Topic: Throttle Lag (Flat Spot) When Hot - '75 CB400F - WTF?

[Senna27]

Hi - This has now entered the "is there nobody who can fix this bike stage"? I've got a '75 CB400F with 25,000 miles on it. I bought it last year and the bike is in good shape.

The problem is, as it's been from day one, is that it develops at rather annoying flat spot, throttle lag, etc. when it gets warm.

I was told the carbs were rebuilt and balanced before I bought it. I had another person check it out and adjust the carbs/mixture. I had a Dyna S ignition kit installed along with new coils, plugs and wires. It runs perfectly when cold and I mean perfectly. I'm able to blip the throttle during downshifts and it's really crisp but when it warms up it runs like a piece of crap. It develops major throttle lag.

Not sure what to do now. If I don't figure this out soon, I may just sell it and let someone else deal with it. Very frustrating!

I've spent a bunch of $$$ on this bike and it still doesn't run right. I've had to cancel a trip with 6 friends next weekend because of it. It's driving me nuts... Thoughts?

[TwoTired]

Do you have stock exhaust and air filter?

[Tower]

If the bike pulls harder at high rpm when cold then when warmed up, it indicates the main jet is too large.  If it pulls better cold in the mid range rpms, then lower the needle.

As twoTired has elluded, improve the air flow by cleaning /replacing the air filter and/or reduce the exhaust pressure (e.g. by replacing stock pipes with 4-1 headers)  Or, reduce the main jet size.

[TwoTired]

The problem described, I've also hear from 400 owners who put pods on the motor, instead of the stock filter.  No mater what jetting changes they made the transition from t idle to higher RPM was never "right".  The throttle slide cutaways were responsible for this and changing their contour is not for the faint of heat.  Most fixed the throttle response lag by returning to stock air filter configuration.

Cheers,

[Tower]

Besides the idle air mix, couldn't you play with float height to fix any lag from idle?  e.g. lower the fuel level if the bike runs well at small throttle when cold, but starts to show signs of lagging as temp rises.  Lowering the fuel level will then lean the mix so its correct at operating temperatures.

Edit: Since he's talking about blipping while downshifting producing lag, that would suggest high rpm low throttle opening issues. (not idle issues). Pilot fuel mixture screw, float level, and pilot jet size also affect high rpm, low throttle snaps.

Edit2: @senna27, what happens to the rpm when you blip the throttle.  Does rpm drop then rises to proper rpm? or lags then drops to the right rpm?  Also, at what rpm and throttle positions are the lags?

[TwoTired]

You should probably tell us what spark plugs you have installed, and what the combustion deposits look like on the spark plug insulators.

[Senna27]

Do you have stock exhaust and air filter?

Thanks for all the input. Now I'll try to answer your questions.

Yes, OEM header, muffler, air filter and air box. The air filter is brand new.

[tsflstb]

What are your valve clearances and when is the last time they were checked?

[Senna27]

Besides the idle air mix, couldn't you play with float height to fix any lag from idle?  e.g. lower the fuel level if the bike runs well at small throttle when cold, but starts to show signs of lagging as temp rises.  Lowering the fuel level will then lean the mix so its correct at operating temperatures.

Maybe the float levels aren't correct. With the bike on the side stand, I started it the other day and let it run for about a minute. After I shut it off, I noticed the two left side carbs were leaking fuel out the drain lines. The petcock was "off" and I checked the drain screws to make sure they were tight.

Is this normal? Maybe the floats are sticking.

[Senna27]

Edit: Since he's talking about blipping while downshifting producing lag, that would suggest high rpm low throttle opening issues. (not idle issues). Pilot fuel mixture screw, float level, and pilot jet size also affect high rpm, low throttle snaps.

Edit2: @senna27, what happens to the rpm when you blip the throttle.  Does rpm drop then rises to proper rpm? or lags then drops to the right rpm?  Also, at what rpm and throttle positions are the lags?

If you want to make a smooth downshift when it's warm, you have to open the throttle up a fair bit (about 1/2 way). If you open it less it just doesn't rev up so you can't match revs.

I think you're right in saying "high rpm low throttle opening issues". For example, I'll be taking an on-ramp with the engine around 5-6000 RPM with the throttle open about 1/4 to a 1/3. If I close the throttle and open it again (like you would feather the throttle through a corner) there is a really noticeable stumble as it opens.

[Senna27]

You should probably tell us what spark plugs you have installed, and what the combustion deposits look like on the spark plug insulators.

To be honest, I'm not sure what plugs my mechanic installed but it had the the same problem before and after the plugs were changed.

I'll have to ride it a bit more and check the plugs to see what they look like.

[Senna27]

What are your valve clearances and when is the last time they were checked?

I had my mechanic adjust the cam chain and valves less than 100 miles ago. He is an experienced 400F mechanic so I assume they were set to factory specs.

[mlinder]

Please tell us your:

Altitude
Kind of plugs you are running
What your plugs look like
What size your main jets are
Needle clip position

Again, sounds like you are running rich.

