Topic: Question for the PD42 carb guys ('77-'78)

[flybox1]

The washers I am using are the same diameter as the circlip, so the "pocket" height is not a player (they sit down inside the pocket, just like the circlip does).

Cool. You're on the right track...

[lucky]

I'm not following your logic here Lucky. From what I have read, one clip on the older carbs is equal to 1mm. Each washer is .0195" which is roughly .5mm so two washers is the equivalent of one clip position, correct? Prior to adding a shim, there is a small circlip that holds the needle in position on the slide. There is a spring that seats it firmly in place. When you add a washer, the needle is raised the thickness of the washer, no more, no less. There is no extra .012" in the equation, at least not on my carbs. 

The M2.5 washers that I am using are not the brass ones in your pic and are the same diameter as the circlip, so  I can't see them getting stuck .

Edit: Now that I have studied your pictures, I think I see the disconnect. There is no way to take a picture like that unless I glued the needle onto the spring or used some bigger washers that sat on top of (or got stuck in) the bracket that screws onto the top of the slide.  The washers I am using are the same diameter as the circlip, so the "pocket" height is not a player (they sit down inside the pocket, just like the circlip does).

You are correct.
In your case if the OD of the washers is the same as the clip you will not have to add the .012 thousandths (deepth of the the pocket) in your case.

So you could use two of your shims .

[Stoli]

The rain stopped this morning so I was able to test out the new configuration.

Recap:

APE Pods
Mac 4-1 header w/ baffled Mac muffler
Slow: 42
Main: 135
Needles: 1mm shim (2 M2.5 washers honed to .0195" each)
Screws: 1/2 -  1 turn out (varies per carb)

It ran surprisingly well.  No hesitation in any throttle range. Has a nice throaty sound that it didn't have before. Not sure if that is the pods or the increase in richness (No doubt it was definitely running too lean before). I opened it up at 60mph and there was no hesitation at all. Still had plenty left at 90mph but I was on a country road and had to back down. Also, it used to pop a bit when I came off the throttle before and it doesn't do that now.

The best part is that there was no problems noted with the pods even with gusty winds. Very stable acceleration and cruise. I suspect that might be because I kept the sidecovers on and my legs are positioned on either side of the pods (my rearsets are set back a ways). Between the two, the pods are sheltered in a sort of natural airbox allowing them to breath consistently. This was especially encouraging due to the fact that in all likelyhood I am running a little rich. Not sure if my logic is correct, but if pods typically suffer from interruptions in airflow caused by crosswinds, that would mean that they get a little starved of air and bog down from the momentary richness. On a bike running rich, this condition would be more prevalent. Ergo, if I am running rich as suspected, I would be more susceptible to this phenomena. The fact that I didn't notice anything like that makes it all the more promising. Someone feel free to jump in and tell me I don't know what I am talking about...

I am going to wait until I put on the ConeEng can next week before I do any plug chops.

I still can't get it to idle very well below 1100 rpm. I re-balanced the carbs at 1100-1200 and did my best fussing with the pilot screws but it tends to hesitate then die at or below 1000. I did check the plugs after it idled at 1100 for a few minutes and they looked a nice tan color so scratching my head there. At least it isn't stalling out at stop signs anymore.  Shouldn't I be able to get this to idle around 750? I know that 1100 is within the specs of the manual, but was hoping I could get that low rpm chugga-chugga.

[martin99]

Hi
If you haven't pulled the slow jets yet, I would suggest you do it. My old F2 had pods and had exactly the same symptoms as you have described. Cleaning the tubes properly made all the difference. As I recall I went to 130 mains and it ran well. I wouldn't worry too much about trying to achieve a low idle speed, 1000-1100 seems fine? I also recall some hesitation/coughing while I was playing with mine, and did consider the cross-wind issue, but in my case it was air leaks at the inlet rubbers through not tightening the clamps properly - easy to overlook if you're regularly pulling the carbs.
Hope this helps.

