No Hot Start Briggs & Stratton Model 100202 (circa 1980s) 4HP, Pulsa-Jet Carb

[jkekenj]

Looking for ideas as to what I may have missed trouble shooting.

Problem Description:
This engine will always Start when it’s Cold and continue to run until shut off.
Once warmed up, most of the time it will not start. When fully hot it does not start at all.
I’m using and electric drill to start it as I nearly wore out my arm with the pull start.

What I have Checked so Far:

Fuel

• Fuel is fresh, regular octane
• Fuel tank is full

Electrics

• Spark from magneto to ground is visible
• Point set at 0.020” as specified
• Magneto to flywheel gap 0.012” (spec 0.010 – 0.014”)
• Spark plug gapped 0.030” as specified
• Spark is also visible with spark tester in series with spark plug.

Carburetor – Pulsa-Jet Slide Choke

• Removed carb, cleaned all orifices and installed new diaphragm & spring kit, set jet screw 1 ½ turns out.
• Carb appears to draw fuel as small upper gas tank chamber had fuel in it when carb removed.

Engine

• Found Valve clearance Intake 0.004”(spec 0.005 - 0.007) Exhaust 0.009” (spec 0.007 – 0.009)
• Removed head, cleaned valves and removed carb, intake set to 0.006”.
• Valve seat and valve faces in very good condition
• Cylinder walls smooth but cross hatches still visible.
• Although compression is not really a good indicator, I measured about 60 PSI.
• B&S manual indicates adequate compression if there is a sharp rebound while turning the flywheel counterclockwise. I feel this rebound.

Replacing the magneto on speculation would cost about $80 (local price) and a brand new higher horsepower (Harbor Freight / Princes Auto) engine can be had for ~ $150.
I’ve put too much time into this thing, but I would really like to find out Root Cause.
If anyone has any idea as to anything else I have missed checking, please let me know and I’ll follow up.

 

[Scrubcadet10]

Will it fire up hot if you choke it? or if you spray starting fluid/parts cleaner in the carb?

 

[jkekenj]

Yes. I tried adding fuel manually.
I have sprayed fuel into the carb (top opening)
Then applied the choke (the rear section slides out and blocks off air from the top, most air comes from a pipe that sucks crankcase air/oil mist)
Most of the time it would not start, but it would start now then if I continued cranking for a longer than normal time.

I've also removed the spark plug and sprayed fuel into the chamber. Again sometime it would start once the fuel/air ratio was withing combustible range.

I've also played with the throttle plate and was able to start sometime. The throttle seems to move very easily and normally sits full open. Its attached to the governor lever via linkage.

 

[Scrubcadet10]

I would start with a simple thing first and try a new spark plug.... Also, have you looked at the color of the spark plug, Brown, White, black?

 

[jkekenj]

Yes I was considering the spark plug, but I did not have the correct one on hand. I'll need to purchase the right one tomorrow.
Thanks for your support.

 

[Rivets]

Please post the type and code numbers. Need to figure out it you have a pulsa-jet or vacu-jet carb.

 

[jkekenj]

Its a 100202 with the pulsa-jet as shown above from the Briggs service manual.

Found another spark plug from another mower.
Started up immediately and I ran it for about 5 minutes. Got to normal operating hot.
Restarted ok after a few turns (more turns than when cold)

Shut it off a few times and then it would not start again.
Tried full choke on but not strat. Did have some backfires
Tried with choke wide open but no start.
Then I rotated the throttle plate with my finger as far as it would close. This started the engine at very low speed and eventually it would stop on its own.
Tried it a few times with throttle closed and it stated but would not keep running. If I allowed the throttle to open, it would stop immediately.

I'm thinking something is closing off in the carb as it heats up. I think I should give it an ultrasonic cleaning.
Any other thoughts?

 

[Rivets]

I know it’s a lot of work to go get those numbers, so I’ll give you this clue as what to look for and then will be ignoring this thread, as you sound you know more than me. After you adjusted the valve clearance, did you relap the valves? 60 psi is low for those older engines, did you check it both dry and wet? Did you check for spark both hot and cold? Did you replace the manifold gasket and check it for leaks when hot?

 

[jkekenj]

This is the carb

 

[jkekenj]

Did not re-lap the valves. They looked good, see pictures. Did not want grit to get inside engine. I would have had to totally disassemble and clean everything.
Torqued the heads to the specified 140 psi with original gasket. I would expect heat to reduce any leak due to expansion, since it works ok from cold and until I shut it off.
Prior to the valve job, I did put oil into cylinder and only notices about 5 psi compression difference. Briggs does not give compression specs. The manual indicated if you have a rebound its good enough.
Spark is good both hot and cold