'72 Carb Tuning

Colt Winchester

Colt Winchester

CB750 Member
Messages
21
Reaction score
2
Points
3
Location
Western Kentucky
Hey Everyone! I wanted to get everyone's opinion on a couple things. I've got a 72 CB750 that I've been working on. I've been working on carbs. I've got all the basics, cam, points, valves and timing adjusted. It's important to note that we're running a 4into1 exhaust and pod filters.

We got an ultrasonic cleaner and took the carbs apart and cleaned them. For the rebuild. I bought a stock rebuild kit. This is where I think I'm getting some issues. Before the carb rebuild, the bike would start up no hesitation even after sitting for several days. It idled well, but seemed to backfire and choke up when riding. After the rebuild, it barely wants to start. I have to fully choke it and cut off all air until it starts to slowly fire up. But it runs great when riding it. There's still a tiny bit of crackling when decelerating, but it's clean and steady.

I've got a few ideas that could be the problem:

1.) Needles: the previous owner had larger needles that we swapped for the original size in the rebuild kit. Do you think we should put the old needles back in?

2.) Air set screws: With this configuration, I've read 1.25 turns out is likely good. But I've seen so many opinions on this... what do you guys think? Is there a proper way to "tune" these to find what the bike actually wants?

3.) Pilot Jet: someone mentioned that the pilot jet that was on the bike may have been bigger and helped to support the bike at idle. I swapped the old pilot back in and it didn't seem to make any difference in how it ran.

I'm curious what you guys all think. I'll include a picture of the carbs so you guys can see which design they are and I'll post a picture of the rebuild kit and old needles (unsure exact size, but they are bigger). Ignore the writing in that picture.
 

Attachments

  • 20240815_114900.jpg
    20240815_114900.jpg
    252.8 KB · Views: 72
  • 20240815_132627.jpg
    20240815_132627.jpg
    169.2 KB · Views: 57
  • 20240815_132617.jpg
    20240815_132617.jpg
    110.2 KB · Views: 59
If you are running pods, you almost definitely need bigger pilot jets. Maybe a jump up by two sizes, even. One you get the bike idling correctly, set the mixture screw via the manual's process. After that, you can start tinkering with your needle and main jets.
 
You should have to use the choke fully to start up, thats what it is for. If the engine is stock with just pods and 4 into 1 then you shouldnt need to go too far from stock jetting. I never have had to go up on pilots with stock displacement and only up one size with my big bore motors. Idle just doesnt use a lot of fuel so normally stock size is enough for stock motor. You shouldnt need much more than 120 mains for your setup unless the exhaust is very inefficient then it will need more than that. I would start with stock needles with a stock motor, again I have never had the need to use a different needle. The needle isnt necessarily bigger it has a different tapper profile to change fuel amounts at a different rate compared to stock..

I tune jetting by using the choke while riding. Pick a main jet size close to stock and needle in center position and go for a ride. get going and go wide open throttle, slowly close the choke while you are wide open, if it is jetted rich the choke will make it worse if it is lean the choke will make it pick up and run better. If its close it will will run ok with part choke but get rich as you close it more Rejet accordingly till mains and wide open are good then do the same thing at half throttle, this will get you going on getting the needle position dialed in. Drop the clip it gets richer raise the clip it gets leaner.

Stock motors rarely need more than one or two main jet sizes and normally drop the needle position one or two slots.

Float level is important and affects jetting in all rpms and easy to get wrong if you dont have the honda float level tool.

When you say stock rebuild kit do you mean genuine honda? Because they never had a kit just individual parts. If you mean a stock aftermarket kit then the new needles and jets can easily be machined wrong, I never use kit jets or needles only clean the oem parts, if I need jets I use jets only from jetsrus.com as they have genuine keihin and good lower cost jets and carb parts.
 
there is minimal air flow at idle, the engine uses the same amount of air at idle to run at an idle speed regardless of what filter. Same at every rpm really, the pods just allow for more potential air flow at any given time though not much more than the factory air box really. At idle you are not even close to maxing out air flow of a stock filter. If pods are over oiled you are creating more restriction than if you stayed with stock filter setup. I very rarely change pilot jets even if I run stock carbs on big bore motors, about the only time is 1000cc area motors with big camshaft and an efficient exhaust system then you might have to go up a size.
 
I ask because what I've seen online is people taping up the pods to decrease air flow through them. But maybe that is meant to help mid and fill throttle.
 
airflow is only an issue with cv type carbs, if they cant get these carbs tuned then they need to sell their tools and bike. Why go through the pain of putting free flowing pods on then undoing the gains by taping the up to restrict the airflow you just put the pods on for lol I laugh every time I see someone post that on sites.
 
