Backfire through carburettor

[Dirtbox]

I have a noise on the carbs where there is a shockwave, coming back from the inlet valve when revving through 2000 RPM. Below and above that it's fine.

I found it by holding the carbs with my hand and you can hear the sharp clack and feel it on the carb. I think its on the Left hand carb more.

I suspected weak valve springs but stuff I have read on line suggest that weak spring problems get worse at higher RPM but mine don't.

The problem is very pronounced when warming up if held at 2000 rpm but gets a bit better as the carb gets warm.

The bike has had a top end overhaul the valves have just been ground in and the valves are spot on for clearances. The garage reports that the valves are sealing very well on vacuum test.

Has anyone had this problem.

By the way the bike has a drop in K&N, a Delkevic Zorst and is re jetted 140 with the needles on the second position. It goes like a rocket compared to when it was standard.

I have a GS500 K2.

The re jetting has only just been done the power and responsiveness is great and the plugs look perfect but could this be a symptom of being a little too rich.

Has anyone got constructive advice ??

[nathaniel2910]

I had exactly the same thing, it was a clack about 1,800-1,900rpm as if the slides were suddenly closing.
I'm not exactly sure what stopped it but I sealed the joints between carb-manifold and carb-airbox.
I set the fuel screws from 2 turns one side and 2.5 turns out on the other, to 3 turns each and balanced the carbs.
As I did it all at the same time I couldn't say which worked the best but it cured it.
Hope that helps, also check the routing of the carb breather pipe, after doing everything I re routed it so it was sticking out below the engine and it caused a surging above 40-50mph.
Make sure it's routed so the end is behind the engine sort of hidden.

[radodrill]

Since it's a low end issue, the first things I'd check are to make sure the carbs are synced, verify the pilot jetting, and adjust the idle/mixture screws,

[BockinBboy]

I had this for about a year on my bike, and it was present when I purchased it.  If I slowly added throttle, right at/just below 2k rpm, it would clunk and rpm would drop slightly... but the rev range everywhere else was just fine.  And as you said, worse when cold, better when warmed up, but still there.  I really couldn't tell you what exactly got rid of it.  I had changed my exhaust and rejetted... which included taking a bunch of things off and cleaning and lubing things up, carb sync, etc... but after all that, the issue was gone. 

I don't think its a richness issue though, because my bike was factory jetted when it was occurring, so maybe leanness on the pilot for some reason?    What is your jet sizing now with filter and exhaust?  Are you using the restrictor ring on the air filter?

- Bboy

[piresito]

You also changed the pilot jet, right?

[steezin_and_wheezin]

I also have this happening on my GS. 06 gsF - full jardine exhaust, k&n lunchbox, PAIR system eliminated, and carbs rejetted(I will have to check my project thread for what jetting I settled on) New plugs beginning of this year. I get the hiccups/backfires only though the left carb as well.

It rarely happened last season(a couple backfires in a full week of commuting), but been getting a few in a row on cold starts this year. Temps have been 30-40*F so it's really putting her to the test. A little wrench weather left before riding season, so now its time to figure out the culprit

Since mine only occurs during idle - I think Radodrill is on the right track with it being associated with the pilot jetting/mixture screw setting. Hopefully I'll get some time in the shop here soon. Going to start with checking all connections for airleaks. Reset my mixture screws. Another carb resync and see where that gets me

[radodrill]

I also have this happening on my GS. 06 gsF - full jardine exhaust, k&n lunchbox, PAIR system eliminated, and carbs rejetted(I will have to check my project thread for what jetting I settled on)

On my '09 F I never had the symptoms the OP is describing.  But with a slightly lean idle mix and a cold engine I could get the exhaust to pop (minor backfire) by quickly blipping the throttle a few times in rapid succession; slightly richening the mix will take care of that.

[Dirtbox]

Thanks to all.

My jets are presently 20/60/140  I did at first re-jet the main at 142.5 but the bike would not rev over 7500rpm until I dropped the jet to 140 after which all was really good except for the backfire.

So in addition to looking for vacuum leaks I will change the mid main jet to 62.5 but I had thought that raising the needle to the second position might be enough.

I will let you know how it goes.

[bremsstrahlung.positron]

Nitrous oxide turbo, I guess 

[Dirtbox]

Total respect to Radodrill.

I have adjusted the idle mixture to 3.5 turns out and the problem has now gone.

My only problem now is holding on to this baby when she goes.

Many thanks to everyone who contributed as all of the answers have helped but it is the idle mixture that was the key to it all.

Dirtbox
Birmingham
England

[Dirtbox]

Thanks again here is the baby

[attachment deleted by admin]

[BockinBboy]

Great to hear, and great lookin' bike!

- Bboy

[Crasm]

Dirtbox - where about in brum are you? Im only down the road from you in west brom   

[Suzuki Stevo]

Anytime I hear about popping in the carbs, tight intake valves come to mind. The flame might be getting to the fuel past the tight/open intake valve, I only say this because I have seen it in person.

[BockinBboy]

Anytime I hear about popping in the carbs, tight intake valves come to mind. The flame might be getting to the fuel past the tight/open intake valve, I only say this because I have seen it in person.

Would richening the idle mix 'mask' that?

- Bboy

[Dirtbox]

The valves are not tight I suspected that first and got 0.06mm ground off the RH shim.  The inlet valves are at now at 0.05 and 0.09 left to right when stone cold.
I know the RH is out of spec but the clearance can only close.

So all is done which is great.

[Suzuki Stevo]

Would richening the idle mix 'mask' that?

- Bboy

I never rule out anything, if it goes away maybe thats all it needed.

[The Buddha]

Lean, sucking air and tight valves can all cause the misfire ...
Its for most practical purposes called lean misfire.
Cool.
Buddha.

[radodrill]

Glad you got it working

[Dirtbox]

Having read the excellent thread on http://gstwins.com/gsboard/index.php?topic=66638.0
I am going to re jet the mids to 62.5 and probably turn in the pilot mixture to 3 turns out.