IS the choke plate fully closing?

As Bill noted the choke plate has to close, intake seal good etc. An easy on the cheap way to remedy your situation is with one of those aftermarket primer gas caps. They look hoaky, but work well and much cheaper than a new carb and tank for an old engine.

Had the same problem.Threw away the Champion plug and put in NGK.No problems now.

There are several things that may affect starting.

A briggs engines from 1977 probably has the old point ignition system. Test the ignition strength by duct taping the sparkplug wire .166 from the cylinder head. Pull the starter cord while looking for a spark. You may need to perform this test in a dark area such as a garage. The color of the spark makes no difference. This test requires about 13000 volts. Starting a cold engine usually requires about 8000 volts. A hot engine usually requires about 4000 volts to run. If the ignition is weak, service the points.

This engine is probably equipped with the old "easy spin" compression release. Starting compression is release by the design of the camshaft lobe. When the engine is running, the loss of compression is not significant. Valve clearance usually decreases on older engines, and compression would be lower. Check the compression and clearance. For an older engine, I would look for 60 - 90 psi depending on your altitude.

Finally the carburetor should be examined carefully. Most probably this is a pulsa jet carburetor (92900 or 94900) with two pick up tubes. The fuel pump spring and cap must be located on the carburetor side - not the tank side of the diaphram. The longer tube works with the built in fuel pump. Fuel is delivered to the upper chamber in the fuel tank. With the carburetor apart, look for a splits in the plastic tubes The screen on the tubes should be clean. Blow through the tubes to make sure there are no obstructions. The diaphram works with the choke as well as the fuel pump. If there is any warpage in the mating surfaces, fuel may not be pumped to the upper compartment. The diaphram spring closes the choke. Intake vacuum is directed to the diaphram to oppose the spring and open the choke. Once fuel is delivered to the upper tank chamber, the short plastic pickup tube draws fuel to the carb venturi area just like a conventional carburetor.

If excession oil is in the cylinder area, the compression may increase. Increased compression may increase the intake vacuum - causing the choke to open while you are pulling the starter rope. To test this theory, observe the choke while pulling the starter rope.

There is a repair kit available for warped surfaces, pn 391413. This kit restricts the vacuum applied - to open the choke slower.

Wow Larry you know your stuff!!

Larry, you're exactly right on about the easy-spin starting vertical pull system; also about points ignition as well as the pulsa jet carb. It's a 92908 model engine. One thing I just noticed today is that the diaphragm does seem to "seep" a film of gasoline. The old one did the same. I installed the diaphragm correctly, as you noted. The area where the choke spring on the diaphragm sets (the recessed area on the tank) was full of gasoline as well, I think that might be another tell-tale sign of a warped tank. I'll check the spark and such tonight; I'd imagine after 30 years it's due for a points and condenser replacement. Just one of those things that drives me nuts, because it really runs great once it's started.

Thanks for the tips!

Same problem on my Toro 20755 with same Briggs engine. Jekyll & Hyde, almost impossible to start and keep running at times, dripping gasoline into the carb keeps it running, wondered about that repair kit 391413 and where it is available, Thanks

'Wow Larry you know your stuff!!'

+1

Just wondering if the choke plate spring is bad and is not holding the plate 'closed' when the engine is off.

ggallo; you probably need a new diaphragm.