Backfire on #4 cylinder

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razorbackvaden
Posts: 2
Joined: Sun Jul 07, 2013 11:44 am

Backfire on #4 cylinder

Postby razorbackvaden » Mon Jul 08, 2013 6:37 am

I recently bought a well maintained 80' CB650. During the test ride and the rides after, I've noticed a small backfire under low rpm load, under 5000 rpm. Whenever I take off hard, the bike picks up well and there is no audible backfire. I've replaced the plugs and wires, and I've done a bench sync because I noticed bluing of pipes 1-3 while 4 remained cooler. I don't have a vacuum sync yet, but thinking that would be a good next step. I'd like some feedback here. Has anyone else had this problem or one similar to it? Am I right to look at the sync or could it be a jetting issue like a piece of trash? I'm open to suggestions to keep this bike running right.

usbear
Posts: 361
Joined: Sun Mar 10, 2013 9:55 pm

Re: Backfire on #4 cylinder

Postby usbear » Mon Jul 08, 2013 9:06 am

carb low speed could be plugged.
Clean properly, sonic best or soak and wire out.
Much post on this subject.

1) verify proper spark- pull plug and hold against head while attached to wire and turn over. good spark then fuel
2) clean carbs and entire fuel system

razorbackvaden
Posts: 2
Joined: Sun Jul 07, 2013 11:44 am

Re: Backfire on #4 cylinder

Postby razorbackvaden » Mon Jul 08, 2013 1:55 pm

I've got new wires and plugs. I've gapped them, and got good spark when grounded. I took the bowls off and didn't find any trash. You think the jet may have something clogging it from the top side?

usbear
Posts: 361
Joined: Sun Mar 10, 2013 9:55 pm

Re: Backfire on #4 cylinder

Postby usbear » Mon Jul 08, 2013 4:07 pm

Yes and possible slow speed/idle jet and circuit!

Idle circuit works off air and vacuum flow thru tiny air passages that plug from evaporating fuel (and other things) which leaves residue and corrosion.
Residue in form of varnish and fuel additives.
This is in Airways and can not be flushed with fuel, not fuel passage. Fuel cleaner will not flush this out, no fuel goes in there or thru there-ever.

Airflow thru TINY air passages siphons fuel up and into carb airflow for mixture and delivery.

No airflow thru Tiny Pass= no suction=no fuel delivery =sputter, back fire, flat spot lean reving and die and so on.

Then there are the tiny passages that fuel does go in that are plugged shut.

Like a paint gun that did not get cleaned out and the paint dried. Put fresh paint and thiner in and try as you might you can not blow it out.

Except these carbs work off Vacuum-not pressure feed (ok some gravity)
But works off suction instead.

Try drinking thru a plugged straw.

So thats how it is.
Trash or rust or water in the fuel can plug jets and ports also.
We had better fuel when these bikes were designed.

Probably plugged.
Check under float needle and seat till no problem.

However always rule out electrical problems.
You did change IGN components so check to be sure of good contacts and firm fitting ign wires and ground.
So I am on the road again for a few days- many good and knowledgable members and moderator here

usbear
Posts: 361
Joined: Sun Mar 10, 2013 9:55 pm

Re: Backfire on #4 cylinder

Postby usbear » Mon Jul 08, 2013 4:12 pm

I had a problem on a new custom build last year that drove me nuts and could not isolate problem.

Friend came over and wanted to help(that was not received well as he knows nothing about mechanics)

Fired it up in a dark garage and he wanted to know if the pretty Blue Spark from the coil to the frame was common as it was very colorful (ugh) and was jumping 3 inches.
I have not lived it down yet.
Nuf Said.

Was all new high intensity ignition system.

It backfired and missed like lean but was intermittent and could not be seen in daylight.

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Volker_P
Posts: 5508
Joined: Mon Mar 06, 2006 1:53 am
Location: southern Germany

Re: Backfire on #4 cylinder

Postby Volker_P » Tue Jul 09, 2013 1:10 am

Welcome here! :D

with a visual sync, it should already run reasonably.
One feelable cooler exhaust sounds like a more severe problem like a wrong/plugged main jet in #4, floater level much too low (mounting floats upside down is possible!) or an air leak or somethíng like that.
Cosky's great (free) online manual: http://cosky0.tripod.com

forum links to common technical issues

If you really like this site and you would not like to see it vanish soon, have a look there: Urgent: Future of HondaCB650.com Forum


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