[Thustle]

Hey everyone so I have a 1978 CB400T “2” and this damn thing won’t pull through my rpms and bogs hard after 1/2 throttle until it dies if I keep it WOT. Low end power is there and goes great. And I am familiar with the “correct” way to pull these apart and clean them, I did read the forum on how to do this. This is what I have done, taken the carbs completely apart and cleaned them. Put the main jets back to stock. 75 and secondary at 105. The bike is COMPLETELY stock. Stock air filter and air box. I backed out the pilot screw 1.5 turns and bench synced the carbs. The inside of my tank is FLAWLESS so not dirt. My petcock works as I should. It starts and idles great. And in neutral it revs fast and perfect. But when it’s under load in gear is when I get the bogging on the throttle. I’m guessing that’s because the secondary jet doesn’t get used until driving. Why idk but I’m sure someone will let me know which would be awesome. Good spark yes, good compression yes, valves adjusted to spec yes! Please help. And yes I have read the other forums about this exact same problem but I still need help!!

 

[longdistancerider]

Check your timing advance, one of the classic CDI ignition failures is lack of advance. If you haven't done it already then run thru the tests listed here Ignition Diagnosis for CB/CM 400/450 and CB450SC manual trans 1978-1986
Have you done a running sync of the carbs yet. Bench sync will get you close usually, enough to get the engine running properly, but it really needs the running sync to be right.
The emulsion tube under the primary main jet may have one or more holes still plugged. You have the press in jet. You can try pushing it out from the throat being careful not to mushroom the tip. Or you can thread a screw into the end with a nut to use as a puller. how to remove keihin pilot jet - Google Search

 

[Thustle]

Check your timing advance, one of the classic CDI ignition failures is lack of advance. If you haven't done it already then run thru the tests listed here Ignition Diagnosis for CB/CM 400/450 and CB450SC manual trans 1978-1986
Have you done a running sync of the carbs yet. Bench sync will get you close usually, enough to get the engine running properly, but it really needs the running sync to be right.
The emulsion tube under the primary main jet may have one or more holes still plugged. You have the press in jet. You can try pushing it out from the throat being careful not to mushroom the tip. Or you can thread a screw into the end with a nut to use as a puller. how to remove keihin pilot jet - Google Search

thank you so much!! I will definately try all this tomorrow and get back to you!! Thank you

 

[Thustle]

Check your timing advance, one of the classic CDI ignition failures is lack of advance. If you haven't done it already then run thru the tests listed here Ignition Diagnosis for CB/CM 400/450 and CB450SC manual trans 1978-1986
Have you done a running sync of the carbs yet. Bench sync will get you close usually, enough to get the engine running properly, but it really needs the running sync to be right.
The emulsion tube under the primary main jet may have one or more holes still plugged. You have the press in jet. You can try pushing it out from the throat being careful not to mushroom the tip. Or you can thread a screw into the end with a nut to use as a puller. how to remove keihin pilot jet - Google Search

okay so I went back to the shop and before I started pulling up my multi meter I decided to go through the carbs one more time and I realized that both of my air cut off valves have little holes and tears in them after holding them up to my flashlight! Could this be my problem? Creating a vacuum leak?

 

[longdistancerider]

Yes, that's a direct vacuum leak.

 

[Thustle]

Yes, that's a direct vacuum leak.

okay So I put my new air cut off valves on and it helped but it definitely did not fix my problem. I realized that I’m flooding out I believe, first gear polls pretty good second gear is worse third gear is worse fourth gear is terrible and fifth gear kills the bike.so I put my new air cut off valves on and it helped but it definitely did not fix my problem. I realized that I’m flooding out I believe, first gear pulls pretty good second gear is worse third gear is worse fourth gear is terrible and fifth gear kills the bike This is all on wide-open throttle the bike maxes out at maybe 50 mph. I’m really starting to get frustrated and I might just buy some Mikuni 30s. Some guy set them up specifically for this bike for 350 bucks. What I rather run stock yes 100%.

 

[longdistancerider]

So you haven't check the ignition system yet? Using already fouled plugs or new ones?

 

[Thustle]

So you haven't check the ignition system yet? Using already fouled plugs or new ones?

new plugs, ignition I checked with timing light today. It was kind of glitchy on idle, when I would rev it would show the two lines pretty good..

 

[longdistancerider]

Well then it's back into the carbs to sort out why you're ending up rich. This is going to require complete attention to detail without using the idea of "I've already checked that before" syndrome that's so easy to fall into.