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Discussion Starter #1
Hello everyone i don't like to ask for help very much.....but i really need some help with my old Mtd gt, aka " Betsey" I just cant seem to get her running right.
She's got a 18.5 b/s model 42a707 type 1238 01, 1997 code, twin opposed i/c engine, The problem is when i mow with it, Does fine for the first 5-10 min,
then i bring it down to about 1/2 throttle or less and it starts to hunt up and down up and down, i can make it stop by pulling the choke out a bit and it will run fine,But when i try to adjust the low speed mixture nothing,Next morning
i go out start her up all is well Intel i start working here a Lil.
IZE]
Things i have tried to resolve the problem...

Rebuilt carb "new gaskets,cleaned,new pump,etc"
New inline fuel-filter
New fuel lines
Cleaned and flushed gas tank
New fuel cap "vent work's fine no vapor lock there"
I put a used ignition on it for a separate problem.
I have checked the crank-case for air leaks around manifold flanges, manifold,
ect..
Cleaned and checked for free movement of the governor and linkages.

Is it possible that a sticking valve can cause this problem,resulting in not enough vacuum to draw in the correct air/fuel ratio? the reason i ask is because i have noticed a lil tick tick tick in the motor, experience tells me thats a clearance issue with the valve and tappet.
But wouldn't the good cylinder make up for that?

Any help would be appreciated, Thanks in advance.:confused::thanku:
 

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It sounds like there is still some contamination somewhere in the carb. Did you remove the welch plug when you cleaned it? Most times when the idle mixture screw doesn't respond, there is something behind the welch plug. I would replace the impulse line to the fuel pump also. A small crack will reduce the vacuum needed. When these carbs are truely clean and good vacuum to the pump they run smooth and accellerate quickly. They are my favorite engine to work on.
 

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I think it's a mixture problem, and lean, since the choke helps. A little ticking from the valves won't cause this problem, but perhaps a tight valve could. That would be rare, though. Does it have a main mixture adjustment on the carb? Is the float level correct, and do you have unrestricted fuel flow into the carb? Also, is the float free to move, or is it binding up on something in the bowl, causing a low fuel level and a lean mixture?
 

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Discussion Starter #4
update......
As of today i was out pulling some stones around almost at full throttle and suddenly had a small amount of power loss, so i moved the throttle down to idle and the engine just
died, This was after the engine was fully warmed up, So i finally got it started again with about half choke "wouldn't even attempt to start without it".
This time i didn't even mess with the low speed mixture screw
and now it hunts at any speed slow all the way up to fast, It gets better though the higher the rpm's? At full throttle its hardly even noticeable.(except for the power loss part of it) It seems to be more quiet when it has these spells as well.
As for everyones questions.....
It does not have a high speed mixture adjustment (wish it did though).
The float seemed fine the last time i took it apart no binding or dragging.
fuel flow to carb is fine.
It doesn't have a adjustable float (i check them anyways to get it in the ball park)
And no i didn't remove the welch plug (to be honest i really don't know how and where to find a replacement)
I hate to say it but i really think this maybe the ignition!? Because of the consistency of the problem,I can almost time when its going to happen, 4-5 minutes at full throttle,with blades engaged.
If it has a weak ignition would this require the engine to need richer mixture?
 

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If you have a weak spark, sometimes a richer mixture will make it run better a bit. Sometimes making the plug gaps smaller will make it run better, and can be an indicator that you do have a weak spark.
You could try making the gaps smaller and see what happens.
If it's raining there like it is here, you may want to start looking for a boat motor....
 

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Discussion Starter #6
Thanks fcubman,:thanku:
I'm going to adjust the gaps and get a spark tester to check the output when it happens again.
I really hope i can get it fixed in time for fall/winter,so i can try out my snow blade.
Whatever the problem is im sure it will be worse in cooler temps.
Don't really know about the boat motor though :), been dry as a bone over here.
highs in the 90's lows in the 60's.And my poor lawn has really took a toll:praying:
Ill post updates on the tractor next time i have her out:fing32:
 

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You're welcome!

You might try using half the recommended gap for a starting point.

Actually, if your spark is weak due to a bad coil(s), the problem may lessen in cooler weather. Heat seems to exacerbate problems with marginal coils. That would explain them running good until they and/or the engine get hot.

It's a well known problem with old Farmall Cubs with ancient coils on them. They quit when the coil gets hot, run fine after the coil cools down. That fact saved me from mowing over a yellow jacket nest - my Cub quit a few feet short of it. In the silence, I heard the LARGE swarm of bees near the ground that I wouldn't have noticed until too late. It's a known fact that a Cub will not outrun a swarm of angry bees. Nor will a Simplicity, I found from personal experience. But that's a story for a different thread...
 

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I am not sure if I fully understand your problem, but here is my two cents. First these engines are designed to ALWAYS run at full throttle. That means 3600 RPMs. If you need to use the choke to keep it running, then it is probably getting more air than fuel. Adjust the mixture or remove the carb and give it a good cleaning.

