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Can the stock cans create decel pop?

Old 08-13-2013, 07:16 PM
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Can the stock cans create decel pop?

Ok guys, bear with me. I'll try to keep this short.

What I've done:
PAIR Removal with Block off plates installed.
Full Baffelectomy
Carb set up by 8541Hawk (He actually came by and did it himself, Thanks Mike!)
TPS adjustment
Carb sync after sync each time I touched the carbs.

So here is the deal. I CAN NOT get rid of the damn backfire/pop on decel! No matter what pilot jets I have tried, no matter how I adjust the shim stack on the needles. It is there. I can minimize it but now (the least I have been able to reduce it) I am on a fine line of losing power when I reduce pop, or increasing power by adjusting the shim stack down which causes more popcorn.

Right now I am running the shim stack Mike put in plus I put in one more very thin aircraft shim I got from our hanger, about 1/2 the thickness I had in there. By adding these very skinny shims I did reduce the pop, but lost power.

I can take them out and go back to how Mike set it up and I am back to (holy **** hang on!) power but a lot of pop when letting off the throttle.

I am pretty sure it is mixture related. When I have Mikes set up in there and it starts to pop on decel I can pull the choke a bit and it quits. That alone tells me (in my mind anyway) that I am still a hair lean, but correctable.

I guess I am looking for other ideas. I can't find my bikes "sweet spot".

Could it be due to the stock cans? By doing the full baffle removal am I creating a problem of trying to make the stock cans high flow cans? Are gasses getting trapped in there and adding to my problem?

I'm thinking out loud and was hoping to get some other opinions. Anyone else down this same road?
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Old 08-13-2013, 07:50 PM
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I don't mind the popping on deceleration. I actually kind of like it. If your bike runs great, why not just leave it? I feel it's just the bikes character.
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Old 08-13-2013, 08:24 PM
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I want to help save people some time. I chased this for a while, having Hawk help me on the phone and everything. Finally i read somewhere that exhaust leaks can cause pop on decel. I went out to check the nuts, and one was loose. I tried tightening it, and the stud spun in the hole! Stripped threads! I also found soot around that headpipe on the cylinder head. I ordered a Keensert from Mcmaster Carr (m8x1.25) and am waiting to install it. We'll see how that helps...

Just for kicks i would replace the copper crush washers behind the headpipes and retorque everything if i were you...

James
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Old 08-14-2013, 05:10 AM
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you`re right about a exhaust leak causing a decal pop. my header developed a crack in the v at the rear of the engine. couldn`t see it unless you used a mirror and flashlight.i got the leak fixed and the pop was gone.
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Old 08-14-2013, 07:18 AM
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Yeah I have heard about the exhaust leaks causing pop. I have searched all over for leaks and have not found any. I will look again.
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Old 08-14-2013, 10:51 AM
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Sick your hand over the end of the can and you will soon hear if there are any leaks. Any intake or exhaust leaks will upset things and cause a bit of popping or carb farts. This is your first port of call before messing with your carbs. Also did you remember to block the outlet on the front carb after the PAIR removal.

(:-})
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Old 08-14-2013, 11:29 AM
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Thanks cybercarl, I'll try the hand trick. I did block it :-)
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Old 08-14-2013, 12:51 PM
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My stock exhaust popped all the time with full bafflectomy and PAIR removed. This was also while running lean at altitude as well. It didn't pop as much when I went to sea level.
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Old 08-14-2013, 01:26 PM
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if you have done a sync on the carbs at home,make sure the nipple on the front head and on the rear head is plugged.
the front one should be capped off and the rear one should be going to the petcock,if it has a T connector make sure its not leaking and it is plugged off.
my bike lost the cap on the front head and it popped really loud,so if one of the hoses is off or leaking,that could be part of the problem.
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Old 08-14-2013, 01:29 PM
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NightHawk, did you change out to aftermarket exhaust? Did it help?

Thanks Saige, I'll check that.
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Old 08-14-2013, 02:31 PM
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Originally Posted by CruxGNZ View Post
I don't mind the popping on deceleration. I actually kind of like it. If your bike runs great, why not just leave it? I feel it's just the bikes character.
I feel the same way.. I like it and think it sounds good... more aggressive or something.
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Old 08-14-2013, 03:09 PM
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It's actually even easier than all the above suggestions...

You say you did a "full bafflectomy", as in removed the baffles completely... That's a really good way of creating the exact condition you have...

A slip-on is a through core exhaust... A cheap and not so good one is nothing but a perforated tube stuck through a larger tube, with some steel wool... but done correctly, you actually have a small amount of back pressure in the can, since they have calculated the volume, and pulses of air, making it create back pressure...

