Topic: Add accelerator pump to a set of 550 PD carbs?

[Flying J]

What do you think? Both my 750 and the 550 i owned had PD carbs. The carb bodies are basically the same. All the holes for the accelerator pump are on the 550 carb they just are not drilled out all the way. So i got to thinking. Could I finish drilling out the holes and put an accelerator pump on my 550 pd carbs? Is it worth it? I have all the parts to make the swap accept the bowl with the accelerator pump. Used a little heat and some pliers and got the parts out of the #4 carb. Here is the spray nozzle and plug as well as the inlet tube.


What are your thoughts?

[harisuluv]

I see what you're saying.  I guess it could be done, but the implementation I think would be a lot harder than the theoreticals. 

[1stgenxxx]

If your carbs are tuned right....You shouldn't need one.

[Flying J]

YEs you dont "need" them, but it would be an added benefit even if your carbs are tuned right. Also im not putting these on a stock motor.

[TwoTired]

I haven't looked at the two carb sets side by side.  But, it seems like the pump actuator might be the hard part.

Still, anything's doable with will, time, and money.

Cheers,

[Flying J]

Right. The pump actuator mints to a hole on carb 2. The same hole is on the 550. The only difference may be the distance from there to the bowl. I'll take some pictures for comparison. Its crazy how similar these 2 are. I don't think it would cost much to convert. I would need a set of small drill bits and the #2 bowl with an accelerator pump.

[mrrch]

Does the throttle shaft have the arm that pushes down on the accellerator pump shaft?

[eastyork1977]

Accelerator pumps are usually found on 550F/750F models. Why not just get an F engine or carbs?

Could be a lot easier. I'd do that or get oversized jets.

[Flying J]

eastyork, welcome to the forum. Have a look around and learn a bit before you start giving advice. The 550 never came with an accelerator pump, K or F. THats why im looking at modifying one. Changing out the engine to an F model? ITs the same engine as the K.
Get oversized jets? Do you even know what an accelerator pump is or does? Tag along and learn something.

So, moving on. THe 750 and 550 carbs have the same body and could even be racked side by side if it were not for the throat diameters. Top bar and choke bar all line up.


To add the accelerator pump you need the linkage that pushes on the accelerator rod. seen here on the 750 carb.Its the thing that mounts in that small hole, right center of the picture.


It mounts to a hole. Same hole same spot on seen here on the 550 carb.


That is connected to the pull cable track which has some extra pieces. But I measured them both and they are the same width and the shaft is the same diameter so you can just swap the 750 pull cable track onto the 550.

750 on top


the 750 an550 have all the same holes. 550s just have not been drilled all the way. THe extra hole on the 550 carb on the top right is the hole for the accelerator pump shaft since this is carb #2


The tube that runs horizontal. this is the 750 #4 carb so it is a dead end.

550 not drilled out


750 this is where the little spray tube goes in and then a plug close off the hole.


This is the same hole on the 550. It alread is drilled out to fit the plug and just needs to be drilled further to push the spray tube in.


I have a set of Extra 550PD carbs on the way i think i will try this on. I do need the bowl from #2 on a 750 or 650 is anybody has one.

[mrrch]

All the workings are there, go at it and say you did it. I don't have a 550 but will be curious to find out the results in the end. I may have a spare #2 bowl (I'll try to remember to check tonight) You could always try member SOHC Digger as well.

[TwoTired]

Are PD 50 carbs that hard to find?  (I haven't tried).

[lucky]

Even if you did all that is needed then you would have to go back over all of your jetting again and you may never get the right combo.

A complete waste of time.
Only the 1977-78 models had that feature.
A ll of the other models 1969- 1976 did nit need that.

BTW do not forget the restrictor tubes that go inside the rubber hose connecting all of the spray jets.

[mrrch]

Nothing wrong with experimentation, don't know the outcome until someone tries.

[Flying J]

I don't think a short spray of fuel when you open the throttle its going to throw off the jetting. I will also have a look in the tubes to see if restrictors are still in there. Should be they are the stock tubes.
Also even if it doesn't work it is any easy mod to reverse.

