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1969 MTD 860
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Discussion Starter · #1 ·
So I'm about at my wits end on this one. Two weeks ago I had the 12hp cast iron Briggs in my MTD 860 running like new. I had it running and went to engage the PTO and it died, so I just figured I engaged it too fast. I haven't been able to get it to start since! I noticed the compression seemed a little low so I did replace the head gasket and properly torqued the head bolts. I have spark, I have compression, fuel is another matter. I can remove the carb and put my hand over the intake and I have plenty of vacuum, air also moves freely through the carb. But when I bolt the carb up and put my hand over it and crank the engine I cannot feel any vacuum, this is what makes no sense to me. I also checked the float and it is moving freely and has no holes in it

My guess at this point is some kind of carb issue but I'm completely stumped. I did rebuild this carb two months ago or so when I bought the tractor, and it was running great up until this incident. Any insight would be greatly appreciated.
 

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I had it running and went to engage the PTO and it died
Is the spark plug getting wet after you crank the engine over for a while? If so then fuel would not be the issue. When you engaged the PTO did the engine shut down quickly?

If you have spark & fuel but it won't run. The next thing to check is the timing. Pull the fly wheel off and check to see if the key sheared. The fly wheel key on a BS is very soft metal. I have seen them shear multiple times. Always use a BS key or aftermarket equivalent, do not try to use a steel key. Torque fly wheel nut to factory specs.
 

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1969 MTD 860
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Discussion Starter · #3 ·
I'm in the process of checking the flywheel key now, yes it shut down quickly when this happened. The plug is not getting wet which is leading me towards thinking it's a carb or timing issue. I don't have a wrench that will fit the flywheel nut and a socket won't do it because it has a shaft about 3" long connected to the nut that held the front PTO pulleys on.

I'm going to measure it today and get a wrench big enough to fit.
 

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1969 MTD 860
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Discussion Starter · #4 ·
I just got the flywheel off. The key was in place and intact. The only thing I can think of now is some sort of carburetor issue. I don’t understand why there’s enough vacuum to hold my hand against the intake but once the carb is bolted on I feel nothing.
 

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1969 MTD 860
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Discussion Starter · #6 · (Edited)
Throttle open and choke off. Plenty of vacuum at the intake elbow, nothing with the carb on. Which is really weird because there is nothing that can block air through the carb. All gaskets are new.

It doesn't make any sense it's like there's a massive leak somewhere but at the same time there isn't. At least I sure can't find it.
 

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1969 MTD 860
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Discussion Starter · #9 ·
I double checked the points I have spark. It won’t fire even with starting fluid sprayed directly into the combustion chamber. It’s almost like it’s way out of time but I don’t know how that would even be possible.
 

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I double checked the points I have spark. It won’t fire even with starting fluid sprayed directly into the combustion chamber. It’s almost like it’s way out of time but I don’t know how that would even be possible.
That's what my engine is doing too! I had gotten it started once, (the first time I ever tried to start it, only took a few pulls as I recall) but then it stalled, and I can't get it started again.
 

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1969 MTD 860
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Discussion Starter · #14 ·
I’ve been waiting on a nice day where I have free time to work on this thing. I have a few more things to try before I give up.

i have a feeling finding a good running cast iron 12 horse or preferably a 16 is going to be difficult and expensive. Would a Briggs horizontal twin bolt up and have the correct shaft size so that I wouldn’t have to do any modifications?
 

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1969 MTD 860
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Discussion Starter · #16 ·
I finally got back to this thing. I pulled the carb off and set the points with a feeler gauge. I have good spark. I have vacuum at the intake elbow. I sprayed some starting fluid up there while cranking and it never fired.

I don’t get it. I don’t see how this thing could possibly have jumped timing when all it did was stall. I double checked everything when I had the head off and everything looked good.
 

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Have you made sure both the intake and exhaust valves are moving up and down and also made sure the exhaust (muffler) is not restricted / blocked? Seems strange that it would not have fired (back fired) on the starting fluid if you have a good spark and compression.

Just a couple of suggestions to check.
 
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