SOHC.co.uk Forums > CB350/400

starting issues

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K2-K6:
Good prospect of a positive conclusion it looks.

These pilot jets are so small though, and easily impaired even if a % of flow is compromised, they'll give this type of effect.

Often it's the jets (well, mostly in reality) as impaired flow matched against the expected vacuum during cranking just doesn't yeald enough to bring easy starting combustion mixture. The "blocking" airbox method obviously increases vacuum while cranking, which gives more confirmation in diagnosis.  That will also pull through main jet if the slides (and needles) are lifted a little too.

It's possible though, with high ambient temperature, that it just may need slightly more of a spin after sitting for a week due to some evaporative effects in float bowls if the fuel is left in those. It's the "nice" volatile aromatic part of petrol that gets out to atmosphere first, via float bowl breather tubes and also the component in fuel that gives the combustion light off it's potency to get going.

Full choke to initially try is likely most successful, even in this weather.

As we see across various thread on here, these idle jet plus their circuit are absolutely key in having these carburettor run to their potential. Also a big part in running without flat spots and other associated ailments experienced.

Mikep328:
One of the BIG items that folks like K2 and others helped me with when I was sorting out my setup is the fact that the "idle" circuit affects a much wider range on these engines/carbs than other bikes I have owned/worked on.

As noted, it is vital that the slow jets/air adjustment screw are working perfectly.  When they are, the engine is responsive at any RPM.  I can roll the throttle on to full from 1500RPM in 3rd gear and the bike will just pull away with no complaint, no lugging, just smoothly accelerate.  Admittedly it not going to do any head snapping at that RPM but it's smooth and comfortable.  Frankly, it can do it much better than my '21 Moto Guzzi V7 850 which, in stock form, was quite recalcitrant below 3000 RPM.  The Honda is a much more comfortable bike trundling through small villages/hamlets at the 20MPH limit than the Guzzi was when stock.

K2-K6:
They are an amazing piece of production engineering, the 400.

50+ years old, near enough to 100bhp / ltr output as makes no difference. Impeccable carburation and combustion characteristics, huge operating rpm range with very simple hardware, giving viceless engine response and smooth running. They've got it all in place really, if, IF, they are set and maintained within the original design specification and tolerancing that Honda brought to the design and verification of them.

The need to be pedantically, obsessively detailed in aiming to have everything chased down to finest inspection and evaluation being paramount in giving their best. Once there, and with regular running, they'll do fine in staying like that. No particular need in respect of constant "fettling" as often projected, just scrupulous initial work and accepting how detailed that has to be approached, then they'll sing their little heart out  :)

Otis:
I have just enjoyed reading this post because I previously spoke about starting issues with the 350F and went through a similar regime and wondered if it was coils. Sadly, because my starter clutch is almost toast I have to kick start from cold and it takes four to five kicks. One tip that I recall I got from here was to check that the plug caps and properly tightened to ensure good contact. I found that one cap on the cylinder that was always last to warm up took a decent tweek and it has been a huge improvement.

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