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MTD 4HP w/ Briggs Replaced Diaphragm, Still No Go


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10-28-14, 12:16 PM   #1  
MTD 4HP w/ Briggs Replaced Diaphragm, Still No Go

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Picked this mower up from craigslist. Did research first, so I knew how to replace the carb gasket and diaphragm before I even bought it. Thoroughly cleaned out the gas tank and carb. Unfortunately, though, she still won't run. I get an occasional pop after yanking the bejesus out of the cord. Starting fluid has no effect. Reset the gaps on the coil to .010 and tested the operation of the brake lever. I can't get a spark that I can see from pulling the recoil and using a tester light on the plug. After taking it all apart to check the coil, I hooked the flywheel up to my drill, cranked it way up, and that seemed to be the only way I could get a proper spark going. Pretty sure I don't want to try to start it this way... I'm leaning towards replacing the coil next, but I'd like to see if anyone has any better ideas. Buying the new coil will probably more than double my so-far investment in this thing, but no big deal.

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10-28-14, 12:24 PM   #2  
Finally located the correct model# (I think)

10A902 2189 B3 02042455

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10-28-14, 12:28 PM   #3  
Did you check the flywheel key, it may be sheared. Have a good one. Geo

 
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10-28-14, 12:54 PM   #4  
Good suggestion. Key looks intact, though.

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10-28-14, 01:07 PM   #5  
Also noticed something I thought was strange but might be normal. The governor is just a paddle that the flywheel fan blows against, so presumably until the motor is running, there really isn't any pressure on the springs on top of the carb. So, it seems I can move the throttle freely without anything presetting where it should be during startup.

 
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10-28-14, 04:23 PM   #6  
Since the engine has a primer it is setting at WOT and when the engine starts the air flow on the governor vane slows the engine. Have a good one. Geo

 
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10-28-14, 04:26 PM   #7  
When you pump the primer do you see fuel being squirted into the intake, if not the priming circuit is not working properly, try pumping it 5-10 times. Have a good one. Geo

 
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10-28-14, 04:37 PM   #8  
Yeah it squirts. Even tried spraying starting fluid in there too but doesn't make much of a difference.

 
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10-28-14, 06:21 PM   #9  
Even though you stated you knew how to change the diaphragm I will post my usual instructions so you can check for anything missed. Have a good one. Geo

You need to install a new diaphram kit, it is Briggs part # 795083 or old # 495770 also Lowe's has a kit #5083 which has the same parts.
Here we go:

Remove air cleaner, there are 2 bolts that hold things together, one on the front of the tank 3/8" and one into the block 1/2", remove these bolts, now "slowly" slide the carb/tank off the intake tube and breather tube, now tilt the tank in to release the throttle linkage and waalaa the carb/tank is removed. Check the intake tube to see if the “O” ring seal and plastic retainer are still on the intake tube, if so remove them and re-install them into the carb. Remove the 5 screws from the carb/tank remove carb(don't loose the spring) now spray all holes, cracks and crevases in both the carb and tank surface with brake parts cleaner, remove the main screen(looks like a thimble), now with a small screwdriver pry out the main jet(carefully) and clean it, check the “O” ring on the main jet for damage, if it is damaged it must be replaced, reinstall the jet, it can be difficult some times to get it to snap back in place(I use the rounded end of a screwdriver handle). Remove and clean the fuel pickup stem(not necessary if the fuel tank was clean). Clean any junk/rust from the tank. Install the diaphram on the tank then the gasket(no goo or sealer) now carefully replace the carb(the spring will try to misalign the diaphram), tighten all screws a little at a time so as not to crimp the diaphram until they are all tight. Install the carb/tank in reverse order and you are done. If I missed something one of the real mechanics will correct me. Have a good one. Geo

 
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10-29-14, 08:10 AM   #10  
So, I gather that the chances that the magneto is bad (given that is does deliver a spark) are low? I'll double-check the carb as you say and also do a little more work cleaning up the surfaces on the magneto because it is a bit rusty.

 
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10-29-14, 09:10 AM   #11  
Problem solved! on closer inspection, the surfaces of the coil facing the flywheel were completely covered in old grease. So a quick scrub with carb cleaner and she fires right up on one easy pull. $20 lawn mower project score!

 
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10-29-14, 09:31 AM   #12  
Success is a wonderful sound. Have a good one. Geo

 
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