backfire question...
02-11-2008, 04:37 PM
#1
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Join Date: Jan 2008
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backfire question...
Ok, I have had my sh for a month this week. It is my second carb'd v-twin the first being a sv 650. My question is this...under 2 conditions it backfires thru the carbs. One being if i let it idle for 10 seconds and then snap the throttle about a quarter to a third open to clear it out. The other is when I decel using engine braking and then clutch and pop the throttle to downshift. The second case is much worse and I can feel it in the tank with my legs.
Now, the funny thing is, my bone stock Sv did the exact same thing only didn't seem as bad since it was only a 650 and just didn't have the wallop the vtr does. I am afraid it is going to blow off a vac line if it hasn't already.
My thoughts are that it is lean either because of jetting or vac leak of some sort. However, my fear is an intake valve not sealing perfectly but I would think if it were a valve I would notice some suffering performance as well. Are there any particular vac lines that commonly come loose that I should look for first? I plan to do some jetting/dyno tuning soon, but not for at least 3 weeks and I don't want to damage anything riding it as a daily if there is a problem.
Any suggestions?
Now, the funny thing is, my bone stock Sv did the exact same thing only didn't seem as bad since it was only a 650 and just didn't have the wallop the vtr does. I am afraid it is going to blow off a vac line if it hasn't already.
My thoughts are that it is lean either because of jetting or vac leak of some sort. However, my fear is an intake valve not sealing perfectly but I would think if it were a valve I would notice some suffering performance as well. Are there any particular vac lines that commonly come loose that I should look for first? I plan to do some jetting/dyno tuning soon, but not for at least 3 weeks and I don't want to damage anything riding it as a daily if there is a problem.
Any suggestions?
02-19-2008, 05:56 PM
#2
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Squid
Join Date: Aug 2007
Posts: 27
Mate, mine did the same till I had it re-jetted with a factory pro kit. Only problem is cruise fuel economy went out the window, but it really hauls *** now. Still fiddling to get it just right and to try bring the fuel economy back a little.
02-19-2008, 06:15 PM
#4
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SuperBike
Join Date: Jan 2007
Posts: 2,116
#1 it is a big V-twin with the largest carbs Honda ever put on ANY production motorcycle or car (I believe).
#2 check your idle speed is 1300~1400 rpm AND the carbs are properly (idle & mid-range) synced (balanced), and maybe turn out your mixture screws a bit.
#3 remove the PAIR system (lots of how-2s on forum) and be sure you have no leaks at the head-to-cyclinder gaskets, and re-route the carb breather lines (again check how-2s)
#4 don't do what you were doing, at least there should be no need to "clear it out" (or check your plugs)
#2 check your idle speed is 1300~1400 rpm AND the carbs are properly (idle & mid-range) synced (balanced), and maybe turn out your mixture screws a bit.
#3 remove the PAIR system (lots of how-2s on forum) and be sure you have no leaks at the head-to-cyclinder gaskets, and re-route the carb breather lines (again check how-2s)
#4 don't do what you were doing, at least there should be no need to "clear it out" (or check your plugs)
02-23-2008, 07:32 PM
#8
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Squid
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02-24-2008, 10:26 AM
#9
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Superstock
Join Date: Jul 2007
Posts: 387
Going up on the mains may get the mixture you need for your engine set-up but has nothing to do with popping on decel. The slides are closed on decel. Any mixture getting by is on the pilot and idle circuit.
No attitude, just info....
No attitude, just info....
02-24-2008, 03:58 PM
#10
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Squid
Join Date: Aug 2005
Location: Tomball, TX
Posts: 57
Whatever the case, I admit to being carb stupid; I'm an EFI guy. I'd still be lost if it wasn't for this site. All I know is it popped just as much after disabling PAIR as it did before then went away entirely after the said rejet.
02-24-2008, 07:19 PM
#11
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MotoGP
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Anyhow, back on topic... I believe he's talking about the carbs spitting through the air filter not the exhaust popping on decel.
In my experience, when the carbs are spitting through the air box it signifies a lean condition. Depending on the rpm range where it happens, you will have to adjust the appropriate jet/needle (pilot/idle - needle/off idle - main/wot
My bike spit through the carbs at idle when the pilot was too lean and also from 1k-5k rpm when the needle was too lean.
As for presetting the mixture screw... IMHO, I think it's bogus (no offense to anyone). I think the best way to set the mixture screw is via idle drop. No 2 carbs or cylinders or any combination are completely identical.
In my experience, when the carbs are spitting through the air box it signifies a lean condition. Depending on the rpm range where it happens, you will have to adjust the appropriate jet/needle (pilot/idle - needle/off idle - main/wot
My bike spit through the carbs at idle when the pilot was too lean and also from 1k-5k rpm when the needle was too lean.
As for presetting the mixture screw... IMHO, I think it's bogus (no offense to anyone). I think the best way to set the mixture screw is via idle drop. No 2 carbs or cylinders or any combination are completely identical.
02-25-2008, 03:08 PM
#12
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World Champion
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The problem with the idle drop method is that it was designed by Honda to be as lean as possible to pass emissions and still be somewhat rideable. Richening the ole girl up is what she really wants.
02-25-2008, 03:08 PM
#13
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Squid
Join Date: Aug 2007
Posts: 27
I'll concur that you have to treat the lean issue wherever it is occuring.
Mine was throttle blipping going down through gears and off idle.
Not knowing much yet I went with "recomendations" to install a Factory pro 1.0 kit.
The recomendations in the kit indicated certain jet sizings depending upon what pipes and filter you have. My spanner guy installed as per those recomendations and it ended up running far to rich mid to top end. Fuel range suffered terribly. So I've slowly been refining myself and have almost ended up back at stock mains, with a 48 pilot and the factory pro needles. So in hindsight I could have saved myself some money in just buying a pair of 48 pilots and shimming the stock needles.
Just my 2 cents worth
Mine was throttle blipping going down through gears and off idle.
Not knowing much yet I went with "recomendations" to install a Factory pro 1.0 kit.
The recomendations in the kit indicated certain jet sizings depending upon what pipes and filter you have. My spanner guy installed as per those recomendations and it ended up running far to rich mid to top end. Fuel range suffered terribly. So I've slowly been refining myself and have almost ended up back at stock mains, with a 48 pilot and the factory pro needles. So in hindsight I could have saved myself some money in just buying a pair of 48 pilots and shimming the stock needles.
Just my 2 cents worth
06-08-2010, 11:19 AM
#16
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Squid
Join Date: Nov 2009
Location: Dublin, Ireland
Posts: 7
Hi all ,hav had this problem since i got mine,, no matter wat u do,, either carbs jetts,, idle speed,, do wat u like,,, am on stage 2 jettin,, n running @ 1300 rpm,, no poppin ,, only when i changing down,, sounds great,, my fuel econ' has imroved,, sine gettin carbs redone after jettin,,, she was 1 thirsty b***h before,, I just leave it now,, my power is up, econ' is good, no probs,,, hey if it aint broke,, dont fix it ,,,lol
06-08-2010, 12:21 PM
#17
Out of my mind, back in 5
MotoGP
Join Date: Nov 2006
Location: Skurup, Sweden
Posts: 6,109
And just clarify... (I know, doubt you do) what is stage 2 jetting?!
Either you re-jet ie you swap jets & needles or you don't... There is no stage 2... But in tuning/(re-)building an engine there might be stage 1 & stage 2 indicating how much you swap out...
Either you re-jet ie you swap jets & needles or you don't... There is no stage 2... But in tuning/(re-)building an engine there might be stage 1 & stage 2 indicating how much you swap out...
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