slight backfire popping

KZ750B4 twin. Pops on deceleration, right side only. Accelerates well, idles well. Power strong. Just annoying pop on right side exhaust. Have cleaned carbs, jets and adjusted. Something not right. Any ideas?

vacuum leak.first suspect would be the carb boots.let bike idle and spray some wd-40 around the carb boots.if idle changes there is your leak.bj

It is caused by a lean condition. It can be a vacuum leak, removing the stock airbox and installing pods or aftermarket headers.

BTW: if you have any small leaks at where the headers bolt to the head, you will hear loud popping.

ALSO: the KZ750B4 comes with Mikuni BS-38 carbs with 45 pilots and even under best conditions, they will pop on decel. If you change to 50 pilots, the popping will stop and it will run better.

Guess how I learned that?:laugh: I run 50 at the pilots, start with pilot screws about two turns out from seated.

FORGOT: anytime you hear popping, you should check valve clearance in case you have one that isn't quite closing.

This is for "bountyhunter", re: slight backfire popping. Tnx for that reply. My stock setup was using 40 pilots. The right side is the popping culprit. I installed a new rebuild kit on the right carb, which came with 45 pilot. That seemed to help, as you indicated, but still has a slight propensity to pop. Where did you source your 50 pilot? And thanks.

bikerjim wrote:

This is for "bountyhunter", re: slight backfire popping. Tnx for that reply. My stock setup was using 40 pilots. The right side is the popping culprit. I installed a new rebuild kit on the right carb, which came with 45 pilot. That seemed to help, as you indicated, but still has a slight propensity to pop. Where did you source your 50 pilot? And thanks.

I believe Z1 Enterprises has the BS30-96 pilots for these Mikuni carbs.

Another place is:

www.psep.biz/store/mikuni_pilot_jet.htm

scroll to the bottom and click on pilot jets, you want the BS30-96 type.

Also, popping can be caused by the fuel level in the carb being too low (check with clear tube) or a vacuum leak.

Hey man, I'm not claiming to know what I'm talking about here, but I do know that I had some popping on my bike. After I replaced some old split tubing, the popping stopped. It seemed to be the same deal as yours, except sometimes it would backfire at highway speeds.
The one way value about the heads regulates the fuel to air mixture in the exhaust gases, and if the rubber hosing is worn out (I've seen it on my kz550 and my friends kz650 now)then the mixture will be off and MIGHT cause it to pop. It's easier than jetting anyway, and a quick check. Good luck figuring it out.

The B twins do have a vacuum line between the intake manifolds. Good to replace it regularly in case it is leaking.

bountyhunter wrote:

The B twins do have a vacuum line between the intake manifolds. Good to replace it regularly in case it is leaking.

B twins do have a vacuum line between the intake manifolds.--- Wondering why they do that, instead of just capping off each vac nipple?

Good Fortune!

Patton wrote:

bountyhunter wrote:

The B twins do have a vacuum line between the intake manifolds. Good to replace it regularly in case it is leaking.

B twins do have a vacuum line between the intake manifolds.--- Wondering why they do that, instead of just capping off each vac nipple?

Good Fortune!

It's an equalization tube between the two intake manifolds. It forces the vacuum to be equal.

bountyhunter wrote:

Patton wrote:

bountyhunter wrote:

The B twins do have a vacuum line between the intake manifolds. Good to replace it regularly in case it is leaking.

B twins do have a vacuum line between the intake manifolds.--- Wondering why they do that, instead of just capping off each vac nipple?

Good Fortune!

It's an equalization tube between the two intake manifolds. It forces the vacuum to be equal.

Thanks.

Now wondering why they don't connect all the vac nipples together on the 4 cylinder CV carb bikes? :lol:

Good Fortune!

Dont re-jet your carbs to fix a popping issue. C'mon!

You think there was a problem with a little pop from high rpm on decel at the factory? With stock airbox, filter and exhaust, the stock jetting is fine. If you didn't want a little pop, all you had to do was add a little idle mixture with the screws.

Going richer on the pilot will make your cruise throttle position shrink, and make the bike seem jumpy at slow speeds.

There is an underlying issue and you will only make things worse by throwing off your carburetion.

Do a compression test.
Make sure that there is absolutely no air leaks from air filter to exhaust tip.
Make sure you have good consistant spark.
Set jets and mixtures to stock, maybe an extra 1/4 turn out.

If still popping, find where extra air is entering the cylinder with the WD40 method and watch the exhaust while you do, it will puff black after the engine sucks it in.

GL man.

No band-aiding!!! :laugh: