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

[Stoli]

'78 CB750K - PD42B Carbs
Stock Slow Jets
120 Main Jets
Stock Air box w/EMGO filter
Mac 4-1 header w/ ugly Mac muffler

Now that I have cleared the cob webs out after the rebuild, I am trying to get the bike to idle properly. The carbs have been balanced and the timing adjusted but with the pilot screws set to 1 1/2 turns out as per the manual, I was experiencing a lot of stalling when idling below 1200 rpm. I went through the procedure in the manual to set the pilot screws and I can now get it to idle well at 1000 rpm. The pilot screws are out 3 to 3 1/2 turns and that seems high to me (I've read a lot of posts where most guys with the same or similar config only have them out 3/4 to 1 turn. So, I was wondering if that is indicative of another problem or is 3 1/2 turns out considered normal? The rest of the throttle range is working fine. No hesitations or flat spots.

[mrrch]

When were the carbs apart for a total cleaning last?

[Stoli]

250 miles ago

Edit:  I cleaned them myself after they sat with gas in them for 20 years and I could have missed something.

[Dave Voss]

The slow jets have very small passages, and they can easily become partially blocked, restricting the flow of fuel such that it affects the idle.  It is best to remove them from the carbs to properly clean them, be sure to look through them with a strong light source from behind to ensure they are not blocked.

I recently cleaned the PD carbs on my 550, and all four slow jets were partially blocked with varnish, after cleaning (and properly adjusting the floats) the engine idled much smoother and at a lower rpm than before, with a few hundred miles of use, it continues to idle smoother.  Also, I've set my screws at 2 turns out, which eliminated a very slight stumble below 1500 rpm that I had when the screws were set at 1-1/2 turns.

[flybox1]

see my sig.
only differences in my setup are my K&N filter and probably more back pressure from my exhaust can.  (these make me run overall richer than you)

more than about 2 turns out and you lose the ability to introduce MORE fuel into the idle circuit.
if you need more than 2 turns out, you'd want to go up in size with your pilot screw. 40's or 42's would work, then you are able to really dial in the idle circuit mixture.

BTW, I have a brand new set of #40's I got from sirius, and I wont be using them.  PM me if you're interested.

[Stoli]

Thanks. The slow jets are pressed in so I didn't remove them as per the note in the manual so I could have missed something there. I just ran a small wire through the hole several times and tried to blow some carb cleaner through them.

[flybox1]

Thanks. The slow jets are pressed in so I didn't remove them as per the note in the manual so I could have missed something there. I just ran a small wire through the hole several times and tried to blow some carb cleaner through them.

those pilot jets have 8(?) holes plus the one thru the middle.
gotta pull/clean them. do this first before changing anything else....

[bollingball]

'78 CB750K - PD42B Carbs
Stock Slow Jets
120 Main Jets
Stock Air box w/EMGO filter
Mac 4-1 header w/ ugly Mac muffler

Now that I have cleared the cob webs out after the rebuild, I am trying to get the bike to idle properly. The carbs have been balanced and the timing adjusted but with the pilot screws set to 1 1/2 turns out as per the manual, I was experiencing a lot of stalling when idling below 1200 rpm. I went through the procedure in the manual to set the pilot screws and I can now get it to idle well at 1000 rpm. The pilot screws are out 3 to 3 1/2 turns and that seems high to me (I've read a lot of posts where most guys with the same or similar config only have them out 3/4 to 1 turn. So, I was wondering if that is indicative of another problem or is 3 1/2 turns out considered normal? The rest of the throttle range is working fine. No hesitations or flat spots.

When the carbs were  balanced was the bike fully warmed up? It is possible the slow jets have been enlarged over the years. What do the tip of the screws look like. I don't know just throwing out some ideas. Do a plug chop at idle then see what they look like.
Ken

[Stoli]

Sounds like it is probably the slow jets then. Thanks for the offer flybox. I actually just ordered some 42s and some 135 mains as I am going to play around with some APE pods and velocity stacks  with one of those unbaffled shortys from Cone Engineering. (I enjoy a challenge). I just wanted to set a baseline with as close to stock as possible  before I started down the slippery slope, and the idle was the only thing that seemed off. The stock exhaust was beyond hope so I had to start with the Mac. I'm guessing even if my stock slow jet was clean, the bike would probably run better with the 40s because of the Mac exhaust.

