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Discussion Starter #1
Hi all!

I've been having an annoying issue with my 2000 sv650; it likes to backfire, a lot!

It backfires (actually after-firing I guess) loads and very loudly whilst using the choke a bit during warm up but if I use no choke it will run but not well at all until its warm (Very lumpy idle, backfiring up the front carburettor). It also backfires tons on the over-run when I'm out riding but especially as it passes through the lower rpms. It does seem to surge a little when cruising along at lower speeds but its not so noticeable at higher speeds (60/70mph). The bike seems to run great when I open the throttle (although there's possibly a small dead spot around 4000rpm). So far I have:

  • Cleaned and rebuilt the carbs twice
  • Had them on and off the bike a million times to mess with the fuel screw settings to no avail
  • Messed with the choke cable and plungers to ensure they aren't stuck
  • Checked the balance of the carbs with a Carbtune tool each time I adjusted the fuel screw
  • Sprayed easy start on all the joints between the airbox, carbs and cylinder heads to try and find vacuum leaks. (I've also checked all the vacuum take off nipples and tubing)
  • Put new plugs in at the correct gap. The colour of them seems to show a good burn (Light tan brown).
  • Tried to track down any exhaust leaks as I know that's a common issue but haven't found any so far. (However the studs do not want to come off without beginning to round off the nuts so I'd rather not remove it unless I absolutely have to)
  • I plan to do the valve clearances this weekend but the bike only has 3500 miles so I'm not expecting to find much out of spec.
My bike is bone stock apart from a slip-on can. The fuel screws are currently set to maybe 2 5/8 turns as this seems to be where it backfires the least at idle. All the other jetting is standard. I'm based in the uk if that helps.

I feel like I'm running out of ideas at this point as I've never had carbs be this difficult so if anyone has any advice it would be greatly appreciated!

Thanks
 

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Discussion Starter #3
Mine only does this if the jets are dirty.


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I have cleaned the carbs a couple times and I've put a little filter after the fuel tap although I am starting to wonder if the tank is rusty as its spent a lot of it's life sitting around. It didn't look too bad inside but maybe worth a clean internally with evaporust or something?
 

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I have cleaned the carbs a couple times and I've put a little filter after the fuel tap although I am starting to wonder if the tank is rusty as its spent a lot of it's life sitting around. It didn't look too bad inside but maybe worth a clean internally with evaporust or something?
I also put a filter after the tank to catch any loose junk.
Ever passage must be %100 clean,which it seems you already did that.
What size jets are you running ?


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Discussion Starter #5
I think they are the stock jets which as far as I'm aware are 17.5 pilots and 137.5 mains?
I'll take the carbs off and clean them again when I check the valve clearances over the weekend.
When I first had the carbs off to clean them they were absolutley filthy, probably the worst I've ever seen so it may be that stuff is still coming from the tank.
 

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Discussion Starter #8
Make sure the float needles and seats move easily.


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Will do!

did you pull and clean the mixture screw passages, blow carb cleaner thru.
I tried to get every passage I could however I could not get the two brass screws that hold the funnel on the intake side of the carb as they wanted to strip out. Are there any passages hidden by the funnel? I did get the little jet next to it and blasted all three small holes with carb cleaner and then compressed air.
 

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sorry don't know whats a funnel.. the mix screw shares the passage with the pilot jet, so you have to pull the screws in order to cean all passages
 

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Check the vacuum line on the fuel peacock for fuel. If the diaphragm is torn it'll allow fuel to be drawn into the intake, causing a rich condition.
 

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Discussion Starter #11
Hi all!

I figured I'd upload a video of the issue in case that helps. Unfortunately I can't really get a video whilst I'm actually riding it at the moment so I've just shown what it sounds like at idle and when I rev it a bit:



I'm gonna check the valve clearances and re clean the carbs tomorrow. However, I'm replacing the water pump housing at the same time as its badly corroded and I'm waiting on the clutch cover gasket so I won't be able to restart the bike until that arrives. I may also have a go at cleaning the tank of rust during this week.

