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I have a Craftsman II 1979 lawn tractor with a horizontal 18HP Briggs and Stratton twin cylinder engine.
It will start on starting fluid and then run just fine on gas.. Once warmed up most of the time will start immediatly on just gas.
Things I have done so far..
Cleaned and gapped and tested plugs.
Tested for spark on both cylinders.
Tested compression in both cyclinders both cold and warmed up. within 10lbs of each other both ways..
rebuilt carb, replaced float, needle, blew out all holes and tanked in safety clean over night.
Replaced vacume fuel pump with 4 psi electric pump and 2 psi regulator (tried with and without regulator, never dumped fuel into the oil and fuel filter is always full)
replaced all fuel lines, filter, and cleaned out gas tank..
It can not be a carb solenoid because this model doesn't have one..

Things I think that might cause it, but want to confirm before taking the entire frontend of the tractor/mower apart)
valve adjustment, coil adjustment (although always has strong spark)

Other then those two things I can not figure it being anything else.. I have mowed for over 2 hours with it once it is started, and then shut it down and it fires right up, but if I wait two or more hours I have to fire it on starting fluid then runs perfectly...

I am an amature small engine guy, I know car engines though and the principles seem the same. So this question may be elementary to you guys, I am looking for either confirmation of my suspected problems or the dr. house in this bunch to come up with something I can't find on google.. lol..

Tanks in advance, and I will post a followup explaining the resolution in detail.. I am retired and fixing riding mowers in now my hobby... I pick them up from barns and garages get them running and then either give them to veterines or sell them and donate the money to non profits.. Or as my wife hates, I just keep them... I have a 200ft by 20 ft yard with 5 working mowers right now, lol..

First, when asking for help please be sure to include all model and serial numbers for both the unit and engine. This will give us a better picture of what you are talking about. Two things come to mind after reading your post. Both have to do with a lean starting mixture. Have you checked to see if the choke is fully closing. If it is try this test, instead of starter fluid. With the palm of your hand choke the air horn and try to start your engine. I think it is going to start, which means to me either the choke is not closing or your float is set on the lean side. If you know the choke is fully closed, I would be removing the carb and checking the float level, setting it slightly rich. With the numbers I would have a better idea of what you are working on, as B & S made about 5 different models of 18 HP engines.

Rivets said:

First, when asking for help please be sure to include all model and serial numbers for both the unit and engine. This will give us a better picture of what you are talking about. Two things come to mind after reading your post. Both have to do with a lean starting mixture. Have you checked to see if the choke is fully closing. If it is try this test, instead of starter fluid. With the palm of your hand choke the air horn and try to start your engine. I think it is going to start, which means to me either the choke is not closing or your float is set on the lean side. If you know the choke is fully closed, I would be removing the carb and checking the float level, setting it slightly rich. With the numbers I would have a better idea of what you are working on, as B & S made about 5 different models of 18 HP engines.

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Thanks for the response..
The closet I can find to my tractor is MODEL NO. 917.255919
The carb gasket part number is (The manufacturer substituted part 271607 with this new part 273186S) from sears.com

The problem is there were no side covers on the motor and the body of the tractor was so rusty all the stickers were practilly gone..
Anyway mine is similar to that one above, the carb gaskets, float, pin, fuel mixer screw and spring part numbers all fit my carb. I was told by my local dealer that all opposed twin 18hp BS engines used the same carb in 1979. But who knows he could be wrong too.. lol.. we always thought all chevy small blocks used the same head gaskets until we ran into the 327 and 400.. lol..

Yes, the choke is fully closed, makes no difference if I suffocate it with my palm.
It will start normally if I pour a little bit of fuel into the carb or use starting fluid.
I adjusted the float a bit richer and it made no difference..
I'll post a picture of the engine and tractor asap, maybe it will help to identify it, remember my side covers are from a different engine that looks exactly the same and bolted right up to this one, but I was told came off a 1980 B&S..

Also, might be interesting to note..

If you adjust the float way too rich.... it dumps fuel into the oil and spews all over the place... BUT, the engine started properly.. lol.. so I changed the oil, flushed all the old stuff out, put in new oil and reset the float so it wouldnt flood the engine.. after an hour it wouldn't start again without either dumping fuel into the carb or spraying starting fluid...

The next curious thing (to me anyway cause this is the first time I am seeing it).

When I put the tractor on full throttle it moves the linkage properly (full motion) and a spring puts tension on what appears to be a governor arm which seems to control the actual fly on the bottom of the carb.
That fly doesn't move though unless I put on the PTO and then it opens up full and stalls the engine..
If I move that fly just a hair (like a millimeter or so) the governor arm pushes it full open and stalls the engine, then holds it there until I crank the engine over a few times and then it reurns to idle and then I spray some starting fluid or gas into the carb and it runs normally..

Update:

I screwed up on my last post.. I didn't tighten the carb down to the manifold properly... so ignore that last post..

After tighting the carb up properly, putting the choke on full and putting my finger in the hole in the choke fly it fired right up...
I tried to readjust the float.. still needed a finger..
I adjusted the fuel meter screw at the bottom of the carb, to almost fully opened and it will start, with the choke full on, and the air clearner must be attached or it will not start..
But now full throttle and all else appears to be fine since tighting the carb to the intake, kinda helps out a bit eh? lol...

