Briggs 19.5 hp motor won’t operate past half throttle / overly rich

[Baymee]

Briggs Model 31Q677 0117 B1 091007ZD
Nikki carb. 795373. G02809. 9817 1B

Problem started recently and got worse. Would not run above half throttle and now doesn't run at idle.

When it was running, and above half throttle, it would backfire and puff black smoke. The spark plug was carboned up.

I checked the valves at TDC and the gaps are .004 and .006 Rotated the pushrods and they seem unbent.

The flywheel key is not sheered. I swapped out the coil and no difference. Gap .012. I tried two different spark testers and both show that the spark jumps a big gap.

Gas is good and I purged the line in case water was in the bottom of the hose. Fuel pump works. The float works and the fuel shuts off when the float is level but the spring loaded needle allows the float to rise high. Submerged the float for an hour and no gas inside the float. The inlet needle seats when the float is parallel to the body but since the needle is spring loaded, the float goes higher.

One unusual event after checking the above: I started up the motor and the engine went to full throttle but with the air filter on, it started to backfire. Swapped another filter and it did the same. Removed the air filter and it smoothed out.

Removed the carb, cleaned all the jets and reinstalled. Had it apart a few times.

Now when it starts, it putts out unburned gas and smoke and stalls with a wet plug. It’s worse than original and won’t run at all. No clue why it ran temporarily at full throttle, above, and now not at all. It’s like a jet is missing.

I can't decide if it's the carb or something Internal. I did a leak down test with 90# supply and about 75# is on the other dial. It's blowing out through the Intake valve.

Please help with ideas. I think I covered all the bases.

This is something that continually got worse which makes me question the carb, but I don’t find any other issues.

 

[StarTech]

Sound like you got at least a couple problems.

1. Make sure the main jet is fitting tight in the plastic fuel transfer assembly. The tiny o-ring tends to shrink and cause fuel mixture problems. Not available separately from Briggs. New jet only.
2. Install a new float bowl gasket set as the separate o-ring will cause fuel problem if loose fitting. Also thin fuel bowl gaskets got to nick free or you will fuel related problems. PN 698781
3. Then you need find why you have an intake valve sealing problem. You go to 1/4" pass TDC compression and back up to TDC compression for the leak down test or simple remove both push rods.
4. Also note these engine are known to blow the head gasket so while you have the valve cover off and doing the leak down test listen to the push galley vs the oil fill tube. If escaping air is load at the push rod galley than the oil fill tube you have a bad head gasket.

 

[Baymee]

Thank you. I will check again. I hooked up a remote fuel tank and ran without the pump. The fuel and compression are visibly pushing out of the carb mouth. If I hold my hand there, it gets wet from gas mist. This time I pulled the flywheel and physically checked the key.....no damage to the key and it’s in the right position.

At the very bottom of the bowl on the inside in the center, is there an o ring?

 

[Baymee]

Pulled the valve cover again. Valve lash good at TDC. Manually turning the flywheel, it definitely pushes out air that I can feel on my hand as it approaches tdc and up to tdc

 

[keakar]

Briggs Model 31Q677 0117 B1 091007ZD
Nikki carb. 795373. G02809. 9817 1B

Problem started recently and got worse. Would not run above half throttle and now doesn't run at idle.

When it was running, and above half throttle, it would backfire and puff black smoke. The spark plug was carboned up.

I checked the valves at TDC and the gaps are .004 and .006 Rotated the pushrods and they seem unbent.

The flywheel key is not sheered. I swapped out the coil and no difference. Gap .012. I tried two different spark testers and both show that the spark jumps a big gap.

Gas is good and I purged the line in case water was in the bottom of the hose. Fuel pump works. The float works and the fuel shuts off when the float is level but the spring loaded needle allows the float to rise high. Submerged the float for an hour and no gas inside the float. The inlet needle seats when the float is parallel to the body but since the needle is spring loaded, the float goes higher.View attachment 52834

One unusual event after checking the above: I started up the motor and the engine went to full throttle but with the air filter on, it started to backfire. Swapped another filter and it did the same. Removed the air filter and it smoothed out.

Removed the carb, cleaned all the jets and reinstalled. Had it apart a few times.

Now when it starts, it putts out unburned gas and smoke and stalls with a wet plug. It’s worse than original and won’t run at all. No clue why it ran temporarily at full throttle, above, and now not at all. It’s like a jet is missing.

I can't decide if it's the carb or something Internal. I did a leak down test with 90# supply and about 75# is on the other dial. It's blowing out through the Intake valve.

Please help with ideas. I think I covered all the bases.

This is something that continually got worse which makes me question the carb, but I don’t find any other issues.

i like to start with the simple things first, if the air filter made it run ok then its a carb fuel delivery issue

the smoking and wet plug on start up is from the carb flooding the engine with gas so go and buy a new carb for it on ebay and that will fix it

 

[Baymee]

I can do that but what about the blowback thru the carb? I guess an mp4 isnt allowed to be uploaded, I tried, but do you think a bad carb would allow gas and air to be expelled out of the mouth of the carb when running?

 

[StarTech]

Thank you. I will check again. I hooked up a remote fuel tank and ran without the pump. The fuel and compression are visibly pushing out of the carb mouth. If I hold my hand there, it gets wet from gas mist. This time I pulled the flywheel and physically checked the key.....no damage to the key and it’s in the right position.

At the very bottom of the bowl on the inside in the center, is there an o ring?

First is only two plain o-rings in the Nikki that you have one is on the 699733 main jet (1.14mm). It is bottom inside the fuel transfer tube.


Fuel Transfer port that has needle valve O-ring.

 

[Baymee]

Thank you. My jet o ring is flattened. Probably could use all the gaskets.

Update...the motor is running full speed with no smoke now. However, it backfires frequently through the carb and fuel mist is visible and felt.

I can fix the carb but is is possible that a burned or badly seated intake valve is causing this? From the leak down test, I know the valve leaks.

 

[slomo]

Cam timing, plastic camshft alert!! Does your engine have a plastic cam? Either you cam timing is waaaay off or your needle and seat is not working in the carb. Only way to get fuel backwards through the carb.

Head gasket

Carb replacement or rebuild kit. Your float level is wrong and excessively high. No wonder it is running so rich. Float should be level with the bottom of the carb body, where the carb bowl seats. Need to bend that tab where the needle sits to make the float level again, with the carb upside down.

Since you have a spring loaded needle, make sure the needle seals (would stop fuel flow) when you blow into the fuel inlet tube, with the float level and carb upside down..

slomo

 

[Baymee]

The float does seal off the fuel parallel to the body. The rest of the upward movement is from the spring. Not sure about the plastic cam. I noticed the starter ring on the flywheel was plastic