Quick test:

Pull out air filter, ride bike 'til it warms up. See if it exhibits the same behavior.

[Senna27]

Please tell us your:

Altitude
Kind of plugs you are running
What your plugs look like
What size your main jets are
Needle clip position

Again, sounds like you are running rich.

Quick test:

Pull out air filter, ride bike 'til it warms up. See if it exhibits the same behavior.

Altitude: 250' - 550'
Kind of plugs you are running: I'll have to check
What your plugs look like: I'll have to check
What size your main jets are: I believe they are stock but I can't confirm that
Needle clip position: Again I believe stock setting but can't confirm

The carbs haven't been apart since I bought the bike last August. I was told by the previous owner that the carbs were cleaned and balanced within the last year.

I will try removing the air filter and see what difference that makes.

[mlinder]

We need the last 3.

Try the quick test.

[Tower]

OK.  Based on @Senna27's answers, I'd recommend slightly lowering the fuel height (not more than 1 mm).  Also, I'd try lowering the needles 1 notch, since its clearly running too rich in the midrange.  (You should be targeting best power between 5000 - 7000 rpm.)

Following these changes, resync the carbs.

Retest at full operating temp and let us know how these adjustments effect your throttle response and we'll go again if not perfect.

Edit:  It would be best if you could report on three test situations.  The first is try blipping to full throttle when engine is at peak power (5000-7000 rpm).  The second test is blip to full throttle with engine at midrange (3500 - 4000 rpm) and the third is to smoothly but quickly throttle up (not as fast as a blip), starting at high idle ranges (2000-2500 rpm).

Edit2: I'm assuming you have the stock jet size and no performance mods such as hotter cam, pistons, valves, pods or exhaust system.

[Senna27]

We need the last 3.

Try the quick test.

I'll see if I can get that info.

[Senna27]

OK.  Based on @Senna27's answers, I'd recommend slightly lowering the fuel height (not more than 1 mm).  Also, I'd try lowering the needles 1 notch, since its clearly running too rich in the midrange.  (You should be targeting best power between 5000 - 7000 rpm.)

Following these changes, resync the carbs.

Retest at full operating temp and let us know how these adjustments effect your throttle response and we'll go again if not perfect.

Edit:  It would be best if you could report on three test situations.  The first is try blipping to full throttle when engine is at peak power (5000-7000 rpm).  The second test is blip to full throttle with engine at midrange (3500 - 4000 rpm) and the third is to smoothly but quickly throttle up (not as fast as a blip), starting at high idle ranges (2000-2500 rpm).

Edit2: I'm assuming you have the stock jet size and no performance mods such as hotter cam, pistons, valves, pods or exhaust system.

I'll look into lowering the fuel height 1mm. How hard is it to lower the needles? I'll also re-sync the carbs.

I'll try the blip test although as I recall the problem only happens under load while riding. When the bike is cold and parked it revs up quite freely with no lag. When it gets warm it doesn't rev up as freely but the big stumble/flat spot doesn't really appear until your riding and it's under load.

Yes, jetting is stock as far I know. Everything else is unmodified. However, I am thinking about going to a louder pipe, perhaps a MAC. I should probably put that on before I do any jetting, syncing, needle adjustment etc.

[Tower]

yes, the final configuration including pipe and main jet should be set before playing with needle height and fuel level.  You will need to remove carbs to get at the slides and needle valves.

The blip tests I am referring to are appliable only under load and when engine is fully warmed up.

[mlinder]

This is an easy one to diagnose:

Please remove the air filter and ride it for a while. After it's hot, does it continue to bog?

[Senna27]

yes, the final configuration including pipe and main jet should be set before playing with needle height and fuel level.  You will need to remove carbs to get at the slides and needle valves.

The blip tests I am referring to are appliable only under load and when engine is fully warmed up.

Yeah, I'll get the pipe installed before I go any further. I'll do the blip test under load when warm.

Thanks for the tips.

[Senna27]

This is an easy one to diagnose:

Please remove the air filter and ride it for a while. After it's hot, does it continue to bog?

I'll do that test, if it ever stops raining! Arrrrrrrrrrrrrrrg...

[Senna27]

Hey guys...

Here's an update. After spending more money to have the carbs sorted out, I was told the problem was my bike needs a complete motor rebuild. Needless to say, I parked the bike and didn't ride it all summer.

About a month ago I was given the number of an excellent mechanic who specializes in vintage bikes, especially the CB400F 550 and 750's.

He picked up my bike just before Christmas and gave it a once over, leakdown test etc. He pulled the head and determined the valve seals were really bad but everything else, valves, pistons, rings, cam, timing chain and tensioner was all okay. He's sending the head out to get freshened up, lap the valve seats etc. He also looked at the carbs and knows exactly what's wrong with them. He guaranteed to have my bike running perfectly come spring.

Sounds promising for next year!

[mlinder]

Good luck!

Pretty bike.

[Senna27]

Good luck!

Pretty bike.

Thanks... I picked up a yellow tank and black side covers which will be going on this spring. The original bits will be stored if I choose to ever sell her, but I don't see that ever happening.