[Stoli]

Thanks oldskool. My original problem was definitely the slow jets. I had the stock 35s in there and didn't pull them when I cleaned the carbs. A few plugged holes combined with undersized jets for my 4-1 exhaust was causing it to run real lean at idle. I just replaced them with new 42s so that problem is solved. At this point, I think I just need to fiddle with the pilot screws some more. The changes in RPM are very subtle so I am not real confident that I have them set perfectly. There also seems to be a difference in opinion of where to set them.

Bring bike to normal operating temp, then:

Honda Manual (start with #2 carb):

  1. Adjust screw to find highest engine speed
  2. Using throttle stop screw, adjust engine back to idle (1000+-100)
  3. Turn screw in until engine speed drops 100rpm
  4. Turn screw 1-1/4 turns open from position obtained in 3 above
  5. Using throttle stop screw, adjust engine back to idle (1000+-100)
  6. Repeat for each carb.

Honda Chopper:

  1. Set RPM to 800-900 (obviously impossible for me so I use 1100)
  2. Turn screw in until engine begins to stumble
  3. Turn screw back out in 1/8 increments (wait 30 secs for change each time)
  4. When rpm no longer increases but starts decreasing again, turn screw back to highest engine speed
  5. Using throttle stop screw, adjust engine back to 800-900 rpm
  6. Repeat for each carb.

So the manual is saying to find the high rpm, then back down 100rpm then open it back up 1-1/4 turn as opposed to Honda Chopper that says to find the high rpm and leave it there. If the plateau of the high rpm range is 1-1/4 turns, then I guess they wind up in the same place?

[Stoli]

Now that I think about it, the procedure at HondaChopper.com says it is for all models of the CB750 (69-78) but it can't possibly be valid for all of them because the screw on the earlier bikes was an air screw that leans out the mixture as you turn it out and the 77-78s is a pilot screw that richens the mixture as you turn it out. They even mention that in the procedure. So how can the same procedure be valid for both types of carbs?

Here is the Honda Chopper procedure which I have paraphrased above:

www.hondachopper.com/garage/carb_info/tuning101/carb_tips.html

Now I've really confused myself 

[lucky]

The pilot screws should all be the same.
Do not use pilot screws to sync carbs.
You are not doing that are you?

Congrats on all you have done so far.
Now you believe pods can work right!

I have told people on this forum and they did not believe me.

[Stoli]

I am syncing the carbs according to the manual (not using pilot screws).

If the pilot screws are all supposed to be in the same place, why do all the manuals say to adjust each one separately? What is your logic there? Wouldn't it make sense that there could be some very slight variance in jet openings and needle (screw tip) heights that can be balanced by fine tuning each one?

I've been flying around the countryside with the pods all afternoon and so far they are running great. They look sweet too.

[flybox1]

The sync screws will not be in the same spot or setting.  EVER   
They are to be used separately to tune each carb to the same vacuum as the others or the slave carb.

[Stoli]

Point taken, but lets not confuse the sync screws with the pilot jet screws. My latest dilemma is with the pilot jet screws. Lucky says to set them the same (7/8 to 1 turn out) and although I am not refuting that, I want to know his logic. The '78 shop manual has a procedure to adjust each pilot screw individually after a carb rebuild, just as it specifies to sync the vacuum with the slide adjustment screws.

[lucky]

Yes you can adjust the pilot air screws the way the manual says but the tach on the motorcycle is not good enough for this and will lead to inaccuracies.

You will need a dwell tachometer that is like this, then that WILL work.
Many owners do not have one of these.
The amount you will have to move the pilot airscrew to get it into compliance is VERY small.

