CB400F Jetting issues and running rich (solved)

Evening gents

Been having some issues with my CB400F. Currently trying to tune for pods but its a total nightmare, iv tried all sorts of combinations from smaller jets to larger jets changing the float heights and even moving the needle, Lots of combinations iv tried but just cant find the sweet spot.

The bike runs really rich so as you can imagine i go through a shit ton of plugs. I finally made a little progress yesterday by moving the needle shim up to the 4th shim, putting a #42 pilot and a #82 main. got the bike started and running for around 2 mins but eventually it died out. iv benched synced the carbs and vacuum synced them and tried to adjust them whilst i had the bike running.
iv attached a picture of the plugs to show you the results... any advise would be greatly appreciated.

Re: CB400F JETTING ISSUES AND RUNNING RICH

So when you say running for 2 minutes....does that mean you are just trying to let it idle? If so, the main jet has nothing to do with that, and the slide won't do anything either. What are you mixture screws set to? If you are running 42 and the stock were 40's, you probably didn't even need to change them. In any case...you might have to lean the crap out of them. Try something a little crazy like 3 or 4 turns out and see if it runs any better. Get us a little more information to how you are running the bike, twisting the throttle? Just idle? All of the above? What pods? Cheap ones? Etc.

Re: CB400F Jetting issues and running rich

Thanks for the quick reply.

So the answer is yes trying to get it to idol properly (currently struggles without a little throttle). Mixing screws are 2 turns out although i have had them out further to see if it makes any difference but the bike wont idol long enough without stalling and then once it stalls it is unlikely that it will start again (fouled plugs are my guess).

The bike has a 4 into 1 exhaust (Delkevic) and after market pods (cheap ones).

Due to the bike being built from the ground up most of the old parts have been replaced with brand new items there isint much that is standard on the bike, including the electrics which has now been replaced with the M unit. All brake hoses have been replaced, Throttle cables, new coils and plugs. Iv done a service on the bike also (new oil filter and oil change) All things on the 3000mi maintenance apart from the ignition timing have been ticked off whilst the bike was being built.

When i finally got round to starting the bike for the first time after i completed the work she fired up and idled fine (this was with the pods fitted and standard jets) i could even give her a bit of throttle and she reacted fine until i got over half throttle where she bogged down a lot (i do have a video of this glorious moment).

After seeing how the bike reacted i thought to myself does it even need jetting ? But i proceeded to re jet the carbs anyway BIG MISTAKE. Ever since then the bike has ran like crap kicking out white smoke followed by black smoke at high revs. The plugs are sooty and get fouled very easily. After taking the carbs off and cleaning them numerous amounts of times and changing jet sizes god knows how many times i finally decided to put a bit more effort into the carbs by taking them apart fully and replacing all the seals, float valve, new standard jets, air flow screws basically all the stuff that comes in a carb rebuild kit.

Thanks in advance

Re: CB400F Jetting issues and running rich

Excellent. You've answered quite a bit.

OK. So let's go back to your stock pilot jet first. Sound like you are flooding out before you even get the damn thing warmed up. A rich condition only gets worse as you get hotter.

I just did a quick search and found this thread at SOHC forums. http://forums.sohc4.net/index.php?topic=56172.0 they said a SMALLER pilot jet will actually be better. Similar setup with more done to the engine is using a 38 pilot. So I would think that with your stock pilot jet (40) you could probably lean it out enough to get her running more than a few minutes at least

Next up is jetting. 75 should have been stock, or at least most likely. Now I'm going to give you general rules here, so don't hold me to them, but they are what most live by I would think...

Pilot jet, deals with idle and off-idle throttle response. Slow response = too lean, or too rich.
Slide needle height deals with the transition to the main jet. Or, mostly the "mid range"
Main jet is really anything from 1/4turn to full throttle. It's what limits your top end and has relatively little to do with the bottom end.

So think about it like this: Each one is a step to the other. Too large of a step or too small of a step will create a stumble. Your goal is to find the smoothest transitions to each with the main jet being how the bike reacts under wide open throttle and top end speed.

Re: CB400F Jetting issues and running rich

Solid advise that i will take in to consideration tonight when working on the bike. I currently have #40 pilot installed but i also have a #38 which i will have a play around with as well. Last time i used the #38 the bike did run a little better

The set up i will use tonight is

#38 Pilot
#75 Main
Needle shim on #5 setting (Drop needle)
New plugs
Pods/velocity stacks
Air screws 3/4 turns out then adjust accordingly
Bench sync
Vacuum sync
Crossed fingers
Wishful thinking

Thanks for the help ill keep you updated.

