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Author Topic: 81 CB650 - cylinders 2 and 3 cold exhaust and backfire  (Read 769 times)

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Offline randomguy81

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81 CB650 - cylinders 2 and 3 cold exhaust and backfire
« on: February 03, 2023, 03:53:20 PM »
Hi all

Have an 81 cb650 with cold exhausts on 2 and 3
Bike backfires

I’ve ran a compression test and I’m seeing 120 on all 4
I’ve switched over coils and cables
I’ve switched over CDI’s
I have spark on all 4 plugs
I’ve checked / adjusted valve clearances

Would really appreciate any advice of my next step - this bike is mentally beating me at every step it seems.
« Last Edit: February 03, 2023, 03:59:15 PM by randomguy81 »
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Offline RAFster122s

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Re: 81 CB650 - cylinders 2 and 3 cold exhaust and backfire
« Reply #1 on: February 03, 2023, 04:09:07 PM »
Sometimes a bad spark plug will spark in free air grounded to the engine but won't fire under compression. Never hurts to throw another plug in to see if the plug is defective.
Long shot but it has been happening with increased frequency past few years...

Are the non firing cylinders plugs wet with fuel?
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Offline scunny

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Re: 81 CB650 - cylinders 2 and 3 cold exhaust and backfire
« Reply #2 on: February 03, 2023, 04:29:19 PM »
"Are the non firing cylinders plugs wet with fuel?"
+1
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Offline scottly

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Re: 81 CB650 - cylinders 2 and 3 cold exhaust and backfire
« Reply #3 on: February 03, 2023, 06:08:33 PM »
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Offline randomguy81

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Re: 81 CB650 - cylinders 2 and 3 cold exhaust and backfire
« Reply #4 on: February 03, 2023, 06:12:23 PM »
Quote from: scottly on February 03, 2023, 06:08:33 PM
Read this thread: ;D
http://forums.sohc4.net/index.php/topic,191710.msg2232615.html#msg2232615

Thank you - the problem I have is I am using an m-unit …

I am dreading working this out  ;D
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Offline scottly

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Re: 81 CB650 - cylinders 2 and 3 cold exhaust and backfire
« Reply #5 on: February 03, 2023, 06:30:59 PM »
Your ignition timing issue has NOTHING to do with a M-unit.
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Offline randomguy81

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Re: 81 CB650 - cylinders 2 and 3 cold exhaust and backfire
« Reply #6 on: February 03, 2023, 07:19:41 PM »
Quote from: scottly on February 03, 2023, 06:30:59 PM
Your ignition timing issue has NOTHING to do with a M-unit.

I figured my wiring of the m-unit was the issue

Will follow that post
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Offline RAFster122s

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Re: 81 CB650 - cylinders 2 and 3 cold exhaust and backfire
« Reply #7 on: February 03, 2023, 10:01:25 PM »
Quote from: randomguy81 on February 03, 2023, 07:19:41 PM
Quote from: scottly on February 03, 2023, 06:30:59 PM
Your ignition timing issue has NOTHING to do with a M-unit.

I figured my wiring of the m-unit was the issue

Will follow that post

Most likely it is not related to your m-unit if it is wired correctly.
Ignition or fuel delivery issues most likely.
If you are getting spark then it is issue of fuel getting to the intakes, easily determined by wet gas on plugs of non firing cylinders.
650 is known to have issues commonly with CDI units and if you swapped them with no change then sounds like they may be fine...
The CV carbs are very picky about being clean and stock airbox to function correctly throughout Rev range. Early 650 didn't use CV carbs.
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Offline randomguy81

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Re: 81 CB650 - cylinders 2 and 3 cold exhaust and backfire
« Reply #8 on: February 03, 2023, 10:28:12 PM »
Have just put a set of Mikuni vm34’s on from Murray’s

Had same issue with old carbs

Bike came with no wiring harness so I found a 750 schematic with the m unit and went to work

Going to try and change the pulsar yellow cables around as I believe from reading the link that was posted they are for 2 and 3
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Offline randomguy81

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Re: 81 CB650 - cylinders 2 and 3 cold exhaust and backfire
« Reply #9 on: February 04, 2023, 07:01:02 AM »
Quote from: randomguy81 on February 03, 2023, 06:12:23 PM
Quote from: scottly on February 03, 2023, 06:08:33 PM
Read this thread: ;D
http://forums.sohc4.net/index.php/topic,191710.msg2232615.html#msg2232615

Thank you - the problem I have is I am using an m-unit …

I am dreading working this out  ;D

My wife refused to let me fire the bike up at 1am so I switched my yellow wires this morning and crossed my fingers - she’s running on all 4!!!

THANKYOU SCOTTLY!
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