So, as many of you know i’m rounding the corner on a cafe of my 1974 cb450k7. I’ve hit a bit of a wall and I am looking for direction. Here is where I am at.
-Good compression on both cylinders (180+/- to temp on both cyl)
-Carbs completely rebuilt, cleaned, CLEANED
-Carb float bowl level set at proper 20mm per spec
-No issues with wiring harness
-No issues with coils
-Points and gap appear fine.
My issue is that the right side cylinder is still poppin/backfiring/missing under load and in neutral. Left side cylinder is hitting just fine. I went as far as to swap the coils just to be sure, and the right side is still missing pretty bad. After I had rebuilt the carbs again with the help of this old bike guy named Jerry who came recommended from WNYsportriders, I went to take the bike around the block and it was still missing on the right pretty bad under load, and backfired so badly that the carb actually popped out of the carb boot on the intake.
Focusinprogress, if you remember, last fall, it was doing the same thing. I’m at a loss. I have Jerry the old bike guy coming over again wednesday after work to take another look at things and continue troubleshooting, but what do you guys think? My thought was maybe valves but at this point i’m not too sure. Once this right side cylinder starts hitting, she’ll be good to go, so i’m eager to get some insight.
Alright, I’ll list some things that could be a contributing factor, in order of probability
-carbs may need to be synchronized, sounds like maybe one carb is fighting for correct vacuum pressure while the other is running A-OK
-Timing may be off. check to be sure the mechanical advancer plate is operating correctly. Each one is a little different, so consult some forums where cb450 knowledge is most plentiful for how to check this
-valve clearances may need to be adjusted. This will only be effective if timing is correct, which is why I didn’t list this before timing
-put in-line spark light/testors on both plugs and run the bike…make sure they appear to be flashing the same.
yeah, I remember now helping you sync them back in the fall. It sounds like it is possibly backfiring through the carb, which is def. a timing issue.
You say the points and condensors are good…are they new? or have you just inspected them? even the slightest bit of corrosion on the faces of the points breakers will cause wierd ignition symptoms…so if it’s not timing, its very obviously something with the ignition circuit for JUST that cylinder that’s giving you trouble.
run a piece of emery paper over them anyways, and then check their gap per the owners manual afterwards with feeler gauges… even if they were brand new, sitting can do funny things to them, so better to be safe than sorry and address all possibilities.
Another thing to check is the intake boot. Had it happen years ago on a bike and just went through it on my quad. With my quad the front cylinder would pop so bad it would knock the carb out of the boot. Bike would run great with choke part way on or if I sprayed carb cleaner around the boot. Even the smallest crack can throw everything off.
FULL coverage…on an agreed value of $1000 with a $300 deductable. SO, if I wreck it…I get a measly $700 for it. LOL. of course if I wreck it bad enough to warrant filing a claim, chances are my organ donor insignia on my license will also be coming in handy…
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do like he says and spray carb cleaner or WD40 around the boot when everything is assembled and see if the engine revs change at all.
Remembering back to a few CB’s I’ve opened up before, they run a single chain, so if you were off a tooth it would affect both cylinders.
Have you had the head apart before? A backfire blowing the carb off means the intake valve isn’t fully closed when the plug fires. If the timing is on its possible you have a bad valve seat or bent valve. A leakdown and/or compression check would show this.