1982 XS650 Heritage Special - Decel popping on one cylinder

fenbrancis

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My 1982 XS650 Heritage special runs very well. It is a little weak on the bottom end when its cold but other than that it runs well (smooths right out after bike warms up). However, when I let off the gas and let the engine breaking do its job, the right cylinder (right when sitting on the bike) has some decel popping. This only occurs whiles actually riding. When I am sitting at idle and rev, as the rpms come back down there is no pop. Everything I have seen looks like either an exhaust leak or some carb adjustment issue.

I have stock carbs, stock air filters, new exhaust gaskets, and stock exhaust. I am running a 145 pilot but the other jets are stock. I went up on pilot as the bike was running very lean at idle. I am running the gonzo ignition, I read that some iginition timing retardation can cause decel pop, however I figured it would cause popping on both cylinders not just one.

I am at 2.5 turns out on the fuel mixture screw. The plug for the popping cylinder was a good color maybe a tad lean, so I went to 3 turns on the fuel screw and it calmed down but still a good amount of popping.

There is no decel pop on the left cylinder. Any and all help is greatly appreciated.
 
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RustiePyles

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It could a few different things, as you stated an exhaust leak could be the cause. That's easy enough to find with a length of garden hose held up to you ear pass the other end over and around the exhaust ports, you'll hear the leak. Could also be valves a little bit out of adjustment. It could be your carbs are a little bit out of sync.
 

fenbrancis

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It could a few different things, as you stated an exhaust leak could be the cause. That's easy enough to find with a length of garden hose held up to you ear pass the other end over and around the exhaust ports, you'll hear the leak. Could also be valves a little bit out of adjustment. It could be your carbs are a little bit out of sync.
I have a good feeling it’s carbs out of sync. I haven’t properly synced them since rebuilt. But took a rough measurement and did the nail trick. They probably need to be synced. Pop only occurs at mid range also. 2-4K rpm on decel
 

RustiePyles

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I have a good feeling it’s carbs out of sync. I haven’t properly synced them since rebuilt. But took a rough measurement and did the nail trick. They probably need to be synced. Pop only occurs at mid range also. 2-4K rpm on decel
If you rebuilt them and didn't sync them I'd be inclined to agree with you.
 

5twins

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Yes, do a proper sync with gauges or a manometer. I always "bench sync" carb sets I rebuild but the last couple were pretty far off once I got them on a bike and checked them, lol. And here I thought I was getting pretty good at that bench syncing thing.

For a stock bike, I've not heard of needing to increase the pilots a size. I know one up on the mains helps. For the idle circuit, you can make it a little richer by decreasing the air jets a couple sizes to a #130. This is the size the BS34 carbs came with in the rest of the world. So, stock pilots (42.5), one up on the mains (to a #135), and two down on the air jets. With stock pilots, the mix screws may need about 3 turns out.
 

fenbrancis

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Yes, do a proper sync with gauges or a manometer. I always "bench sync" carb sets I rebuild but the last couple were pretty far off once I got them on a bike and checked them, lol. And here I thought I was getting pretty good at that bench syncing thing.

For a stock bike, I've not heard of needing to increase the pilots a size. I know one up on the mains helps. For the idle circuit, you can make it a little richer by decreasing the air jets a couple sizes to a #130. This is the size the BS34 carbs came with in the rest of the world. So, stock pilots (42.5), one up on the mains (to a #135), and two down on the air jets. With stock pilots, the mix screws may need about 3 turns out.
Yeah, I will definitely look into proper sync. My bike was running pretty lean at idle with the fuel mixture screws at 3.5-4 out (but this may also be becasue they were out of sync), so I bumped up the pilot to have more range on the fuel mixture screw. Running both now at 2.5, one plug is a nice toasty brown the other is more rich. (the toast brown plug is the right carb (the one that gets adjusted with with screw) So I have a feeling once I sync though they will both go to rich so I migh have to step back down to the 142.5 pilot and try your method. Thank you for all of the information!

All in all, sounds like the carbs need to be synced before throwing new jets in.
 

