Running issues. I'm out of ideas

[MGTS]

Anyone?

[datzenmike]

I don't think a rotten egg smell is definitive that your catalytic converter is damaged or not working. An emissions test might tell you. I wouldn't give it another thought.

[A guy named Rick]

On 4/3/2021 at 10:30 AM, MGTS said:

When I did the test, It was with a brand new cat, literally 1 hour after it was installed 

 

Very interesting that your max and average numbers differ from mine. without posting a pic, i'll list my numbers as well as average and max where i got my smog done. i'm also in california and have an 86 720

I put the 15mph/25mph separated by a comma

CO2: 15.1, 15.2
O2: 0, 0
HC PPM: (MAX) 136, 108 (AVE) 26, 20 (MEAS) 74, 37
CO: (MAX) 0.92, 1.17 (AVE) 279, 234 (MEAS) 297, 248

By the way, i have the original cat too and running a mixture control solenoid from a Nissan Sentra 1.8, i found out

 

Edited June 16, 2021 by A guy named Rick

[A guy named Rick]

 

 

Do you think my HC is slightly elevated due to cat or maybe a little blow-by? compression is off by a little, and i do notice a little puffing from the vent tube on the valve cover.

Edited June 16, 2021 by A guy named Rick

[datzenmike]

HC is unburnt fuel. (hydrocarbons)

 

This could be slightly over rich to begin with.

Burning oil from bad rings and or bad valve guide seals or wear.

Intermittent plug misfire.

 

Catalytic converter needs oxygen in the exhaust to work. The 720 has an air injection system AIS. If disconnected of blocked or something and not working...

 

[A guy named Rick]

makes sense. i have a miss at idle. gonna change the plugs again. you're right about intermittent misfire, when setting timing i notice the timing light isn't 100% consistent. i suspect the coils, which look original. they're cheap, i may as well replace, along with cap and rotor.

[datzenmike]

Coils never fail and if they do they quit. The don't work some of the time. Don't change something unless it's at fault or you waste time and money and add inferior untested parts. Check the wires and not loose on the terminals.

 

More likely plugs are dirty or the wiring is old and cracked. Cab and rotor old and dirty/cracked. If 5 or more years old you could change them, otherwise a visual check is all you need. I wipe all wires down with WD-40. It cleans them and you get to examine every inch for wear, cracks or damage. (chaffing)  Inspect the rotor and inside and outside the distributor cap for cracks and wear on the contacts.

 

 

[A guy named Rick]

i would leave the coils as a last case scenario anyways. i will start with wires, plugs, cap, and rotor. plugs are new, just threw them in earlier this year when getting it running and tuning it. they just need to be cleaned up at best. the wires i don't trust though, should be replaced.

[Charlie69]

I have a miss in my 86 720 and it is a burned valve.  This engine is over 450,000 miles.

[A guy named Rick]

14 hours ago, Charlie69 said:

I have a miss in my 86 720 and it is a burned valve.  This engine is over 450,000 miles.

i have a pic around here somewhere you can see how nasty the crud buildup is on my intake valves. i wonder what they look like now that i sprayed some of the seafoam aerosol in there for a few minutes. it didn't smoke at all when i did it. maybe could be because the coolant (just water at the time) that was getting into the intake (due to not scraping old gasket off thoroughly) steam cleaned out all the carbon in the engine? idk. i had some black soot water out the tailpipe when that was going on.