The increased air flow you mentioned is exactly what leads me to needing an increased pilot jet. Truth be told, I've never played with pods, so I'm working off historical experience with hyperchargers, big air kits, etc.
 
brettp said:
The increased air flow you mentioned is exactly what leads me to needing an increased pilot jet. Truth be told, I've never played with pods, so I'm working off historical experience with hyperchargers, big air kits, etc.
Click to expand...
I did some looking and found one potential issue. My floats were all over the place. A buddy whose been a Honda Mechanic for the past 40 years came by with some old guage tools and helped me set the float and I swapped the retainer down one position from stock (second from the bottom). After we bench synced and reconnected everything, I took it for a drive and it was phenomenal. It was getting a little rich at idle, but an air adjustment fixed that up. I drove it about two hours and it did great. Then randomly, it started cutting up and spitting gas out the first carb. I put it in the garage and wanted to check it out the next day after it cooled down. I found that it was completely dead which was odd because it wasn't left on. It still ran poorly and cut up. I'm really not sure what could be wrong now.
 
Sounds like the first carbs float didn't seat. Could be junk from the tank got to it (do you run an inline filter?).
When you say it was completely dead when cooled down, do you mean the battery?
 
brettp said:
Sounds like the first carbs float didn't seat. Could be junk from the tank got to it (do you run an inline filter?).
When you say it was completely dead when cooled down, do you mean the battery?
Click to expand...
Yes. The battery was completely drained. I rode it for an hour, pulled it into the garage and it was dead. It's almost like the altenator isn't charging the bike completely. I also thought that, so I swapped in a never used original Honda float that we got and it did the same thing. All the needles are new and swapping needles from didn't carbs doesn't stop it. It still leaks... I'm not sure what else would cause that...
 
Could be crud on the brass fuel inlet of the carb bowl. I use brasso (brass polish) to clean that area. You should also inspect it to make sure it isn't nicked or damaged. If you have a nicked edge, no seat will seal. But that is unlikely. It is more likely you just need to clean that inlet.
Oh, have you made sure the leak is coming from the nipple on the bowl? Sometimes the fuel rail leaks and the fuel runs down the side of the carb to that nipple, making it look like it is an overflow issue.
Always a good idea to bench test the the carbs before installation.
The charging system could be a host of things; do you have a manual to begin testing?
 
brettp said:
Could be crud on the brass fuel inlet of the carb bowl. I use brasso (brass polish) to clean that area. You should also inspect it to make sure it isn't nicked or damaged. If you have a nicked edge, no seat will seal. But that is unlikely. It is more likely you just need to clean that inlet.
The charging system could be a host of things; do you have a manual to begin testing?
Click to expand...
I have digital copies. There should be any crud on it because all the brass is new and I dismantled each carb and ultrasonically cleaned them.
 
The brass overflow tube in the bowl have a tendency to crack and that lets fuel run out. If the needles are non rubber tipped like oem in the round tops they tend to leak whenever they feel like it. I always swap in the rubber tipped needles from the keyhole carbs, they are a bit longer but the tab on the float will bend enough to use them. The rubber tips dont leak near as much as the metal tip style.
 
dirtdigger said:
The brass overflow tube in the bowl have a tendency to crack and that lets fuel run out. If the needles are non rubber tipped like oem in the round tops they tend to leak whenever they feel like it. I always swap in the rubber tipped needles from the keyhole carbs, they are a bit longer but the tab on the float will bend enough to use them. The rubber tips dont leak near as much as the metal tip style.
Click to expand...
Someone else mentioned that... I just figured that wouldn't be my issue since I've swapped multiple needles and float and still have the issue.
 
dirtdigger said:
either float is sinking or overflow tube in bowl is cracked
Click to expand...
I almost don't want to tell you guys what the problem was... I pulled all the spark plugs to look at then and it was running a bit rich, but noticed #1 looked like it wasn't firing... I wiggled the wire and the plastic cap popped off... I screwed it back in, plugged everything back up and that was the issue...
 

Similar Threads

Colt Winchester
Sparkplug Health
Replies
1
Views
390
brettp
B
S
Cb750f1 1978 carb over revving
Replies
14
Views
630
Stevew63
S
M
Sticky Carb
Replies
5
Views
458
hinksy
hinksy
J
Slider problem on Carb
Replies
4
Views
1K
JtgHawk
J
I
HELP! 78 CB750F won't run right
Replies
18
Views
612
brettp
B
Back
Top