5twins

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Fuel level (float setting) is also very important on these. To high or too low will make the carb run rich or lean. The manuals were behind on listing the level spec for the plastic floats (22 ± 1mm). Many still listed the brass float spec (27.3 ± .5mm).
 

fenbrancis

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Fuel level (float setting) is also very important on these. To high or too low will make the carb run rich or lean. The manuals were behind on listing the level spec for the plastic floats (22 ± 1mm). Many still listed the brass float spec (27.3 ± .5mm).
Ill also give this a check. I have always wanted to make my own carb synchronizer as it seems simple and effective. Howevers, the precision of gauges may be a better bet. What do you recommend/use?

Looking at the motion pro one is very similar to the home made ones
 

RustiePyles

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Ill also give this a check. I have always wanted to make my own carb synchronizer as it seems simple and effective. Howevers, the precision of gauges may be a better bet. What do you recommend/use?

Looking at the motion pro one is very similar to the home made ones
I have had the Motion Pro quad set for years and have never had a problem with it. I've synchronized countless dual and quad carb bikes with it.
 

5twins

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I have and use an old Carb Stix, the one with mercury in it. It's always worked well but I'm tempted to build one of those homemade ones, if for no other reason just for comparison sake.

CarbStix3.jpg


CarbStix.jpg


CarbStix2.jpg
 

fenbrancis

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I have and use an old Carb Stix, the one with mercury in it. It's always worked well but I'm tempted to build one of those homemade ones, if for no other reason just for comparison sake.
Ive got all the stuff for a homemade one, want to give it a try and see how it does. Will likely purchase one as well. See how they compare
 

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Alright, I put my homemade manometer on the carbs, cylinder that is decel popping and running more lean immediately drained off water (had less of a vacuum)

I would think that would cause the cylinder to run rich, why is plug more lean?

And holy hell, trying to adjust is TOUCHY
 

Kevin Werner

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Alright, I put my homemade manometer on the carbs, cylinder that is decel popping and running more lean immediately drained off water (had less of a vacuum)

I would think that would cause the cylinder to run rich, why is plug more lean?

And holy hell, trying to adjust is TOUCHY
I think commercial manometer have a restrictor to a very small orifice to help with that touchyness. Others on this site have clamped the hoses down a bit. Put them between 2 blocks of wood and clamp or use a welding vicegrip clamp and evenly throttle them down a bit. I just did mine first time and could watch the columns dance opposite eachother as each cylinder fires/breathes.
 
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jpdevol

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Alright, I put my homemade manometer on the carbs, cylinder that is decel popping and running more lean immediately drained off water (had less of a vacuum)

I would think that would cause the cylinder to run rich, why is plug more lean?

And holy hell, trying to adjust is TOUCHY
Perhaps there is a leak at intake manifold on that side, causing both less vacuum and lean condition.

On the manometer set-up; a valve in the lines is helpful to dampen pulses (especially if using gauges) - aquarium air supply valves work well:
https://www.amazon.com/Pawfly-Aquar...3939&sprefix=aquarium+air+valve,aps,88&sr=8-5
 

fenbrancis

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I got them balanced. Right exhaust still popping.

Set both mixtures to stock (3 turns out) and then ran the popping one to 3.5 turns out. Popping stopped. But now wondering if I have put a band aid on real issue.

Brand new filters, brand new intake boots, new intake boot gaskets and barb cover on right side carb boot.
 

Kevin Werner

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I got them balanced. Right exhaust still popping.

Set both mixtures to stock (3 turns out) and then ran the popping one to 3.5 turns out. Popping stopped. But now wondering if I have put a band aid on real issue.

Brand new filters, brand new intake boots, new intake boot gaskets and barb cover on right side carb boot.
A. What filters did you install? Pods? Pleated or foam or stock? When I had popping, upping the pilot jet helped. That makes me think, since you upped the pilots already, that you need to run carburetor/fuel injector cleaner for a couple of tanks of fuel because it sounds to me like one side might be gummed a little. Did you ride all winter or is this new this season?
 

jpdevol

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intake or exhaust valve leaking?
 

fenbrancis

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A. What filters did you install? Pods? Pleated or foam or stock? When I had popping, upping the pilot jet helped. That makes me think, since you upped the pilots already, that you need to run carburetor/fuel injector cleaner for a couple of tanks of fuel because it sounds to me like one side might be gummed a little. Did you ride all winter or is this new this season?
Stock filters, stock exhaust, went up to 145 pilot.

Carbs were completely rebuilt in spring (when bike was bought after sitting for 10 years).
 
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