Topic: Carb PD51A. to be, or not to be aircut

[Robert]

Hi,

I'm confused. There are some carb related problems with my CB 650. Keihin PD51A (piston valve type) Today I opened the cap of carb #1 where the aircut valve is supposed to be, and well, there's nothing in there  . I do not want to tear that thing apart to look into the other carbs, I guess this makes more problems than good.
I consulted two exploded drawings of that carb, one of them says there's an aircut, the other says there's none. Does anyone know?

Thanks a lot! Rob. 

[Zeke]

What year?

My 650c 1980 has them in every carb.  I don't know what they do though.

I wish I knew more about these as they're giving me an f'ing headache right now.

Zeke

[Robert]

Thanks a lot Zeke for answering  . The bike is a CB 650 C, 1980 as yours, but I'm not sure if the carb is the original one. If you could take a look at the carb, the number is stamped on one of the outer carbs (#1 or #4). Mine reads  PD51(F)A  If you could tell me what you see, that would be a GREAT help to me.



I read your post "Post Rebuild Carb Tuning" While I'm absolutely no specialist, especially not on carbs, I do have some ideas.
I agree with your friend, that the bike should run OK with pilots 1 1/2 out. According to most literature, the pilot screw is the very last thing to adjust. So it should run good - given everything else is cool - with the default setting. For the PD51(F)A this is exactly 1 1/2 turns out. I also agree on Harry, that you need to dynamically check the ignition with a strobe light if you haven't already.

You wrote that 1/4 sparks are black, while 2/3 are OK. From my experience, this is a very strong indication that there's something wrong with the ignition system. If you have checked all the ignition system already you don't need to bother what I write next.
There are two complete ignition circuits on the CB 650 C: one for cylinder 1/4 and one for cylinder 2/3. In about 80 % of all ignition problems, the problem manifests itself on two cylinders and can be checked by swapping one part a time (pick up coil, ignition unit AKA CDI sometimes (while it is technically not a CDI) and ignition coil). There's one case, where problems manifest only on one cylinder: A partly faulty ignition coil. It has two seperate connectors for 1/4 alternatively 2/3 and it is perfectly possible that #2 fires just fine, while #3 misses.


The strobe is not only good for setting up the timing, it also shows ignition related problems very fast. as you SEE when the ignition system misses (No constant strobe light).

The pickup coil can also be checked this way: Disconnect the 4 pole connector coming from the pick up coils (under the right cover), there should be 500 Ohm of resistance between the two yellow wires and the two blue wires. If possible this test is also done when the machine is hot.

The spark units below the seat can be checked by just swapping the connectors. problems then wander from cyl 1/4 to 2/3 or vice versa.

ignition coil can be checked by swapping the left coil with the right coil.

And one last thing: Since the CB 650 C has a factory preset solid state ignition system that does not misadjust itself easily, it is an absolute necessity to get the timing exactly right. Static won't to it. below 1050 rpm it should look like this:



If machine won't idle at 1050, use the starter for a few seconds with the spark plugs removed, eventually hooked up to a car batterie. Again, the machine is *very* picky when the timing is off.


Robert

[Robert]

Hi Zeke.

I scrolled thru your posts in this forum, and found that you have made very nice picture of your carbs 
According to the pics it seems to me that you have a PD50A. The pilot screw factory setting for the PD50A is 1 5/8 turns out (or 1.625 or 1 1/2 turns and 1 hour 30 minutes - my special clock system, seems easier to me ;-), just in case if you haven't known already.

[Zeke]

Hey Guys, been out in the shop too much.

My carbs are on the bench again.  PD50DWSB -- this is what it says.  I checked them all to make sure they were the same.

The addendum in my shop manual says that for the 1980 the starting point for pilot is 2-1/8 turns out.

Doesn't really matter, as the "idle-drop" procedure will have you turning them out further, then screwing in #1 until just right, then matching all the others to that one.

Anyway, what the hell are the diaphragms for -- then seem to act as a pressure regulator or something for the float bowl -- I don't know, I just had them apart again last night to "make sure" that my carbs are clean, so I paid attention to where the carb clean was coming out of the passages.

Robert, thanks for your suggestions -- I did in fact switch the coils to see if I'd get dark plugs on 2 and 3.  There is a way to test the spark detailed in my manual but I haven't tried this yet -- spark looked OK at the plugs and as I say my bike ran awesome before the valve guide died and I was forced to rebuild.

Zeke

[Robert]

The shop manual is always right! The pilot setting I wrote is from a german repair manual (not from Honda). There are propably differences between the countries and continents too.

Anyway, what the hell are the diaphragms for -- then seem to act as a pressure regulator or something for the float bowl -- I don't know, I just had them apart again last night to "make sure" that my carbs are clean, so I paid attention to where the carb clean was coming out of the passages.

Me to was wondering. I found this greenspun article interesting: http://www.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=00AygV
Especially an article from Mike Nixon:

The air-cut valve interrupts the idle circuit's air bleed, to richen the mixture available from the idle circuit, on trailing thottle. The benefit is that an otherwise ultra-lean mixture is not sent to the exhaust to mix with its constituents and then afterburn (not "backfire"), causing the familiar "pop" sound. The air-cut valve began appearing on Hondas during the 70s, when idle mixtures began to be leaned (I think the XL250 was first). When the diaphragm fails, manifold vacuum can access the idle circuit directly, resulting in a leaner than normal mixture at idle and a richer than normal mixture at high rpm (difference is manifold vacuum can access both air and fuel sides of the circuit, but one is stronger at different rpm due to the circuit's small size). These symptoms of course make the problem very hard to troubleshoot, so write this down somewhere. :-) Happily, the air-cut valve is redundant on properly adjusted carburetors, that is, when the mixture screw is richened up to about 3% CO (carbon monoxide). My advice is to disable the system, thereby permanently eliminating it as a possible problem. This is easily done by simply blocking off the hole in the carb casting that comes from the intake manifold, with a suitably sized piece of rubber inner tube or something similar. Incidentally, the air-cut valve is not an emissions device, but rather a consumer confidence device.

-- Mike Nixon

Couldn't find the article here in the greenspun archives though, but I am thankful that the articles are still available here and catagorized too, must have been a lot of work, so thanks to the good souls behind this work.

Regarding the air cut, there was some more info in the "Honda Common Service Manual" I once found googling.

I have an aircut valve on order and expect it to be here the next days. Will post if it affects idle.

My problem is, I can't sync carb #1 without getting ridiculously high rpm at Idle. (found no way to get it down to 1100 rpm apart from de-syncing carb#1, have tried everything). Know everything about throttle free play, cable problems, fast idle, idle adjustment, Robert knows everything  (yeah right) do now hope it is because someone stole my air-cut valve!!!!

Have made a nice diagramm with a desynced carb#1. (Propably of no use at all, but it's  chic to look at it 
"Vergaser 4" means carb 2-4, "Vergaser 1" means ...ja, the rest ;-)