Posted: 9/9/2008 9:05:15 AM EDT
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I have a Briggs & Stratton 4.75 horse pushmower with a bit of a problem. It ran fine 2 weeks ago, but now it won't start. I can prime it, and try it, it fires 2 or 3 times, enough to burn out the primer gas, but after that, it quits. Clogged fuel line, right? Tore it apart this morning, got rid of the old gas in the tank, cleaned the crap out of the carb, cleaned the air filter, and removed the tank. Maybe a clogged pickup? Nope, screen is clean as a whistle. It's almost like when a fuel pump goes out, but this thing just has a pickup tube, not any sort of pump anywhere that I can see. Any suggestions? |
remove the pickup tube and lean it out. could be small pebbles in the pickup tube. had that happen to me with a rotiller. |
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Pickup tube appears to be sealed. Screen on the bottom and the top end run into the carb. Looks like there's no way for anything but liquid to get into the tube without completely disassembling the (plastic) carb. Shanking it produces no rattling sounds or anything like that either. Gas appears(!) to flow freely down it when poured into the carb, so I can't say that it's not going up into it when it's supposed to. Edit: I'm going to try to remove the pickup tube yet again, since it's the only place I can see that this might cause the problem. |
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You say you cleaned the crap out of the carb. Assuming(!) that the 4.5 HP engine still uses the old 5 HP carb. Did you remove the brass jet and make sure that the idle/run fuel orifices were clear? Sometimes this is overlooked. Don in Ohio Never mind. Just saw that it is a plastic carb. I know nothing about! |
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Does that carb have a diaphragm fuel pump? Are there air leaks, loose fuel lines, pinholes in fuel lines, old cracked fuel lines, etc.? Those lines are pretty cheap to replace. What about a cracked or otherwise damaged primer bulb? That can create a massive air leak. |
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There are no fuel lines to speak of. Pickup tube runs into tank, gas goes from tank directly into carb, no other lines. From carb directly into engine. Primer bulb is in good shape. There is a metal tune extedning from the engine that has a rubber elbow and connecting to the carb above the pickup tube. I suspect this is a vacuum tube and causes the pickup tube to suck up the gas into the carb. If this is the case, I strongly suspect the rubber elbow of not providing vacuum, though it appears to be in good shape, outwardly. No cracks. Perhaps it's not sealing up and not providing suction. |
That may just be your crankcase breather. |
That tube is the crankcase breather tube. No vacuum there. I am thinking that there should be some kind of rubber diaphragm between the tank and carb. If so change it. |
Roger that. There is. Off to the lawn mower house to look for a diaphragm. |
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Okay, the only way to determine what's going on in the cylinder is to pull the plug and read it. Yes, we know you have spark, but the plug color will indicate whether it is lean or rich. My guess is the plug will be sooty (and too narrowly gapped if original, never pulled) from burning off the rich mixture from you priming it. Clean the plug electrodes with a wire brush and some spray cleaner, dry off, check the gap, and reinstall. Then see what happens. Also look for soot coming from the exhaust which will indicate a rich mixture. |
I don't have a plug wrench to fit this stupid ass weird fucking plug or I'd have already pulled it. Something like a 19.5 mm plug.  New diaphragm, same old shit.  Edit: finally located my multi-adjustable nut fucker (adjustable wrench) and pulled the plug. Yep. Sooty. |
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Does the carb also have a tube that goes into a bowl at the top of the tank? Does it have a screen that just fits over it? If it does, pull the screen off and check to see if there is debris in the main jet. I have seen flakes of rust or dirt get stuck in that jet and cause your symptoms. You have to run a little piece of wire through there to clean it out. Good luck. 
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Oil looks normal
Checked already. Looks good. I hate to admit defeat, but I'm about done wtih this thing. I've got a toothache and this piece of shit has me about ready to sling it into the back yard. |
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I have this problem every year. I take the bowl off the carb. Under the bolt that held the bowl on is a hole and I poke a tooth pick through it till it bottoms out. Twist a little and blow through it. Put it back together and it runs fine after that. This last year was the first year it didn't work and I had to buy a new carb, but the mower is 9 years old. |
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Okay, you've got the plug good to go. Here's the next step. I know you've pulled the carb and cleaned it. I am speaking generally here, not make/model specific. At this point, I want you to pull the main jet (it probably only has one - sometimes it is part of the bolt holding the float bowl on - it would be in the center of the bowl. Have something to catch the gas. Dry off the jet and hold it up to a light. Can you see through it? If you cannot, the jet is clogged. Pull a piece of wire from a brush (or a drill bit will work if small enough) and use it to clear the obsruction. There will be one hole in the center of the jet and generally two holes on the side where the fuel transfers from the bowl. Over time, these jets can clog up hard and cleaner just won't budge it. Anyway, clean it until you can see light through all of the holes and reinstall. Hopefully this is it. I wish I was there to help in person. |
| Check the priming bulb hose for a crack or a break. It'll leak some when you prime but will still start but will die once it tries to draw from the tank. Also, make sure wasps have not clogged the exhaust [mud daubers] it'll produce the exact same symptoms. Check this well, sometimes it's not really noticable at first glance. |
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It's either the float closing and not allowing fuel to go through...check #19 of the diagrahm, they sometimes have a hole/tube inside that can get clogged..shoot some cleaner into that screw if it does have the hole into it. Then check #12 shoot some cleaner through it also. Hope that helps. 
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