I'll report back with the results of the work when I can!
 

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When I read the title of the post I suspected you had an aftermarket muffler. First, the sound you are hearing is legit, but it is not backfiring. A stock muffler has more internal baffling than aftermarket (in most cases) and the stock mufflers mask this sound so well, you just don't hear it. What you are hearing is a normal sound for a low restriction aftermarket exhaust. The cause is some excess fuel getting into the pipe at various moments of operation. Deceleration being a prime time for such events, but you can hear it a little most of the time to lesser degree. If you go to the track and listen to bikes that have low restriction exhausts (cars too) you will hear many of the do the same thing. Its normal. Its not a problem. The sounds reminds me of almost every low restriction track car exhaust system I've ever heard.

Backfiring is a huge load of fuge in the pipe that when ignited is deafening and if large enough can blow things apart.
 

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You want to do the idle,pilot , and main jet passages don't disasmble the carb.squirt cleaner thru any accessable passage.
 

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When I read the title of the post I suspected you had an aftermarket muffler. First, the sound you are hearing is legit, but it is not backfiring. A stock muffler has more internal baffling than aftermarket (in most cases) and the stock mufflers mask this sound so well, you just don't hear it. What you are hearing is a normal sound for a low restriction aftermarket exhaust. The cause is some excess fuel getting into the pipe at various moments of operation. Deceleration being a prime time for such events, but you can hear it a little most of the time to lesser degree. If you go to the track and listen to bikes that have low restriction exhausts (cars too) you will hear many of the do the same thing. Its normal. Its not a problem. The sounds reminds me of almost every low restriction track car exhaust system I've ever heard.

Backfiring is a huge load of fuge in the pipe that when ignited is deafening and if large enough can blow things apart.
Chased after the same thing when I put a Muzzy on my ZRX. Agree, totally normal.
 

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I have a cheap after market end-can and it doesn't do this.,

Soundbite,


The most common causes of these kinds of things are,

Air-leaks into the Carbs and or exhaust but an often overlooked part of CV(Constant Velocity) carbs are the Air-Cut-Off mechanisms/diaphragms.


Parts numbers 34 to 37 on this Partsfiche(Worth a bookmark of your model),


They are designed to enrich the fuel-to-air Ration when the throttle is closed as the mixture becomes weak at this point in CV carbs which can cause popping(Common fault on Harleys). If the mechanism is faulty or the Diaphragm becomes porous/brittle or any tiny Venturi become blocked time to service and or change.


When I fitted my End-can I used an exhaust/Muffler paste like this on the joints for a Gas-tight seal,


HTH :)
 

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Does your bike have a PAIR system? If so....they are notorious for causing exhaust popping as they add extra air especially during decel conditions. If you have such equipment....the easy out is to just block it off which should stop the popping.
 

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Discussion Starter #18
Hi everyone!

Sorry for the late reply, life (and of course coronavirus) got in the way.

Have you checked valve clearance yet? I bet your exhaust valves are tight.
I checked my valve clearances and all were in the middle of the spec except for one of the front intake valves which was bang on 0.1mm so very close to being tight. Unfortunately I don't have access to a torque wrench where I'm staying and I don't feel comfortable tightening down cam cap bolts without it so I'm going to leave it as is for now.

You want to do the idle,pilot , and main jet passages don't disasmble the carb.squirt cleaner thru any accessable passage.
I have a cheap after market end-can and it doesn't do this.,

Soundbite,


The most common causes of these kinds of things are,

Air-leaks into the Carbs and or exhaust but an often overlooked part of CV(Constant Velocity) carbs are the Air-Cut-Off mechanisms/diaphragms.


Parts numbers 34 to 37 on this Partsfiche(Worth a bookmark of your model),


They are designed to enrich the fuel-to-air Ration when the throttle is closed as the mixture becomes weak at this point in CV carbs which can cause popping(Common fault on Harleys). If the mechanism is faulty or the Diaphragm becomes porous/brittle or any tiny Venturi become blocked time to service and or change.


