starting issues

[K2-K6]

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?

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! 

[Ken4004]

Hi Aerial

Have you resolved your cold start issue or is it still under investigation ?

Regards Ken

[Ariel-KH-500]

I removed the carbs yesterday and put them through the ultrasonic cleaner again at 60 degrees and for a 20 minutes , replaced the carbs and bike started ok I then rebalanced the carbs using my vacuum gauges. Bike start fine and ran ok on the bench. today I will start the bike again to see if all is cured although the real problem was when it had stood for a few days if it stars o k today I'm then going to leave it for a few days and see if it will start as it should. One question for the experts is there anywhere where I can get some new felt seals for the slide lifting spindles. will repost in a few days with an update.

[Ken4004]

Hi Aerial

When you took the carbs off the bike and open them up did you notice anything in the bowls like jelly ? , just wondering if the ethanol in the fuel has reacted with any moisture and this is clogging your pilot jets.

I had this once where I filled up with fuel at Sainsbury’s and within a mile of riding from the station the bike stopped and fuel was pouring out of all four carb over flows .

I removed the carbs to clean them and found lots of this jelly substance .

It would be nice to see something blocking the jets when you have taken the trouble to remove the cards from the bike !

[Ariel-KH-500]

No the float bowls were spotless inside,I always use e5 petrol (Esso) in all my bikes always from the same garage. After re fitting the carbs it started fine then left it for 24 hours and again started fine I am now going to leave it for 4/5 days and then see if it starts ok, the problem has always been when the bike has not been started  for while if started every day no problems. I will post again next week with the result after it has stood un started.

[Ariel-KH-500]

An update on the starting issues, after the 2nd ultrasonic clean and carb balance the bike started 1st press of the starter it was then left over night and again a 1st press start. the bike was the left untouched for 4 days again 1st press start it was left again for 5 days and again started right up I've been out on it yesterday and did approx 80 miles with a stop at around half distance. I am now going to let the bike stand for a week and see if it still starts ok after not being started as this was always the main problem so far so good will report again when I next start the bike.

[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.