Re: CB400F Jetting issues and running rich

Hi
All good advice. I have been rejetting my XS750 after fitting pods. I also fitted rebuild kits when I went up 2 sizes on mains. Big mistake. The primary jet and the primary air supplied in the rebuild kit were not correct. My carbies need

Sent from my GT-I9506 using Tapatalk

Re: CB400F Jetting issues and running rich

XS750AU said:

Hi
All good advice. I have been rejetting my XS750 after fitting pods. I also fitted rebuild kits when I went up 2 sizes on mains. Big mistake. The primary jet and the primary air supplied in the rebuild kit were not correct. My carbies need

Sent from my GT-I9506 using Tapatalk

Click to expand...

Sorry hit the wrong button.
The pilot jets in the kits were 42 non bleed type, they should have been 42 bleed design. The primary air vents in the kit were half the size of the originals. Result was way too rich. Don't assume rebuild kits are 100% correct.
One other trap could be that the inner flange on the pods are covering the primary air entry which would make it rich, just check they have free air.
Otherwise just take one step at a time.
Good luck.

Sent from my GT-I9506 using Tapatalk

Re: CB400F Jetting issues and running rich

If you have any brass in your carbs from rebuild kits, take it all out, throw it in the bin and replace with OEM parts. There are no original tuning parts in your carbs that will ever wear out. Use factory Keihin jets only. Make certain that the pilot air circuit is clean. With absolutely stock carbs your bike should idle fine and be drivable. These carbs are immune to the pilot air being blocked by the filter, so as long as the filters flow decent air that isn't it. Most of the carbs I see are simply dirty. Yes, those are the ones "professionally, personally, ultra-sonically etc, etc" cleaned. These are pretty simple carbs but if the pilot air is restricted you will get the symptoms you describe. It is a bit odd for all the carbs to be running so rich. Make sure the ignition has plenty of battery power.

Re: CB400F Jetting issues and running rich

Update

So went back through the carbs stripped them again put all OEM parts back in the bike along with original jets, float heights set to 22mm, Bench synced and vacuum synced, the only thing iv changed is the needle height which is on setting 5 (top setting)

Bike is still running bad and plugs are coming out worse than before just super carbon fouled within 1 or 2 mins of running the bike. bike will start and it will idol but only with quite a bit of choke and even when you give it some choke its either one way or the other super high revving or low almost about to stall.

Iv attached a video so you can see the bike running (almost running)

https://www.youtube.com/watch?v=6xjImnlExOI&feature=youtu.be

Re: CB400F Jetting issues and running rich

When you say needle position #5 do you mean needle fully up (rich) or needle fully down (lean).


Sent from my GT-I9506 using Tapatalk

Re: CB400F Jetting issues and running rich

Video is now un privatised.

XS750AU said:

When you say needle position #5 do you mean needle fully up (rich) or needle fully down (lean).


Sent from my GT-I9506 using Tapatalk

Click to expand...

So the needle is fully down inside the carb. So the shim position is at the top of the needle (furthest away from the needle tip)

Re: CB400F Jetting issues and running rich

Hi
Downloaded the CB400F workshop manual. Standard slow jet is #35, it also says that with the choke lever and air screw properly adjusted the engine speed should reach 3,500 to 4,500 rpm! The air screw opened 7/8 and the float height 21mm.
Hope it helps to get a base point.

Sent from my GT-I9506 using Tapatalk

Re: CB400F Jetting issues and running rich

UPDATE !

Seemed to have turned a slight corner today. After cleaning the carbs again taking them apart completely and going over everything with a fine tooth comb (copper wires actually) put them back together new plugs, i also benched synced them a little more open than usual. I also set the floats to 24mm

Set up as follows

40 pilot jet
75 main jet
air screw 2 turns out
Needle on number 4 shim (1 down from the top)

Turned the bike over and it ran really smooth. As you can see from the video it actually idols so I'm guessing the next step is to start dialling it in, i started to do that but i ran out of fuel. The plugs are slightly black not carbon fouled but have fuel on them i can see slight patches of brown on them so I'm guess I'm getting close.

The bike seemed to idol fine but bogged down slightly when i got to half throttle. I guess a larger main jet will sort this out. What do you suggest would be the next steps and is there a certain way i should dial the carbs in, as in one cylinder at a time ?

