Topic: CB400F CR26 carb questions

[Speedy Siegl Racing]

Hi,

I am having a hell of a time figuring out how to make this bike run properly!

Specs:

1977 CB400F, Yoshimura 466 kit, lightened flywheel, oil cooler, Web Cams 303 grind "hot street cam" (0.328/0.328 lift, 270/270 duration, 248/248 duration at 0.050"), Dyna S ignition, Dyna 3 ohm coils, accel plug wires, Shorai battery, CR26 carbs, yoshimura header/exhaust.

Carb specs:

65 pilot, 230 air jet, 100 main jet, 2.0 slides, yy0 needles with clip all the way down

Symptom:

1/4 throttle "bog": Motor has a ton of torque at the bottom end (3-5k rpm), but when you give it throttle anything but very very slowly it will fall on its face. On the dyno, my air fuel ratio at neutral throttle (not accelerating or decelerating) at about 10:1 which isnt great, then when I roll on the throttle and it bogs the ratio spikes lean to 18:1.

I have tried 3.0 and 4.0 slides, all different needle positions, YY7 and YY6 needles, and different idle air mixture screw settings to try and even this weird scenario out and nothing seems to work!

Any input would be appreciated.

Thanks!

[Powderman]

What air filters are you running?
10:1 is too rich, and 18:1 is lean enough to burn holes in pistons. Should be around  14:1 for naturally aspirated gas engines.

[Tim2005]

Hi, luckily a lot of work has been done on this before. Have a read through of this to start with:

http://forums.sohc4.net/index.php?topic=42903.0

[Speedy Siegl Racing]

Thanks guys.

Powderman, I know where the ratio needs to be, do you have any input on how to get it there?

Tim2005, I have read and reread that thread many times. Unfortunately, my cam is different than theirs and neither of the two detailed setups yield a positive result on my bike.

Any other ideas?

Thanks!

[flybox1]

Whats your air screws setting now?
Have you tried just upping the mains(110/115) to get the 18.1 down, first? (all else stays the same)

[Speedy Siegl Racing]

Flybox1, I have had air screws all over the place to maintain a 12-13:1 idle air mixture through all the other changes. With the 2.0 slides it was at 0.5-1 turn out, with the 4.0 slides its around 3 turns out. My initial setup was with a 110 main and that did not affect the spike. After I pull through the bog zone it pulls awesome and the main jet seems a little rich even with the 100 main: 12:1 at 10k.

Unless anyone has other ideas, Im going to try to turn my needles down and make a custom profile to get me a lot of fuel where it spikes. At this point its either custom needles or put the stock cam back in so its rideable and that would be sad!

[flybox1]

good luck...keep us posted.

[Tim2005]

Hi again. Yes one of the two bikes in that long thread is mine, and indeed the cam is different, other things too. Anyway, 4 suggestions; firstly I see you are in Seattle. Also in Seattle somewhere is Kevin Mullin, who has done a ton of work on 400s over the years, he's in the facebook group "Vintage Honda CB400f Club" too. It would be well worth you looking him up. OK, on with other ideas, in reverse order of likeliness.  Firstly, are you certain you have sufficient fuel supply to the carbs? The symptoms do sound like they could just be fuel starvation. Secondly, what era Yoshimura exhaust is it? There are 3 types I'm aware of- (a) vintage flat collector type, (b)vintage square collector type,  (c) modern.   Type (b) are quite hard to jet, they flip rich-lean in the midrange.  Type (a) are worth a fortune, and I've not tried a modern (c) one yet. Thirdly, that cam. from my info those numbers are a Web "Cafe Race" cam, which has an extremely long duration, 40 degrees more than a Yoshimura ST2 which was a popular cam for racing here in the UK, I've never heard of anyone using one.  I think I'd try swopping that out & see if you can get it running cleanly on a stock cam, then look at a Yoshi ST1 or ST2 instead.

[SOHC4 Cafe Racer Fan]

Tower (and Isaac),

Ian H says: Try some longer velocity stacks or a plenum to control the air flow, along with a smaller pilot jet.

[Speedy Siegl Racing]

Thanks guys!

CB750 fan, this is Isaac, and I am unable to fit any longer velocity stacks without hitting the frame.

Tim2005, I will look for Kevin. Is that 400fourstore Kevin? I am 100% positive I am not running out of fuel. I have a pingel petcock and have been doing most of these tests with the tank off and using a test bottle instead while on the dyno. For the exhausts, the collector is round, but the header pipes mate to the collector in a square pattern... is that what you were thinking? After the collector it has the hard angles out to the side of the bike, then straight back instead of the curved ones I have seen. Regarding the cam, would it be worth re-grinding it to the ST2 duration if the lift is ok? I would hate to have done all this work to put a stock cam back in it!