The stock exhaust is a chamber type... It creates it's back pressure and the noise reduction by a series of chambers which the air, and noise pass through... Removing the baffle, you have made the exhaust into a single, large chamber, like the through core, with no steel wool, no perforated tube, and the entirely wrong chamber size and shape to create any back pressure at all...

Plain and simple... You get the same noise level as an slip-on, but with more hard and tinny noises that are harmful to hearing (yeah, yeah, but it's true... It's the difference in frequencies...) and ZERO back pressure... And while it sounds cool, it also produces an engine that is hard to tune...

Partial bafflectomy retains the chambers, and an amount of back pressure... It just lets about half the air and noise take the direct route, the other half goes into the chambers... So, in the end, about the same back pressure as a good slip-on... that's a great compromise, and a good idea...

Full bafflectomy, not so much...
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Old 08-14-2013, 08:46 PM
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Originally Posted by Tweety View Post
It's actually even easier than all the above suggestions...

You say you did a "full bafflectomy", as in removed the baffles completely... That's a really good way of creating the exact condition you have...

A slip-on is a through core exhaust... A cheap and not so good one is nothing but a perforated tube stuck through a larger tube, with some steel wool... but done correctly, you actually have a small amount of back pressure in the can, since they have calculated the volume, and pulses of air, making it create back pressure...

The stock exhaust is a chamber type... It creates it's back pressure and the noise reduction by a series of chambers which the air, and noise pass through... Removing the baffle, you have made the exhaust into a single, large chamber, like the through core, with no steel wool, no perforated tube, and the entirely wrong chamber size and shape to create any back pressure at all...

Plain and simple... You get the same noise level as an slip-on, but with more hard and tinny noises that are harmful to hearing (yeah, yeah, but it's true... It's the difference in frequencies...) and ZERO back pressure... And while it sounds cool, it also produces an engine that is hard to tune...

Partial bafflectomy retains the chambers, and an amount of back pressure... It just lets about half the air and noise take the direct route, the other half goes into the chambers... So, in the end, about the same back pressure as a good slip-on... that's a great compromise, and a good idea...

Full bafflectomy, not so much...

So would you suggest getting rid of the stock cans and getting some slip ons?

This may be a dumb question but that's why I ask here, many smarter, more mechanically minded riders than I am: Will any slip on work? Or does it have to be made for a Superhawk?

Example, I have seen some Devil slip ons on Ebay lately for pretty good prices. Could I take a dual exhaust set up like this: CBR 929 954 Devil Shotgun Exhaust Race Slip on Carbon Fiber Muffler Universal | eBay

and fit them to the pipes on my SH?
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Old 08-14-2013, 08:51 PM
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Originally Posted by eersfanpilot View Post
NightHawk, did you change out to aftermarket exhaust? Did it help?

Thanks Saige, I'll check that.
When I went to a DanMoto carbon exhaust it eliminated the decel pop. Sounds pretty good too! Got them on Ebay for around 200 if I remember correctly.
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Old 08-14-2013, 08:53 PM
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Originally Posted by NightHawk1911 View Post
When I went to a DanMoto carbon exhaust it eliminated the decel pop. Sounds pretty good too! Got them on Ebay for around 200 if I remember correctly.
That's kind of what I'm leaning toward trying.

I think I may part out my '72 XL250 and use that money towards setting up the Hawk more toward sport touring.
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Old 08-15-2013, 09:53 AM
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Originally Posted by eersfanpilot View Post
So would you suggest getting rid of the stock cans and getting some slip ons?

This may be a dumb question but that's why I ask here, many smarter, more mechanically minded riders than I am: Will any slip on work? Or does it have to be made for a Superhawk?

Example, I have seen some Devil slip ons on Ebay lately for pretty good prices. Could I take a dual exhaust set up like this: CBR 929 954 Devil Shotgun Exhaust Race Slip on Carbon Fiber Muffler Universal | eBay

and fit them to the pipes on my SH?
I would suggest not ruining the stock can in the first place, but then, that's just me...

Yes, you can take whatever can you like/have/can make fit, and stick it on there... It might be a royal pain in the behind to tune, but you can do that...

The reason that the larger, more reputable manufacturers actually make different cans for different bikes, is that they calculate the flow/pulses and a lot of other things, to make an exhaust with the right characteristics... Cheap manufacturers, just pick a stock item, and makes a mid pipe for each bike...

So, it all boils down to a choice between paying for someone else to do the thinking, and buying a exhaust made for the bike...

Or doing the thinking yourself, by finding an exhaust from a similar bike perhaps, or used... Or, the other option, with a higher risk, but potentially a bigger pay-off if it works... Trial-n-error...
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Old 08-15-2013, 10:05 AM
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Good points Tweety. Thanks.
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