Topic: PD46 Seats

[bwaller]

Has anyone ever found replacement float needle seats for these 77-78 PD carbs on a CB550. They are stamped with a number 2.0. Must be some other model carb using the same seat??

When I went through these carbs they had been sitting a while. Since there were no replacement seats listed I just installed new float needles. The bike will idle well, but every so often it'll hiccup in the intake and lose rpm before returning to a decent idle. I just rechecked the pilots and they were not blocked, the floats are set to 14mm, and the bike is in good tune.

I'm wondering if there is an o-ring on those seats and if they could be leaking? I checked an extra carb set by spraying a little oil around the seat blocking it with a pencil eraser and blowing air in the gas line... no leaks so I'm not sure if there is an o-ring or if they are pressed in.

Any ideas?

[paulages]

brent- aren't the seats pressed in on the PD46 carbs? i'd guess that if it was leaking you'd most likely notice it while sitting. you're thinking the fuel level is momentarily getting too high, then re-leveling? hmm. i have a junk set at the shop- i'll try and remove the seats and see if there's anything beneath it. no worry if i damage it.

a "hiccup" almost sounds the opposite to me, like maybe the float or valve is sticking up and starving the carb for a minute, then refilling.

[bwaller]

They may well be pressed in and no o-ring because to remove would indeed ruin them. I tested a couple extra carbs and couldn't find one to leak at all.
There is never any gas overflow but it won't sit there and purr at idle either so I'm trying to fiqure it out. The engine is "tight" but as you know not exactly stock either.
The floats definitely don't stick, and haven't seen a needle hang up when checking the rack off the bike but running who knows?

[paulages]

is there a reason you're running the PD carbs? kinda seems a little pointless IMO, as you don't get the advantage of the accelerator pump like on the equivalent 650 and 750 carbs. i do think the PD46s are a better design, but the limited adjust ability seems like it would be frustrating on a non-stock engine. those carbs run so lean i could see the slightest variation in any of the forces at work here causing the hiccup. i've noticed similar rough idle when i was jetted too lean and the bike got a little warm (with 022A carbs). your float vents are running behind the swingarm? is the rest of the carb setup stock?

[bwaller]

Paul,  the biggest reason I used the PD's was that I had a set of NOS insulators for them. I had read they were a leaner set-up than the previous carbs but fiqured surely they could be jetted correctly. In all honesty to complete the package larger carbs would be needed, but it's my wifes bike and it's very quick as is.

When the engine is warm enough to not require any choke it idles perfectly. After it has been run and up to temperature it idles for a bit and then as I called it produces this "intake hiccup" and loses rpm and rattles the primary chain but won't stall.

If I seat any of the pilot screws the bike will stall, but there doesn't seem to be a sweet spot that will help idle anywhere from 1/2 -3 turns out.

In case there are some helpful ideas this engine is 592cc, runs a mild Web street cam with some mild porting, 10.1:1 CR,  PD carbs with the stock intake plenum, but no airbox, rather a single large K&N attached to the back of the plenum, 4 into 1 with a modern can. 

Needles in the middle clip, 107.5 mains, stock #42 pilots. The bike runs very well except for this idle issue which only seems to have become a bit of an issue this the second riding season.

[paulages]

The bike runs very well except for this idle issue which only seems to have become a bit of an issue this the second riding season.

i can't help but feel like that's a key statement. if it were something regarding the "hardware," it seems like it would have been consistent from the beginning, rather than something being a little out of tune but i guess that's what you're getting at by suspecting the float valve seats. you're running a dyna too, right? do all your plugs look the same? that might clue you into something.

[bwaller]

Last night I sat back after searching the subject for a long while and was embarrased for just having asked another bloody carb question just like so many others. I quess I was just lookin for a quick fix. What I'll do now is get to the bottom of it and post back with the solution(s)

To answer your questions though, this bike has a Dyna S, the plugs are all identical and look good. I did remove a float valve seat and of course they are pressed in and wouldn't leak. I talked to a friend this am who has a set of tapered reamers so I'm going to increase the orifice on a set of #42 pilots. It's too easy to fiqure it's a lean condition.....I'll let you know.