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12hp Briggs & Stratton won’t start

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Kyle M.

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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.

I’ve also checked the flywheel key and it is in place and intact

My guess at this point is some kind of carb issue but I'm completely stumped. I rebuilt 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.
 
#2 ·
Other than the choke and throttle plates there is nothing in these carbs that would block air flow. Is the bowl filling with fuel? A quick puff of ether will tell you if it is not getting fuel.
Is the spark strong? Did you check spark at the plug gap or the wire? It's possible the spark plug fouled.
 
#3 ·
Sure seems really odd that you get plenty of vacuum at the bare intake port but not at the carb intake when you bolt it on. Can't think of anything on the carb that could leak that much to reduce the carb intakes vacuum to nil, other than something is caught between the carb and block mating surfaces. Or perhaps the carb mounting bolts are not the correct ones and bottoming out before tightening.

As you crank with the carb on, can you hear any "hissing" that would indicate a massive air leak in the carb or its mounting? Perhaps just remove the plug wire, and completely block the carb's intake, and turn the engine fairly slowly by hand and listen for any hissing noises.

With little or no air passing through the carb's venturi, it isn't going to run even with good fuel flow, spark and compression. Did you try squirting a little gas into the plug hole then try to start it? It should pop and run a few revolutions on that gas in the cylinder. Maybe observe the carb as it does so to see if you can spot anything.
 
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Discussion starter · #4 ·
nojgib said:
Other than the choke and throttle plates there is nothing in these carbs that would block air flow. Is the bowl filling with fuel? A quick puff of ether will tell you if it is not getting fuel.
Is the spark strong? Did you check spark at the plug gap or the wire? It's possible the spark plug fouled.
Click to expand...
The bowl is filling with fuel, spark appears good. I still had the intake elbow from the carb to the head bolted on with a new gasket when I checked for vacuum. The gasket between the carb and intake elbow is brand new as well. Throttle open, choke off I can fell no vacuum through the carb. Which as you already mentioned is very strange. The gasket between the two halves of the carb is new as well.
 
Discussion starter · #5 ·
Alien5044 said:
Sure seems really odd that you get plenty of vacuum at the bare intake port but not at the carb intake when you bolt it on. Can't think of anything on the carb that could leak that much to reduce the carb intakes vacuum to nil, other than something is caught between the carb and block mating surfaces. Or perhaps the carb mounting bolts are not the correct ones and bottoming out before tightening.

As you crank with the carb on, can you hear any "hissing" that would indicate a massive air leak in the carb or its mounting? Perhaps just remove the plug wire, and completely block the carb's intake, and turn the engine fairly slowly by hand and listen for any hissing noises.
With little or no air passing through the carb's venturi, it isn't going to run even with good fuel flow, spark and compression. Did you try squirting a little gas into the plug hole then try to start it? It should pop and run a few revolutions on that gas in the cylinder. Maybe observe the carb as it does so to see if you can spot anything.
Click to expand...
I have plenty of vacuum with the intake elbow bolted to the head, also a brand new gasket. It’s when I put the carb itself on that I seem to lose all vacuum.

No hissing that I can determine. Everything appears to be sealed up just fine.

I haven’t tried any gas or ether. The spark appears strong but I guess it’s possible that it’s not. The points were set by eye by the previous owner and I haven’t touched them. What are the chances of the spark being any stronger if I set them properly with a feeler gauge?
 
#6 ·
Hmmm.

Is this the engine intake and carb you have or is it newer?
Image
 
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#9 ·

Antique Engine Manual and Specifications | Briggs & Stratton

Looking for antique engine specifications, manuals or parts from Briggs & Stratton? Learn where to find them here!
www.briggsandstratton.com www.briggsandstratton.com

Kyle M. said:
The points were set by eye by the previous owner and I haven’t touched them. What are the chances of the spark being any stronger if I set them properly with a feeler gauge?
Click to expand...
Chances are very good of increasing spark with correctly cleaned and adjusted points. Points will also adjust the timing of the spark. Do yourself a favor and start there.
Can you post photos of your carb? Or tell us which of the 3 you have from post #4 here?
www.mytractorforum.com

Briggs & Stratton cast iron carbs.

