Topic: 77-78 carb tuning

[750K]

Due to some float level/overflowing problems with the pd41a's on my 77 I cleaned a set of 78 pd48b bodies and swapped internals from the pd41's. Float levels set to spec, no overflowing now. After getting the carbs back on I could only start it up with the choke off, knob in the down position. At anything over half choke it would sputter and die.

120 mains,
42 slows
mixture screw set 1 1/4 out from seated
Needle clip in the middle, which should be one richer than stock
Stock airbox with a clean filter
Accelerator pump working strong
Baffled 4-1 header

Common sence says I'm rich at idle with it dying at half to full choke, so I went mixture screw 1 turn out this morning and then 3/4 with a bit of improvement. I have not yet checked if the fast idle cam is set properly.

With the pd41's it would fire up with full choke and sit around 2500-3000 rpm like the manual says, I had all the same jeting etc but with mixture screws at 1 3/4 at the max.

Compression is good, 140 cold across the board and last time I checked it hot it was 150 on all 4. Points set, timed with a light, advance dead on, new plugs, plug wires and boots. I've been chasing a lean condition with plugs 1 & 2, 3 & 4 look fine, but the lean plugs didn't change after I went for a ride staying in the 1/4 throttle range on the way home. Other than it needing a proper sync it ran fine, pulls hard through the range, no stumbling under load. Highway ride was nice, lots of power over 3/4 throttle with no popping on decel.

Any opinions? I might put the 40 slows back in if checking the fast idle cam and a proper sync don't solve the choke issue.

[flybox1]

Any opinions? I might put the 40 slows back in if checking the fast idle cam and a proper sync don't solve the choke issue.

^ this. but only one change at a time.  start with your fast idle cam.  get it working properly in lifting the slides when choke pulled.  still might not start well, but get it sorted.  then. do your carb sync
only after ^ make the change to 40's if needed.

you had a good idle with the 41a's....why not just get the float valves working properly.  i'm lazy    seems like too much work, and now tuning, for some cheap float valves   

[750K]

Any opinions? I might put the 40 slows back in if checking the fast idle cam and a proper sync don't solve the choke issue.

^ this. but only one change at a time.  start with your fast idle cam.  get it working properly in lifting the slides when choke pulled.  still might not start well, but get it sorted.  then. do your carb sync
only after ^ make the change to 40's if needed.

you had a good idle with the 41a's....why not just get the float valves working properly.  i'm lazy    seems like too much work, and now tuning, for some cheap float valves 

Trust me, I've tried. I think the root cause of my float overflow is something other than my float needles and float which are all brand new. I've been fighting with them for months and months, this is way easier to do afaik...

[BobbyR]

One thing I did notice was you mentioned needle clip position. The 78 carb did not have a needle clip. Did you swap that from the 77 carbs?

[750K]

@BobbyR, correct. I swapped the adjustable needles from the 77 carbs into the 78 bodies, along with the rest of the jets, gaskets etc etc that are brand new. I did use the 78's slides but everything internal is in tip top shape, super clean passage ways. I was very thorough when I cleaned everything.

[flybox1]

throw the 40's in, mixture screws at 1 turn, adjust your FIC so it engages with choke, and start'er up 

[750K]

@flybox, that's what I was thinking. I'll report back once I have the time to check and synch, at least I'm getting down pat the song and dance of taking off and putting on the stock air box haha. I'll be able to do it blind folded soon...

[mrrch]

Your carburation looks too rich for a more or less stock set up.
Stock pilots are 38 or 35's (i cannot remember offhand)

[750K]

@mrrch, I agree now that I'm at this point.

35's are stock slows for the 77-78k's, I've been trying to nail down a lean running #1,2 cyl. A PO never re jetted for the 4-1 so I went up to 120's & 40's from 115's & 35's, after playing with mixture screw settings plug color didn't change. Thats the only reason I stepped up to 42 slows, I've got new intake rubbers on #'s 1 & 3 and good used rubbers on #2 & 4 with new clamps. Anytime I've checked for vacuum leaks with wd40 nothing changes, I ran through ignition, points, plugs & wires...... and I still come up lean on 1 & 2. I dont think I'm running overly lean and burning a valve, compression tests are always high and even across all 4 and when I check valve lash I'm good as well. While running all 4 pipes at the head are all about the same temp to the touch, it's frustrating. 

[Eydugstr]

750k - If this will help - These folks have the inbetween size push in slow jets for those carbs, in case you need them.  http://www.siriusconinc.com/search_result.php?make=1&model=cb750&part=1&partno=&search=search&x=51&y=19&start=25

[750K]

^^^thanks, I have 38, 40, 42 from them & stock 35's as well. Should be good in the jet dept , just need to find some spare time...

