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Discussion Starter #1
I just rebuilt my 20hp briggs 461707-0145-E3 engine along with the carburetor that came on it. This engine is for my LT-1500 craftsman lawn tractor built in 2000. I could not get it to run unless I poured small amount of gasoline in carb but it ran nicely until the fuel burnt out. I found that a gasket was backwards in the fuel pump assembly. I corrected that and it cranks up now but backfires terribly, pop pop bang! What could be wrong? I noticed with the new flywheel key in place the flywheel could be moved back and forth a small amount. before I torqued it down would this cause the backfire? Do you think the carb could be off a little or would the carb cause a backfire? It seems strange that it runs nicely when I pour gas in the carb and get no backfire. There are no loose wires that I can see. Can any one tell me what to check next. Your help is greatly appreciated.
 

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I believe the timing is advanced to much causing it to backfire. There shouldn't be any movement on the key. Is it the right size key or the wrong flywheel. Also check for a cracked flywheel, that could cause the key way to be too wide.

We ran off set keys in 5 HP Briggs gokart engines they would backfire some times and about pull your arm out of joint.
 

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Discussion Starter #3
The key fits tight in the flywheel but kinda loose in the crank. " thought about putting a drink can thickness shim beside the key in the crank, but which side of the key should I put the shim on ? Its just a very very small amount of back and forth like a 32nd or so. I noticed it is hard for the starter to spinn off sometimes, just like the timing is off but I am sure I lined up the dots on the gears. "just about sure"Is there any way you know of that I could check to see if the timing is off without tearing the engine back down? do you think the 32nd of a inch play in the key way would make that much difference? Thanks
 

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That will make a lot of difference 1/32 is .031. Shim it on the front side of the rotation. That would be clockwise between 12 and 1.
 

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" I poured small amount of gasoline in carb but it ran nicely until the fuel burnt out. "

That says to me that you still have carb trouble. Have you tried adding choke? May have water in carb bowl.

Walt Conner
 

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That says to me that you still have carb trouble. Have you tried adding choke? May have water in carb bowl.
When he adds fuel to the carb perhaps it is causing it to run rich enough that it doesn't backfire?

I'd go with DonF's suggestion of shimming or replacing that key.
 

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Discussion Starter #8
I will try to answer , yes I had the valves adjusted at a automobile motor machine shop, they replaced all the seals and everything I could buy new except the valve itself . only small problem was two of the valve guides were semi loose but they had no tools to replace them. I rebuilt the carb and when I hooked it up no fuel would go in. I took it back apart and found that diaphragm between the pump body and outer plate was turned the wrong way from what I could deduct by looking at the parts. I put it in like the rebuild instructions said but it leaked fuel out of the outer plate holes so I turned it around and it quit leaking and the fuel would go into the carb now. There is no water in bowl and it has a choke lever. I have another fly wheel I will check it for key fit and see if it works. I am also going to check the carb gaskets again. I think it might be the timing like DonF said. I never knew about the extra gasoline in carb making it run rich enough to get by the off timing if that is what it is. One more thing, the muffler is shot the exhaust pipes dont touch either sides of the holes they go in. I ordered a new muffler and should be here in a couple of days. I dont know if that will help but maybe. Any more thoughts from yall will be greatly appreciated. With the backfire like it is , is there any chance of damage to anything? Thanks
 

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"fuel pump gasket backwards"...makes me think this is an opposed twin briggs, as they has the unitized pump/carb. So valve adjustment wouldnt be a "common" problem.

The timing key should not make it backfire unless it was sheared considerably...its common in racing to delete the key, or to shave it, to shift the flywheel for more power, Ive never experienced a problem starting or running with as much as 6-8* of advance.
 

