Backfires through carbs

I just got my bike runnig after a rebuild(new rings& hone,relaped valve seats).I also added a Dyna 2000 with grey coils and set the timming at 39*.The valves are shimmed ok.My bike is backfiring a bit through the carbs and pipe at about 1/8th trottle id say(pilot system).What causes this and why?? Could my selection of timing curve be an issue? 17.5 pilots, needle on mid clip, 140 mains.I think i'll try 22.5 pilots but it ran ok last year with 17's ?

Post edited by: mark1122, at: 2007/06/21 09:39

That is a nasty looking business! Wow...

Oh yeah... backfiring... you didn't mention which carbs you are using. It kind of makes a difference. The slide cut out dictates pretty much which pilot jet you should use and most VM aftermarket carbs on most KZs are smoothbores and most have 1.5 slide cut outs which mean a #17.5 pilot jet with the air screws mostly closed OR a #20 with them open a tad. It looks like you have 33s? They are almost ALL 1.5 slide cut outs... If you have a set of 2.5 slide cut out 29s, like they sold for Suzukis, the pilot jet will need to be at LEAST #27.5.

OK... If the bike WAS running OK with out the backfire prior to mucking with it... AIR IN THE COMBUSTION CHAMBER is what is causing it... in other words too much air as in either:

a. LEAN from pilot air/fuel mixture (air screws mucked up or wrong pilot jet)
b. somebody removed the pipes and didn't use new crush gaskets
c. carb holders leaking a bit of air.
d. vacuum ports leaking (if you have 33s, that ain't it, is it?)
e. poor seal (reseat carbs in carb holders and retighten clamps.
f. float levels low
g. fuel delivery obstructed
h. dirty air filters or over-oiled, etc

Anyway, that is about all I can come up with off the top of my head.

wiredgeorge wrote:

Anyway, that is about all I can come up with off the top of my head.

The top of you head must be the size of an aircraft carrier's flight deck!:woohoo:

I was going to put in my $0.02 but I think WG covered it all.

Later, Doug

My bike was doing the same thing.. I was running 17.5 pilots too.. I put the 20's in like WG said and it went away.. Sounds like your running lean like I was..

I live with these stinky ol' carbs about 12 hours a day... I see EVERYTHING that can go wrong and am expected to be able to fix it. Once I figure something out one time, it is easier to remember how to do the fix again, rather than figure it out again. The neat thing about the carb business is having technical issues. You see stuff that makes you scratch your head. The only real way to have any fun fixing technical issues is to only do the figuring ONCE hehe

WOW, you did a hell of a job on that bike, what color of red is that, I like it!!

Thanks for the help guys.I took it down the road today at 1/8th throttle for 2 miles.It had a bit of stumble and was not smooth,some backfireing through the exhaust apon deceleration but not through the carbs today.I turned the air screws in from 1 turn to 1/4 turn and didnt feel any dif.In is richer right?Air screws are in the intake side of the bodies.I did a plug chop on a new plug and it looked lean.You make some good point George ,thank you .I will check for leaks.They are vm33mm smoothbores with 2.0 slide cuts.I am going to try 22.5 pilots next.oh yea the floats are a little high at 25mm, i think.should be 22 to 24mm.
BSKZ650.Thanks the color is a candy red over a silver base coat.I did it myself.firt time .Looks ok .

Oh yeah.George ,I did put in new crush washers on the exht. but i noticed when i did the first oil change(after 3 miles of use on fresh motor)that my oil drain plug now hits the pipe .So maybe the pipe is not on quite rite and is getting a bad seal??.Thanks again.more things to consider.

sweet bike

Different pipe manufacturers make the flanges different lengths. If you have a hard time making a good seal with the copper crush gaskets, www.z1enterprises.com does sell a variety of exhaust gaskets... some are MUCH thick (and seal a LOT better) than the copper ones. Some exhausts work fine with the copper gaskets but some just don't crush at all... take a look, next time you have the pipes off to see if your crush gaskets are getting crushed and of course, have the new gaskets ready and on hand as they only crush once.

thanks George.I'll check that out.Do u think adding the dana 2000 and coils could burn well enough to lean it out more than the stock ciols and points?

Post edited by: mark1122, at: 2007/06/24 01:42

I put 22.5 pilots in and it runs way better.More work to do but were moving forward.Thanks