actually they are both 26mm on the engine side.

You don't understand.  The inlet and the outlets are not the venturi.  The venturi is located where the slide is.  And it is bigger for the PD50 by 4mm.  The venturi section is the smallest restriction in the carb air duct when the slides are wide open.

I'm afraid it is very unwise to mix the two carb sizes on the same engine. 
Further, unless you are boring the engine nearer to 650cc, or intend to run at higher RPMs (a lot), a bigger carb throat will diminish streetability for the lower throttle positions.  I'm not saying it won't work at all.  It just won't have as good throttle response at lower RPMs.  The bigger venturis lower the airspeed through the carbs if keeping the displacement and cam the same.

Now it should be possible to adapt the accelerator pump circuits from the PD 50 to the PD 46.  You'll have to drill the bodies to make the distribution work.  Install the squirters, etc.  Then there is the linkage to adapt.  What it will get you in the end is a throttle that will take any sudden throttle of any magnitude of throttle position change.

[MiGhost]

The PD 46 carbs have 22mm venturis.  Aren't the PD 50 carbs 26mm?

Yes, and for reference.

PD50 Carb Specs CB650 79-80, CB650C 80
PD50A JET, MAIN (#90) SLOW (#35) slide is 56 B (small air cut out) needle - 59 A (high clip position)
PD50B JET, MAIN (#90) SLOW (#35) slide is 81 B (larger air cut out) needle - 35 B ( lower clip position)
PD50C
PD50D JET, MAIN (#95) SLOW (#35) slide ?? needle ??

Actually they are both 26mm on the engine side.

The venturi is in the center of the carb where the slide is. If you look into the throat of the 46's you will see it narrow down from both ends.

The Pd50b is a few mm larger than the pd46a on the intake (choke) side.

Air filter opening specs
PD50B
OD 46.0
ID 42.5
V  26.0

PD46C
OD 38.0
ID 33.5
V  22.0

Due to the differences in airflow, and capacity. It is not a good idea to try to mix the carbs. It will be impossible to vacuum sync the rack correctly, and the imbalance will cause erratic running issues.

Check with member harisuluv for the carb body that you need.

[MiGhost]

SO  - questions:
The pd50b have a bunch of extra air lines and tubes flowing into the venturi (smog stuff i believe). Not sure how much that would affect running.  the jet positions, bowls, needles, seats, etc otherwise seem identical.

To explain some of the extra stuff that you find on the PD50 carbs

Accelerator Pump:
The brass tube sticking straight up just to the right of center will be the discharge nozzle for the accel pump. Fuel is supplied from the pump which is located on the #2 carb float bowl, and fed through the small gray tubes located just below the throat opening.

Float bowl vents:
These will be the rubber T's located between carbs 1-2, and 3-4. The two hoses are then routed back to the top of the air box area. Just like the PD46's These vents supply atmospheric pressure to the float bowls.

Air Cut off Valve:
The brass tube along the left edge of the throat opening at approx the 9 o'clock position is the air feed for the cut off valve. This valve is located under the cover on the left side of the carb body, and is made up of the diaphragm, return spring, vacuum passage o-ring, and cover.

There will be 3 passages from this chamber into the carb body. 1. The air feed from the carb throat is in the center hole of the chamber. 2. The vacuum source located to the top left of the chamber. 3. The slow jet passage. This connects with the slow jet air bleed (Left side of the 2 air jets in the bottom of the throat)

The air cut off is an emissions device that allows extra fuel to be supplied when the throttle is suddenly snapped closed creating a momentary lean condition in the cylinder that does not allow the F/A mixture to be burned completely. This unburned mixture then ignites in the exhaust to cause a popping sound.

Once the carbs are tuned correctly, and the lean burn condition is corrected. This cut off valve is no longer necessary. To disable the cut off valve operation you simply block the vacuum source passage in the top left. If the diaphragms are worn, and you do not wish to replace them (They are expensive, and not necessary any more). Block off the slow jet air passage in the bottom of the chamber also. It is not necessary to close off the brass tube in the carb throat. Once the other two passages are blocked off. The tube only supplies air to the empty, nonfunctional chamber.

All other jets will be located the same on all the PD series carbs.

[alacrity]

Thank you all.   
I should have mentioned that the engine is now 600cc and I am running the cb650 cam.

But in any case. The dumb frankencarb rack is an officially dead idea.   I hope it provided some amusement.

[alacrity]

Ok... so thank you again.
Fully chastened am I.

I so have a lingering question for MrTired.
You wrote:

" What it will get you in the end is a throttle that will take any sudden throttle of any magnitude of throttle position change."

If I understand your point.. This is a good thing.  As opposed to the way the pre-accel pump carbs would allow the engine to fall on its face if you wound in too much throttle too fast at the "wrong" rpm and load.
Right?
Maybe I should get a used #2 pd46a from harris as a spare in case I mess up the one I have trying to make it work

[TwoTired]

I so have a lingering question for MrTired.
You wrote:

" What it will get you in the end is a throttle that will take any sudden throttle of any magnitude of throttle position change."

If I understand your point.. This is a good thing.  As opposed to the way the pre-accel pump carbs would allow the engine to fall on its face if you wound in too much throttle too fast at the "wrong" rpm and load.
Right?

Yes. 
It also allows you to set the idle for the very leanest setting, which would save fuel and keep the plugs pretty clean during prolonged idle, too.

Maybe I should get a used #2 pd46a from harris as a spare in case I mess up the one I have trying to make it work

Take a better look at the PD50s you have.  Notice that while the pump is at #2 station, all the carbs have a squirt nozzle in the inlet ahead of the choke plates.  That one pump feeds all four carbs.  So, if you wanted to mod the PD46's for accel pump operation, all the carbs would have to be modified with drilled passageways, to accommodate the routing of accel pumped fuel and to accept those squirt nozzles.  The carbs would also need an extra accel pump fuel rail linking the carbs together.  These fuel links would most likely need restrictor orifices, as the PD 41 and PD42s have on the Cb750s (probably the PD50, too).  These restrictors even the pressure for each carb so they will shoot the same volume of fuel into each carb throat.  Without them, # 2 carb will get more enrichment than 1 and 3, and 4 will get less than 1&3.

Did I say it would be easy? Ha Ha   

Since you have the engine punched to 600, and have a better breathing cam, the PD50s will probably work pretty well.  Do note that the inside walls of the intake ducts are all the same diameter, rubber couplers included.  Any steps in the duct walls will create turbulence and erratic  tune characteristics for certain RPM ranges.  I really don't know how well the PD50s mate with the 550 intake manifolds and couplers.  I do know that simple straight rubber tubes that fit the outside of the carb flanges and manifolds, won't get you a smooth walled internal duct surface.

[alacrity]

Thanks for taking the time ...   I am looking at it all very closely and it seems like I will at least need to create some sort of adaptive manifold/"boot" system to connect the PD50 carbs...

So what I am gonna do FIRST is get the beast running well with the stock carbs.  And then, later, see about making this pd50 rack work correctly.

Pulled an all nighter... got the front end and the back end on.  Finishing the electrical is next.

[754]

You want to do all that to..save..100 bux..MINUS the price of a single carb..?

 I am not believing this..