Topic: 1978 sohc CB750 straight pipes carb issues- HELP!

[freezerburn]

I’m a lurker here but I’ve gained a ton of help from all posters here and I do thank you folks. Progress has been pretty good I’ve mounted both tires, installed a header conversion to run studs, set the valves, rebuilt the carbs and built a nice manometer for syncing the carbs up.
The bike has had a hesitation off of idle and has grayish/tan plug readings and I’m thinking the jets are a tad too small. All other rpms operate perfectly. After removing the jets (pictured) I see a “125” embossed on one and I thought that that was the number I needed but apparently bigger is needed to get it right. The exhaust is a 4 into 4 straight pipes and are really great quality compared to a lot of them that I’ve seen around on the interweb.

[freezerburn]

More pics to gawk at while you reflect on what jetting I need to try. I’m new to rejecting (properly, lol) and this bike has been run long enough too Lean:(

[freezerburn]

Bike-

[freezerburn]

Manometer stuffs-

[freezerburn]

Manometer completed. I haven’t used it yet because of the rejetting.
FYI it’s running the Dyna ignition and Super Coils but here-

[freezerburn]

On the backside by the stainless bracket are a couple sardine cans with tin lids for the adapters, plugs and stuff.

[freezerburn]

The tube lengths, chamber volumes and everything else I could think of are identical to each other. Fun project, I can’t wait to use it once I get carbs right.
Upon reflection I haven’t checked the timing of the ignition where the points were.
Be cool, freezerburn

Please help as Alaska summers are kinda short, lol. fb

[Old Scrambler]

Looks a bit rich...............with pods and those pipes............maybe 115s but check the needles for proper height. Check your timing.........the pipes are blue from burning fuel in the exhaust. I was in Alaska for all of last summer...........it was GREAT

[freezerburn]

I will definitely need to learn how to check the Cdi timing.
I thought that grayish color plugs indicates a lean condition..?
Can anyone tell me if “125” is something that would be embossed on the Stock jetting as I have on mine I wonder?

[754]

They made them and may have used them or 120,s on earlier 750s with the other carbs. Your came stock with smaller ones..

[freezerburn]

Well sheesh. I thought that I would be needing to get bigger jets but I may need smaller ones. As I recall, the the carbs were screwed out 1 1/2 turns  from bottom and that's where I set them back after rebuild. It hesitated before rebuild as well as after.. so maybe turn the screws back in to 1 turn out and go from there maybe. Plus I'll need to confirm the timing is correct first and then I can sync the carbs with my new custom tool:)
Any more contributions or suggestions would be great.

BUT... why are my plugs grey/light tan if I'm running rich? I can't get my brain around that yet   -fb

[BobbyR]

Here is a video on reading spark plugs from NGK

[freezerburn]

Nice one BobbyR, thanks! I guess im running more or less right then on the carbs maybe turn the screws in a bit.. now I'm thinking timing of that electronic ignition. I haven't done that before but I do know it's a Dynas Ignition (Dyantek) out of Glendora, Cally. Super Coils too.

[freezerburn]

A shot of the coils (after I put the tank back on) I was rushed at the last moment.

[freezerburn]

Float level may be an issue as well I'm thinking. I replaced the current bank of carbs with a cleaner es to begin with and rebuilt them but I didn't check the floats   I've noticed checking around the forum that the float levels for the pdB's are 12.5mm and the A's are 14.5mm (mine are the B's) but I've noticed that some of the owners get the two get confused fairly commonly andwe, it is an old bike that has been through many hands. They may be set at the incorrect 14.5 level.. once this was corrected the online/forum owners reported no hesitation and good throttle responce which are my only problems with the bike. As Old Scrambler mentioned it is probably a bit rich but my needlles are non adjustable for '78/pdB's. It appears no money may be involved which would be great!         

Let's see then-
timing,
re do valves post timing change if any,
check float heights,
try out my new manometer, yeehaw!

Can anyone think of any thing to add to check? The input has given me quite a bit of direction and hope let me tell you, thank you all again with any additional guidance and I'll update. I'm gathering info for when I rotate back home in a week or so. fb


 

[freezerburn]

pdB carb bank-

[POPS 911]

You can SHIM the needles if you need more fuel, think about it that pointed needle end will be higher in the tube, thin shim. Do you have a stock air box ? You can ride the bike to check lean = pull choke out a little at 3/4 throttle and if bike picks up and run fine = your lean on the main if it blubbers and slows down your to fat. U-TUBE has many sites for tuning carbs for free watching. Make sure those jets are all clean out we use jet drills to clean out the junk from bad gas = soft copper wire will clean most jets but do not run steel wire in those holes. Good JAPAN jets have numbers on the top, CHINA made jets are on the side and hard to read.

