400/4 carb flat spot

[NE0]

Hi guys,
My 400/4 has always had a flat spot on the carb, literally at the point when you just open the throttle.
I've put up with it for years and now I've decided I've had enough and need to fix it.

I've replaced all the rubber parts and blown out all the passages with an airline, sync'd the carbs and resynch'd until i was blue in the face!......yet it still persists.

What am I missing, or where am i going wrong?

All suggestions welcome, i don't want to replace them but if thats what you recommend then so be it....hopefully you'll have some cheaper suggestions!..............

[SteveD CB500K0]

Get yourself a cup of coffee (and maybe a sandwich) and read through this.

I've been collecting FAQs for the last 5 years. Some of the comments may seem a little patronising (that's TwoTired for you...) but stick with it.

Then you'll know as much about SOHC4 carbs as anyone in the world!

Steve

[NE0]

Thanks for that Steve,
(The vast majority of the links are now all dead, but you probably know that)
i managed a big cup of tea and a bowl of cereal..

it would appear that its all down to how rich/lean the carbs are then for the flat spot.

Like i said, its always been there. I've owned the bike since the early 80's.
It was built in 76 and by 1984 it had 6 owners, maybe the flat spot was why it was sold on so many times! But I must confess it does seem to be worse these days.

I'm about to take the engine out : the cylinder base gasket is leaking. so I might as well change the seals while its out and the carbs will get another clean and refurb. Only last year I did all the rubbers on them, trouble is I've had an occasional fuel leak from the one of the overflow pipes ever since.
We all know how difficult it is to remove and replace the carbs, so i decided it could wait until they come off again to correct it! Now its winter, the bike is off the road while my Yam DT does all the work!

if i cure it this time , I'll write it up for others to read.

[totty]

In my experience a flat spot when just starting to open the throttle can often be cured by adjusting the pilot jets, try richening them first by 1/4 turn. It's worth a shot before doing anything more complex.

[NE0]

thanks for that totty,
 I think i'll do that before I remove the carb bank, If I can eliminate the flat spot BEFORE taking them off, then all I'll have to do is sort the fuel overflow problem, and not worry too much about taking them all apart!

The flat spot is literally as you open the throttle, as in.... you drop the revs to turn into a side street, reopen the throttle and then comes the  flat spot...everytime!...a little more throttle and off you go, at times you think its not going to pick up! but it always does!

[totty]

That's typical lean idle in my opinion, either the pilot jetting/adjustment is too lean, or part blocked, or a air leak.

Sometimes accompanied by popping and banging on overrun.

[Tomb]

Wouldn't it be easier doing a plug chop at those revs, takes the guessing out of it.

Ride the bike at the flat spot revs as long as you can, then kill the engine - clutch in - shut throttle all at the same time, coast to a stop and take the plugs out and they'll show you how the mixture is. (apologies if I'm teaching granny to suck eggs )

If you can't get a colour on the plugs richen them (screws wound out) for a start point

[bigfoot44]

I had exactly the same problem with my CB350f. I finally cured it by changing the main jets to 80's instead of the standard 75's. Modern fuels make the engine effectively run weaker so upping the jet size should combat this. The larger jets are available from David Silver on special order.

[K2-K6]

If it runs ok when you get past the flat spot then that suggests the main jetting is fairly accurate (plug coulour in general use will confirm this).

If it idles ok without stopping or hunting, then the slow jets can't be far away.

A question; do you get the flat spot when you pull away from standstill in first gear or is it smooth?

When you have the engine on overun with throttles closed then the engine will obviously be turned by the wheel without much fuel demand to keep it going.  if the idle circuit is below optimum during this phase, then you get the combustion chambers going lean so that to speed up again you then have to open the throttles initially before you get a response (i.e. a flatspot) in that to convert the combustion back to optimum to make the motor accelerate you are relying just on the mains being openend more than really needed for the road speed you want and giving you that "kick" that ends the flatspot.
To blend this more smoothly the idle circuit needs to deliver more, so the advice to open out the idle screws is worth trying but go out by at least one whole turn, the objective is to see if you get any improvement (not just an accurate change) if so then it would confirm the idle jets are responding to adjustment and then let you make a desicion as to what final change is needed.

I know that's a bit of a long winded explanation but hope it makes sense.

[NE0]

Thanks for the response guys but I posted the question back in january.

I increased the mixture screws back then and solved the problem.

However, shortly afterwards I took the bike off the road, stripped the engine, replaced all the seals, primary chain, transmission bearings, gearbox parts, forks and a shed load of aother parts!!! and its due its MOT tomorrow at 9am.
The rebuilt motor is nice and quiet but until i get it on the road I won't know if the flat spot has returned.......all being well it's still fixed.

[NE0]

well folks it passed the test. nice new MOT.

Took it for a spin and the flat spot seems to be resolved, got a bit of a tune up to do though!
will retighten the head tomorrow and then do the tappets.
Nice and quiet now, amazing what a new primary and can chain can do!

[K2-K6]

That's good news on both counts then for the Mot pass and still have the flat spot missing.

As a point of reference for anyone who subsequently looks up this thread about flat spots; one of the few minor adjustments you are given to trim this area of running is the idle screws as the manual gives the start point but it is fairly normal to tweak this to get what you need for your bike to run well in the ambient conditions you have.

As you've indicated the problem has been there for years and I certainly remember people trying to ride round this in the past, it's one of the most annoying places to have that poor response so if you ca trim it out it's well worthwhile.