slight backfire popping

loudhvx wrote (in another post):

At idle, if the carbs are out of sync, but you put a balance tube to connect all four intakes together through ports in the manifolds, you have a mismatch based on firing order and distances between cylinders. The air in the tube has momentum so depending on which way the air happens to flow will affect how much each cylinder gets. This will run as uneven as having the carbs out of sync and no tube.

If you run a tube from each manifold to a central chamber so each intake has equal access to the chamber, it still won't work since the differenctial from one cylinder's carb, relative to the average vacuum in the chamber, will be different for each cylinder. Each cylinder then gets a different amount of charge.

Obviously both options will work if the carbs are all in sync, but then you don't need the contraption at all.

The only way it would work is if the 4 carbs feed into a large box, then the large box is connected to the cylinders. But you'd never do this because then you could just use one big carb to feed the box... much simpler.

If there was a way to eliminate the need to sync the carbs for $2 worth of vacuum hose and some cheap Tee's, wouldn't Kawasaki have done it? That would have saved hours of factory setup.

The length of hose between the vac nips is not stock on many twins. It actually lowers the intake vacuum at idle, which means more idle throttle input is required to achieve a given idle. This allows two slightly out of sync carbs to sound better at idle becuase they are burning slightly more fuel and creating more resistive potential to stopping.

YAY!

:silly:

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!

Lucky for me when they went to the 34s they didn't need the nipples for vacuum testing on the intake boots anymore. (Even luckier for me they never changed the intake ports on the motors. Bonus for anyone who wants to lose 50 lbs and run a slightly more modern carb.) Hose tossed and nipples capped makes balancing more precise, and trouble shooting easier on motors with the 38s imo.

After many years I'm still deciding, if that hose is one of Kawasaki's worst attempts at cheapness, or an afterthought use for them nipples as a balancer unit for the idle circuit. Heh. Easier/sloppier Air screw setting without gauges is all I'd give it credit for.

For high volume intake. Feeding two 3" square cylinders through those nipples, I give the hose a fail.. lol

The fact that you are only getting decell backfiring from one muffler leads me to suspect an exhaust leak. Poissibly the header gasket, or maybe a rusted hole underneath.

B4 is a '79, right? Got an air injection? Temporarily cap off the air intake tube from the air box to the exhaust ports. If it goes away, you might have a stuck reed valve allowing air to flow freely in the exhaust ports instead of around idle (which should be the only time the reeds ever are able to open).

Many thanks for the posts regarding effects from interconnecting vac nipples.

Good Fortune!

TeK9iNe wrote:

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.

I guarantee that this is not true for 750 B4 twins. many were shipped with 40 pilots which are too lean to run right EVER. Believe me, I know. The 45's that were stock on most bikes and all bikes not made in the US run sort of "OK" but are still a little weak. 50's definitely ran better with every set of pipes I have had including stock.

You think there was a problem with a little pop from high rpm on decel at the factory?

Yes, I guarantee it as well as other much worse problems like misfiring and bucking at several RPM points between 2k and 2800 RPM which could not be fixed running the 40 pilots. The dealer told me they knew the problem but, by law, could not install richer jets because it increased emissions. So, I installed them and problem was fixed.

sparkn wrote:

B4 is a '79, right? Got an air injection?

Nope, no air injection.