Export thread

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

#1

B

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.F704180C-26EE-43E2-A355-2360C2A5C54E.jpeg

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.


#2

StarTech

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.


#3

B

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?


#4

B

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


#5

K

keakar

Baymee said:
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.
Click to expand...
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


#6

B

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?


#7

StarTech

StarTech

Baymee said:
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?
Click to expand...
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.
699733 jet.JPG

Fuel Transfer port that has needle valve O-ring.

Fuel O-ring.png


#8

B

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.


#9

S

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


#10

B

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


#11

StarTech

StarTech

And the 793880 camshaft only has one tiny piece plastic on the ACR. The lobes and the rest on the camshaft is all metal. I know as I replaced enough of them for failed ACRs.

thumb.png


#12

S

slomo

What was that old wordage? If it backfires through the carb it's ignition timing? Backfires through the exhaust it was cam timing? Where's the Chevy guys at?

slomo


#13

S

slomo

StarTech said:
And the 793880 camshaft only has one tiny piece plastic on the ACR. The lobes and the rest on the camshaft is all metal. I know as I replaced enough of them for failed ACRs.

thumb.png
Click to expand...
Great so we have a steel cam. Nice find StarTech. Hopefully it hasn't jumped time?


#14

S

slomo

Still sounds like it's overly rich. You said the float was closing proper. Put a fuel cut-off valve in the fuel line. Start the engine and close the fuel valve. See how it runs. Should run for 20 or so seconds and begin to sputter. Fuel valve closed should stop the excess fuel from coming through the carb when the engine is running. If in fact this is an excess fuel issue??

You might have a valve seat not where it belongs holding a valve open. You said you can hear air leaking out on leak-down. Head gasket or valve seat maybe?

slomo


#15

B

Baymee

Question. I don't know if this engine has a plastic cam but if it did, would the valve clearances at TDC always be correct like they are?


#16

B

Baymee

Slomo, not sure what you mean about the shut off valve but I do have a remote tank hooked up to bypass the fuel pump and it has a shut off.

But even if the carb was rich I would expect to see black smoke but would I expect to see mist and feel pressure coming back through the carb inlet?


#17

B

Baymee

See

Attachments





#18

StarTech

StarTech

You need to find out why, according to leak down test you did, the intake valve is not sealing. What I see is blow back. It also can be cause by a worn exhaust valve cam lobe but you said failed the leak down test on the sealing of the intake valve. It going to require to remove the cylinder head as the intake seat may be loose and not allowing the intake valve to close properly.

I just recently repair a Ariens Zoom 34 where the exhaust was nearly worn completely round and the engine was running like your engine but the engine did not fail the leak down test; just very little movement of the exhaust rocker arm.


#19

S

slomo

Baymee said:
Question. I don't know if this engine has a plastic cam but if it did, would the valve clearances at TDC always be correct like they are?
Click to expand...
StarTech found you have a normal steel camshaft. Forget about a plastic camshaft.

You might be 180 degrees out on your valves or a flat lobe as StarTech said.

slomo


#20

S

slomo

Baymee said:
See
Click to expand...
Throttle is surging like a mo-fo in your attached .gif file.

slomo


#21

S

slomo

Baymee said:
Slomo, not sure what you mean about the shut off valve but I do have a remote tank hooked up to bypass the fuel pump and it has a shut off.

But even if the carb was rich I would expect to see black smoke but would I expect to see mist and feel pressure coming back through the carb inlet?
Click to expand...
With that much blow-back, either the needle is dumping fuel constantly flooding the cylinder or you have a cam/valve/head gasket issue of some kind.

You can install a valve. that shuts off the fuel, after you mow, inline to the carb. Doing this, you never have to worry about your carb needle dumping fuel into the cylinder and then into the crankcase contaminating your oil with gas.

slomo


#22

S

slomo

Baymee said:
Question. I don't know if this engine has a plastic cam but if it did, would the valve clearances at TDC always be correct like they are?
Click to expand...
Don't forget there is more than 1 TDC. I was shocked that you found the valves IN SPEC. I've never see any used mower with proper spec valves. Course I get them thay way and not running LOL.

slomo


#23

B

Baymee

I think I found it. Should have done this sooner. The intake valve rocker moves the valve stem inward 7/16" in a cycle while the exhaust valve only moves 1/8". Cam lobe?


