Topic: CB400F 1977 Throttle Lag

[palong]

I've just bought a 1977 CB400F 44,000 miles that has a throttle lag (bogged down feeling) up to about 1/4 throttle in all gears, then revs freely throughout the rev range and really pulls well at higher revs. It's had a top end overhaul by a Honda mechanic around 1000 miles ago, head skimmed, valves reground and valve guides replaced. Starts on the button and idles smoothly at 1200rpm. Advise please on where to start looking for the problem. Thanks Paul

[Trevor from Warragul]

I'll probably get flamed for this, but try opening the airscrews to 1 1/2 turns out.  Standard for my 350/4 is 7/8th turns out, but it bogs something awful at that setting, runs like a dream at just over 1 1/2 turns...

Trevor

[trueblue]

Sounds a little like it is running lean on the idle circuit, try richening it up a little, but a similar bogging can occur if it is way too rich, a little hard to tell which without actually being there, so if the aforementioned suggestion doesn't work try leaning a little, turn the screw 1/4 turn at a time then try, if it improves you are on the right track, keep going until no improvement is noticed there will be a small range of adjustment where you won't notice any difference in how it runs, tune it to the rich side of this range.

[Trevor from Warragul]

I agree with trueblue - adjust a little, test ride, adjust a little, test ride..

[jessezm]

Are you running with the stock airbox and filter set up or do you have individual pod filters?

[palong]

Thanks for the advice. The bike is totally standard, except for the exhaust, it's a David Silver pattern part.

[camelman]

Your main and pilot jets will affect carb operation all the way down to just cracking the throttle open. The typical carb charts don't show those two circuits having much, if any, affect at that point, but they definitely do on the 350/400 carbs. You mentioned having the top end rebuilt, but did not mention the carbs. With as many miles as your bike has, I would imagine your jets and emulsifier tubes...and emulsifier tube housings need to be cleaned. You'll likely find a fine mud in them with a claylike consistency.

Other things to check are timing, which will cause those exact issues if retarded, and cam chain stretch. A stretched cam chain will not allow the engine to run efficiently with the most noticeable performance issues at lower RPM.

I'd recommend checking timing first (make sure the engine is at the correct idle speed), then cleaning your carbs, and then replacing the cam chain if the problem is not fixed.

Camelman

[palong]

Thanks for the advice, I'll check the timing first. Are there any posts on carb cleaning ? Also when you adjust the air screws is it a tank off job ? 

[camelman]

Tank on for idle screws, or maybe just lift the rear a little. The FAQ section has a good carb cleaning writeup.

[palong]

Thanks for the advice so far, timing is OK so I'm moving me attention to the carbs. My air screws have nylon caps on them. I want to pop them off to set the screws at 1 1/2 turns out as a starting point. The outer carbs are easy to get at but I cant get the caps of the centre carbs. Is there a quick way to do this ?

[Bodi]

If your carbs still have the plastic knobs on the idle screws... they are definitely due for a good cleaning! I've never seen anyone put them back on a carb set (OK, for a showroom resto). Even if well cared for, after 35 years it's time to get in there.

[lucky]

Same old problem.

You want us to help get you bike running right without hesitation etc.,, but we need all of the facts at one time.
Like this:

Year of bike?
Size of bike?
idle jets?
main jets?
Air intake system?
Exhaust system and muffler?
Mixture screw setting.

Just copy and paste into a post and fill in the information.

Then it is much easier to help you.
BTW adjusting the mixture screws richer is the first remedy.As mentioned by MORINI.

If that won't do it then larger idle jets may be needed if you have a modified air box or modified exhaust.

[Doctorlumen]

+1 on pulling the carbies and cleaning. If the bleed screws were still capped, then cleaning at the very least is on order. Do some plug chops (also good info to look into on the site) and see where she sits. Also, do check that timing and insure it is correct. Past all that, you are looking at an increase in jets, electronic ignition...overbore, porting...the list goes on!

As a side note, I found that dropping a size on the idle jet helped the "take-off bog". But I'm running pod filters and bored carbs. And she still bogs a touch. 

[RickB]

palong,

I'd be interested to hear how you go about solving this. I have a very similar issue on my CB400F, I trying increasing the idle jets. Helped a little, but not as much as I'd like. I'm going to try playing with the float height when I get back to my bike. I have an aftermarket exhaust on mine though.

Keep us up to date and good luck,
Rick.

[classic bike tuner]

lower the needle position one clip  to first from above

gerrit

[RickB]

palong,

Any luck?

Rick.

[vames]

Was this bike a good runner at any time recently? Any reason to believe that anybody seriously messed with any major settings since it was a good runner?

All of this advice above may or may not be any good, but you're just going to frustrate yourself in the short term if your problem is just dirty carbs. Clean the carbs really well before you even think about stabbing blindly at the problem with changes in needle position, turning out the idle screws, changing jet sizes or anything else. Clean the sh*t out of them --  but don't go changing the settings to try and work around the sh*t. Do a full 3000 mile tune-up -- set timing, change plugs, oil, filter, synch carbs etc.

If the bike still doesn't run right after doing all that, then you might think about whether your carbs need to be changed from stock settings.