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16.5 Hp Lawn Tractor runs only on (nearly) full choke

I have an MTD lawn tractor (pretty old) with a B & S Model: 42A707
Type: 123801 engine (Opposed L Head Twin Cylinder).
It was running poorly, so I pulled the carburetor and did a complete rebuild;
I replaced all the part that were included in the rebuild kit except for the Welsh Plug,
which seemed tricky and I didn't see what replacing it would do.
I cleaned out all of the passages with aerosol carb cleaner and everything seemed in great shape.
I reinstalled the carb and fired up the engine.
It started right up and ran great, unless I tried to open the choke.
It seemed to me that it must be either starved for fuel on the carburetor intake,
or was leaking air into the cylinders downstream of the carburetor.
The carburetor rebuild went so well you would have a hard time convincing me (though it wouldn't be impossible)
that there was a clogged fuel passage somewhere in it.
On the other hand, I sprayed starter fluid around the carb, intake manifold connection points, etc while it was running
to see if it caused the engine to sputter, but it had no effect.
I considered that the head gaskets might be leaking air, so I tightened those bolts.
They were "not particularly tight" (pardon my technical language) when I got to them. Some were looser than others.
I tightened them all. I did not use a torque wrench. I didn't know what the proper torque would be.
I let the engine run for about ten minutes and it sounded great the whole time (with the choke on almost full).
It was not, however, under load operating the blades or the wheel drive.

I am pretty much at a loss now for what to do.
I suppose I can re-assemble the tractor and just (try to) use it while leaving the choke mostly on full.
That somehow seems wrong to me.

Anyone have some advice?

I have read that closing the choke and operating the machine causes lean fuel/air and overheating. Probably gotta do the carb, again. Or, seek professional help with the Welsh plug.

pythons37 said:

I have read that closing the choke and operating the machine causes lean fuel/air and overheating. Probably gotta do the carb, again. Or, seek professional help with the Welsh plug.

Click to expand...

Thanks, Pythons37.
I have ordered a pair of head gaskets and will try that first.
I should probably also talk to a local small engine repair shop about the Welsh plug.
Chartman

Probably didn't get a orifice cleaned out good enough. If it runs with the choke in, than that means it is getting to much air vs fuel with it open. Pulling the choke restricts the air, and makes the air/fuel mixture correct. Or at least close to correct. Go through the carb again.

Go here http://outdoorpowerinfo.com/repairs/#id_briggs
Download the two threads on the opposed twin carb then reflect upon what you did with yours
They are a particularly tricky carb to work on & having the fuel pump combined does not make them any easier.
The welch plug you did not remove accesses the idle mixing chamber which one will assume is working on your mower.

Chartman said:

Thanks, Pythons37.
I have ordered a pair of head gaskets and will try that first.
I should probably also talk to a local small engine repair shop about the Welsh plug.
Chartman

Click to expand...

https://www.amazon.com/Briggs-Strat...coding=UTF8&psc=1&refRID=WZVHJE23JPQMXJ87F8W1

This might help.

helomech said:

Probably didn't get a orifice cleaned out good enough. If it runs with the choke in, than that means it is getting to much air vs fuel with it open. Pulling the choke restricts the air, and makes the air/fuel mixture correct. Or at least close to correct. Go through the carb again.

Click to expand...

Thank you, helomech.
I really thought that everything in the carb was cleaned out well when I worked on it at the bench.
The prevailing opinion, however, seems to be that there is still a carb problem.
The tractor is still "open", so it will be a small deal to pull the carburetor again.
I appreciate your input!

bertsmobile1 said:

Go here http://outdoorpowerinfo.com/repairs/#id_briggs
Download the two threads on the opposed twin carb then reflect upon what you did with yours
They are a particularly tricky carb to work on & having the fuel pump combined does not make them any easier.
The welch plug you did not remove accesses the idle mixing chamber which one will assume is working on your mower.

Click to expand...

Hi bertsmobile1,

That is a FABULOUS link you referenced.
I can't thank you enough!

I am looking forward to pulling the carburetor again and going through it with a renewed understanding of how the ports should be connected,
and where there may be blockages that need to be addressed.
I may not get to it for a few days, but I will certainly report back on what I find after I do.

...and thanks to everyone who responded.
Chartman

Chartman said:

Thank you, helomech.
I really thought that everything in the carb was cleaned out well when I worked on it at the bench.
The prevailing opinion, however, seems to be that there is still a carb problem.
The tractor is still "open", so it will be a small deal to pull the carburetor again.
I appreciate your input!

Click to expand...

I had a boat I fought all the time. Thought I cleaned everything and it still ran like crap. Brought it to a mechanic and he cleaned the carb and it was all good.

Chartman said:

Hi bertsmobile1,

That is a FABULOUS link you referenced.
I can't thank you enough!

I am looking forward to pulling the carburetor again and going through it with a renewed understanding of how the ports should be connected,
and where there may be blockages that need to be addressed.
I may not get to it for a few days, but I will certainly report back on what I find after I do.

...and thanks to everyone who responded.
Chartman

Click to expand...

DOn't thank me I got it from some one here when they were talking about cross refferences for belts but you will have to go to the home page & navigate to it.
Yes a very good site, a lot of work has gone into it

helomech said:

I had a boat I fought all the time. Thought I cleaned everything and it still ran like crap. Brought it to a mechanic and he cleaned the carb and it was all good.

Click to expand...

Yep, I used to soak & spray with cleaning solutions , then I bought an ultrasonic cleaner.
'Chalk & cheese

Final Entry (If anyone cares)
I gave up.

I pulled the carb again and cleaned it thoroughly; and I mean THOROUGHLY!
I cleaned all passages with carb cleaner and made sure they were clear with a thin wire.
I traced every port from an entrance to exit (except what was behind the Welch Plug, which seemed to be a wide open area)
I put everything back on the tractor and primed the carb with "Quick Start" spray.
It fired up right away and ran with the choke off.
It sounded great.
I then shut it off and reassembled the tractor to give it a try.
It still started up fine, but in a couple of minutes it started to lose rpms and lacked power.
Putting the choke on helped, but it still wasn't running well.
I tried mowing with it anyway, but it didn't seem to have much power.
I tried playing with the choke but could not get it to run well.
At one point I thought that maybe the governor was at fault, since it had trouble under load.
Maybe the governor was not opening up the throttle when the engine was being strained.
So I tried to open the throttle manually while the engine was on and the lower unit was engaged.
Before I could find it, the belt for the lower unit suddenly broke.
I took that as a bad omen and the last straw.
I did some shopping around for a new tractor and was pretty much ready to take the plunge.
Then, two days ago, I saw a fairly new used one for sale on a neighbor's lawn and ended up buying it.

In the interest of full disclosure, I should point out that last fall, while I was working on the fallen leaves,
the tractor caught fire. I had to put it out with a fire extinguisher.
This Spring I replaced a number of melted plastic parts and a lot of wiring.
The truth be told, the tractor has been running about the same as it was before the fire and I don't think it was a factor.

Thanks to everyone who responded.
This is a great forum.
I had this item posted on another forum for weeks.
There were about 100 views and NO responses.
On this forum I had several responses the same day I posted.
Thanks, again.
Chartman