Feeling a bit better about my progress and iv definitely turned a corner.

https://www.youtube.com/watch?v=O6AmKxvU7lc

Re: CB400F Jetting issues and running rich

You must change all 4 carbies with any modifications.
As you are running ram tubes and depending on your exhaust system you could probably go up 3 or 4 sizes on your mains. You should move the needles to mid position, so you have room to tune when you get close with the main.
If you are not going to ride it to test then you need a good air supply to keep the engine close to the right temp. I use one of those cheap electric leaf blowers. When you get close you must ride the bike.
Remember one step at a time now it is running.

Sent from my GT-I9506 using Tapatalk

Re: CB400F Jetting issues and running rich

Ok, there's one more thing to check for. This is going to be kind of hard to explain....

You say you've adjusted sync quite a few times. I worked on a guy's set of carbs a few weeks ago that had the mixture screws all so tight that it was causing a high idle and all sorts of other gremlins. I adjusted them loose, and got them as close as I could by eye and noticed when I did that, his idle screw adjustment was able to come down 10 turns or so...yeah..a LOT. So make sure you aren't pushing the adjustments to their limits. They will all be "in sync" but will not be the proper sync if that makes sense?

I was out of town and didn't get to reply sooner. Good work so far!!!

Did I miss anything about your timing? Is that dialed in? If not, now that you have it idling, you HAVE TO set that properly. Hopefully you, a friend or an automotive place can rent you a timing gun. You want to make sure that at idle, you are on the F mark and when revving up (2.5k+) you are at the 2 hash marks for the advanced timing. If not, those things can get stuck in and need to be lubed properly. Again, great work so far! Still getting too much fuel it sounds like, so open up those air screws (mixture screws technically on this carb) another 1/2 turn or so.

Re: CB400F Jetting issues and running rich

UPDATE !

Seemed to have turned a slight corner today. After cleaning the carbs again taking them apart completely and going over everything with a fine tooth comb (copper wires actually) put them back together new plugs, i also benched synced them a little more open than usual. I also took advise from another thread about float heights being 24mm set mine to that.

Set up as follows

40 pilot jet
75 main jet
air screw 2 turns out
Needle on number 4 shim (1 down from the top)

Turned the bike over and it ran really smooth. As you can see from the video it actually idols so I'm guessing the next step is to start dialling it in, i started to do that but i ran out of fuel. The plugs are slightly black not carbon fouled but have fuel on them i can see slight patches of brown on them so I'm guess I'm getting close.

The bike seemed to idol fine but bogged down slightly when i got to half throttle. I guess a larger main jet will sort this out. What do you suggest would be the next steps and is there a certain way i should dial the carbs in, as in one cylinder at a time ?

Feeling a bit better about my progress and iv definitely turned a corner.

https://www.youtube.com/watch?v=O6AmKxvU7lc

Re: CB400F Jetting issues and running rich

So i managed to get the bike idling really nicely, got her all warmed up and put some new plugs in. Started her back up and ran the bike for 10 mins (with fan in front) pulled the plugs and here is the results.

Weirdly cylinder 1 and 4 took a while to warm (don't think there firing)
Cylinder 2 was really hot
Cylinder 3 was hot but not scorching.

Pics to follow

Re: CB400F Jetting issues and running rich

plug color doesn't tell us much about mixture, but the left plugs look like they did not fire at all or at least not much. That could be c=deceptive though because at idle plugs barely get warm and don't color much if at all.

Are 1 and 2 on the same coil by any chance or is it 1-4 and 2-3 or something else? 1 and 4 look like they may have run a little, and 3 is working hard and getting much hotter, and two is asleep still.

Re: CB400F Jetting issues and running rich

teazer said:

plug color doesn't tell us much about mixture, but the left plugs look like they did not fire at all or at least not much. That could be c=deceptive though because at idle plugs barely get warm and don't color much if at all.

Are 1 and 2 on the same coil by any chance or is it 1-4 and 2-3 or something else? 1 and 4 look like they may have run a little, and 3 is working hard and getting much hotter, and two is asleep still.

Click to expand...

The coils operate as follows 1,4 - 2,3.

Iv currently got 2,3 running perfect but 1,4 are my issues currently, Took them both off to see if they have a spark and they DO have a spark so no issues with firing. my next idea is to have a look at possible blocked pilots.

Re: CB400F Jetting issues and running rich

Timing though? Looks like 1/4 are still a little "wet" looking from the photo.