CB350 Popping on deacceleration


1970 Honda CL 350
I just finished an engine rebuild and got the bike running through idle to full rev with no problems. One thing I am having an issue with is on deaccerlation I get tons of popping. The plugs don't look lean or rich but will post a pic to show the group.

I am running Mikuni vm30 with filters and custom exhaust 2 into 1.

Slides: 2.5
Needle: 6F4
Needle Jet: 159-p4
Pilot Jet: 22.5
Main Jet: 170
Jet Needle clip: 2nd position from top

I am wondering if it is still running rich where should I start in making the system rich. The pilot jet first?

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Popping on decel is completely normal for any carbureted engine. "Engine braking". It is a lean condition caused by rolling off the throttle (closing butterflies) and cutting off the air, using the engine vacuum to slow down. Its simply more noticeable w some exhaust systems than others.

If you pull in the clutch while decelerating the popping will stop, because the engine rpms will quickly drop to normal idle.

If you try to overcome the popping through jetting or A/F adjustment you will probably find it runs too rich at idle and carbon fouls plugs.

The exhaust on your bike is pretty neat looking. May need to add some heat shielding. :eek:

If you find you have rich issues w the right carb. You may find its boiling fuel in the bowl.
Thanks Trek for the info. It doesn't pop as much when I pull in the clutch.

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Popping is usually the result of an air leak in the exhaust allowing unburned furl to ignite.

Try sealing the joints in that beautiful exhaust and see if that helps
Teazer: I put the clamps on there to help with that on the jointing areas.

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It may just be that with that exhaust (which looks to be a loud one) popping that you could not hear before is now loud enough to annoy you. You still should make sure that the exhaust is tight, from the exhaust seals to the last joint.
Thanks guys, will check again and see how fit the joints are in the exhaust

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After a 30 minute ride the bike temperature from the cylinder heads is about 300 degrees. Is this bike running to hot or is that a normal temperature?

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Running Pamco V2 ignition with full advance between the marks.

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Doe it look to be running lean from the spark plug? I just don't know which area of the throttle position I need to make rich as it runs from idle to full throttle really good.

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5 degrees of advance is 10 - 15° of plug temp. I'd consider your timing being too advanced as well. One adjustment at a time, but I'd try dialing the timing back for some runs before I dug into the carbs. Then go back to carbs. You're definitely dealing with unburnt fuel in igniting in your exhaust, but why is a question. I'm inclined to think it's your timing.
Retarded timing would cause that.

It's hard to see that plug, but it looks like the ground strap has just about the right amount burned clean and that's odd. What temperature range is the plug? And what is stock?
Teazer: Retard timing would cause overheating? Dumb question but retard is what? Advancing the timing?

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