Timing problem?Air coming out of intake n carbs

I have recently built a new motor with gpz cams and milled polished and ported head. I had a previous post about not getting gas in motor but when i tried to pull start the bike i noticed the carbs were not sucking but spitting gas and air. Is this a timing problem i had my friend help me degree the cams in but maybe we did it wrong? Any help or answer will be greatly appreciated thanks!

Would normally think valve clearance might be an issue (usually intake is too tight), or valve seat interface where valve fails to fully close (carbon or burnt), but would here also consider possible cam timing issue because of the recent engine work.

Good Luck!

The head should have been rebuilt at least the character i bought it from said so. Also after getting the head milled at a machine shop they put in all my valves. Also valve shims were done after i put togeather motor. Any other suggesdtions. Thanks!

When reinstalling the camshafts, precise orientation and camchain position is essential to correct cam timing. The procedure is detailed in the shop manual, and probably in other KZR forum threads. With under-bucket shims used in the KZ650's, the camshafts are first removed in order to allow access for removal of the buckets. Other members are more qualified to render advice on correctly replacing KZ650 camshafts.

Could do a forum search for a procedural walk-through covered in previous threads. (see "Search Box" above).

Am also thinking there's a KZ650 shop manual available in Filebase under "KZ Information" (see green ribbon toward top of screen).

With proper cam timing, decent compression, correct valve clearances, and good valve seating, the cylinder should suck mixture from the carb, and not push anything back through the carb. Imo, anything from the combustion chamber going back through the carb means the intake valve is not tightly closing when it's supposed to.

If had to guess now, would suspect an error during the last camshaft installation.

Good Luck!

Due a compression test.

May of found the problem my valve clearences seemed to be pretty out of wack i could only get the smallest feeler guage (0.038mm) in on one intake valve. The shim I had in was a 235, I am going to go ahead and change the shim to a smaller size. Does this make sense of what my problem could be? Thanks for all the help.

odl666 wrote:

May of found the problem my valve clearences seemed to be pretty out of wack i could only get the smallest feeler guage (0.038mm) in on one intake valve. The shim I had in was a 235, I am going to go ahead and change the shim to a smaller size. Does this make sense of what my problem could be? Thanks for all the help.

Any gap at all when cold should allow complete closure of the valve. So imo the 0.038mm gap isn't causing the present symptoms.

odl666 wrote:

...Also valve shims were done after i put togeather motor.....

Am thinking the camshafts were incorrectly re-installed following the most recent shim exchange.

Good Luck!

But that meadurment was only on one valve the rest had no clearence at all sound like a problem?

odl666 wrote:

But that meadurment was only on one valve the rest had no clearence at all sound like a problem?

Insufficient valve clearance is definitely a problem, and should be corrected before again running the engine.

As earlier noted, too tight intake valve clearance may prevent complete closure when supposed to be seated, thus allowing upward piston movement to push whatever's inside the combustion chamber back out through the carb (even with perfect cam tming).

Good Luck!

Consider each of the 4 strokes in a 4-stroke engine.

These strokes describe the process that's happening in a running engine as the crankshaft rotates and the pistons move up and down.

1 = intake
2 = compression
3 = power
4 = exhaust


stroke 1 = intake
Intake valve is open.
Piston is moving down and sucking mixture from the carb into the combustion chamber through the open intake valve.
Flow through the open intake valve is supposed to always travel toward and into the combustion chamber.
Exhaust valve is closed to prevent unwanted partial sucking from the exhaust.

stroke 2 = compression
Both valves are closed.
Piston is moving up and compressing the mixture that was inhaled in the previous stroke 1.

stroke 3 = power
Both valves remain closed.
Sparkplug fires and ignites the compressed mixture which "explodes" and pushes the piston downward.

stroke 4 = exhaust
Exhaust valve is open.
Piston is moving upward and pushing remains of the exploded mixture from the combustion chamber out through the open exhaust valve.
Flow through the exhaust valve is supposed to always travel away from and out of the combustion chamber.
Intake valve is closed to prevent anything from being pushed through it backward.

If during strokes 2-3-4, the intake valve fails for any reason to completely close against its seat as its supposed to, pressure from inside the combustion chamber is pushed out "backward" through the valve seat and toward the carb. Such failure to seat may be caused by insufficient valve clearance (gap), or a burnt valve, or some other reason.

Correct positioning of the camshaft is necessary to correctly "time" opening and closing of the valves.

Correct valve clearance (gap) -- set with shims -- is to assure that a valve is allowed to fully close in its seat, especially after the engine reaches normal operating temperature.

To see the four strokes in action, click here > Varoom!

To hopefully facilitate easier understanding of the basics involved, have purposefully ignored all the related matters such as overlap, cam profile, duration, ignition timing, exhaust pulses, etc. And thankfully so, as am barely ankle deep in that ocean of knowledge (maybe actually still on dry sand.)

Good Luck!

That cam timing is way off. Friend may have been high. Might have used the wrong piston as number one. The cylinders are expelling the fuel, all of it. There should be flames shooting out those intake ports every time spark occurs. The is no fuel. There is no true friends. But there's lots of bud! So cheer up. You've got a unique motor, and it can be fixed. But first! Check the cam timing. Take off the valve cover, roll the motor and watch when the intake starts to open. Where by god is that piston NOW! Is it at the bottom of the cylinder and rising as the valve opens? Is it even IN there? Go from there. And when u get it straightened out, give it a comp. test so you know what is happening if any cylinders not firing,this eliminates alot of confusion wether the carbs are to blame for whatever. If it tests bad. Roll it over till both valves are closed and shoot some air into the plug hole on each cylinder and listen to the sound of the air whooshing out. You'll be able to tell where it's going (but not when it's coming back) havnt a clue there. And get back to us. Soon. Whose to say the world isn't coming to an end.