Topic: '82 CB650SC. Popping on deceleration with new exhaust. Possible lean condition?

[fizzlebottom]

I replaced my 40yo 4-into-4 pipes with a 4-2-1 system from Ripple Rock Racers. Everything is great, but I am now getting a bit of popping on deceleration. I've read quite a bit and folks tend to agree that this usually indicates a lean condition. These bikes were already setup pretty lean from the factory to comply with federal emissions regulations.

My carbs are all freshly rebuilt with the original #118 mains. The bike starts fast and idles great once warm. What's odd is that the '81 model had nearly identical carbs (press-in slow jets vs screw-in slow jets) but came with #120 mains.

So I'm wondering if it'll be worthwhile just ordering a set of Keihin #120 mains from Jets R Us? Aside from deceleration the bike spins up really nicely through the whole RPM range once it is up to temp. Before that it gets a little finnicky if I snap the throttle too quickly.

Do you think #120 jets are the right place to head next?

[Flyin900]

If those are the CV carbs they do have an air cut valve on each carb to assist in BF on deacceleration. It still may not be enough though with the exhaust change. I would look to the slow jet side before the main jets and go up from say a # 40 to a #42 so one size above the present jetting. I would also look at enrichening the idle circuit mixture screw a little more 1/2 to 1 turn further out.

As with any changes to a motor there is no one size fits all... it is a process. You're hopefully still running the stock air box and not pods?

[fizzlebottom]

If those are the CV carbs they do have an air cut valve on each carb to assist in BF on deacceleration. It still may not be enough though with the exhaust change. I would look to the slow jet side before the main jets and go up from say a # 40 to a #42 so one size above the present jetting. I would also look at enrichening the idle circuit mixture screw a little more 1/2 to 1 turn further out.

As with any changes to a motor there is no one size fits all... it is a process. You're hopefully still running the stock air box and not pods?

They are CV carbs, VB44C to be exact. But they do not have air cutoff valves. These units, as well as the VB44A carbs from the '81 never came with air cutoffs.

Regarding the slow jets: Originals are #35, so 38 would be 1 size up.

And yes, I'm running the stock airbox & filter. No funny business here.

Lastly, I will definitely give the idle mix screws a little turn out before ordering any jets. They're currently out 2 1/4 by the book and I haven't touched since. I'll turn them out by 1/4 turn at a time and see where things improve or get worse.

[fizzlebottom]

Just in case, I ordered a set of #38 slow jets and #120 mains from Jets R Us. I also had a realization:

I've taken these carbs apart several times for cleaning and always think about this one odd part in each carb. Under the vacuum piston housing, there is a black kidney-shaped plastic piece screwed into the body of the carb. Under that piece is a similarly shaped gasket. I've never thought twice about these pieces; I just put them back in place after cleaning the carbs and call it a day. Replacements have never shown up in any rebuild kits I've purchased.

Well, apparently these are called "air jet covers". That came as a surprise to me because I have never seen replaceable air jets for the carbs. Not only that, but the gasket was originally a uniformly thick rubber piece that has clearly been compressed long enough to lose its elasticity. I looked across several other similarly designed Keihin CV carbs and most have only included the plastic cover but not the gasket. Odd, but I'm not questioning carb design at all.

I've ordered a sheet of Buna-N rubber and am going to fashion some new air jet cover gaskets before I go replacing any jets. I'm really just curious to see if fresh gaskets will have any significant impact on carb operation. If it doesn't, and I'm still running lean, then I'll start with turning the pilots out and proceed to #38 pilots and then #120 mains if I'm still lean.

[Flyin900]

Your right about the CV carbs on your bike not having a air cut circuit. That is one bike set I haven't worked on so as the saying goes "never assume anything" 

The air jets under that kidney shaped plastic are not replaceable, so that I why you have never seen replacements. They just need to be verified as clean when doing the rebuild. That is where the air cut valve circuit would have been if your carbs had that feature. Most CV carbs don't have those paper gaskets in there, just the plastic kidney cover on later models.

It doesn't hurt in replacing them.

I think the up jetting and some fuel screw tinkering should hopefully solve your lean issue.

[RAFster122s]

Spending nearly $300 for a good 30L commercial grade ultrasonic cleaner off a good eBay seller is worth it if you are going to be living with and maintaining carburetor equipped motorcycles. The CV carbs are truly a PITA as they are so picky and do not tolerate deposited in their tiny air and fuel passages. So the typical, $90 now?, Harbor Freight Ultrasonic cleaner with it's one smaller size transducer just does not cut it.

[Stev-o]

Popping on decel can also be caused by an exhaust leak.  Did you replace the exhaust port gaskets when the new system was installed?

How do your plugs look?

[fizzlebottom]

Popping on decel can also be caused by an exhaust leak.  Did you replace the exhaust port gaskets when the new system was installed?

How do your plugs look?

I was as meticulous as I could possibly be when installing the new pipes. New gaskets and made sure the old crushed ones were out. Tightened them as evenly as possible and not so tight that it crushed them to death. I've checked all the pipe joints a few times now for leaks and can't find any, but I will definitely go through again and see if I'm missing something.

[Stev-o]

Plugs?

[Deltarider]

Spending nearly $300 for a good 30L commercial grade ultrasonic cleaner off a good eBay seller is worth it if you are going to be living with and maintaining carburetor equipped motorcycles. [...]

That's a lot of money. Personally I never needed an ultrasonic cleaner. In the past I've communicated with quite a few dealers that offer the service of ultrasonically cleaning carbs. All were honest enough to admit that many carbs can succesfully be cleaned by just using your hands and some carb cleaner. Moreover none of them that offer the service, will give guarantee for success...
Before anyone starts drooling and projects all kinds of ideal properties. And yes, I have seen plenty pics of very clean, shiny carbs. Doesn't tell me a bit.

[fizzlebottom]

Spending nearly $300 for a good 30L commercial grade ultrasonic cleaner off a good eBay seller is worth it if you are going to be living with and maintaining carburetor equipped motorcycles. [...]

I'm feeling confident that they're as squeaky clean as I can get them. At the beginning of last summer, they were fully disassembled and rebuilt with fresh o-rings across the board and every passage has been blasted to pieces with carb cleaner, including the accelerator pump circuit. Floats and float needles were confirmed free of leaks and sitting at correct heights. Reassembled with everything according to book specs and the bike fired up immediately. Only needed up adjust idle.

Plugs?

Haven't had the chance to pull them yet, so I'll try to get to that tonight or this weekend. New jets are showing up Friday so I'll check plugs before I change any parts at all.