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General Category >> Rubber Side Down! >> Carb tuning to eliminate backfiring (at sea level)
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Message started by David Bookbinder on 09/07/12 at 04:53:22

Title: Carb tuning to eliminate backfiring (at sea level)
Post by David Bookbinder on 09/07/12 at 04:53:22

Recentlly received a 2007 S40 as a gift from my brother, who bought but did not install the Dynojet kit for this bike. Bike backfires badly when rolling off the throttle, particularly when riding above 40mph, and "poofs" when shutting it off. Brother drilled out the idle mix plug and adjusted, no improvement in backfiring.

I gather from the Dynojet site that the primary gain of installing this kit boosts horsepower about 5%, but doesn't specifically address the backfiring problem. From my readings online, I am wondering if changing the pilot jet would do it. I live near the ocean. Anyone have success eliminating backfiring, with the standard exhaust and air filter, at sea level?

Thanks in advance -
David

Title: Re: Carb tuning to eliminate backfiring (at sea le
Post by LANCER on 09/07/12 at 10:59:58

4B4D4040444D46414B4A5D2F0 wrote:

Backfiring is primarily an issue of a TOO LEAN pilot circuit &/or air leak in the exhaust system.
Check your pilot screw position...how many turns out from full bottom. Turn it clockwise (CW) and count the number of turns until it bottoms out. This is a brass screw (soft) so be sure your screw driver fits the small slot as perfectly as possible and when turning it go easy so as not to damage the screw when it bottoms. Use your finger tips when turning and that will help avoid damage.
The screw can be turned OUT (CCW) to richen the mixture but do not go beyond 3 turns out. When the engine is completely warmed up ... like riding a good 10 min. before adjust the screw ... then turn the pilot screw OUT until the engine at highest rpm. If it continues to increase rpm all the way to 3 turns OUT then you need the next larger pilot jet; a #55. After changing the jet, you will need to start the pilot screw tuning procedure all over again at about 1-1.5 turns OUT.

Be sure your exhaust system is without air leaks or else your tuning efforts will be in vain. IT WILL NOT TUNE PROPERLY OTHERWISE.

THE Dyno-jet kit does not contain a pilot jet. The reason is that they made the kit for a stock engine with stock exhaust, in which case a larger pilot jet is not required ... or at least rarely.
The main jets they include are their own brand and are numbered differently than the Mikuni main jet, making tuning a bit confusing. It is easier to use Mikuni jets.
The needle in the kit is useful though. It has 5 grooves for adjusting the mid range mixture, making it unnecessary to remove the white spacer for adjustments.

Title: Re: Carb tuning to eliminate backfiring (at sea le
Post by David Bookbinder on 09/07/12 at 18:37:50

Thanks for the quick reply. I will re-check the idle mix screw; my brother did the adjustment, but he doesn't live at sea level, and he also may have maxed the screw out.

How can I check for an exhaust leak?

Thanks again,
David

Title: Re: Carb tuning to eliminate backfiring (at sea le
Post by justin_o_guy2 on 09/08/12 at 07:03:54

If you can find a place where the wind isnt blowing, a candle helps.
YOu could just snug the header bolts at the head, theyre a common issue.
This isnt a place to get macho, a short 1/4"ratchet & one hand will get it to spec.

Title: Re: Carb tuning to eliminate backfiring (at sea le
Post by David Bookbinder on 09/08/12 at 07:47:05

Thanks. Header bolts are tight. Will try readjusting the idle mix screw and if that doesn't do it, replace the pilot jet with one larger as suggested above.

Is it necessary to take the carb off the bike in order to get to the jets, or will I be able to wiggle it out once I remove the tank? (Sorry for the basic questions, but it's been 35 years since I tinkered with bike maintenance/repair/modifications and I'd prefer to disturb as little as possible as I get back into it.)

Thanks again,
David

Title: Re: Carb tuning to eliminate backfiring (at sea le
Post by justin_o_guy2 on 09/08/12 at 08:15:19

Jet access is easy enough by just loosening the clamps & tilting the carb. Thats IF you can get the bowl off. If the bolts have stuck in the aluminum, youre in for a rasslin match.
A #2 phillips with the tip shortened till the X fills the slots or get the right tip. I cant remember what its called,sorry.
Antisieze is your friend.
The main jet has a washer on it.