[Deltarider]

Interesting project. I admire your initiative.

[Don R]

Might just work. sounds like you've thought it through. I'm wondering why the carbs were cast with a provision for something not used. Maybe it got cancelled due to bean counters and cost cutting.

[TwoTired]

You will be able to greatly increase the pilot circuit air supply.

I once tweaked the air screws on 022a carbs for max idle speed and minimum exhaust gas hydrocarbons at each exhaust outlet.  Setting was outward about 5 turns.  Beautiful idle.  Completely awful throttle response as ANY twist made the engine wheeze and essentially shut off.  Accelerator pumps would have cured this, I'm certain.
I had to go back to factory air screw setting to be able to actually drive the bike.  The older carbs are actually quite piggy at idle.  Which is why they were phased out of production.  Simple and forgiving of untrained meddling. But, horribly inaccurate for providing exact engine needs.

Cheers,

[Lostboy Steve]

Well, seeing as though I accidentally have a set of 77-78 750 pd's from a purchase on ebay, maybe I too should jump into this fire with my 77 550k. I don't know what the real benefit will be though.

[lucky]

Well, seeing as though I accidentally have a set of 77-78 750 pd's from a purchase on ebay, maybe I too should jump into this fire with my 77 550k. I don't know what the real benefit will be though.

I am sure someone will jump and say DO IT.
They have all the fun of sitting back watching thee show.
Remember if you drill one hole wrong or something cracks or
breaks there goes a good carb body.

The stock 1969 carbs ran GREAT!! I was a dealership mechanic at that time, on these bikes. I know.

[Flying J]

If you drill a hole wrong it won't matter. It may make the carb useless for the conversion but will not affect the function of the rest of the carb.

[Retro Rocket]

Geeze Lucky, if everyone bailed out at the thought of something going wrong we would all still be living in caves. Good project FFJ, this is the sort of stuff that peaks my interest in this forum, sometimes the most rewarding part of fettling anything is making something yourself, whether or not it is successful isn't really the point, its the satisfaction of using your mind and skill to do it yourself, continue on the road to discovery mate.....

Sometimes the journey is more important than the destination...

[Flying J]

Well I picked up a spare set of PD carbs for this experiment as well as a #2 bowl for the accelerator pump. I also picked up the mill bits i need. They dont work well in my dril chuck so I'm waiting on some collets to come in the mail. Then I will try the bits out on some scrap to make sure they are the right diameter for all the different holes. After that i will practice on an old carb body. If all goes well Ill take apart my spare rack and go to town.

[alacrity]

I am picking up this old thread on purpose because I am considering precisely this same modification.

I got a rack of 77 cb750 pd42a carbs basically for free with the accel pump on the #2 float bowl.

I have a 77 cb550k with its pd 46a carbs.
I have bored the barrels to 600cc total, put compression ratio should be no higher than 9.5:1.
I will be running one of Carpy's 4-1 pipes and (mostlikely) will use the stock airbox with a UNI or similar filter. I might consider adding some holes to the airbox.
I am thinking I'll start with a set of 120 mains (but I have 115, 110 and 105 mains standing by).
I have the #42 pilot/slow jets installed.

So... I wonder what happened to Flying J's project.  I've been a bit frustrated with the slow way my cb500 takes throttle from standstill when it's otherwise correctly tuned.  Once the juice is flowing, it does what it's supposed to do... and in fact, given the physics involved it's ALREADY doing what it's supposed to do, right? I just don't like it when I'm used to the RIGHT NOW WFO experience of modern bikes.

This could be an elegant (and basically free) solution that gets a small step closer to approximating that twist bang go feeling I've come to love.

What am I missing logic-wise here? 

thanks

[crazypj]

Accelerator pumps are usually found on 550F/750F models. Why not just get an F engine or carbs?

Could be a lot easier. I'd do that or get oversized jets.

 That's a first for me, never seen a 550 F1 or F2 with accelerator pump

[MRieck]

Even if you did all that is needed then you would have to go back over all of your jetting again and you may never get the right combo.

A complete waste of time.
Only the 1977-78 models had that feature.
A ll of the other models 1969- 1976 did nit need that.

