Topic: CB400f flat spot at 2K-3K

[mca2]

I've finally got my CB400f running after sitting in a damp shed for 18years. I had to fix a couple of things (refurbish the master cylinder, brake piston, unstick the clutch plates, replace the ignition bits, swap some tires and clean out the carbs and replace al the rubbers inside), but now it's running. Running stationary at 1200rmp is fine and it can also rev up to 9000rpm, but i have this massive flat spot between 2000 and 3000rpm. If i gently increase the throttle, the revs go up to about 2000, then it hangs a bit even if i increase the throttle and then it jumps to about 3000rpm. If i engage first gear and try to drive off, i have to make sure revs stay well above 3000 otherwise it nearly dies on  me.

The problem is worse when the engine is warm. After reading a bit on the forum, it could be that the engine is running too rich. I didnt have the chance to inspect my spark plugs at these rmps yet, but at stationary revs the plugs are nicely brown as they should be.

As the flat spot occurs at low revs i wonder what could help me. Dropping the needle one step? Opening the air screws a bit further? Or should i start spending money on new main/slow jets and needles?

Any help or hints would be appreciated.

[Gordon]

As the flat spot occurs at low revs i wonder what could help me. Dropping the needle one step? Opening the air screws a bit further? Or should i start spending money on new main/slow jets and needles?

Don't go throwing cash at the problem or making any changes until you know what's actually wrong.  Do a plug chop at the engine speed where the problem occurs so you know what's going on first. 

And make sure the mechanical timing advance behind the points is opening and closing smoothly. 

[adamrice]

I have a '76 cb400f also just returned to running order after 7 to 8 years storage and with exactly the same problem as listed above. I would be very interested if the suggested solutions worked (what is a plug chop?) or if there are any other suggestions that may solve the problem.

[jessezm]

Did you clean the emulsifier tubes in the carbs thoroughly?  I've heard that the very tiny holes can get clogged and starve the gas supply at certain rpms.  Just a thought.

[camelman]

Gents,

I just went through this with my CB350F (everything is basically the same as yours).  The first thing you should look at (after cleaning the carbs), is to get a new air filter.  The old one is shot... period, and it will completely screw up the mixture.  Also, make sure you have the lid on the stock airbox.

I spent a lot of time screwing around with mine before getting a new air filter and finding a lid for the airbox.  It made it into a whole new motorcycle.

Camelman

[flybox1]

Gents,

I just went through this with my CB350F (everything is basically the same as yours).  The first thing you should look at (after cleaning the carbs), is to get a new air filter.  The old one is shot... period, and it will completely screw up the mixture.  Also, make sure you have the lid on the stock airbox.

I spent a lot of time screwing around with mine before getting a new air filter and finding a lid for the airbox.  It made it into a whole new motorcycle.

Camelman

+1
new air filter anD clean your carbs again.
be sure to pull your main jets, and slide out the emulsifier tubes hidden underneath them.
clean them up really well, and be sure the tiny holes along it are clean and clear.
its probably gunked up and the fuel has a tough time atomizing when sucked thru these holes, giving you the flat spot.

[fmctm1sw]

And make sure the mechanical timing advance behind the points is opening and closing smoothly. 

+1.  The manual on my 350F states it should open starting around 1400 RPM if I recall.

[fasturd]

+1 on the re-cleaning of the carbs and then make sure the timing is spot on. Obviously the air cleaner needs to be clean and the factory air box in place. 400s and CBs in general are notorious for not running well if not specifically tuned for lack of an airbox or while running pods.
400s are not known for any obvious flat spots so something just isnt right. Shouldn't be hard to solve.

Good luck.

[Tim2005]

+1 on cleaning the carbs, particularly the pilot jets - the centre hole through them is only 0.4mm and easily gets clogged.

[bwaller]

Tim has a point. I had one that I struggled with a while. I had cleaned the pilot jets (a couple times) & they were open, but not completely. It sounds like the same problem you're having. In the end to get them clean I heated them to "glowing" and turned whatever was left in them to ash and then carb cleaner finished the job. No more stumble off idle.

[flybox1]

heated them to "glowing" ........and then carb cleaner finished the job.

hopefully not when they were "red"!

[adamrice]

Thanks for all the great information, there are several things here to try. The air intake and exhaust are complete and stock standard (including the airbox lid luckily) - air filter is unknown age so here is the first step, I will get a new one.
The carbs were ultrasonically cleaned and I did have to reclean the pilot jets as they were still partly clogged even after the first clean. I have missed the aerators behind the main jets so will go back to check them & clean them out if necessary.
I will also check the advance/retard as other than checking the points (which look fine) I have not done anything in that area.
Thanks again, I will report back in a few days.
 

[adamrice]

I have now gone through all of your suggestions and the problem is so much better as to be almost but not totally cured.
1. New air cleaner:- made no noticeable difference at all to the flat spot but I now know I have the air intake just right.
2. Checked the timing and although it was OK the flash on the timing gun was slightly intermittent. So I got new points and one condenser (David Silver sent only one although I asked for 2). My  Haynes manual says that a misfire that gets worse as the engine heats up may be a faulty condenser. Fitted points and condenser and carefully re-timed and the problem seems a bit better but not gone.
3. Took out the carbs again and cleaned everything again with carb cleaner. This time I took out the main jet emulsifier tubes as recommended and they were very gummed up so cleaned them thoroughly too. By the way I tapped them out very carefully using a piece of brass modellers tube (5/32")which just fitted onto the top of the tube and did not damage the surfaces at all. Just been for a ride and the improvement is very marked.

I will replace the other condenser and give the bike a few decent runs to see if it settles down on its own. I will also check plug condition after some runs to see if this tells me anything further. Suffice to say that the flat spot is now so slight as to be barely noticeable so thanks alot for all the advice and the successful outcome.

[camelman]

Once you are all done, I'd recommend a carb sync.  That should cure the last of it.

What shape are your plug wires in?

Camelman

[flybox1]

Once you are all done, I'd recommend a carb sync. 

Camelman

+1!!!
(as long as your valves are gapped and your timing is spot on...)


then ride it like you stole it! 

[adamrice]

I got my license in July and went for a ride to try the bike out and found some more problems to solve.
 
First there was an oil leak from the rocker cover caused by a flattened and perished gasket that I will replace. While I have the rockers off I am going to replace the cam chain & tensioners. I have a soft link join for the new chain so intend to run it through the crankcase attached to the old one and then re-join it with the soft link. Any hints will be well received!

Second the oil pressure light came on from 2500 rpm & below when the engine was hot. I have taken out the oil pump and checked it out. All well within wear tolerances but again all the O rings were hard and flat so I have replaced them all (except for the tiny gearbox oil pump which I didn't touch). I hope this sorts out that problem as it seems logical that oil bypassing the O rings will be worse when it is hot and thin and in percentage terms will be worse when the revs are low. We shall see...

Also took off the sump and cleaned out some smallish amount of black sludge and bits of gasket but nothing that looked worrying.

Took off the exhaust headers to get to the sump and found that there were no exhaust port gaskets! I like to think that this will have a big influence on my incompletely resolved flat spot problem so have got new gaskets and will fit them when I re-assemble.

Because of work commitments the bike is going slow at present but I will report again when I have got all the above done. Of course I am also cleaning and polishing everything that comes off so that also makes for slower progress!

PS Synced the carbs, a couple of months ago and I am fairly sure that all the wiring is good.

[Industrial Rat400f Killer]

Clean your gas tank and replace fuel lines too, after sitting for that long there is most likely some gunk all the way through the whole system. It would suck to clean the carbs and then have gunk from the tank go right in there on the first run.