Need some help on 8N puffing noise diagnosis

I'm trying to help a fellow forum member diagnose the puffing he's
getting out of his exhaust and so far we're not getting very far.

Bob has done a lot of troubleshooting and has found that if he
runs a propane torch (unlit) across the front of his carb intake
the noise clears up, but the noise couldn't be adjusted out.
The propane torch does not help anywhere else around the
manifold or gaskets for example.

So he sent me the carb, I bolted it on my 8N and it ran just fine.
While it was here I took a short video of it on my tractor.
I apologize for the sound quality, I had a belt slipping.
Here is that short [b:fc6a4b19bd]video[/b:fc6a4b19bd]

When Bob got it back, he made a couple videos of it running on
his tractor. Much better videos by the way! As you can see
in this first one, he has great spark quality. [b:fc6a4b19bd]Video2[/b:fc6a4b19bd]

His compression is 110-120 on all four cylinders. Excellent.
New Autolite 437's gapped at .025.

Here is the noise we are trying to eliminate. It isn't there on
initial startup, but shows up after a couple minutes run time. [b:fc6a4b19bd]Video3[/b:fc6a4b19bd]

Help/suggestions appreciated! Bob, feel free to chime in as
I'm sure I left many of the troubleshooting steps out.

Disconnect exhaust pipe at ex manifold, If it
still huffs and puffs its a valve. If there is no
puff at manifold you have a baffle in the muffler
problem and thats no biggy. I say its the muffler.

Maybe a cracked manifold or a bad manifold where the carb bolts to it causing it to have a vacuum leak

Old I am glad you said maybe cause the more I thought about it if he shoots propane into the equation that means its a mixture air and gas not correct, but when the air is replaced with a flammable it would not show up as a dead spot of a mixture issue. So air coming in could cause an interruption. That said I have had cars,trucks and tractors do the same puff puff and ran fine the only time you are ever aware of it is when your at the end of the exhaust pipe, just no more than the way it is, no real problem. Somebody will nail it-my work here is done. <9>>>>><{

My recent experiance with puffing was diagnosed very similar Except I sprayed Kroil on the intake
If this is the same thing I fixed mine by milling my manifold surface flat. Of you cant mill it a wide file wrapped with the appropriate grit or bench type belt sander will work. I cant hear so good so if it is a chirp, let me know what fixes that as I have the same issue.

Have you eliminated the off-chance of a weak plug (even though new) or plug wire?
The induced propane might make such a culprit less prone to missing. (?)

Thanks Royse, you got everything we have done so far right. The propane does seem to help a little when I put it into the carb but it does not fix the problem. I've tried going around the manifold with the propane with no affect. I'm going to try disconnecting the exhaust first, hopefully its just a bad baffle in the muffler. If that doesn't work I guess I will pull the manifold and check its surface. It is a fairly new aftermarket manifold, maybe has 50hrs on it, I'm pretty sure it did the puffing noise before I replaced the manifold. I replaced the original manifold because the flange where the pipe connects to rusted off. The exhaust pipe and muffler on the other hand has been on the tractor since my dad changed it at least 20yrs ago.
I don't think there is a problem with the plugs or wires, I'm on the second set of plugs and a brand new set of wires. I also checked the spark at all four plugs and they are all very good.

You can't do much work at idle speed and that
little puffing will go away soon as you start to
work it a little.
Probably could stop it by retarding the spark a
bit but that might rob it of some power. Probably
not noticeable loss if light work.

Zane

I'm no expert but sounds like a baffle in muffler rattling to me.

(quoted from post at 06:49:25 03/22/15) You can't do much work at idle speed and that
little puffing will go away soon as you start to
work it a little.
Probably could stop it by retarding the spark a
bit but that might rob it of some power. Probably
not noticeable loss if light work.

Zane

Click to expand...

It still has the puffing noise even at full throttle but much less noticeable. I have the timing set with TOH timing jig. I did try moving the timing plate with it running in both directions but the puffing never went away. Then I took the dist. off again and retimed it with the jig. It has tons of power and starts up on the first crank its more of an annoyance than anything else. I'm going to check the muffler out tomorrow hopefully that's it!

could it be that the valve springs are weak or one is broken?

Update: Today I removed the exhaust, still has the puffing sound but much more noticeable without the muffler. I used a mechanics stethoscope around the manifold, head, and plugs trying to tell which cylinder the sound was coming from but I couldn't tell. I used a propane torch all around the manifold with no change I also tried using it again directly in the carb and it did not straighten it out either. While I had the exhaust off I figured I would pull the manifold and have it checked. It was not warped but I did a light surfacing on it anyway. I bought new gaskets, can't remember which side goes toward the block, the flat side or concave side?
Could a valve out of adjustment be doing this? The motor only has 5-10 hours on it since full rebuild which included new valve guides, springs and machining the valves and seats and lapping them in. The only thing not replaced in the block was the cam and the tappets which were is good shape. The motor made this same noise before the rebuild too, I always thought the carb needed a little tweaking that I could never get right.
I don't know if I should go into checking the valves or not. They have to be seating if I'm getting 120psi compression right?
Could something else be wrong in the dist? Last time I changed the points I installed new bushing and new weights, this time I put points, condenser, cap and rotor. I did try adjusting the timing while it was running and could not get it to straighten out. Any other ideas? Thanks