Cb360/390 pickup hesitation when fully hot

Sderbyshire

Into Sailing, classic Triumph cars and motorbikes.
Hi folks, seeking some carb setting advice.

My cb360/390 is running beautifully, starts well, idles well, pulls to 10k in every gear :)

I can get a solid idle at around 1200 revs too.

However when the bike is fully HOT after a good blast and i stop at a junction, then the engine hesitates and may even stall on pickup.

Eg, i open the throttle a tiny bit to pick up the idle speed ready to let the clutch out, and there’s a drop in revs and even a stall. It does this in neutral too so its not a clutch drag issue.

Being as this is happening only when the engine is hot, have i set the idle mixture too rich and this is causing the issue, i do seem to need 2 turns out on the left carb and more like 3 on the right carb to get the solid idle.

Thoughts?

Steve
 
Hi Steve
Have you checked the float hight in the bowls (20-22) ?
Are you running pod filters ? Check that the rubbers that joins the bell mouth on the carbs isn’t over lapping or obscureing any of the holes ?
You could try making a heat Shield between the engine and the carbs as these bike do get very hot you might be getting vaporising of the fuel while you sat at a junction are lights
Hope you sort it out
Jeff
 
Hi, I’m having the same issue on a CB250. Idles fine and can run up the revs well, but if left idling will die when the throttle is opened slightly.

What carbs do you have?

I have stock Keihin on mine. They have a primary and secondary main jet. The primary helps to supply fuel in the transition between the idle circuit and the needle jet. Somewhere in that transition I either have no fuel getting through, blocked jet, or too much air getting, leaky manifold?

Other than blocked jets or air leaks, could the tip of your needles be worn letting more fuel through as it lifts?

Are they slide carbs? Could the slides be wrong with a large cutaway?
 
Hesitation on take off, running stock CV Kehien carbs on the 360 is usually due to a lean condition. (pod filters installed ?)

If you try to overcome this by richening the A/F adjusters she will be hard starting (flooding - fouling plugs at idle)

You are cracking the throttle open introducing a "gulp" of fresh air into the system. It goes too lean and wants to die.

A larger pilot will help w the issue, However on the down side it MAY also cause a rich condition between 2500-4500krpm (bogging)

I was able to work through this annoyance by employing smaller main primaries along w the larger pilots.

The smaller primaries I have are custom made, because at that time I couldn't find them on the shelf.

Its a delicate balance because the smaller main primaries can, in turn, cause it to die on deceleration.

Im sorry I cant provide the #s off the top of my head, it been some years ago. Given time I could probably find the jetting recipe that worked best for me.

The largest obstacle to making it run right w pods is the lack of needle selection.

I was just getting started making custom needles when I got sidetracked w other builds so never got around to finishing that experiment.

To make it simple...I now have the stock boxes w factory filters installed, along w factory jetting and she runs perfect.

good luck and if you do find the fix please let me know.
 
So, what you really need to make it run great w pods is a larger pilot, stock or larger main secondary and a lean needle w a more aggressive taper.

and a selection of main primary jets to really dial her in. (both, larger and smaller)

Also, the first and most important thing when running pod filters the carbs need to be modified by crazypj
 
Hi trek97, thanks for the reply. I'm not running pods, just the stock boxes and filters. Thought it wise to get the bike running properly as stock before messing about with airflow and other performance mods. I have a 350 with velocity stacks and found it easier to put some Mikuni VM28's on it as I can get the various jets and needles for it easily.

The Keihin on the 250's and 350's use a #35 idle jet, #70 primary jet and #100 secondary main, AFAIK. I think this may be the same for the 360's but the secondary is a #110 instead? I will see if there are alternatives to the primary jet, but think maybe only #68 is a possibility?

I'm going to strip them back again and look for a fuel blockage. Run them through the ultrasonic cleaner again.

I have read that the primary emulsion tube needs to be installed correctly with a screwdriver to make sure it does not block a crossover hole. I did not do this so that may be an issue?

Also, the first and most important thing when running pod filters the carbs need to be modified by crazypj

Ah... and miss all this fun ;)
 
Well then If you got stock boxes, thats something we can work with.

Im getting ready to install pistons and jugs right now.

I will do a little thinking on it.

Be absolutely the 3 pilot holes in the carb throats are clear and flow good.

This circuit is incorporated in w the A/F mix.