Pops

[freezerburn]

Thanks pops911 for the write up. I will certainly be trying the choke test! Nice one! I’ve run the bike for at least 500 miles and that tiny stumble off idle is the only problem. A sweet machine and very smooth running.
So far I’ve been told I’m rich, lean and in between plus potential timing is improperly done as well as float level check needed. ALL observations are very much appreciated to compile and assert when I get back home.
No intake boot leaks as ether checks indicated none
There was a 4 into 1 header with clamps and I changed them out and put this 4 into 4 on using a diy stud conversion. Anyways, now I can put a centerstand on the bike! I love centerstand sand I just got one on fleabay for $41 free shipping. The 4 into 4 that I have is a nice and high profile and sturdy design as well so I can ride around on the logging roads and dirt. After all this is the big AK.
I think I’ll order the jet set- 105’s x4 through 130’s x4 to have on hand. Are there only those jets plus needles required only or.. are there slow jets, screws etc of various sizes to gather as well? My first time dialing in carbs.. thanks a TON, freeze

[POPS 911]

Do not mess with needles, main  jets and idle - 1 1/5 TO 2  is a good start turning jet in and out should make a change. Stock air box or [ PODS call for a main jet change ] many u-tube sites show you how to make a change = type in TUNING PODS ON A MOTORCYCLE.

POPS BK911

[freezerburn]

A quick update- changed out the pods for k&n filters, I went back into the carbs and indeed the floats heights were all off, air fuel screw o-rings were all hard, #2 carb o-rings were hard, mickey ears needed to be drilled out on one ear. timing was okay but I (static) adjusted it anyway and disassembled, cleaned and lubed plus reassembled the advance mechanism. The three pieces of tubing that were between the carbs (the smaller ones) were loosey goosey and spinning on the nipples. Adjusted valves. All was replaced/adjusted, repaired etc and then I called it a day before attempting to start it up in case I missed something. Haven't used my diy manometer carb sync tool yet.. that will hopefully be the grand finally. Stay tuned and please don't hesitate to contribute. 'freezer

[madmtnmotors]

I know the PDA carbs have two sets of rubber crossover tubes, not sure about the PDB. One set of rubber tubes is for the accelerator pump and the other is for the bowl vents (looseness would likely not be an issue with these). If you replaced the accelerator pump crossover tubes then you need to fish the old ones out of the trash and cut out the little brass restrictor so it can be reused in the new tubing. The accelerator pump crossover tubes would be the ones closest to the rear of the carbs.

[freezerburn]

The ones that I took off were already replacement tubes alass, no inserts to be had. fb

[madmtnmotors]

The ones that I took off were already replacement tubes alass, no inserts to be had. fb

You can cut new ones from brass or s.s. tubing of the appropriate size if necessary. Whether they are absolutely needed may be open for debate, but I figure Honda had a reason for using them and they are easy enough to replace. I'll have to do some digging, but I know the metal restrictor tubing size has been posted on this forum before.

[freezerburn]

Cool, I used clear fuel line and will be able to see if it pinches, etc. That was the plan anyway. About to head to the shop and will post any updates this evening. Keep it coming friends 

[madmtnmotors]

Cool, I used clear fuel line and will be able to see if it pinches, etc. That was the plan anyway. About to head to the shop and will post any updates this evening. Keep it coming friends 

The restrictors in the accel pump tubes I believe were used to balance the flow between the "squirters" in the carb throats. I don't think there is any issue with the tube collapsing, just the potential for uneven flow to the accel pump nozzles? IIRC the accel pump was used to remedy the potential off-idle stumble due to the idle circuit being leaned out a little to meet emissions regulations of the era.

[freezerburn]

Thanks for that.. a coincidence that I'm having off idle stumble? I'll post up Sunday evening after I go through things. As mentioned, I remedied and corrected many things (I hope!) at recent tear down but haven't had time to lite it off and get a hard copy report as of yet:) Will post up and will keep an eye on the tubes madmtnmotors!

[BobbyR]

You can adjust the tab on the carb linkage that pushes on the accel pump to come in early. This can eliminate stumble and give you better general throttle response.

[freezerburn]

Thanks BobbyR for that. Everything above idle is excellent except for that bit of stumble. At any rate I’m sure I needed to go in the carbs again anyway so I’m glad I did but I will be adjusting that tab as well for sure! fb