Farmall A Exhaust Sucking Air

My son recently spent his 4-H money on a 1945 Farmall A, and we're working on it together. (Both of us are novices!) The previous owner said it was running 5 years ago, but has been sitting in the barn since. We have spark, and we have rebuilt the carburater, but still can't get it started. Yesterday we realized that the exhaust is sucking air in, which seems like maybe a problem with a valve. (Or a head gasket, but how would that have blown sitting in the barn?). So we opened up the valve cover and adjusted the clearance on the valves per the manual, but aren't sure what else to do in there, or what else we might try. Any suggestions? Many thanks! p.s. we haven't done a compression check, since we don't have a gauge.
 
I would suspect a bad valve.

Were any of the valves real loose, if so that would be the one.
 
Many auto parts stores rent compression gauges etc. You pay for them use them then take them back and they give you your $$ back. Or you can also do the thumb test by putting your thumb over each plug hole and see if it try's to push your thumb off. Good chance you have a valve or 3 stuck open just little bit. I would since it sat that long fill each cylinder with ATF. Put the plugs back in and also pour as much ATF down the exhaust as you can till it starts to leak out some place. Let it sit a week and then pull the plugs out and spin it over and see if it is still sucking air. Good chance the ATF will have freed up things plus also helped compression
 
Could well be a stuck valve or it's even possible mice could have carried food or nesting materials in there.
 
In my experience when these old tractors sit many were close to needing a valve job before sitting and the the seats continue to corrode from not running and you end up with no compression. I bought an A like
that and a Cub like that and once they had a valve job they ran fine. My A had 10 psi on a couple of cylinders and nothing on the others. The valves were in real bad shape when I removed the head.
 
If you have no compression gauge you take your thumb and place it over the spark plug hole then spin engine and it should blow air past your thumb. Poor mans gauge. If you thumb seals it -- no compression.
 
Thanks to everyone for these ideas. We will be pursuing the valve problem this weekend, and I'll post the results. One question - is there any downside to using ATF? I assume we would change the oil afterwards. Thanks again!
 
No need to change the oil after you do that treatment. Just crack open the top oil check petcock so as to drain any excess oil out. ATF is just a very high detergent oil and will help clean up and free up parts.
 
Just a quick note to close out this thread. Thanks for all the suggestions - we tried a number of them but couldn't get the darn thing started until a friend noticed that the carbon was missing from our distributor cap! Fixed it with pencil lead and the spring from a ballpoint pen. So we're still notsure how much of a problem the compression/air sucking thing was . . . but we're up and running!
 
If you can put a dollar bill over the exhaust pipe and it's sucking the bill in as much as blowing out you have an exhaust valve that isn't seating right. If you have a compression tester it might be a good time to see where the problem is.
 

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