So you can pull the A/F mix adj and spray cleaner in through that hole and see it come out the 3 little holes directly under the closed butterfly.

let me find a pic.
 
Three small hole located above the large A/F hole in this pic.

if they aren't clear this can cause hesitation on take off.

11494-170917122114-26811790.jpeg


tilt your head right and you can see the cleaner spraying through the large A/F hole along w 3 pilot holes.

11494-160917153040-26801818.jpeg
 
When the butterflies are closed only one of the 3 will be visible.

Also that jet feed hole you are talking about lining up, I dont think its critical but yes I always line them up on mine. :)

Heres a couple links to a couple pages I wrote about rebuilding carbs w pics...

http://www.dotheton.com/forum/index.php?topic=27417.msg827012#msg827012

Tis one is pretty old...Its got some different info.

http://www.dotheton.com/forum/index.php?topic=63133.0
 
trek97 said:
Heres a couple links to a couple pages I wrote about rebuilding carbs w pics...

http://www.dotheton.com/forum/index.php?topic=27417.msg827012#msg827012

Tis one is pretty old...Its got some different info.

http://www.dotheton.com/forum/index.php?topic=63133.0

Wow! Thanks for the help and links. These are great. Lots of interesting points that could do with some investigating other than cleaning and clearing all passages..

1 - Aligning holes in slide on the filter side. I was wondering if the holes make a difference to the speed of lift of the slide and whether their position or size could result in a lean mix as it begins to lift?

2 - Checking diaphragm top covers are sealing correctly.

3 - Checking throttle butterfly is seating correctly. I stripped these so may have done something silly putting them back. The hinged flap can only be orientated one way if I remember?

4 - Replacing o-rings on primary and secondary jets. Some are already on order. The Keyster emulsion tunes would fall out of the orifices, so I have put the original ones back in, with the original jets, but the o-rings could do with upgrading.

5 - Float level??? I have had them at 26mm, as per the advice on off the Honda Twin forum, but then did the clear tube method and found the levels were around 7mm below the gasket level. So to get the level up to 3mm below the gasket line I have adjusted to 22mm, but felt this was a bit too high as the tang was getting caught on the rest of the float. Maybe 26mm measured with my float level gauge was cool anyway?

6 - check for air leaks at the butterfly throttle lever pivot. Could this let air in when it moves? How would I check, squirt some plusgas at it when I blip the throttle.

Blimey! That’s probably enough to be going on with. New intake rubbers and o-rings on order. I’ll grab a fresh can of carb cleaner today. Last time I used the stuff I mnaged to squirt myself in the eye... not sure how the jet was angled! ::)

It’ll be a couple of days before I can get ‘em back on again after this, but will post up results as soon as I have some progress.
 
Isn't carb cleaner in your eye just horrible.
I now wear goggles when doing anything like that. Once was enough for me.
 
I wear (prescription safety) glasses all of the time working on the bikes, but still have managed to squirt carb cleaner, brake cleaner, and brake fluid in my eyes all within the last two months.
 
Okay, so I flushed them with carb cleaner, checked everything was clear and in the right position. No air leaks. Replaced intake rubber insulators for new Honda ones. Started it up and with a little adjustment it idles. Blipped the throttle... no hesitation or bogging whatsoever. With the bike in neutral it runs up the revs perfectly.

However.... put the bike in gear and try to accelerate and it does nothing. No power under load whatsoever. There’s no misfiring or bogging, just no power at all. Had to sling it back in the garage as I ran out of time, but completing stumped. Will attempt to look again tomorrow.

I had the carbs balanced before I took them off and exhaust presssure was the same. Now the left is noticeably weaker and the throttle stop screw didn’t appear to make any difference so have a feeling the left cylinder’s not firing? Maybe only running on the right?
 
Just had a quick check and the left plug’s not getting any spark. Will run some tests later and hope to get both firing again!
 
The terminal on the left points was grounding on the inside of the points cover. A little adjustment sorted it and now the bike runs great. Having removed the intake rubbers I can see a thin hairline crack, so replacing those was worthwhile. Hopefully I can get some miles on it this weekend, open up the throttle fully and tweak the settings whilst hot, but I had a quick blast this evening and it’s responding really well and accelerated cleanly with no flat spots or misfires. Thanks for the advice!

Now to sort out the clunky rear brake, knackered splines on the gear change spindle, the slight front wheel wobble, the dodgy indicator switch........
 
Back
Top Bottom