collapse

www.dimecitycycles.com

https://www.townmoto.com/collections/vanson-leathers


www.restocycle.com

www.CITYLIMITMOTO.com

www.jadusmotorcycleparts.com

www.bisonmotorsports.com

www.speedmotoco.com

www.lostapostlejewelry.com

www.cognitomoto.com

www.sparckmoto.com

www.pistonsociety.com

www.steeltowngarage.com


Author Topic: Cb360/390 pickup hesitation when fully hot  (Read 581 times)

Offline Sderbyshire

  • Posts: 254
  • Into Sailing, classic Triumph cars and motorbikes.
Cb360/390 pickup hesitation when fully hot
« on: Sep 16, 2018, 05:56:31 »
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 thereís 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

Offline jeff4uk

  • Posts: 367
    • 71 triumph T100c
Re: Cb360/390 pickup hesitation when fully hot
« Reply #1 on: Oct 18, 2018, 03:37:25 »
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 isnít 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

Offline scott_jangers

  • Posts: 68
Re: Cb360/390 pickup hesitation when fully hot
« Reply #2 on: Nov 10, 2018, 03:17:41 »
Hi, Iím 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?

Offline trek97

  • DTT SUPPORTER
  • *
  • DTT BOTM WINNER
  • *
  • Posts: 6025
Re: Cb360/390 pickup hesitation when fully hot
« Reply #3 on: Nov 10, 2018, 07:44:15 »
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. 
« Last Edit: Nov 10, 2018, 08:14:10 by trek97 »

Offline trek97

  • DTT SUPPORTER
  • *
  • DTT BOTM WINNER
  • *
  • Posts: 6025
Re: Cb360/390 pickup hesitation when fully hot
« Reply #4 on: Nov 10, 2018, 08:02:55 »
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
« Last Edit: Nov 10, 2018, 08:06:51 by trek97 »

Offline scott_jangers

  • Posts: 68
Re: Cb360/390 pickup hesitation when fully hot
« Reply #5 on: Nov 10, 2018, 16:48:35 »
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?

Quote
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  ;)

Offline trek97

  • DTT SUPPORTER
  • *
  • DTT BOTM WINNER
  • *
  • Posts: 6025
Re: Cb360/390 pickup hesitation when fully hot
« Reply #6 on: Nov 10, 2018, 20:20:54 »
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.

Offline trek97

  • DTT SUPPORTER
  • *
  • DTT BOTM WINNER
  • *
  • Posts: 6025
Re: Cb360/390 pickup hesitation when fully hot
« Reply #7 on: Nov 10, 2018, 20:24:37 »
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.



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


Offline trek97

  • DTT SUPPORTER
  • *
  • DTT BOTM WINNER
  • *
  • Posts: 6025
Re: Cb360/390 pickup hesitation when fully hot
« Reply #8 on: Nov 10, 2018, 20:27:12 »
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




 
« Last Edit: Nov 10, 2018, 20:39:46 by trek97 »

Offline trek97

  • DTT SUPPORTER
  • *
  • DTT BOTM WINNER
  • *
  • Posts: 6025
Re: Cb360/390 pickup hesitation when fully hot
« Reply #9 on: Nov 10, 2018, 21:33:25 »
you need stock jetting.