BTW do not forget the restrictor tubes that go inside the rubber hose connecting all of the spray jets.

Really?

[Bootlegger56]

I like the sound of this and definitely will follow.  My thinking is it would help the hesitation present on aggressive throttle opening that seems to plague the older style carbs.  But with my limited knowledge I could be way off mark.  I apologize for not having anything to contribute but look forward to being edjecated!!

[Flying J]

Mission has been aborted. I bought all the mill bits in the sizes i could but after drilling some test holes they are not the right size. With holes that small they have to be perfect or the little jets slip right out. It is absolutly possible i just lack the correct tooling. 

[crazypj]

probably need to drill undersize and ream?

[bollingball]

probably need to drill undersize and ream?

+1^
ffj You have come so close try again. The place for your tooling is a watch repair shop and they are pretty reasonable the old timers are few and the new guys are just like most bike mech. they just don't know how to work on them I will dig up a link. A used set is the way to go Most of the old timers took good care of there tools. Google watch repair tools I know this because my father in-law bought a set awhile back.
Ken

[Bankerdanny]

Accelerator pumps are usually found on 550F/750F models . Why not just get an F engine or carbs?

Could be a lot easier. I'd do that or get oversized jets.

Actually no. All years of the 550F have the same carbs as the 74-76 550k's (well, not exactly the same, but functionally identical), no AP. '77 was the last year of the F and it didn't get the PD46's that were on the K bikes that year.

[bollingball]

Even if you did all that is needed then you would have to go back over all of your jetting again and you may never get the right combo.

A complete waste of time.
Only the 1977-78 models had that feature.
A ll of the other models 1969- 1976 did nit need that.

BTW do not forget the restrictor tubes that go inside the rubber hose connecting all of the spray jets.

Really?

He did write this last year maybe he has learned something in that time. I sure don't see how the jet size has anything to do with this. Lucky if you read this could you come back and explain?
Ken

[iron_worker]

It won't affect your main jet selection, however, it will most certainly affect your idle jet size and probably neelde position too. You may even need to mess with your slide cutaway to get the transition just right.

As TwoTired pointed out, the idle circuits on carbs with no accelerator pump are *intentionally* set over-rich. This is so when you whack open the throttle quickly there is enough fuel there to maintain combustion. If the idle were set to a stoichiometric ratio of 14.7:1 then as soon as you whack the throttle, the vacuum in the carb throats drops off very suddenly. The differential pressure between the low pressure in the carb throat and the atmospheric pressure in the carb bowl (vented) is what drives the fuel up and through the jets. Thus as you quickly open the throttle the bike has a tendancy to go lean suddenly ... and if not compensated for it will hesitate due to a lean misfire.

The accelerator pump compensates for this effect by adding extra fuel whenever the throttle position opens up.

There would most definitelely be some trial and error in tuning the off-idle transition.

IW

[bollingball]

It won't affect your main jet selection, however, it will most certainly affect your idle jet size and probably neelde position too. You may even need to mess with your slide cutaway to get the transition just right.

As TwoTired pointed out, the idle circuits on carbs with no accelerator pump are *intentionally* set over-rich. This is so when you whack open the throttle quickly there is enough fuel there to maintain combustion. If the idle were set to a stoichiometric ratio of 14.7:1 then as soon as you whack the throttle, the vacuum in the carb throats drops off very suddenly. The differential pressure between the low pressure in the carb throat and the atmospheric pressure in the carb bowl (vented) is what drives the fuel up and through the jets. Thus as you quickly open the throttle the bike has a tendancy to go lean suddenly ... and if not compensated for it will hesitate due to a lean misfire.

The accelerator pump compensates for this effect by adding extra fuel whenever the throttle position opens up.

There would most definitelely be some trial and error in tuning the off-idle transition.

IW

IW Thanks for explaining that. So I guess when the EPA came in on the 77 lean setting of the pilot circuits of different bikes that is why Honda put on the pump. Would that be correct?
Ken

[754]

FWIW. I think it was Sudco sold an accellerator pump kit, mounts outside carbs , and you need a spltter for the added cable..