Compression, Air, Fuel, Ignition

[Merkava_4]

An engine needs all four of those to run. Remove one of those four, the engine will not run. All four of those need to be in the correct quantities too. Out of those four, the Fuel is the most likely element to be in insufficient quantity. The other three are relatively constant and less prone to being removed from the equation. A spec of debris blocking the carburetor jet will put the engine out of commission. Fuel contamination is the number one cause of mowers not running. Not all mowers can have an inline fuel filter. If they have a tank mounted carburetor, there's no place to put an inline fuel filter. The responsibility falls on you to keep the fuel clean.

If your mower won't start, check the following:

1. Fuel delivery - at the correct amount
2. Air delivery - at the correct amount
3. Ignition
4. Compression

A compression gauge and a spark tester are valuable tools to have on hand.

Your engine needs compression in order to suck the air through the carburetor and also to compress the fuel/ air mixture once it gets there.

Your engine needs approximately 12-14 parts of air to 1 part of fuel to sustain combustion.

Finally, your engine needs ignition to ignite the fuel/ air mixture.

I encourage you guys to read up on carburetors and the principles they operate by. Basically, they work off the principles of vacuum and pressure. They must provide air and fuel to the engine at the correct ratio at all times.

 

[PhilsSmallEngine]

If you have a tank mounted carburetor and you have an approximately 2" opening, use a tank drop in screen like this one from Honda:
Honda 17672-880-000 on Repair Clinic
I've used this one with great success on a number of my Briggs engines with a tank mounted carb without the foam insert. This keeps the big chunks out of the tank.

Compression testers are good to a point. Most engines have a compression release so you won't read much over 60psi on most engines before the release kicks in. The new Briggs Ready Start doesn't even show compression until the flywheel hits nearly 250rpm. It's making the job of checking for compression quite difficult.

The rest is very important and I agree to the order of checking unless the recoil has no resistance.

 

[Merkava_4]

I forgot about the Ready Start compression release. Will compression show up if I turn the engine over fast enough with a drill? I've done that in the past when I didn't have a spark tester.

 

[PhilsSmallEngine]

You'll get the approx. 60psi as long as you exceed the 250rpm. A drill on the flywheel will give you the speed you need for as long as you need for a reading. It still isn't a great gauge of compression though. My general rule of thumb is if I find ~60psi, the engine is most likely workable. It's not foolproof, but it's the best basis I've found.

The other caveat of the Ready Start is that the ignition is controlled by a microcontroller which creates problems checking for spark. Briggs really has created a troubleshooting nightmare for technicians. It's like working on a car from the 1980's. Not enough data to easily determine the problem easily.

 

[Dean85]

Originally Posted By: Merkava_4
An engine needs all four of those to run. Remove one of those four, the engine will not run. All four of those need to be in the correct quantities too. Out of those four, the Fuel is the most likely element to be in insufficient quantity. The other three are relatively constant and less prone to being removed from the equation. A spec of debris blocking the carburetor jet will put the engine out of commission. Fuel contamination is the number one cause of mowers not running. Not all mowers can have an inline fuel filter. If they have a tank mounted carburetor, there's no place to put an inline fuel filter. The responsibility falls on you to keep the fuel clean.

If your mower won't start, check the following:

1. Fuel delivery - at the correct amount
2. Air delivery - at the correct amount
3. Ignition
4. Compression

A compression gauge and a spark tester are valuable tools to have on hand.

Your engine needs compression in order to suck the air through the carburetor and also to compress the fuel/ air mixture once it gets there.

Your engine needs approximately 12-14 parts of air to 1 part of fuel to sustain combustion.

Finally, your engine needs ignition to ignite the fuel/ air mixture.

I encourage you guys to read up on carburetors and the principles they operate by. Basically, they work off the principles of vacuum and pressure. They must provide air and fuel to the engine at the correct ratio at all times.



Good explination. The first thing I check when working on a small engine that will not start is the air filter (if equipped) then spark. If no spark, replace spark plug. If new plug does not work, then I check the ignition system. If spark resumes with no start, then fuel is replaced. If engine still does not start, carb is cleaned and compression is checked.

 

[Astro14]

I would add timing. Though it doesn't often change on small engines, I once had a Ford 302 that stopped running right. It had fuel, air, compression and spark. All were perfect.

But the timing chain had jumped a tooth...so both spark and valve timing kept the engine from running...

 

[Clevy]

12-14/1 air/fuel isn't right. You can run engines a bit richer and much leaner.
My bike goes into lean burn once it senses minimal throttle inputs and speed stabilized and iirc it's 17/1 or leaner.
107hp-115tq and 50mpg on the highway.