SOHC.co.uk Forums > CB350/400

What a Shame..!!

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z1100r:
Well I got that wrong....It was seriously rich on the primary circuit not lean. I still dont know why. 2 turns on the air screw as my manual suggests leaves it 'hunting' away with sooty black plugs. 3 turns and its settled down to a nice throttle response, even tickover, and much cleaner plugs.
   I noticed when running it on my plastic bottle that as the fuel runs out (less in float bowls) it switched from eratic and lumpy to running lovely. Float heights are set at 21mm as manual. Its a strange problem. When cold the bike ran great, no choke was ever neede though. It all suggests a blockage in the air feed to the mixture screws, but i cant find any problem.

Turn your fuel off and see if yours improves (cleans up) as the petrol runs out (after say a minute on tickover). Makes a huge difference to this one. Suddenly it will purr at 800-1000 rev perfectly. Turn the fuel back on and it starts the lumpy horibleness again after a short while. The weird thing is that its all 4 cylinders affected exactly the same. I could accept one carb having an issue but not all 4 with exactly the same problem. It like someone has put a pin hole in all the floats. haha

I've not bothered connecting any throttle cables or airbox rubbers while I sort this so I can get them on and off quick. They are off again this afternoon for a final inspection and to set float heights to 22mm. 

At one point last night an awful lot of 'steam' was comming from the engine breather, mind you I had had it on tickover in the garage on and off for about 4 hours - engine was very hot.

Final go later today...on 3 turns out its very good, so if I cant find anything else thats what I'll leave it at and give the bike a good whizz around - do hundred miles on it and see if things settle down.


Edit:-

oh another thing....that foam breather filter at the bottom of the airbox...blimey that was just wet oily dust...it was horible.

Hmmmmm.. Ive just had a search around and it seems this is a very common problem with these carbs. It has to be floats. It has to be. haha. Well fuel levels in bowl..arrgh...I'll check that with a tube later.

PatM:
Its a shame the float bowls are not glass or plastic! I remember in the 70's when NIKI carbs first came over here, in my Anglia 1500/1600 (the latter for the rich-kids!)- the Nikki was a good cheap substitute for the Webber 28/32 twin-choke and 1/3rd of the price.
Anyway, back to the bikes... Ill be interested to know how yours runs on 3 turns out (the Haynes says 2 1/2, Ive had mine set at that but di once have them at 1 1/2 and it ran poor) Ive a feeling mine will start bnetter as I treated it to a full fuel tank yesterday as it had passed it MOT- If that's the case and it does start better from cold- then again, Ill have to remove and empty the tank as I put a picture of my fuel filter on another page on this site last week- that's what it looked like 2 years ago when I first got the bike running- and Ill be doing the cam-chain and nuts/bolts dance around the garden.

z1100r:
Float height seems a controversial issue on the 350f and 400f. People that have taken carbs off bikes that have not been tampered with previously have all measured the float height to be 24mm using the exact method I use (Vernier caliper set to float height value and float just resting on spring tip but not compressing it). It was imposible to tell what they were originally because they were seized solid on these carbs. So far I've read about 3 people stripping std 350f previously unmolested carbs and finding float heights to be 24mm. I've also read about loads of problems with rich slow running from people who have rebuilt carbs and set them to 21mm.

So I'll set them to 24mm later and see...24mm so rings a bell, I sort of remember an issue like this in 1983'ish before I had a manual when i raced the 460. yeah 24mm..Thats right..!!!

Dan:
I'm having a similar issue with my 400 - runs cleanly when cold but after the engine has warmed up it bogs down when you pull away, needs revving more than necessary to get moving but when the revs build up it pulls strongly and revs freely. If you shut off the throttle when riding along then open it again you can feel a hesitation, then the bike accelerates normally. I was going to have a play with the mixture screws and after reading this thread it looks like I may have to clean carbs again or check float heights. @ Z1100R - would value your opinion on this as you seem to have a good knowledge on the matter.

z1100r:
This is doing my swede. Those dammed carbs. I cant understand it, i've had them apart so many times now its stupid. There seems nothing wrong with them. The bike pulls and drives away absolutely perfectly when cold or warm but as soon as it gets hot the problem starts. The exact problem Dan and lots of others have.

What I cant fathom is that its all 4 carbs. Plug chopping the bike flat out and at all speeds the plugs are the most perfect light tan colour. Leave it  ticking over though and they go sooty black.

There is one thing of concern and that is when the bike is hot an awful lot of steam comes out the breather. Way too much. So I pulled the airbox breather pipe off and left it dangling, things improved, but the steam was still pouring out. It suggests that there is blow past on the pistons and maybe i have a stuck ring or something, but this thing runs so smooth and doesn't smoke at all. 

The air screws seem to have little effect past 2 turns out yet if if i screw them in to 1 turn the bike will stall. 3 turns or even 4 turns makes no difference. Normally twiddling an air screw makes the engine speed up or down.

I have to say its been getting better every day and it is now quite nice but its definately still there.

Floats are 24mm, airscrews are 3 turns. Number 2 carb does occasionally overflow. Its always number 2, the others are fine, so it looks like I'll have to re-examine no2.   

Oh well more head scratching....hahaha

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