[flybox1]

Point taken, but lets not confuse the sync screws with the pilot jet screws. My latest dilemma is with the pilot jet screws. Lucky says to set them the same (7/8 to 1 turn out) and although I am not refuting that, I want to know his logic. The '78 shop manual has a procedure to adjust each pilot screw individually after a carb rebuild, just as it specifies to sync the vacuum with the slide adjustment screws.

not confusing them at all.    The pilot screws should be set the same in each carb. Turns out is based on either the stock exhaust/intake setup, or, to a standard known to produce the correct air/fuel mixture for the intake and exhaust being used. In the case of your intake/exhaust setup, 1 turn out is really close.

[Stoli]

Took it out for another 80+ miles today. I was able to get it to hesitate and bog down a bit by starting at about 50MPH (3500 rpm) and then snapping the throttle to full as fast as I could. A quick but smooth advance in the same situation was not a problem and the bike responds well. Snapping the throttle in the lower gears is not a problem at all. There was a really gusty crosswind while I was testing it.

Can't help but wonder if the 42s are a little big. The reason being the inability to idle below 1100 and a starting irregularity. When I had the slightly clogged 35s on, it started as advertised: full choke + cold engine would result in a fast idle up around 2500-3000. Now, I have to take the choke down  to about 1/3 almost right away or it runs real rough when cold and it will only get up to 1500 or so initially. Of course, when I do take the choke off, I have to hold the throttle up for a minute or so or it will die. Once it has warmed up, it is OK but like I said, it will always die at an idle below 1100 (warm or cold) . 

[flybox1]

Have you dialed in your fast idle cam and choke cable operation?

[TwoTired]

One of the reasons that the 77-78 CB750s have accelerator pumps was the EPA entry requirements for idle emissions limits.  The idle screws could be peak leaned for each cylinder's minimum emission, and the accelerator pump shot gas in as the slide were raised to get enough fuel to increase RPM and power (throttle response).

You do need a sensitive tach capable of 50 RPM change indications to properly adjust the pilot screws.  When new, they were mostly set at the same turn count when done.  But, with needle and seat wear, I don't think that is as reliable anymore.

Of course, it was all tailored for the stock induction and exhaust back then.

[Stoli]

Have you dialed in your fast idle cam and choke cable operation?

I haven't messed with it since I pulled the carbs to replace the jets but it was spot on before and I can see the mechanism move when I pull the choke. The problem is that I can't leave the choke on full or the engine stumbles and dies. I think I should take another look at it because I may be able to adjust it so the cam moves the same amount at 1/3 choke as it did at full choke before. Thanks for that suggestion. Hadn't thought of that.

I ordered a dwell/tach meter off of Amazon today so I think I will follow Lucky and TwoTired's suggestions and try to fine tune the needles, then revisit the fast idle cam and hopefully I'll be in business. I'm glad TwoTired included the comment about the stock induction and exhaust. That's been nagging me in the back of my mind but maybe I'll be able to get close after introducing mods that the Honda Engineers never intended.

[martin99]

Done a plug chop yet? Gas isn't what it was thirty-five years ago either. A cooler grade of plug (if you still think you're running lean) might help things. I have three 30 year+ bikes, and none of them run properly on the grade of plug originally intended.

[flybox1]

Stoli, if you've bench sync'd your carb slides, and or had your carbs on and off a few times,  and NOT also (re)adjusted your fast idle cam set screw, this could account for the current condition of low/poor idle at start up(slides might be lower), and you having to tend to the throttle until it warms enough.  with a full pull of the choke knob, it should hold at 2000-2500, and then climb as it warms letting you know she's ready to get after it

[lucky]

Pull all 4 of the pilot air screws out and make sure that there is a washer, spring, and O ring.

Many times the o-ring is all mangled up.
The O=ring goes in first , then the washer,  and finally the spring.

This can make a huge difference.
Also make sure to use a bright LED light and paperclip with little bend on the end to fish things out of the hole. Make sure there is not an old O ring or washer stuck down in the hole.

[flybox1]

Pull all 4 of the pilot airFUEL screws out and make sure that there is a washer, spring, and O ring.