#24

StarTech

StarTech

Baymee said:
I think I found it. Should have done this sooner. The intake valve rocker moves the valve stem inward 7/16" in a cycle while the exhaust valve only moves 1/8". Cam lobe?
Click to expand...
Sound like it.
slomo said:
StarTech found you have a normal steel camshaft. Forget about a plastic camshaft.

You might be 180 degrees out on your valves or a flat lobe as StarTech said.
slomo
Click to expand...
Impossible to have the camshaft 180 as the turn ratio is 2:1 (2 crankshaft rotations to 1 camshaft rotation). Even if you actual put in a 180 out to the timing mark the very next full crankshaft the timing will line up. Plus being a wasted spark magneto system, the plug fire on every complete rotation of the crankshaft.

Now that said you can install the camshaft up +-90 degrees and have the engine out of time. I have time a few old 12 hp L head engines out by one tooth of camshaft gear and have more HP.


#25

Scrubcadet10

Scrubcadet10

Baymee said:
I think I found it. Should have done this sooner. The intake valve rocker moves the valve stem inward 7/16" in a cycle while the exhaust valve only moves 1/8". Cam lobe?
Click to expand...
I'm sure you know what you're doing, are you sure it isn't the exhaust vale opening for the compression relief on the cam? Just wanted to point this out to further help confirm a cam problem.


#26

B

Baymee

Here are the pics. Look at the timing marks, then see the piston position. How did that happen? Then see the cam.



The word "cam" was mentioned enough and that was correct. Thanks to all that opined.

Attachments









#27

Scrubcadet10

Scrubcadet10

To me the closest lobe, (intake?) Looks worn more than the other


#28

S

slomo

Baymee said:
I think I found it. Should have done this sooner. The intake valve rocker moves the valve stem inward 7/16" in a cycle while the exhaust valve only moves 1/8". Cam lobe?
Click to expand...
Yes, valves should have similar
StarTech said:
Sound like it.

Impossible to have the camshaft 180 as the turn ratio is 2:1 (2 crankshaft rotations to 1 camshaft rotation). Even if you actual put in a 180 out to the timing mark the very next full crankshaft the timing will line up. Plus being a wasted spark magneto system, the plug fire on every complete rotation of the crankshaft.

Now that said you can install the camshaft up +-90 degrees and have the engine out of time. I have time a few old 12 hp L head engines out by one tooth of camshaft gear and have more HP.
Click to expand...
I was talking about setting valve lash and being on the wrong stroke. Sorry for the poorly written info LOL.

slomo


#29

S

slomo

Baymee said:
Here are the pics. Look at the timing marks, then see the piston position. How did that happen? Then see the cam.



The word "cam" was mentioned enough and that was correct. Thanks to all that opined.
Click to expand...
Was I correct with cam timing off? Sure looks like it. That cam lobe resembles a drag racing solid roller, minus the missing lift LOL.

When you have blow back out of the carb, you are dealing with a huge racing style camshaft. Lots of valve overlap on those. A mower uses the exact opposite cam profile. Time to call Briggs and get a new camshaft.

slomo


#30

S

slomo

StarTech and ScrubCadet10, could THAT cam gear rotate on the shaft and cause this faulty cam timing? Or someone got it off by mistake? I see one lobe splined up a bit. Those lobes shouldn't move unless stripped.

slomo


#31

Scrubcadet10

Scrubcadet10

It's possible, I've never heard of it happening, but after more research, it seems there are alot of problems with the 793880 cams and the cams it replaces, Taryl did a video where he replaced one, and the mower would run okay, but be popping and backfiring. Worn cam lobe was the problem....
When the cam timing dots are lined up on the crank gear and cam gear, there should be no pressure on the valves and be able to pull that cam out easily on most engines, if the gear slipped, i suspect you would have to find the sweet spot where it is not lifting a tappet.
But looking at a stock picture of the cam compared to OP's cam, the lobes look to be in the correct position.


