SOHC.co.uk Forums > CB350/400

400/4 carb flat spot

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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! ;D 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!

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