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Author Topic: CB360 right cylinder cold on idle  (Read 938 times)

Offline hillset

  • Posts: 91
CB360 right cylinder cold on idle
« on: Nov 27, 2018, 12:46:06 »
Hi Everyone,
     I have a 74 CL360 with mikuni vm28's and a pamco electronic ignition.  Just had the bike in the shop for a couple of weeks (problems getting the pamco to work and had them install the vm28's for me) and when I got the bike back she was up and running again.  The day after I got the bike back I noticed the idle was set pretty high (bike was idling around 3k rpm) so I adjusted the idle screws on the vm28s to about 4 turns out and got the idle down to about 1.2-1.4k rpm (which I believe is spec).

     The problem I'm having now is that the right cylinder is cold on idle, but warms up when I take the bike out for a ride.  At idle, the right cylinder is blowing clear exhaust but its cold and the exhaust pipes don't heat up like the left cylinder.  After riding for about 5 min, the right exhaust warms up, but the idle speed increases to about 2k.

     I've checked compression on both cylinders (right ~ 140, left ~135 but this was without throttle open), verified the timing is correct for left and right cylinders, changed the plugs, and adjusted the cam chain tension.  I've also made sure fuel is adequately flowing from the tank through the petcock to both carbs.  Does anyone have any ideas on what could be going on on the right cylinder?  I know my compression numbers are out of spec and I'm planning on doing a top end rebuild later this winter, but I feel like if the right cylinder is warming up after riding and the left cylinder is running fine, compression shouldn't be causing one cylinder to be cold at idle, especially if that cylinder has better compression than the one that's firing?  Thanks for your help. 

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Re: CB360 right cylinder cold on idle
« Reply #1 on: Nov 27, 2018, 13:02:34 »
That compression is fine. Its not accurate anyways since you didn't have the throttle open. Should be no need for a top end rebuild.

The carbs are most likely just out of sync, on VMs you basically just need to pop the filters off and make sure that the slides are opening the same. There are a few different ways to do it, but that's what it sounds like to me.

I would also check for a vacuum leak around the carb boots (use an unlit propane torch or starting fluid etc around the boots) and listen for a surge in RPMs.

Offline irk miller

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Re: CB360 right cylinder cold on idle
« Reply #2 on: Nov 27, 2018, 13:14:03 »
You didn't mention your air mix.  Where is your air mix on each side?

Offline hillset

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Re: CB360 right cylinder cold on idle
« Reply #3 on: Nov 27, 2018, 13:23:24 »
Thanks for the quick replies everyone.  I'll check to make sure the slides are opening the same on both sides with the popsickle stick trick later today, and I'll try spraying some starter fluid around the carb boot as well.

I haven't touched the air screws on either carb since I got the bike back.  I'll check that as well and make sure they are the same on both sides.  Thanks again for all of the help.

Offline hillset

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Re: CB360 right cylinder cold on idle
« Reply #4 on: Nov 27, 2018, 13:42:58 »
Ok just checked everything:

-Slides are opening at almost exactly the same time.  Right slide is opening a HAIR earlier than the left, but it's like as soon as I see the right side slide move, if I touch the throttle the left side moves too so they're really close.

-I sprayed the right intake boot with starter fluid and did not see the idle increase or hear the engine rev.

-Air screws were at 1.25 on the left and 1.75 on the right.  I assumed this wasn't on purpose and adjusted the right air screw to 1.25 turns out.  Right cylinder is still cold when I was idling.  Anything else I can check? 

Could it be a difference in the internal jetting of the carbs?  I'm thinking my next step would be taking the carbs apart and making sure there isn't anything loose or gummed up in them?

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Re: CB360 right cylinder cold on idle
« Reply #5 on: Nov 27, 2018, 14:01:01 »
Make sure you're getting spark on that side.

Offline hillset

  • Posts: 91
Re: CB360 right cylinder cold on idle
« Reply #6 on: Nov 27, 2018, 14:27:29 »
Yes I have a nice bright spark on the right.  The plug is blacker than on the left side (left side looks brownish-white), and the right side spark is yellow but bright and visible in the sunlight.

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Re: CB360 right cylinder cold on idle
« Reply #7 on: Nov 27, 2018, 14:54:22 »
Yes I have a nice bright spark on the right.  The plug is blacker than on the left side (left side looks brownish-white), and the right side spark is yellow but bright and visible in the sunlight.

Is your battery fully charged? You want a nice fat, blue spark.

Offline hillset

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Re: CB360 right cylinder cold on idle
« Reply #8 on: Nov 27, 2018, 15:29:09 »
Yes the battery is fully charged.  I can try putting in new plugs tonight and see how it does with those.  Could fouled plugs cause a cylinder to not fire at idle but fire when riding?

Online teazer

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Re: CB360 right cylinder cold on idle
« Reply #9 on: Nov 27, 2018, 19:57:35 »
Sounds like something is not right on that right carb.  The fact that the mechanic set the air screw to 1.75 turns suggests that he had trouble getting it right. There are many possible causes of a cylinder being dead at idel and it could be ignition or it could be carbs.  Others have addressed some of the ignition possibilities, but seems to me to me more likely at this stage to be related to the carbs.

I would pull the carbs and check fuel level and after that I would throw in a pair of new pilot jets, or at least I would check for flow between the two.  You can do that with a can of carb cleaner or WD40.  Spray through one jet and then the other and compare flow rates.  It's not an accurate comparison but will give you an idea if they are close or very different.

I'd repeat that process on the air passages and fuel passages.  One side then the other.

But start with the FUEL levels.