1999 Briggs 303447 Vanguard 16 hp head

[Jerry In Maine]

been chasing a problem with this engine despite not really knowing what I'm doing...


Engine ran poorly. Low power, backfired through carb. Carb was filthy so I rebuilt it with no change. adjusted valves, etc. with no results. Noticed that one plug would develop a thick coating of dry black soot while the other came out clean with the insulator a nice brownish tan. I cleaned the dirty plug but it would foul after a few minutes of run time and performance would quickly go down hill.


Pulled the head thinking I'd find a burnt valve but actually it didn't look bad as best I could tell, but the intake valve had carbon buildup between it and the seat. Kinda tells me it might not have been sealing properly, and maybe this is why fire was coming up through the carb (?). Might also have caused poor combustion which might have been while the plug was fouling. (?)


Spark was equal on both sides so I think the coils are OK.


The plan is to clean the head up, lap the valves, and replace the seal on the exhaust while I'm in there.


Am I on the right track?

[dirtdobber]

The plan is to clean the head up, lap the valves, and replace the seal on the exhaust while I'm in there.

Since you have already torn it apart, then yes you are on the right track. Get it all back together and run a compression test. If you have good compression on both cylinders and it still runs bad, go back to the carb. You said you rebuilt the carb, but did you thoroughly clean it in the process? There are tiny passageways that need to be clean.

You may know this, but the black soot is an indicator of excessive fuel.

[Jerry In Maine]

Pretty sure carb is clean. Soaked it for two days then blew out each passage with the little tube on a can of carb cleaner. If the carb was providing excess fuel wouldn't I be fouling both plugs? In my case it's just the front one. Back plug very clean.

[racerone3]

If the problem persists when you get it back together, try unhooking the small black wire that connects the 2 ignition coils. (unhook from both). sometimes when the diode goes bad you will get what looks like good spark with no load, but once you load it spark falls off. I doubt it is carb. both plugs would look more similar if it was. If you are looking at the flywheel side of the engine, is it the cylinder to the left? that you are having the issue with?

You may have also just had a valve lash issue. if the intake was never closing fully, you would have fire out the carb, and not have proper compression to get a full burn... hence the soot

[Jerry In Maine]

looking at the flywheel trouble is to the right, same as where the fuel pump gets its vacuum. I did adjust the valves to .004 but problem persisted. As mentioned the seat of the intake was covered in soot...not sure if this would have prevented it from closing fully. Im glad I pulled the head if for no other reason than to rule it out and for the experience. I will try disconnecting the black coil wire. thank you!

[ARGOJIM]

If the problem persists when you get it back together, try unhooking the small black wire that connects the 2 ignition coils. (unhook from both). sometimes when the diode goes bad you will get what looks like good spark with no load, but once you load it spark falls off. I doubt it is carb. both plugs would look more similar if it was. If you are looking at the flywheel side of the engine, is it the cylinder to the left? that you are having the issue with?

You may have also just had a valve lash issue. if the intake was never closing fully, you would have fire out the carb, and not have proper compression to get a full burn... hence the soot

I wrote near the same thing but my either my phone or the internet ate it.

[Jerry In Maine]

I read differing procedures for adjusting the valves. Some say to set the piston to .25" below TDC...others say just to ensure the opposite valves is fully down. Which is correct?