[SOHC4 Cafe Racer Fan]

Thanks guys!

CB750 fan, this is Isaac, and I am unable to fit any longer velocity stacks without hitting the frame.

Tim2005, I will look for Kevin. Is that 400fourstore Kevin? I am 100% positive I am not running out of fuel. I have a pingel petcock and have been doing most of these tests with the tank off and using a test bottle instead while on the dyno. For the exhausts, the collector is round, but the header pipes mate to the collector in a square pattern... is that what you were thinking? After the collector it has the hard angles out to the side of the bike, then straight back instead of the curved ones I have seen. Regarding the cam, would it be worth re-grinding it to the ST2 duration if the lift is ok? I would hate to have done all this work to put a stock cam back in it!

Isaac,
If you're looking for Kevin the CB400F restoration and parts guru from South America, I think his handle is ttr400.  stonej@mweb.co.za
Don

[Tim2005]

Hi Isaac, yes that is 400fourstore/ KSM Enterprises Kevin, he went abroard for a while but is back in Seattle now I believe. I'll send him a link to this on facebook.   Pingel tap situation sounds good. Not so sure about that header, with an angle after the collector, could be another type I've not seen before (any pics?). As for the cam, see if you can find Kevin first before making any major decisions, it'd be much better to get views from someone who can actually get to see the bike & get a feel for what the motor is doing. Regards

[Kevin]

Hi Guys,
I'm honored you'd think of me :-)
But I'm afraid I won't have much to add that hasn't already been tried before.
I'm not presently living in Seattle, but do plan on returning there eventually. I'm on the East coast now.
Kevin Bidgood, TTR400, my brother from another mother lives in Pretoria, South Africa, not South America.

I have two CB400F's with CR carbs.
One is basicly a full race motor and runs great with the stock setup the way the carbs were delivered from Sudco, and if memory serves me correctly this one has a Webcam. It does have the aluminum velocity stacks from Dragon Industries ( I am the one who provided them a stock stack from the CR so they could duplicate).

My second 400F with CR's does not run that great. It also will fall flat on its face if you try to roll on the throttle too quickly.
I just bought this bike and it is also one of the bikes mentioned in the link that Tim provided Reply #37 is this bike, with tech details). Dave Sprinkle had modified the CR's throttle linkage for a mutual friend of ours. This one has the K&N pods on it.
I think it ran better before sitting for a while. I have cleaned the gunk out of the CR's and that made a huge difference, but I think it has room for improvement.
I'm slowly working on this bike, it is here on the East coast with me and I hope to have it running a bit better.
Idle is about 2100, below that it slowly starts dying.
It will scream along at 12K rpm but you have to roll on it slowly to get there.
I don't have access to a dyno, so I don't have anything technical I can supply ~ sorry.

Good luck and please keep us posted on what you find ~ I will do the same.

[SOHC4 Cafe Racer Fan]

Hi Guys,
I'm honored you'd think of me :-)
But I'm afraid I won't have much to add that hasn't already been tried before.
I'm not presently living in Seattle, but do plan on returning there eventually. I'm on the East coast now.
Kevin Bidgood, TTR400, my brother from another mother lives in Pretoria, South Africa, not South America.

I have two CB400F's with CR carbs.
One is basicly a full race motor and runs great with the stock setup the way the carbs were delivered from Sudco, and if memory serves me correctly this one has a Webcam. It does have the aluminum velocity stacks from Dragon Industries ( I am the one who provided them a stock stack from the CR so they could duplicate).

My second 400F with CR's does not run that great. It also will fall flat on its face if you try to roll on the throttle too quickly.
I just bought this bike and it is also one of the bikes mentioned in the link that Tim provided Reply #37 is this bike, with tech details). Dave Sprinkle had modified the CR's throttle linkage for a mutual friend of ours. This one has the K&N pods on it.
I think it ran better before sitting for a while. I have cleaned the gunk out of the CR's and that made a huge difference, but I think it has room for improvement.
I'm slowly working on this bike, it is here on the East coast with me and I hope to have it running a bit better.
Idle is about 2100, below that it slowly starts dying.
It will scream along at 12K rpm but you have to roll on it slowly to get there.
I don't have access to a dyno, so I don't have anything technical I can supply ~ sorry.

Good luck and please keep us posted on what you find ~ I will do the same.

Don't know why I said South America.  Yes indeed that Kevin is from Pretoria, S. Africa.

[scottly]

Isaac, please post your dyno graphs with the A/F data.