[Stoli]

I have been careful to make all the adjustments with the bike fully warm. As I recall, the tip of the pilot screws looked OK to me but I really didn't know what I was looking at to be honest. The bike was put away with 8K on it, so most parts had very little wear.

[mrrch]

I think your going to be too rich with those jets if the engine is stock.
My engine is stock, black mac exhaust with a few more holes in the baffle, steel dragon velocity stacks and I'm running 115 mains and stock 38 pilots (my 77 PD's have the adjustable needles 2nd clip from top)

[flybox1]

some PO's can get ham fisted pounding those brass pilot jets in.  it will do OK chamferred a bit as the main body ID of the pilot controls flow.  this way at least there isnt a blockage.

dont forget that you'll need to do a chop at mid to 3/4 throttle for your needle jet/clip position.  kinda surprised you havent noticed lean running as is. 
you'll deninitely need to change clip position with stacks/coneeng can and 42/135's 

[Stoli]

Could be running a little lean right now. Especially at lower RPM but I've been trying to avoid that range  .

I've been doing a lot of searching and reading and it seems like the sweet spot for the '78K with stock carbs and little to no restriction on the intake and exhaust is around 42/135 with a couple of those M2.5 washers to shim the needles, so that is my starting point. I'd rather start too rich than too lean. 115 mains seem small, but I guess that they aren't really a factor unless you are more than 3/4 throttle and the average street rider doesn't spend too much time up there.

Supposed to be a lousy day weatherwise tomorrow so I think I'll pull the slow jets (will have to do it eventually anyways) and see how those horizontal holes look. Thanks for all the advise. I am slowly figuring out how these carbs work.

Does it help to soak the slow jets in some kind of lubricant or should I just start twisting and pulling?

[Dave Voss]

Does it help to soak the slow jets in some kind of lubricant or should I just start twisting and pulling?

It shouldn't require a lot of force to remove them, they are only supposed to be lightly pressed in, just hard enough that they won't fall out on their own.  I recommend pulling them straight out by grabing them with a pair of soft grip pliers (the type without sharp gripping teeth).  Don't worry too much if you leave a mark on the outside surface from gripping and pulling, although it may look unsightly, it won't affect their function.  If you twist them to aid removal, the carb body and the jet can become marred, which may affect the way they fit afterward.  Once clear and clean, a good method for installation is to lightly insert the jet into the carb body, and then give it a moderate tap with the plastic handle of a screwdriver.  One good tap should set it in place.  Of course, this is easiest with the carb rack off the engine.

[flybox1]

Straight pull, light twist.  They'll pull right out.

[lucky]

Thanks. The slow jets are pressed in so I didn't remove them as per the note in the manual so I could have missed something there. I just ran a small wire through the hole several times and tried to blow some carb cleaner through them.

Thanks for giving the the jetting info and intake and exhaust type to us first.
With your set up all you should need is a mixture adjustment.

2-1/2 turns on the mixture screw is excessive.


The way you cleaned them will not work.
 Remove the floats and needles FIRST before taking out the slow jets so you do not damage the floats.
Do not mix them up.
If the float needles have not been replaced for two years then replace them.
Use a .010 electric guitar string because the stock jet is .013 thousandths.
Then clean with cleaner and use compressed air.
Hold each one up to the light and make sure you can see daylight through them.

The accelerator pump system must also be working on the 77-78 carbs.

Remove the air cleaner and look into the intake with a bright LED pocket flashlight.
With the engine off and the gas on turn the throttle a couple times and you should see the brass post in the intake of the carb squirt a shot of fuel into the engine.
Make sure to check all 4 carbs.
Do not assume that because one works, that they all work.

[lucky]

250 miles ago

Edit:  I cleaned them myself after they sat with gas in them for 20 years and I could have missed something.

How many miles were on it???

[Stoli]

The bike had 8K on it, owner died, brother threw it under a tarp and there is sat for 20+ years.

I have read the other posts regarding the accel pump and it works fine. My son has a 77F and the PO had put the accel pump back together wrong, so I learned all about that when we fixed his a few months ago. Thanks goes to Lucky and Flybox for a lot of good info there.

