Does anybody know what causes the primer bulb on a flat head Briggs to stick in after it has squirted in one shot of fuel. I assume it is the flap on the diaphragm that must be sticking but I have never really bothered looking into it, I just put a new kit through them
Hi Norm, usually its quicker to grab another used carby but if its just a degraded rubber bulb you can get them working again .
If its a sprint motor then its usually a stiff, aged rubber bulb that has lost its natural spring-back tension or If a check valve is stuck, gummed up with old fuel varnish, or deformed, it fails to vent or seal properly.
As we know because it delivered that initial squirt, the core fuel delivery is doing its job, pointing right back to a stiff bulb, a sticky internal check valve, or a bit of varnish in the primer passages.
Most people may just buy a new primer bulb but I've always found you can easily get them working again to bring the flexibility back to them if that's the problem.
How to Restore a Stiff Briggs & Stratton Primer Bulb 1. What You Need Motor oil (standard engine oil)
A small brush, rag, or an oil can
The primer bulb still attached to the engine (or removed if you prefer working on the bench)
2. Step-by-Step Instructions Clean the Area: Wipe away any dirt, old fuel residue, or debris from the outside of the rubber primer bulb.
Apply the Oil: Use an oil can or a brush to thoroughly coat the outside of the rubber bulb with motor oil. Make sure it is completely covered.
Work It In: Keep pushing the primer bulb in and letting it pop back out (or manually pulling it back if it's stuck) while ensuring the rubber stays soaked in oil. This forces the oil into the pores of the dried rubber.
Let It Soak: Leave the engine sitting for a few hours (or up to a couple of days) with the oil pooled or coating the bulb so it can fully soften the material.
Repeat if Needed: Give it a few extra pumps while coated in oil during the soak period to help flex the rubber back into shape.
Put Back in Service: Once the rubber is fully flexible and springy again, wipe off the excess oil. Fill the gas tank with fresh fuel and run the mower; the constant contact with gasoline will keep the rubber conditioned and pliable for years.
For a classic flathead Briggs, spending time trying to surgically clean, free up, or rebuild the tiny internal check valves hidden behind the primer port is rarely worth the headache.
Between cheap replacement carburetors widely available online and the fact that you can often harvest a good used one off a donor engine, swapping the carb entirely is the most efficient and practical route if the internal check valve is shot. Why waste time chasing micro-passages when you can just bolt on a fresh unit and get right back to mowing?
Thanks Max, Yes I wasn't going to bother too much with it, it is obviously a sticky diaghragm , so I will stick a kit in it and see how that goes. It belongs to a mate of mine, I wouldn't bother with it normally but I will get it sorted for him
Yes it can't be the Diaphragm Norm,Why it's not the diaphragm:If the diaphragm valve is open or closed the primer bulb should still return after being pushed , it must be a blockage on the pickup screen or blockage in the circuit to primer.
From memory that plastic fuel pickup tube in some of these has a check valve ball but most don't ,I usually remove the plastic tube under the carby and blow the tube and carby out with compressed air.
I used this drawing below to show the red fuel flow to primer on left side.
Thanks Max, If I pull the carby down I would give evrything a good clean so that should fix it. I always give the tube a good clean with carby cleaner. I was trying to avoid it because I couldn't be bothered