I guess now I just gotta find the right float and mixture adjustments and I'll be all set..
Thanks a million, if I didn't try what you said I would have been chasing my tail for weeks and probably just given up and bought a new carb for it... lol...

Next problem will be getting my craftsman lt1000 and dlt5000 to not pop when I shut er down.. (also a carb adjustment and/or vlave I believe)..

I’ll get back to you sometime this weekend with a manual for your engine, which should help you out. From your pictures I’ve got a pretty good idea of the model. Did you look for an engine model number on the fan shroud?

The low speed circuit in your carburetor is dirty. That's where the engine gets fuel during startup.

When you use ether, you're supplying the fuel that would ordinarily come from the low speed circuit.

'Circuit' in this case is not referring to anything electrical. It's simply the routing of fuel from the carburetor bowl through the drilled passages and jets to the point where it's discharged into the throat of the carburetor. Usually that is very close to where the butterfly sits when closed.

mitchstein443 said:

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Update:

I screwed up on my last post.. I didn't tighten the carb down to the manifold properly... so ignore that last post..

After tighting the carb up properly, putting the choke on full and putting my finger in the hole in the choke fly it fired right up...
I tried to readjust the float.. still needed a finger..
I adjusted the fuel meter screw at the bottom of the carb, to almost fully opened and it will start, with the choke full on, and the air clearner must be attached or it will not start..
But now full throttle and all else appears to be fine since tighting the carb to the intake, kinda helps out a bit eh? lol...

I guess now I just gotta find the right float and mixture adjustments and I'll be all set..
Thanks a million, if I didn't try what you said I would have been chasing my tail for weeks and probably just given up and bought a new carb for it... lol...

Next problem will be getting my craftsman lt1000 and dlt5000 to not pop when I shut er down.. (also a carb adjustment and/or vlave I believe)..

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Idle the engines down to low idle for at least 5 seconds then turn the key off . No backfire like that.... If you still get a backfire then there is a issue........

Let us know Mon Ami ~!~!

Boudreaux In Eunice La. said:

Idle the engines down to low idle for at least 5 seconds then turn the key off . No backfire like that.... If you still get a backfire then there is a issue........

Let us know Mon Ami ~!~!

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Yeah I've tried that, always throttle it down and do my 10 Mississippis, lol.. it still pops.. The 5000 didn't do it before I put the new carb on, so I'm guessing the non adjustable carb needs to be adjusted.. lol.. The longer I run the mower the louder the bang is.. I actually have this issue on three of the, I forgot my MTD does it as well.. two are brigs engines and one is a kohler.. All are single piston and all have a new style carb (brand new) after market on them... I believe they are running rich.. Compression ratios do not change more then 5 from cold to hot, so I doubt it to be a valve issue... I considered putting a shutoff switch for the carb solenoid, can kill the solenoid 5-10 seconds before shutting off the key maybe that would keep them from dumping fuel into the muffler.. I've blown up two brand new mufflers in less then a season, and they get expensive.. lol.

The newer carbs have a solenoid valve hat jambs rod up the main jets rectum to stop the carb passing fuel through the venturi while it is spinning with the magneto turned off.
The unburned fuel sits in the red hot muffler till it dilutes to a combustible ratio at air pressure then ignites - bang.
The new carb is probably jetted leaner but just does not have the solenoid.
So to avoid the after fire you need to run the engine for long enough at low speeds to cool the muffler down far enough ( and risk overheating the engine ) or live with the bang.
The other way to avoid it is to fit a tap in the fuel line and starve the engine off

bertsmobile1 said:

The newer carbs have a solenoid valve hat jambs rod up the main jets rectum to stop the carb passing fuel through the venturi while it is spinning with the magneto turned off.
The unburned fuel sits in the red hot muffler till it dilutes to a combustible ratio at air pressure then ignites - bang.
The new carb is probably jetted leaner but just does not have the solenoid.
So to avoid the after fire you need to run the engine for long enough at low speeds to cool the muffler down far enough ( and risk overheating the engine ) or live with the bang.
The other way to avoid it is to fit a tap in the fuel line and starve the engine off

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The new carbs have the solenoid.. I was thinking I could put a switch to turn the solenoid off before shutting down the ignition... no fuel in should result in no bang out... my hythesis anyway (I'd say theory but I don't have enough experience for that yet lol)

Here is a manual that should help you out. As to my opinion on afterburner in the muffler, I say idle down and live with it. It’s not going to hurt anything, except some ears.

https://drive.google.com/file/d/0B6NaqjIxWV1ybHphbUNTQUNNR2s/view

mitchstein443 said:

The new carbs have the solenoid.. I was thinking I could put a switch to turn the solenoid off before shutting down the ignition... no fuel in should result in no bang out... my hythesis anyway (I'd say theory but I don't have enough experience for that yet lol)

Click to expand...

Sounds reasonable, they are just a strait drain of 12V off the battery.
only down side of an external switch is leaving it on because if the solenoid drains the battery completely, most times the battery will not be able to be recharged.