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Discussion Starter #10
Red it is a opposed twin briggs 20 hp built in 2000. The diaphragm has 4 bolt holes and one hole between the bolt holes on the side. When I installed the diaphragm holes like the instructions said to it would not let fuel in so I turned the diaphragm around and it started letting fuel in and cranked up for the first time but backfired like mentioned above. I know its hard for yall to diagnose my problem without seeing in person but suggestions will help. IF not the timing would the carb make it backfire like that? Thanks, HIII
 

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Are you sure you got the two little bitty springs in the fuel pump? I can send you a Service Manual IF you like, address below, put in proper format and remind me, model number and what you want. I do not believe it is the flywheel key, actually it sounds like your crankshaft keyway may be wallowed out a little but not causing your problem.

Walt Conner

PS: They put instructions in the pump kit for a reason.
 

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Discussion Starter #12 (Edited)
Walt I put a different flywheel with key on it this morning and it didnt help. I took the key out and moved the flywheel half key over and half key before torqued it down and tried it in both positions it wouldn't fire.I took the pump back apart and put the outer diaphragm in place like the schematic said to and what happens now is gasoline comes out of the 2 holes in the front plate when I turn it over and gasoline spews out of the carb throat. My carb only has one spring in the pump area that pushes against a round cupped washer. Should there be 2? I will try anything yall suggest, I am getting desperate and my grass is getting to tall. Thanks for taking time to answer me. HIII

PS: My carb is in the briggs&stratton twin cyl L head manual #271172
It is in section 3 page 2 fig.1 and section 3 page 4 fig.3

20hp briggs 461707-0145-E3

carb # 692815
27-421
0327
 

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I am sure I lined up the dots on the gears. "just about sure"Is there any way you know of that I could check to see if the timing is off without tearing the engine back down?
i was aked this question once, and the more i swore i lined them up i had to know, i tore the engine down again and they were aligned as i thought, but i did it to get that ease of mind knowing that wasnt the problem :goodl:
 

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Is your fuel inlet line hooked up right? There will be a fitting for pulse t othe engine, and a fitting for fuel from the tank...is it possible they are reversed?
 

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There is one large spring and two little bitty springs in the fuel pump, one behind each flapper valve. I offered you a Service Manual to tell you how to do things. You have the fuel pump put together wrong. I think if the vacuum and fuel lines are improperly attached, it just will not pump gas. Vacuum line is the bottom one, fuel inlet is on the side.

Walt Conner
 

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There is one large spring and two little bitty springs in the fuel pump, one behind each flapper valve. I offered you a Service Manual to tell you how to do things. You have the fuel pump put together wrong. I think if the vacuum and fuel lines are improperly attached, it just will not pump gas. Vacuum line is the bottom one, fuel inlet is on the side.

Walt Conner
Walt , don't have ipl in front of me but i thought only the 3 bolt pump had the small springs not the 4bolt
 

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Discussion Starter #17 (Edited)
Walt I really need the service manual. I will give you all the info about my model numbers.( engine :461707-0145-E3 code 0002165A ) and ( carburetor: 692815 27-421 0327 ) 4 screw fuel pump . carb part # 693479 . is this what you need for the service manual information. My fuel line is connected to the plastic body on the side of the pump and the vacuum is connected to the bottom brass of the pump. Yesterday it started blowing gasoline out of the two holes on the outer cover on the fuel pump when I turned it over. and it seems to want to start up without backfire until something makes it flood out and gasoline pour out. I hope it is a carb problem , I remember the small spring on the inside the pump body but my kit did not have replacements for them . Thanks for hanging in and giving your help.
 

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Discussion Starter #18
Walt do you have all the information you asked for to get me that service manual you are offering ? I just ordered another gasket kit from briggs so maybe I can see if I did anything wrong, something is wrong because it is blowing gasoline out of the outer fuel pump case holes. Thanks, HIII
 

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Discussion Starter #19
I found the problem, The machine shop that did my valve job only put one keeper one of the valves. No compression on that cylinder. I pulled head and saw valve stuck open. Put keeper on cranked right up but still get fuel spraying out of fuel pump cover. I just ordered a new carb so maybe that will be that. Thanks for yalls help, HIII
 
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