#32

S

slomo

So how would the piston get 1/2 to 3/4" down into the bore with the dots lined up? Crazy.... The piston should be TDC with the dots in line right?

slomo


#33

Scrubcadet10

Scrubcadet10

slomo said:
So how would the piston get 1/2 to 3/4" down into the bore with the dots lined up? Crazy.... The piston should be TDC with the dots in line right?

slomo
Click to expand...
Oh yeah *bangs head* very odd....


#34

B

Baymee

Original engine. Never opened up. The marks aligned each revolution but the piston wasn't at TDC. I wonder how it happened


#35

B

Baymee

793880 is the cam that Briggs sells. Is there an upgrade or is this the only one? And will it hold up?


#36

Scrubcadet10

Scrubcadet10

I think all the new 793880 are supposedly better made, I think the problem was an issue with materials, too soft and wore quickly.
please correct me if I am wrong.


#37

S

slomo

Scrubcadet10 said:
I think all the new 793880 are supposedly better made, I think the problem was an issue with materials, too soft and wore quickly.
please correct me if I am wrong.
Click to expand...
Definitely looked soft and wore quickly.

slomo


#38

B

Baymee

Hold on here. If the piston isn't at TDC then the pinion gear must have slipped on the crank.


#39

Scrubcadet10

Scrubcadet10

Looking at the IPL, the gear is keyed to the crank..


#40

B

Baymee

I just saw that too. When it rains.....


#41

B

Baymee

On the Briggs website it states that if the timing marks line up, it's not important where the piston is. It will not usually be at TDC. News to me. Anyway, the crank gear and key are good and carb and cam on order.


#42

B

Baymee

Regarding the ipl above, there is no spring shown; but the service manual shows one. Anybody have a part number on this ?

Attachments









#43

StarTech

StarTech

slomo said:
StarTech and ScrubCadet10, could THAT cam gear rotate on the shaft and cause this faulty cam timing? Or someone got it off by mistake? I see one lobe splined up a bit. Those lobes shouldn't move unless stripped.

slomo
Click to expand...
If Cam gear rotate on the shaft then the ACR pin won't be centered up on the intake lobe and possibly be broken off. BUt from image the lobes are in the correct positions according to the camshaft I just looked at.
Scrubcadet10 said:
To me the closest lobe, (intake?) Looks worn more than the other
Click to expand...
That is the exhaust lobe that is heavy worn. Still not as bad the one I just recently replaced on an Ariens ZTR. They case harden but once that case harden area wear though it is soft and will wear quickly. BTW save that ACR components from that worn out camshaft for later use.

As for piston position I will double check that in the morning as I got governor and oil pump coming in late today for a 310000 series engine. I just haven't paid that much attention as to the piston position on these engines. But I think it should be at TDC and if so then the matching crank gear key is probably damage but I will know for sure in the morning where the piston is position with the timing marks lined up.


#44

S

slomo

Normally when you advance cam timing, you build more cylinder pressure. Gives you more low and mid punch torque.

slomo


#45

StarTech

StarTech

Okay got the answer to piston position by actually looking at an engine here that I know is running okay after a lockup oil pump. It is down (about a third to half of the stroke) like shown pass TDC.

Baymee said:
Regarding the ipl above, there is no spring shown; but the service manual shows one. Anybody have a part number on this ?
Click to expand...
Not all the Nikki carburetors have that spring. More don't than does.


#46

S

slomo

Guess the engine manuals have no specifics on the piston and cam timing marks?

slomo


#47

StarTech

StarTech

Slomo, The question was piston position when both camshaft and crankshaft gears are timed. I can see why if is the way it is as it make changing the camshaft a lot easier without have to disable the valve train otherwise. On these it is a common thing to change out the camshaft due to ACR failures.

The service does the markings including the piston marks:

The notch or arrow on the piston and “MAG” on the rod (B) must face the same direction. When installed in engine, the notch will face the flywheel.


#48

B

Baymee

Installed the new cam. The piston is not at TDC, it's position is still like the picture showed. Ran for half hour test.

Thanks all. I'm glad it's running well again.


#49

Scrubcadet10

Scrubcadet10

Great news!!!


Footer Links 1

Top