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Has anyone tried this? (Read 2259 times)
JESPOKER4FUN
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Has anyone tried this?
03/04/10 at 21:45:08
 
Just read this on a different forum. Sorry I know what you are thinking.
Yes I wandered off in search of new ideas.

The cure for s-40 or ls650 savages backfiring on decel. is not to change needle or plastic ring or jet kit, The carb has whats called an anti backfire valve, also called decel valve,or cut valve,it is located on side of carb with 3 screws. The problem is the spring in the diaphragm is to stiff, remove diaphragm and cut one full round off spring this will allow diaphragm to open sooner on decel and allow more fuel to motor stopping backfire problems.
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siriusjoe
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Re: Has anyone tried this?
Reply #1 - 03/04/10 at 22:16:59
 
JESPOKER4FUN wrote on 03/04/10 at 21:45:08:
The cure for s-40 or ls650 savages backfiring on decel. is not to change needle or plastic ring or jet kit, The carb has whats called an anti backfire valve


I don't know if that's true or not, but I do know one thing... The stock S40 carb setup is so lean that the engine simply does not run properly at ANY throttle position, and no "anti backfire valve" adjustment will fix that.  

The air screw adjustment helps the decel backfire quite a bit, but even at 3 turns it's still too lean, and it doesn't address the mid-range or top end issues at all.  

I just got a Dyna muffler from Ebay and I'm expecting a jet kit from Lancer shortly.

Joe
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dasch
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Re: Has anyone tried this?
Reply #2 - 03/05/10 at 03:14:06
 
transient enrichement valve... some here (me included) blame that thing for some other stuff too. Idea seems valid in theory.
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LS-Rich
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Re: Has anyone tried this?
Reply #3 - 03/07/10 at 01:04:19
 
I'm really curious about this. Anyone else? Lancer????
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justin_o_guy2
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Re: Has anyone tried this?
Reply #4 - 03/07/10 at 06:37:31
 
I could stand some education on this.
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thumperclone
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Re: Has anyone tried this?
Reply #5 - 03/07/10 at 07:03:33
 
TEV just a guess
part of the"choke"
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dasch
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Re: Has anyone tried this?
Reply #6 - 03/07/10 at 09:42:07
 
Found it someplace online, it eliminates backfire during sudden deceleration  - engine braking.
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justin_o_guy2
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Re: Has anyone tried this?
Reply #7 - 03/07/10 at 09:44:36
 
Based on how mine used to buck & snort, Id say it is not just real god at doing its job.
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Re: Has anyone tried this?
Reply #8 - 03/07/10 at 09:56:01
 
Only thing I've seen on this was to clean it out if you have troubles.
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Re: Has anyone tried this?
Reply #9 - 03/07/10 at 15:51:19
 
It's definitely worth pulling the diaphragm cover from the side of the carb and inspecting. It gets ignored for long periods and may be difficult to get off - I had to remove the carb and use an impact driver the first time.

The diaphragm must be clean and have NO cracks or holes. If it can't hold a vacuum, it cant work.

The plunger attached to the diaphragm needs to perform reasonably well as an on/off valve for the enrichment circuit - so it should be clean and show no more than mild signs of wear.

The cover has a very small drilling to connect the vacuum side of the diaphragm to the engine side of the carb throat. This has to be as small as possible to damp out pressure pulsing, but that makes it prone to blockage. Don't use wire or anything hard to clean it - I use copious blasts of CRC carb cleaner.

If when you took it apart it was pretty ugly, after reassembly you may have to re-tune your idle circuit/air screw as this may have been set wrong to compensate for the enrichment circuit not working properly.

The suggestion about redcing the diaphragm spring tension might be worth considering, but I think all that'll do is shift the point of snap crackle and pop to a lower rev range.

I had an old Ducati Pantah for years and to try and keep the *%#$#* Dellorto's in tune I spent a fair bit of time riding around with vacuum guages taped to the tank. That showed there's three basic states happening in the carb:
  • Idle
  • throttle-on
  • throttle-off

At idle the throttle's barely open and the idle circuit controls mixture strength. There's big pressure-pulsing in the carb making the air pass back and forth through the throttle, so gas is sucked twice and a correctly tuned idle circuit actually delivers a "lean" amount of gas for each suck.

During throttle-on there's lower vacuum and reduced pressure pulsing at the throttle, and the idle circuit is less important because other jets/circuits come into play.

During throttle-off the throttle's barely open and the idle circuit is again the major player. But now, while there's high vacuum at the throttle, there's little pressure pulsing and gas only gets sucked once. A correctly tuned idle circuit will be delivering a hugely lean mixture until revs drop back close to idle.

Hence the enrichment circuit - which gets pulled open by the high vacuumof throttle-off and closes again when revs drop closer to the point where the idle state returns.

Softening the diaphragm spring will keep the enrichment circuit open longer, but there's a risk the circuit may be held open when you don't want it.

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Re: Has anyone tried this?
Reply #10 - 03/07/10 at 17:04:50
 
Thank you. This was quite helpful. Nobody really even mentions that thing. Turns out it's rather important and can cause problems.
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Re: Has anyone tried this?
Reply #11 - 03/07/10 at 17:21:23
 
Mark, excellent detailed info on the functional steps involved.

This valve is connected to several parts of the carb; the engine side of the carb throat as mentioned; the choke/enricher system; the filter side of the carb intake and the pilot circuit of course.
As mentioned, the very tiny passageway from the under the 3 screw cover but outside of the diaphragm,  is very easily clogged and will cause problems.  If you are one of those who do their own carb work then I suggest you invest in a carb tool that is about 8-10 various sized cleaner wires.  The set is less than $10 and is invaluable when needing to clean out clogged passageways.  The wires have a bit of a rough surface which aides the cleaning process.  It is sort of like a tiny round file.
I have never cut the spring so cannot comment on the effect of that mod, but assuming the carb is clean throughout and functioning normally, I find it much easier to cure the backfire issues with adjusting the pilot screw and/or rejetting as well.  Do it correctly and the backfiring WILL STOP.
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Re: Has anyone tried this?
Reply #12 - 03/07/10 at 20:11:18
 
LANCER wrote on 03/07/10 at 17:21:23:
....If you are one of those who do their own carb work then I suggest you invest in a carb tool that is about 8-10 various sized cleaner wires.  The set is less than $10 and is invaluable when needing to clean out clogged passageways.  The wires have a bit of a rough surface which aides the cleaning process.  It is sort of like a tiny round file....


Lancer,

You mean like one of these:

K&L Carb Cleaner Wire Set?

I've got one but haven't used it yet.  I'm sorta worried it will hog out the jets.
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miker
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Re: Has anyone tried this?
Reply #13 - 03/08/10 at 04:52:19
 
You can pick that tool up at any welding or plumbing supply house. They will call it a "torch tip cleaner", and it may cost less than a carb cleaner.
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Charon - FSO
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Re: Has anyone tried this?
Reply #14 - 03/08/10 at 13:34:05
 
Most of the instructions I have seen for cleaning carburetors say you should not use wires to clean jets, because the wires are harder than the jets and can damage them. I have seen it suggested that you use nylon guitar strings instead. If you are (or know) a fisherman, you can use monofilament fishing line in whatever size seems appropriate.
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