[bollingball]

I'm with Fly on this part (you had a good idle with the 41a's....why not just get the float valves working properly.  i'm lazy )

Plus I would love to see the ones you could not get to work. But you have made up your mind. What is the brand name of both set?

I have not yet checked if the fast idle cam is set properly.
I would get this working first.

750K Trust me, I've tried. I think the root cause of my float overflow is something other than my float needles and float.
I do trust that you tried. Now you need to know why you could not make it work. Listen to Fly he is pretty good by now on these carbs.
Ken

[750K]

Nothing's changed idle wise, idles equally smooth with the new carbs as with the old, it just dies past half choke from a cold start. I'm probably running to rich at idle but I'll start with the fast idle cam and go from there.


Here's what's been happening Ken, I'd set the floats with a caliper per manual. When I put them on a rack on my bench to verify with the clear tubes on the overflow tube #'s 1,2,4 all sit level and steady when I undo drain screw on #3 the float level won't stop rising, it pours out of the jet orifices and I mean pours. I've cleaned everything multiple times including the needle seat. New rubber tipped float needles, new floats. No binding on the float towers or bowls, swapping floats and needles from one bodie to the other doesn't change anything.

I think it has something to do with the atmosphic venting on the 41's I have, it's like a pressurized overflow on it. My 41's have no vent nipple like the 42's do, they vent through the overflow tubes and not a separate system. When I switched the jets, needles and floats & needles into the 42's all 4 float levels were dead on with no overflowing on the first clear tube try. So it's either the venting path or the brass pressed in float needle seats are worn. I highly doubt the seats are worn as I pulled old rubber tipped float needles out when I rebuilt them.

I ran through everything with a fine tooth comb, my tank is spotless and I run an inline filter. The external gas tank I run for bench testing is clean and has an inline filter. After fighting with them over and over I'd rather re jet than use the 41's. As far as I'm concerned they're a paperweight.

[flybox1]

I'm with Fly on this part (you had a good idle with the 41a's....why not just get the float valves working properly.  i'm lazy )
Listen to Fly he is pretty good by now on these carbs.
Ken

Ha! I had a good sensei

[750K]

Alright, flybox & ken

Adjusted fast idle cam per manual, that helped a bit. I then set mixture screws at 1/2 out and was able to get the bike to run with the choke out almost to 3/4 choke, so back in with the 40's.

Update:
So I think I got it sorted out, I'll know better in the morning when I do a true cold start. I pulled the rack and put the 40 slows back in, double checked the fast idle cam. Then set the mixture screws 1 turn out and fired it up with bit more than half choke, ran decent with a pop and crackle now and then. Did a proper carb sync which smoothed things out, still the odd pop here and there. Played with this mixture screws a bit and ended up at 1 1/2 out and went for a ride, still some popping on decel. Mixture screws out to 1 3/4, much better no popping! Nice smooth idle, pulls nice through the throttle range. Doesn't fall on its face when you crack the throttle wide open, cruised around for a bit and took the highway back and stretched its legs a bit.

Still have to check plugs and most likely do a bit of fine tuning but so far so good, Put the stock seat back on. I forgot how comfortable it really is, think I'll leave it on till I get around to adding more foam and re covering the one I had made a while back.

[toytuff]

Played with this mixture screws a bit and ended up at 1 1/2 out and went for a ride, still some popping on decel. Mixture screws out to 1 3/4, much better no popping! Nice smooth idle, pulls nice through the throttle range. Doesn't fall on its face when you crack the throttle wide open, cruised around for a bit and took the highway back and stretched its legs a bit.

I had to go 1 3/4 plus on mine also. Same results. They ran lean back in the day and today's fuel doesn't help matters. Manual is a starting point.

tt

[MOONDOGNYC]

What is the stock IMS supposed to be at ?
I thought for the PD's on an F model it was 1 3/4 ?

[flybox1]

Good news.  Keep us posted.

[750K]

Indeed good news, it fired up from cold this morning at around 3/4 choke. Will try and die at full choke, I'm ok with this actually. This bike has always run lean since I've owned it so if it's richer finally that's a good thing, I pulled the #1 plug which has always been the leanest looking of the bunch and after my short ride last night it's showing a bit of grey/tan color. So I think she's close and I gotta say the new carbs seem to work nicer, the main return spring has more of a snap shut feel to it than the old 41's.

Jetting and tuning is way better than fighting the old carbs in my book, I always felt they were the root of my problem. The more I got the bikes different systems run through the more I came back to the carbs. I think this finally proves it. An added bonus to all this carb bs is I no longer think removing the stock airbox is a pain, it's a snap now haha.

[BobbyR]

Great news! Considering the age of these bikes and their multiple POs, and general wear the factory settings may no longer be valid. Sometimes you just adjust to function.