Starts with a vaccum, ends with pressure

Hey folks. Having a perplexing issue and hoping one of you might have a better idea than mine.

The short version is: when I start the bike, it's produces a vacuum (at the carb vacuum ports). As soon as I give it throttle, it starts producing pressure (again, at the carb vacuum ports)!!!

The longer version: I am a novice, but handy enough that my work has been effective up to this point. My bike is a 75 KZ400-D. I recently got it running after taking apart the engine to replace the gaskets. I'm pretty sure I put the engine together correctly (specifically, the camchain and cam sprocket, timing wise). While it was apart I replaced the piston rings, lapped the valves a bit (which I now know is not recommended) but not much and right to the limit of spec, greased the timing advance mechanism and replaced a nearly dead regulator/rectifier.

After that, adjusted the valves, the point, synced the carb and after riding it for a week or so, noticed smoke where it shouldn't be and learned (hopefully correctly) that I should have sealed the gasket between the header and the engine with 700 degree copper permatex rtv. Did that a few days ago and when I fire up the bike it now sounds like it should - at idle. But as soon as I give it gas, my carb sync tool shows (very mild but nonetheless) pressure instead of vacuum, which only confirmed what my very poor test ride suggested after reinstalling the tail pipes. The carbs, by the way, seem to be moving in sync.

I plan on checking the valves again and the timing, but... has anyone heard of this? I think the exhaust thing is not a cause, just a coincidence. It happened before on this bike - two things break simultaneously and I notice one first, thinking the other is related. I'm not happy with the state of the carbs (couldn't get one of two slow jets out to check it) but I boiled the carbs and put a can of seafoam through the tank, so I can't imagine one of the carbs is flowing substantially different than the other enough to cause this problem.

Any insight would be appreciated. Having just started riding last year and not having worked on engines before (except to change oil in my previous vehicle, one with four wheels) I've learned alot from this bike.. This lesson, though, is weird and I'm not sure I'm thinking about it right. Would appreciate any ideas.

Carb sync tools should be designed to measure average vacuum/pressure relative to atmospheric pressure. There are some other designs that may measure instantaneous vacuum/pressure or may be measuring relative to the other cylinder. These other designs may give a strange indication.

Obviously, if your intake manifold pressure was positive, on average, then gasoline and air would be blowing out of the intake, not to mention that the engine could not run like that.

If there are strange intake vacuum readings, then valve timing and valve adjustment would be the first thing to check.
Then you would also want to do a compression check.

A pressure in the intake port is cause by a leaking intake valve (@compression/ignition stroke blow back).
As loudhvx suggest do a compression test and check valve clearance.
Could be a burned intake valve caused by a to tight valve clearance.

A leak-down test injects air pressure through the spark plug hole to pressurize the combustion chamber.

Can perform a poor-man's leak-down test by introducing compressed air into a spark plug hole and listening to where it escapes from the combustion clamber. This may be done at TDC or any other cam positions where both valves are fully closed. Top dead center (TDC) is when the piston is at its highest position. An easy way to tell TDC is by alignment of the T mark with the case mark when viewed through the timing window.

May introduce a spurt of compressed air by using a rubber cone-tip blow gun (rubber air nozzle) held into the spark plug hole. And of course keep holding it in position to keep air from coming back out the spark plug hole while listening for escaping air at other places. The air compressor should not be running while listening because the noise will likely drown out any sound of escaping air. Or just use a portable compressed air tank.

Air heard escaping from exhaust port indicates exhaust valve not fully closing (perhaps too tight clearance or damaged valve or valve seat).

Air heard escaping from carb intake indicates intake valve not fully closing (perhaps too tight clearance or damaged valve or valve seat).

Air heard escaping from crankcase breather indicates loss of compression past rings into crankcase (perhaps worn piston rings or cylinders).

Air heard escaping from head gasket area indicates loss of compression past head gasket (perhaps due to a blown head gasket).

A leaking valve may sometimes be resolved by adjusting the clearance to within specs.

A leaking head gasket may sometimes be resolved by torquing the head fasteners when the engine is stone cold (such as after sitting overnight).

Good Fortune!

Thanks for the generous feedback.

My best guess was a faulty timing advancer - I guess I didn't expect that the valves might be to blame spec-wise, since I set them at max height less than 250 miles ago. Checking those is easy and will do that first. Resetting valves would be nicer than buying a new used timing advancer, for sure.

A leaky gasket is definitely a possibility - there's a bit of oil leaking from left and right side of the combustion chamber gasket. I am able to torque the engine bolts ok but the ones right under the spark plugs are an awkward position for my wrench - I retorqued once after 50 miles on the new gasket and was going to wait for 500 to do one last one. Maybe I'll do it now and give these last two bolts an extra little push by hand. The oil, tho, was weeping too back when I had compression.

The air test is clever. Think i can manufacture a similar test with the tools at hand.

My weekend is two days away. Will report back when my tools and motorcycle meet again. Thanks for the help you guys.

The timing advancer is functioning properly, the valves are set correctly. The air test, while good in concept, didn't provide useful feedback - mostly heard air pushing past the butterfly valves, I think. Compression test - 90 for both, 120 with oil through the plug holes - tells me that the valves are sealing enough, though the rings not great. Even so, 90 should be able to get me to and from work.

Spoke with a mechanic outside this forum he said "check the intake manifold for leaks" but the boots are on tight and I don't see how a leaky boot would push air BACKWARDS through the carb - leaky valves seem to make more sense.

While I haven't kept as good a watch to know for sure, the head gasket now seems to be leaking from the front as well (sides before) and only since running it for about ten minutes since the poor running began. When I was putting the engine back together I thought the gaskets would be the least of my problems for the next few years (coated them in copper, replaced necessary o rings, thread lock on the engine studs, black rtv here and there). Something's going on inside and I just have no idea. Ordered new gaskets yesterday. Going to take apart the engine once more. Maybe the second time through I'll notice if maybe I was cutting corners I shouldn't have.

You should not hear air in the butterfly valves during the leakdown test. That means the intake valves are open a lot. You need to adjust the valves or check the valve timing or get a valve job. 90 psi is very low for a compression test.

So as I mentioned I would do I pulled apart the engine. Flipped the cylinder head upside down and put half a cap of mineral spirits around each valve. No leaking. I was really hoping this would be the problem. I hesitate to pull the valves out if they're not leaking - can't clean them better if they already don't leak. That being said, can valves be sticky?

The only way compressed air inside the cylinder can get from inside the cylinder to the carb is by getting past a supposedly closed intake valve. When valves are indeed sealing on the bench as evidenced by the fluid leak test, but are failing the leak-down test after reassembly, I would expect some error in reassembly such as valve timing or an error in setting the valve clearance. Because compressed air is moving from inside the cylinder to the carb by getting past a supposedly closed intake valve.

Possibly a weak or broken valve spring, or bent valve, or imperfect valve guide (damaged or too tight) or scratched valve stem -- am thinking that "sticky" is referring to anything that might impair free movement of the valve stem back and forth inside the valve guide -- any of which might allow fluid sealing on the bench under some circumstances (say tapping the valve onto its seat before adding fluid).

Good Fortune!

That's pretty much where my thinking has dead ended. I've started the search for a local shop that'll service bike valves/cylinder head.