I'm sure the issue here is some clogged slow jets which I did not remove when I tore apart the carbs over the winter.

My new jets got here yesterday so I think I will just put them in today when I pull the carbs off the bike instead of cleaning the 35s and putting them back in. I also have the pods but won't have my Cone Engineering shorty until next week, so I think my new baseline will be:

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)
Screw: 1 turn
 
I suspect that the 135s might be a little big for the current muffler (which has a baffle) but hoping the ConeEng shorty (which is straight through) will lean it out. Figure it won't hurt to run it a little rich on the top end for a few days while I wait for that to come in. Based on what I have read, the 42s should be about right. Shim is another question mark, but thinking I might need to add another washer when I put on the shorty (bringing the shim to 1.5mm).

[flybox1]

good luck over the next few days, and keep us posted on how she runs.

[Stoli]

good luck over the next few days, and keep us posted on how she runs.

Will do. Looks like the roads around here won't be dry until Sunday.

I could be wrong, but the jetting issues seem pretty straight forward (solvable) when you tinker with the airflow on both ends. Most guys switch back to the airbox because of issues with laminar flow which pods and stacks can introduce. Is that correct?

Flybox - Did you try out larger slow jets at one time, or did you decide the stock 35s were good based on idle performance and a plug chop?

[flybox1]

(I cant speak to your question about pods/stacks as i've always used my airbox)

I've never needed larger slow jets as my plug readings were always within the tuneable range of my fuel mixture screw.

The new cone engineering 18" quiet core can I installed, didnt have many reviews, so, I assumed it had less back pressure  than the insert i was running on my kerker (but less than the 12" open core you'll use), but to my surprise, it actually had more resistance.  I was still able to tune idle leaner(@ 1.0 turns out) with my fuel mix screw, and I wouldnt need the 40's I ordered for it.   Its been warm here, and I got some decel popping after a brisk ride home from work yesterday, so i think im maybe an 1/8th turn too lean.  Plugs will tell.  I'm installing new exhaust studs and re-wrapping my kerker this weekend.  I'll get another look at them tonight after my ride home.

with stacks and your open 12" can, 42's @ 1 turn should be pretty darn close.

[Stoli]

How do you like the quiet core? I came very close to going that route when I was making the order at ConeEng the other day. I suspect that the open core will be too loud for me but figured I'd give it a try and switch to the quiet core when my neighbors start complaining.

Also, how did you suspend it?  Got any pics?

[flybox1]

heavy duty clamp at the overlap, and a rear footpeg mount bracket welded
pics and stuff are here...
http://forums.sohc4.net/index.php?topic=105018.msg1378502#msg1378502

mellow enough in the hood to keep the peeps happy.
6k rpm+ and it makes noise 

[lucky]

The bike had 8K on it, owner died, brother threw it under a tarp and there is sat for 20+ years.

I have read the other posts regarding the accel pump and it works fine. My son has a 77F and the PO had put the accel pump back together wrong, so I learned all about that when we fixed his a few months ago. Thanks goes to Lucky and Flybox for a lot of good info there.

I'm sure the issue here is some clogged slow jets which I did not remove when I tore apart the carbs over the winter.

My new jets got here yesterday so I think I will just put them in today when I pull the carbs off the bike instead of cleaning the 38s and putting them back in. I also have the pods but won't have my Cone Engineering shorty until next week, so I think my new baseline will be:

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)
Screw: 1 turn
 
I suspect that the 135s might be a little big for the current muffler (which has a baffle) but hoping the ConeEng shorty (which is straight through) will lean it out. Figure it won't hurt to run it a little rich on the top end for a few days while I wait for that to come in. Based on what I have read, the 42s should be about right. Shim is another question mark, but thinking I might need to add another washer when I put on the shorty (bringing the shim to 1.5mm).

Two shims will be too much.
IF the shim sits on top of the pocket inside the bottom of the slide that pocket is .012 thousandths.
So your total amount will be .195 thousandths + .012= .0315 thousandths or lets just call it 32 thousandths. Just right.


Don't let this happen: It can get stuck and cause the needle to be crooked in the holder.

[Stoli]

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).