'51 8n side mount question

[boler76]

this tractor arrived in bad shape, the engine oil pick up tube came loose and sucked air damaging engine badly, I installed new cam, had crank redone , installed new sleeves pistons , new valve train etc, engine now runs very nice and smooth when cold but as it starts to warm up there is popping sound in exhaust, which gets worse as motor gets up to temp, have changed coil, condenser, plugs but no difference, there is 120 - 122 psi compression and 18 in vacuum , when choke is pulled slightly engine smooths out some and speeds up, the small carb screw is adjusted in as far as it will go, backing it out makes it run worse, I need suggestions thanks

 

[old]

this tractor arrived in bad shape, the engine oil pick up tube came loose and sucked air damaging engine badly, I installed new cam, had crank redone , installed new sleeves pistons , new valve train etc, engine now runs very nice and smooth when cold but as it starts to warm up there is popping sound in exhaust, which gets worse as motor gets up to temp, have changed coil, condenser, plugs but no difference, there is 120 - 122 psi compression and 18 in vacuum , when choke is pulled slightly engine smooths out some and speeds up, the small carb screw is adjusted in as far as it will go, backing it out makes it run worse, I need suggestions thanks

Sounds like you need to adjust the valves sound like you have exhaust valves not closing all the way

 

[Jim L WA]

My first thought is a crack in the manifold opening up as it gets hot. That would cause it to pull in air which is why it runs better when you choke it a little. You could spray some starting fluid around the manifold and listen for a change in the engine.

 

[HarryVanderpool]

Agree with Jim. Also check throttle shaft. This is a common issue.
Also a good carb cleaning is in order.
Why only when hot?
Because cold air is more dense than hot.

 

[soaked]

Hi boler76,
Is the popping only at idle?
When its warmed up, does it hesitate, surge, or bog down like its running lean, at higher than idle rpms, or does it run fine?
A restricted carb idle circuit would only cause low rpm issues

 

[boler76]

Hi boler76,
Is the popping only at idle?
When its warmed up, does it hesitate, surge, or bog down like its running lean, at higher than idle rpms, or does it run fine?
A restricted carb idle circuit would only cause low rpm issues

runs fine at higher than medium idle

 

[soaked]

Could be dirty idle curcuit in the carb
Most of my 8ns--- clone or MS carb, run with idle mix screw about 1/2 turn out.

Idle curcuit headwaters are where the .048 wire pokes out just above the main jet. (Sometimes the stepped passage is partially blocked with sediments)

These pics show poking a wire through the primary idle orifce and down to the spot the idle mix screw lives.

Not sure if the lower secondary orifice can take a wire through also. (Been awhile.)
Get a cheep clone to try, if you dont have a spare already. Spares are good!