3600 RPMs insures that the engine runs at the top of its power curve and is critical to cooling the engine properly. Running it slower will put stress on all parts of the engine and insure that it fails sooner than it should. these engines need full RPMs to get proper lubrication as there is no real oil pump in them and they are lubricated by splash.

Nick
 

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Discussion Starter #9 (Edited)
Update.....
Well its been almost a week since i last posted and still no rain so, i haven't had a chance to mow the lawn and do some troubleshooting.
But i did have some time to make a service road in the woods for my mtd witch failed horribly I slid all the way down a 30ft valley with the rear brake's locked up
trying desperately not to flip or hit anything.witch i didn't thank God!:fing32:
and slid to a stop at the bottom,all i was trying to do was test how good the traction was by going down a lil bit at a time then backing up to test for traction.So i made it back up halve way and then the tractor decides to do its lil vroom vroom vroom thing...Anyway to make a long story short i was stuck halfway up the valley spinning out,When i came to the conclusion that i was gonna have to get the come-along out. So lil by lil i finely got to a flat spot and got it out. I was so relived to have it back out i didn't even feel like doing any troubleshooting :fing20:.
But one thing i did notice.... When it had cooled and i got it to the flat spot and restarted it was doing the same thing again and had to give it some choke (more then normal for a cool engine) and then a min later it started to run rich so i pushed the choke back in
and it ran normally.
So whatever the problem is, its definitively heat related and takes some time to reset itself.
-rayboy
 

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I agree with ACSam, I have a 19.5 with the 3 screw pump, it had blockage and we had to reclean the carb a few times for it to smooth out. have you double check the pump springs?
 

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I think I would be running to the outhouse after that one. That was one scary experience. If you are having a heat problem, I would remove the heat shields and see if some critter has built a nest under them. It is a common problem which will prevent air flow and cause heat to build up. A mouse will get up under the smallest places and build a nest. This will definitely cause the engine to over heat very quickly.

Does it start quickly or does the starter sound like it is dragging. Might be the timing. I found one engine had the key pin bent and cause it to be out of time, which caused that to happen. Put a new one in and problem solved. As I said before, you must run these engines at full throttle or they will overheat. Air flow is critical to proper cooling, as is timing.

Question, does the starter spin freely when cold and after it dies on you, does it drag until the engine gets cool again?
 

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Discussion Starter #12
Well i'm finally going to try and get to the bottom of this within the next couple of days.
My uncle who's a retired small engine mechanic will be over tomorrow.
I took the carb apart again today,and cleaned it the best i possibly could.
I'm satisfied the carburetor is fine now.But still the same problem.
The engine spins over fine hot or cold,and always starts fine.
Tomorrow i will have a definite solution to the problem.
Hes bringing some tools with him based on what i told him,cylinder Balance tester, ignition tester,valve spring compressor,and some measurement tools.
I appreciate everyones help and suggestions.
And i will post what we find out tomorrow!
 

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Discussion Starter #13 (Edited)
Well today i have got an answer to the problem....
My uncle came over today and hooked up a balance tester to the engine both where making good enough compression to fire and run.But the cylinder on the left hand side (when siting on the tractor) is leaking air Thur the exhaust valve badly,and both had a lil air leaking past the rings and valves.
But my uncle said the air leaking past the rings is normal for an engine this old.
So he had me start it up and work it in full throttle with the mower deck on practically scraping the ground (grass is dead) So i ran it in till i heard the power loss he had me bring it back in, And he noticed something i never did the lil tick tick in the motor goes away when it warms up.
He said he don't think its the igntion, but he tested it anyway to be sure and it was fine.
Now from here on i didn't full comprehend what he was saying,But the best i can describe it is that when the engine is cold the valve parts contract and when the engine is warm they expand,(i knew that at least ways for rings) but he said that when the valve guides get worn on these engines the valves tend to get cok-eyed and stick intermittently and become tight in the guide, just enough to let in a lil extra air and cause that cylinder to run lean.
And he also said that when the intake valve on the bad cylinder starts to go bad it can shoot a puff of air up the manifold on its compression stroke and cause the other cylinder to have problems.
He said he likes these engines he had one with over 1800 hours on it with the same problem and i have another one with about 1500 hrs (rough estimate) but the valves have to be perfect on them to run just right.
So now i have to figure out what to do he doesn't do machining work anymore
so i will have to find someone else to do it, He recommended that both cylinders be checked because both where less that perfect.
I asked him if it would hurt it if i just used it like it is and he said yes...but it should last for a good lil while before something finnaly breaks.
So now i have some finale questions.....
The guys a lil old and kind of nutty :rolleyes:...lol does every thing that i told you he said make since?
Should i try to get a valve job done on it.Or use as is?
:trink39::trink40: -Rayboy
 

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I wouldn't throw dirt on it just yet--did you say how many hrs.you have on it?did you ck the compression on both sides,these are the things I look for when in question about a rebuild or replacement.You have one of the best eng.B&S ever built--imho--so ck it out good before you do anything rash.:trink40:
 
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