FTFY 
cb350f/400f's have pilot AIR screws 

[Stoli]

Thanks for all the advice and suggestions guys. I think I am good to go. At least good enough for now. I put on the Cone Engineering "Shorty"  and then did a full tune-up with the help of a dwell/tach meter and she is idling smoothly at 1000 rpm with the needles out 1 turn.

Not sure what the dwell is supposed to be exactly but the meter I got was pretty cheap so I set the point gaps to the lower end (.3mm) and then made sure the dwell of 2-3 was the same as 1-4. The meter read 25 but that was for an 8 cylinder engine, so I think that means it is actually 200° according to the cheapo meter. Ignition timing and carb vacuums are spot on.

Will try to assess the mixture that the new jets are providing when I get a chance.

Do any of you guys with the PD42 carbs get a hesitation during rapid throttle advances in the 3/4+ range at speeds greater than 40MPH? A steady roll-on is fine but a rapid twist causes a slight bogging. Not sure if it is due to a:

1.  Momentary rich mixture brought on by the accel pump and an oversized main jet (135) or

2. Fuel starvation brought on by the rapid opening of the slides that the accel pump can't compensate for because it was designed for the induction of the stock airbox and not pods.

I can live with it, but was just wondering what thoughts/experience you guys have had with the 77/78 bikes.

[flybox1]

Stoli, gotta say.  your bike looks awesome! 

i cant say that ive noticed the hesitation, but then i dont hammer it up there.  roll on always.
what you are experiencing has nothing to do with the accel pump.   its all your mains, and possibly clip shims.
the travel of the diaphragm shaft is used up and cannot introduce more fuel if you are on the throttle any more than 1/4....
to check this i'd do a run in 3rd at ~75mph for a few miles, chop, coast to the side, and check the plugs.

[Stoli]

... the travel of the diaphragm shaft is used up and cannot introduce more fuel if you are on the throttle any more than 1/4....

Of Course!! I should have realized that. It's obvious when you look at the mechanism istelf on carb 2. Thank you for cluing me in there. Also, thanks for the complement  . Those Cone Engineering guys do a great job and their prices are very reasonable.

[lucky]

Forget about the travel of the accelerator pump diaphram rod.
No adjustment really and none really needed.
The diaphram is very thick and it take very little movement to pump the fuel.

The reason you have a hesitation when gibing it quick throttle on this 78 is because the slide needle needs to be one step richer.
Otherwise the clip on the needle needs to be moved down from the top one notch.

The 1978 does not have adjustable needles. No clip grooves.
You can either use shims/washers or get 1977 or 76 needles with the five grooves.
If you use shims/washers you can use two .020 thousandths washers on each needle ONLY if they fit inside the pocket at the bottom of the slide.
Otherwise the little washers must be the same OD as the clip. If you get washers that are larger that the clip  and they will not fit inside the pocket at the bottom of the slide then only use ONE .020 thousands shim so that the shim plus the .012 thousandths pocket depth will add up to .032 thousandths (1/32 inch.

Then you will get rid of that hesitation.
The #135 main jets are a little large.
I would use #42 pilot jets.
#44's are too large and it will not run right.

You have pods and 4 into 1 exhaust.

Jetting for pods or velocity stacks is almost exactly the same. Very little difference.
Maybe a minot mixture screw adjustment.

[lucky]

You are getting some very valuable information and saving a lot of time and frustration.
Just think if you had to remove the carbs for every jet size and combo that you wanted to try.

That  is whet I had to do when I wanted to make velocity stacks and 4 into one work.
I had to try idle jets step by step all the way up to #44's. Taking the carbs of each time.
To get the needle position right it was the same thing. Take the carbs off and take them apart to change the needles for each clip position tested.

So you may get tired of taking the carbs off and on but it is better than starting from scratch.

When I figured my 78 carbs out many people just said it cannot be done -forget about it.


IF you had 1969 carbs you would not have to take them off for any changes.