How to Diagnose and Fix a Hard-to-Start Briggs & Stratton Lawn Mower Engine: Valve Adjustment and Compression Release
Is your Briggs & Stratton engine hard to start? Does the engine struggle to turn over, and it seems as though the starter must be bad or the battery dead? Is it time to pay big bucks for lawnmower repair? How will you get that lawn tractor to the repair shop?
Before you do anything too drastic, however, keep in mind that it may not be that serious of a problem. If this is an overhead valve (OHV) engine, then it may be that you have just overlooked a basic maintenance procedure. Sometimes you have an easy repair on your lawn tractor.
Most of the overhead valve engines have a compression release feature that allows for the starter to not have to work hard to crank the engine. It's a slight raising, or bump, on the lobe of the camshaft. What it does is open the valve a little early to allow excess compression to bleed off. At speeds over 300 rpm, the tappet just sort of floats over this bump, and it has no effect on engine performance. But at starting speeds, it's essential, otherwise the starter just can't turn the engine over.
If the engine will turn over easily and hiccups, won't rev or make power, or just won't start, then you may be looking at a fuel supply problem.
This guide will help you diagnose and fix your hard-to-start Briggs & Stratton engine.
How to Diagnose Your Lawn Mower Engine's Problem
To diagnose this problem, you will need a voltmeter.
First, check the battery voltage by using the voltmeter on each battery terminal. You should have about 12 volts.
Next, check the voltage at the starter with the key in the start position. You may need a helper at this point, depending on the equipment. What you have to do is ensure that all safety features are activated just as though you are starting the engine. With one lead from the voltmeter on the post on the starter where the cable connects and the other lead to a good ground such as the engine block, there should be about the same voltage as there was at the battery. A drop of a volt isn't a concern. If all this checks out okay, then it's a reasonable diagnosis to suspect the compression release.
Proper performance of the compression release requires proper valve lash adjustment. Adjusting the valves is a relatively simple project requiring only a few tools and once you learn how to do it, you should include it with your annual maintenance program.
Twin Cylinder Engines Require Treating Each Cylinder Separately
If you have a twin cylinder engine, you have to treat each cylinder separately. You have to bring each piston to its respective TDC to adjust each cylinder's valves.
- Ratchet with extension and 3/8", 7/16" and 1/2" sockets
- Feeler gauge in .004" and .006"
- 3/8" wooden dowel or substitute
- Torx or Allen driver to fit lock screw
How to Fix a Compression Release Problem in Your Briggs & Stratton Engine
- First, you need to access the valve cover. This will be the silver cover on top of the engine head. It will have OHV stamped into it and is held on with four screws/bolts. These screws are usually 3/8" or 7/16" hex head and are removed using a socket and ratchet. There is a gasket between the valve cover and head. If you are careful, you can usually re-use this. Then remove the spark plug.
- Now with the cover off, you can see the rockers, push rods, valve springs, and valve stems. The valves are the ones with the springs on them. If you turn the flywheel by hand, you will see the valves move up and down and see how the assembly works.
- Find the intake valve. This will be the one that the carburetor feeds fuel and air into. If you remove the pushrods, typically the intake valve will have an aluminum pushrod, while the exhaust valve has a steel pushrod.
- You will need to turn the flywheel clockwise until you see this valve open (move downwards) and then close again. Once the intake valve closes, the piston will be headed to the top of its travel, or Top Dead Center (TDC).
- At this point, take the wooden dowel and insert it into the spark plug hole. You should feel the top of the piston.
- Slowly turn the flywheel by hand while holding the dowel against the piston, and you will feel the piston move. Be careful doing this. If the dowel starts to become caught in a bind, back the piston up and remove the dowel. You only want the dowel in the cylinder when the piston is at or near the top of its travel.
- With the piston at TDC, rock the flywheel back and forth to find the highest spot of the piston travel using your dowel.
- Find a spot on the head that's easy to see, and mark the dowel where it lines up with that spot. Now remove the dowel and make another mark 1/4" above the first mark.
- The valve lash should be set with the piston at 1/4" past TDC. What you are doing is making a gauge to line up the piston travel with a reference point. Now put the dowel back in, and rotate the flywheel so that the piston is now 1/4" past TDC.
- On the rockers, the stamped metal pieces that open the valves, there will be a nut and a lock screw inside that nut. Usually this screw will be a Torx head, but whatever it is, loosen it and then loosen the retaining nut. You will notice the slack in the rockers, the amount of slack is what you are adjusting.
- For the intake valve, take the .004" feeler gauge and insert it between the valve stem and rocker. Then tighten the retaining nut until the feeler gauge will just barely slide in and out. It should be a snug fit, and the gauge should have some resistance when trying to pull it out.
- Then tighten the lock screw back down. That valve is finished.
- Repeat the feeler gauge process on the exhaust valve using the .006" gauge, and replace the valve cover.
- If the gasket tore when you removed it or it isn't in the best of condition, you can either purchase a new gasket or do what we do in the shop and use PermaTex Ultra Black to re-seal the valve cover. We also use anti-seize on the valve cover screws. Both PermaTex and anti-seize are available at any auto parts store, as well as the tools required. (Note: If you have a twin cylinder engine, you have to treat each cylinder separately. You have to bring each piston to its respective TDC to adjust each cylinder's valves.)
- Once you have the valve cover back on, you're ready to start the engine. Don't worry if it's still doing the same as before. It usually take a few tries to do this right if you've never adjusted valve lash. Just start over and try it a couple more times if necessary. If you can't get it right after several tries, you may have a different problem which will require further diagnosis.
Thank you for reading this, and I hope it is of some help. Several people have sent me messages asking for further help in diagnosis. But please understand, due to constraints within this type of forum, there is just no way for me to do that here.
What is your level of mechanical aptitude?
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.