Engine misses at low RPM

[NCMau]

MF 202, Continental Z134 engine. I can not keep engine running smoothly under 1000 RPM.

Here is what I have done so far:
Valve job (significant change in compression readings and runs lot better)
* New head gasket (discovered 3 hairline cracks between cylinder walls, top of the block)
* Applied block seal
No visible oil in water, or water in oil.
No visible smoke or water odor at exhaust.
No bubbling in radiator.
Installed new plugs, wires, points, rotor and coil.
Healthy 1/4" spark with test plug
Installed carburetor kit and adjusted (not lean).
Oil pressure is 20 psi
Compression dry/wet: 115/125 psi on all cylinders (using 20W50 oil).
The distributor has a small shaft play of 7-8 mils by pushing the shaft with a screw driver. Also there are small rotor shavings (brass dust) inside. Could this be a problem?
Although the manifold is pitted, there are no visible cracks or holes. Also a new gasket was installed.
Changing timing does not fix the problem.

Plugs are a fouled but the points of the electrodes are clean, indicating that they do fire, at least sometimes. Maybe oil rings? when I removed the head, the cylinder walls looked good with hardly any ridges, It seems that the sleeves didn't have a record hours.
As I stated before, I installed a carburetor kit and did a good job in cleaning it. At this point, however, the problem would indicates a lean gas flow at low RPM. I don't know. I hate to invest $250 on a new carb, and may not even be problem. I wish I had a spare to try it.
Are there any opinions on using a 14-18mm adapter with a modern hot plug to replace the 386s. Or for that matter using a non-foul adapter like they used in the days of the gas burners?. The tractor does not smoke.

I am running out of ideas. I don't want to do engine internal surgery. If I have to, I'll drive it like it is. I am hoping to get a stimulus of ideas from the gurus. Any ideas are welcome.

 

[maybar]

MF 202, Continental Z134 engine. I can not keep engine running smoothly under 1000 RPM.

Here is what I have done so far:
Valve job (significant change in compression readings and runs lot better)
* New head gasket (discovered 3 hairline cracks between cylinder walls, top of the block)
* Applied block seal
No visible oil in water, or water in oil.
No visible smoke or water odor at exhaust.
No bubbling in radiator.
Installed new plugs, wires, points, rotor and coil.
Healthy 1/4" spark with test plug
Installed carburetor kit and adjusted (not lean).
Oil pressure is 20 psi
Compression dry/wet: 115/125 psi on all cylinders (using 20W50 oil).
The distributor has a small shaft play of 7-8 mils by pushing the shaft with a screw driver. Also there are small rotor shavings (brass dust) inside. Could this be a problem?
Although the manifold is pitted, there are no visible cracks or holes. Also a new gasket was installed.
Changing timing does not fix the problem.

Plugs are a fouled but the points of the electrodes are clean, indicating that they do fire, at least sometimes. Maybe oil rings? when I removed the head, the cylinder walls looked good with hardly any ridges, It seems that the sleeves didn't have a record hours.
As I stated before, I installed a carburetor kit and did a good job in cleaning it. At this point, however, the problem would indicates a lean gas flow at low RPM. I don't know. I hate to invest $250 on a new carb, and may not even be problem. I wish I had a spare to try it.
Are there any opinions on using a 14-18mm adapter with a modern hot plug to replace the 386s. Or for that matter using a non-foul adapter like they used in the days of the gas burners?. The tractor does not smoke.

I am running out of ideas. I don't want to do engine internal surgery. If I have to, I'll drive it like it is. I am hoping to get a stimulus of ideas from the gurus. Any ideas are welcome.

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For starters, at low idle rpm, pull the plug wires one at a time to see if you can narrow the problem down to an individual cylinder.

 

[NCMau]

Done that. It is a random occurrence.

 

[banjodunn]

G'day if you have got 7 - 8 mm of sideways movement on the distibutor shaft then i think that is a real good place to start it would be jumping around all over the place




Jon

 

[Willl]

Ditto on the distributor shaft slop, had a old mail truck w/ same.