When I fitted my End-can I used an exhaust/Muffler paste like this on the joints for a Gas-tight seal,


HTH :)
I've had the carbs apart again and as far as I can tell the air cut off valves look ok. Theres no obvious damage to the diaphragm but do you think it's worth replacing anyway? It's £30 so I'd rather save the money if it's not needed. I did notice that the carbs have a lot of brown rust staining on them even though they've only done 70ish miles since I last cleaned them so I'm beginning to strongly suspect its rust constantly clogging the carbs. I've started cleaning the inside of the tank with evaporust in hope of curing this. I've also put some exhaust sealant around the end can joint but I haven't been able to start the bike yet to test it.

When I read the title of the post I suspected you had an aftermarket muffler. First, the sound you are hearing is legit, but it is not backfiring. A stock muffler has more internal baffling than aftermarket (in most cases) and the stock mufflers mask this sound so well, you just don't hear it. What you are hearing is a normal sound for a low restriction aftermarket exhaust. The cause is some excess fuel getting into the pipe at various moments of operation. Deceleration being a prime time for such events, but you can hear it a little most of the time to lesser degree. If you go to the track and listen to bikes that have low restriction exhausts (cars too) you will hear many of the do the same thing. Its normal. Its not a problem. The sounds reminds me of almost every low restriction track car exhaust system I've ever heard.

Backfiring is a huge load of fuge in the pipe that when ignited is deafening and if large enough can blow things apart.
The strange thing is though I'm using an exhaust I had for an identical SV that I owned a few years ago and it never use to backfire anywhere near as bad as this one does.

Chased after the same thing when I put a Muzzy on my ZRX. Agree, totally normal.
Weirdly enough I also own a ZRX(1100) and that does pop a lot with the full exhaust I've got on it! But the video doesn't show how loud this SV when I'm out riding it.

Does your bike have a PAIR system? If so....they are notorious for causing exhaust popping as they add extra air especially during decel conditions. If you have such equipment....the easy out is to just block it off which should stop the popping.
I don't think it has a pair system it's a 2000 uk model and I can't see anything that looks like a pair system on it.

And finally, whilst I was taking the cooling system apart during the valve clearances I had the thermostat housing off to replace the hoses as they were old and crusty. I had a little look inside the housing to check if it was clean and look what I found:

20200420_114257.jpg

Now correct me if I'm wrong but there should be a thermostat in there shouldn't there? The bike only has 3500 miles on it so I think it may have just never had one installed in the factory. I'm not sure that this would be the cause of the issue though. Unless it's possibly making the bike run cold? Either way I'm going to put a thermostat in there to rectify it.

Thanks everyone for the replies, I'm gonna finish cleaning the tank and put the bike back together and I'll report back with any progress!
 

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If you think the ACV is ok then leave it.

I would however renew the Spark plug caps. These can be a sneaky problem maker and cause your symptoms and even limit top-end speeds.

I use this type and you can get them in black.


as the OEM ones are a rip-off price.



The piece of card is just temporary when I work on the Front plug to stop damage to the Rad fins and my knuckles. Also I've removed my Horn bracket and put my horn on the Front left of the bike. This means I can just undo the 3 Rad bolts and move the Rad forward to get to my Front plug and not have to drain the coolant,


 

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Discussion Starter #20
If you think the ACV is ok then leave it.

I would however renew the Spark plug caps. These can be a sneaky problem maker and cause your symptoms and even limit top-end speeds.

I use this type and you can get them in black.


as the OEM ones are a rip-off price.



The piece of card is just temporary when I work on the Front plug to stop damage to the Rad fins and my knuckles. Also I've removed my Horn bracket and put my horn on the Front left of the bike. This means I can just undo the 3 Rad bolts and move the Rad forward to get to my Front plug and not have to drain the coolant,


Thanks for the advice I'll get a pair of those caps on order!
 
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