Can any one tell me if there is a difference in carbs on cast iron B& S engines I am talking about 10 hp 12 hp 13 hp 14 hp or the 16 hp they all look alike to me I have carbs from 10 12 13 & 16 I have been told they are all the same what is the real story
www.mytractorforum.com www.mytractorforum.com

I have the 32ci and the large carb and my end cap fell out once. If you have the large carb yours could be loose and that would be the only thing I can think of that would allow that much of an air leak in the carb.
 
#10 ·
Nojgib,
Good comment there!

Kyle,
I would guess that you already know this but Nojgib makes a point. It would also support the "no vacuum" observation you are seeing.

The horizontal opening of the carb's intake (red arrow in pic) must have the cap in place. It cannot handle air coming in from both the (correct) vertical intake, and the horizontal opening if it us unplugged, or it will not not function.

I have heard stories of backfires blowing this out but cannot confirm if that is possible. Check yours.

Image
 
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Discussion starter · #11 ·
The plug is still in place, I'll check tonight and make sure it's tight. I have the large flow jet carb with 4 bolts. I set the points last night using a feeler gauge and not much seemed to change. If it weren't for the fact that all of them I've found were $250-$350 I'd just buy another B&S carb and try that. I've had mine apart multiple times before and after this incident and I can't find anything wrong. I'd rather not buy one of the Chinese carbs that's out there for around $70.
 
Discussion starter · #12 ·
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?
 
#13 ·
Kyle M. said:
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?
Click to expand...
There has been many different successful motor swaps into these. I don't know that any are a direct swap since the output shaft must be in the same location as original. You may need to make motor mounts, some electrical modifications and a different fuel tank will most likely be needed.
 
Discussion starter · #14 ·
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.
 
#15 ·
You might try another spark plug, sometimes they can produce a nice spark in the open air, but can't under compression.

You might also do a compression test on the engine. And when you had the head off, did you check the valves for any carb buildup near where the valve mates to the ports?
 
Discussion starter · #16 ·
dave_r said:
You might try another spark plug, sometimes they can produce a nice spark in the open air, but can't under compression.

You might also do a compression test on the engine. And when you had the head off, did you check the valves for any carb buildup near where the valve mates to the ports?
Click to expand...
I’ll give it a try tomorrow with a different plug. What’s the correct gap for these? Valves were nice and clean when I had the head off.I’ve read it’s hard to do a compression test on these due to the compression release but I tried anyway and was getting 65-70.
 
#17 ·
Photo is from my '79 16HP manual. I believe yours should take the same, but I gave a link to Briggs' site for antique manuals in post #9 if you want to double check for your specific motor.
Even with a weak spark I think ether should have at least given you some sort of indication of trying to start.
How fast is the motor turning when you try to start it? If your battery or starter is weak or there is a bad connection somewhere the motor could be turning too slowly to start it.
You didn't say if you were checking for spark at the wire or plug gap. If the plug is fouled you would still see spark at the wire, but it will not be firing at the electrode.

Image
 
Discussion starter · #19 ·
I finally got back around to messing with this thing last night. I pulled the head off, everything looks good, checked the points, timing seems right on. I have good compression, strong spark with the spark tester. It won't even attempt to fire. The only thing at all I can think of is that I fouled out the plug and the replacement plug I bought was bad off the shelf.

The other thing that stumps me is if I pull the carb off and crank the engine you can feel a ton of vacuum putting your hand over the intake elbow above the carb. Put the carb on and put your hand over the carbs intake and nothing. There are no blockages or holes in this carburetor. It was freshly rebuilt in April and the engine was running like new at the time.

I guess I'm going to try a new plug, I have absolutely zero idea beyond that. Anybody know the actual correct spark plug for this engine? I've been using a Champion 868.
 
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Kyle M.
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