SOHC.co.uk Forums > CB350/400
starting issues
K2-K6:
--- Quote from: Rayzerman on May 20, 2026, 08:32:45 PM ---Many seem to have difficulties with these 350's.... I've had my own share of grief with getting these carbs bench sync'd then set properly on the bike.... by chance has anyone decided to go up a size on the idle jets? And of course, did it help or hinder?
--- End quote ---
A good question, and worthwhile considering as to ultimate aim of improvement.
Would that be to give larger bore in jet in trying to mitigate blocking ? Or something else.
I'd caveat my view with not wanting to run fuel excessively as there's downside in lubrication, bore washing etc from unburnt fuel going somewhere its not wanted. I'd be keen to observe care in that regard.
The flip side though, and built into strategy of injection system (along with exhaust monitoring in closed loop) is that a flexible approach could be worth consideration into further evaluation.
One size up for idle jet should bring a need to individually assess the air screw set point in utilising that flow, along with understanding what, if any, the gain will be.
Judging by setting that air to fuel ratio dynamically, as the manual instructs, will show how far you need to to move it in bringing the mixture in range. This is echoed in fuel injection strategy with long & short term trim values from running experience within the environment the vehicle is used. This effectively reinforces the method of rejetting and then judgment as well established theory and practice.
It may end up with broadly similar fuel flow, but facilitate cold start by enhancing choke effect against cranking vacuum by pulling initially more fuel through the slightly larger jet to get the combustion lit up in that first fire phase.
Ariel-KH-500:
Another update on the 400 starting issues. After reading the suggestion of blocking off the air intake and turning over with the kill switch turned to the off position I tried this after not starting the bike for four days as the problem has always been when its been left a while, so fuel on kill switch off air box intake covered three four second bursts on the starter and low and behold it fired straight up. So according to previous post this indicates primary circuit I will now remove the carbs and investigate in the meantime anyone with previous experience of this problem or inspecting the primary circuit I would be very grateful for any advice thanks in advance.
McCabe-Thiele (Ted):
I found this link is handy when taking the carbs apart - the photos are pretty good at illustrating how things go back together.
.https://www.vintagebikebuilder.com/cb350-400f-carb-rebuild-demonstration.html
I use DP60 or WD40 to check the airways in the body are clear, spray in at one end until it emerges at the other end of the carb body etc.
exvalvesetdabbler:
Or use a syringe and white spirit to verify that the internal passageways are clear.
While you have things apart , verify that the choke flaps are fully closing.
Regards
Dave
Mikep328:
FWIW, my 400 has always been a hesitant cold starter from dead cold- with outside temps say below 10C/50F. My method nowadays is to turn the kill switch to the off position, put the choke fully on and spin the starter for 2-3 seconds. Then turn the kill switch on, hit start button. Bike starts immediately and choke lever is moved down a bit to maintain a fast idle - 1500 RPM or thereabouts. Choke can be off completely a minutes after start. In warmer weather it will start immediately from cold with the choke on - no need for "pre-spinning." It always needs the choke to start from cold, regardless of ambient temps.
In cold temps if I DON"T pre spin, just put the choke on, it typically takes at least 4-5 start attempts (each one a 2-3 second push of start button) before it will start/keep running.
I've always assumed it's just a cold blooded little sucker!
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