 

[Eagleview]

The distributor has a small shaft play of 7-8 mils by pushing the shaft with a screw driver. Also there are small rotor shavings (brass dust) inside. Could this be a problem?

Did you mean 7-8 thou of sideways movement ?

If you did mean 7-8 mm , the points would be opening that much as well as the gap you had set and erratically at that (it could not run with that much gap).

Also any metallic dust or carbon dust will cause spark tracking inside the cap which will cause miss firing .

Also look at the Earth strap on the breaker plate as they break sometimes .

Miss firing can also be caused by sticky valves and loose valve seats .

 

[NCMau]

7-8 is not mm, it is thou/inch. It is side movement when I push it with some degree of force with a large screwdriver. I don't really think this sloppiness is the problem, but I don't really know. That's what I am asking.
I need to pin point it in one area, if I can. Because of the new valve job, we can safely exclude the valves.
Carb or distributor?
(The ground strap is fine)

 

[Willl]

Can you move it side to side at all just by hand ?

 

[NCMau]

Can you move it side to side at all just by hand ?

Yes, very little. Just don't if it is significant. Pushing with a screw driver would be the max.

 

[Jerry/MT]

MF 202, Continental Z134 engine. I can not keep engine running smoothly under 1000 RPM.

Here is what I have done so far:
Valve job (significant change in compression readings and runs lot better)
* New head gasket (discovered 3 hairline cracks between cylinder walls, top of the block)
* Applied block seal
No visible oil in water, or water in oil.
No visible smoke or water odor at exhaust.
No bubbling in radiator.
Installed new plugs, wires, points, rotor and coil.
Healthy 1/4" spark with test plug
Installed carburetor kit and adjusted (not lean).
Oil pressure is 20 psi
Compression dry/wet: 115/125 psi on all cylinders (using 20W50 oil).
The distributor has a small shaft play of 7-8 mils by pushing the shaft with a screw driver. Also there are small rotor shavings (brass dust) inside. Could this be a problem?
Although the manifold is pitted, there are no visible cracks or holes. Also a new gasket was installed.
Changing timing does not fix the problem.

Plugs are a fouled but the points of the electrodes are clean, indicating that they do fire, at least sometimes. Maybe oil rings? when I removed the head, the cylinder walls looked good with hardly any ridges, It seems that the sleeves didn't have a record hours.
As I stated before, I installed a carburetor kit and did a good job in cleaning it. At this point, however, the problem would indicates a lean gas flow at low RPM. I don't know. I hate to invest $250 on a new carb, and may not even be problem. I wish I had a spare to try it.
Are there any opinions on using a 14-18mm adapter with a modern hot plug to replace the 386s. Or for that matter using a non-foul adapter like they used in the days of the gas burners?. The tractor does not smoke.

I am running out of ideas. I don't want to do engine internal surgery. If I have to, I'll drive it like it is. I am hoping to get a stimulus of ideas from the gurus. Any ideas are welcome.

What's the fouling on the plugs look like? Black and sooty or black and oily? Sooty and it's too rich a mixture. Oily and the oil rings have lost coil control.
Make sure you don't have an air leak around the carb's metering section. Idle and slightly off idle has the high manifold vacuum and therefore the highest pressure differential to drive an air leak. get some starting flui an CAREFULLY spray it around the throttle shaft(both sides) , the intake to carb connect flange and the intake manifold to cylinderhead connections. When you do this, the engine will speed up when you hit the leak.

The spec for radial runout on the distributor shaft is like 0.002". Rotate the engine til the points are closed and then push radially on the distributor shaft watching the points. Do it from all direction and you should not see the points move. I suspect this is your problem since the distributor cap terminals and the rotor are often made of brass and you're finding brass shavings. The fix is to turn the shaft and fitr a new bushing. An auto electric shop can do this for you.
Don't replace the carb. Make sure it is really clean by soaking it a minimum of 24 hrs in real carb cleaner( not the spray can stuff) and blowing all the passages to make sue tey are clear. Did you replace the throttle shaft seals and throttle shaft when you rebuilt it?