12.5 IC Flathead blowing air out through intake

So I was working on reviving one of my 12.5 I/Cs I had to put on an MTD mower im working on repairing to mow grass. I got it all set up, hooked fuel up to it and starter fluid and let it turn over, but I noticed it was blowing air out through both the exhaust and the intake. Is something wrong with my timing? Too much compression? I just replaced the head gasket but it was an OEM replacement.

if its blowing out both ends (no pun) the valves could be getting stuck, or timing could be off a little. assuming its a flat head.

Its a flathead.

I was Googling what was wrong and something said it could be the valves. Ill try pulling the head off and getting a wire brush in there.

my gokart motor does the same thing. but i think its more from over heating and lack of lube. i ran it without oil yesterday for 2 hrs. it was blowing as much out as it was in.

Just fixed my kids 12hp with this problem, not enough valve clearance on the intake valve. took valve out and ground a little off, all is well now. Actually took a little too much off, has a slight valve tick now. But runs good.

So the valve needs cleaned. Can I do it without taking it apart?

If its the same as mine, no. Need to remove valve, and grind off stem end until you have .010 in clearance between valve and lifter, when piston is on top.

Did you take a look under the head?

Not recently no. It snowed today and the MTD its on is outside so im not gonna touch it till it gets warmer out.