Carb tuning question

[ShawnNYC]

So, I bought my first bike in quite a few years and it's my first XS (1983). I've spent the last year cleaning it up and fixing some odds and ends. All with much help from posts here on this forum, so thank you.
The bike runs fairly well but it's sputtering and popping in the top end of every gear. I've gotten to where the mixture screws are out about 8 turns and it now runs a little better. I'm no carb guru but I'm guessing the jets may need to be changed? I'll try to get them off tomorrow to see whats in there but I was hoping someone may have the same exhaust etc and some insight on what I may need to do. Carbs are spotless and I've recently replaced the exhaust gaskets...

 

[angus67]

Wow. 8 turns out. You need to take the carbs off and clean them out, put them back on, and see how it runs, then maybe reject. Depends on mods

 

[5twins]

You're lucky the mix screws didn't fall right out, lol. Yes, you need to rejet. I see you have pod filters and what I'm thinking is an open exhaust. I'd start out with one up on the pilots and 2 up on the mains.

 

[Superjet]

Have you checked your plugs for any indication how it is running?

 

[ShawnNYC]

I took the carbs off and rebuilt them when I first got the bike, so they’re clean.

Yeah. I was keeping an eye on the screws while I took it for a ride. They stayed in but my tail light mounting screws vibrated out. lol

I pulled the plugs a week ago and they were dry. I’ll run it around the block take a look again tomorrow.

 

[ShawnNYC]

And yes, they are open pipes. Not very long long ones either... Double D from TC Bros from the looks of them.

 

[650Skull]

I took the carbs off and rebuilt them when I first got the bike, so they’re clean.

Not necessarily true. Check and clean again, especially the pilot circuit...........

 

[ShawnNYC]

Thanks Skull. The weather has me sidelined for a few days so I’m going to take them off. Again, and go through them once more while I identify what size jets are in there.

 

[5twins]

Write everything down so you can remember it in the future, particularly the changes you make. Here's a stock listing, your carbs are the last in the list .....

Those particular carbs were set up very lean from the factory to meet E.P.A. specs. Any changes you make to intake and/or exhaust usually require jetting changes because of that.

 

[ShawnNYC]

Thanks for the info 5twins. I had a few minutes today and pulled the carbs.
Going by the chart, the jets were upped a couple of sizes already. If I have the head for it tonight after work, I’ll scrub them down again. Maybe I missed something or some crud made it past the filter since I cleaned them last.
Here’s what’s in there now...

 

[5twins]

Jets increase in size by 2.5 number increments so you're up one size on the main and pilot. That's probably fine for the pilot but the main may need more. Try a 137.5 (2 sizes up) and then a 140 if that isn't enough.

 

[ShawnNYC]

Thanks 5twins. Ive read the carb guide and hundreds of posts, but I appreciate the direct feedback. My time to tinker is limited and so are riding opportunities. I had to run out of town, but I ordered both of the larger jets before I split. I’m hoping to get the carbs apart this weekend to clean and try out the new jets. Hopefully that does it.

 

[ShawnNYC]

Quick update...
Pulled off the carbs, scrubbed everything and put in the 137.5 mains. I was able to squeeze in a short ride between rain showers. The popping is a little less and it’s not sputtering quite as much at the top end. But it’s still there. I got the mix screws out to 4 turns before I had to head back.
I suppose trying the 140 mains would be a good idea considering I have them.

 

[5twins]

Yes, try them. Carb jetting for mods is all about experimenting, stepping up/down incrementally until you find what works best. No two of these 650s are exactly alike (motor condition) so the best we can do is give you some general recommendations within a few jet sizes. This is based on what has worked for others with similar mods. Be aware, though, that open, short straight pipes can be difficult to tune for. As far as an exhaust choice goes, they are the absolute worst. They look cool is about the only thing you could say good about them.

 

[ShawnNYC]

I’ll try the 140s. Honestly, I hate the pipes. lol
Just haven’t had the time to find something quieter and more efficient. The PO had them on there so I’ve kept them. And they scare the deer...
The speedster pipes from Hugh look nice but the bike is kinda low.

 

[Superjet]

Yeppp not much for bafflin' on those pipes you got on er' now!

 

[ShawnNYC]

So I had some time this morning and put the 140s in. Big difference. The acceleration is much smoother. Unless I really wind out the gear, it’s good. There’s still a fair amount or popping on decel, but the pipes are a little leaky despite new gaskets. I’m pretty sure that’s why.
I also adjusted the cam chain and the valves. Big day. lol Ended up being beautiful out and my test ride turned into an all day ride in the mountains.
Thanks 5twins, and everyone else who replied. Best ride since I bought it.

 

[5twins]

Don't stop now, try 142.5's. That's how you jet for mods, step up one size at a time until you get too big and it runs worse, then step back down a size or 2, back to what ran best. But overall carb jetting is a combo of all the jets and the needle setting. As you work the mains up larger and larger, you may also need to change pilots and the needle setting along the way. Unfortunately, your needle is not adjustable so it will probably be the limiting factor on how large you can go on the mains. But, most running the BS34 carbs find it works out OK. You can get large enough on the mains before the midrange gets too rich because of them. On an earlier carb set, you would lean the needle setting when that happened, but you can't and most usually don't need to.

 

[ShawnNYC]

Seems one size up might be the trick. I’ll grab them. Then I’ll prob change the pipes and have to start over. lol
It’s been almost as much fun getting this thing right as it is riding it. Almost.

 

[peanut]

popping on deceleration indicates that you need to up your Pilot jets too especially if your mix screw is 4x turns out !. On deceleration with a closed throttle the engine only has the fuel and air supplied by the Pilot jet . I ended up with 35 pilots on mine having tried 30 and 32.5 with straight thru pipes and foam filters

A simple but potentially dangerous test would be to apply a very small amount of choke whilst decelerating. The choke supplies an independant fuel enrichment and is a good diagnostic tool. if the popping stops you have your confirmation but make sure you do it safely lol