Valve Adjustment, Compression Release and a Hard to Start Briggs and Stratton Engine
90Briggs and Stratton Valve Adjustment and the Automatic Compression Release
Is your Briggs and 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?
It could be that it's not that serious of a problem.
If this is an Over Head 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 which 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 300rpm, the tappet just sort of floats over this bump and it has no effect on engine performance. But at starting speeds, it's essential or 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 and you need to read the article on basic fuel system and carburetor repair.
Find the Problem
To diagnose this problem, you 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.
Getting Started
Tools Needed
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
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 how the assembly works. Find the intake valve, this will be the one that the carburetor feeds fuel and air into. Should 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.
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The Fix Is In
With the piston at TDC, rock the flwheel back and forth to find the highest spot of the piston travel using your dowel. Then 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, so 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 there's 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, the gauge should have some resistance when trying to pull it out.
Then tighten the lock screw back down and 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 of 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.
Please 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.
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Thanks for reading it.
Thank you, very helpful and save me $$. I appreciate it, best written article on the subject I could find.
Thanks! I'm glad it helped.
does the piston need to be in same place for the intake and exhaust valve to be adj.?
Hi, yes, the engine needs to be at 1/4" past Top Dead Center (TDC). At TDC, the intake valve has just closed and the exhaust has not begun to open. The piston must be at 1/4" past TDC, so be sure to follow the instructions above for how to find that spot.
Thanks a ton. Brought my Rider in twice to have the issue looked at. They just kept telling me the battery was bad. It took me 15 minutes start to finish and the engine starts and runs like a top. Yee Ha!
Glad it was of some help. It's surprising the number of shops which fail to catch this one.
The engine on my Craftsman lawn tractor would only crank once and not start. I was thinking it was the starter but found your article, followed the steps, and it started right up. Thank you.
That did it! Intake valve adjustment was .oo6" instead of .004", once adjusted correctly it made all the difference in the world.
Thank you for posting!
Have had starting problems with my Sears riding mower for over 2 years. Last summer (2010) paid some small engine "expert" $180, and he replaced the starter and battery. Same problem after he left.
Saw your article and tried it. Had never adjusted valves on any type of engine. Took me a few tries to "get the feel" of what I was doing, but finally put the valve cover back on and gave it a try.
ENGINE STARTED IMMEDIATELY AND CONTINUES TO DO SO. Thanks very much for your valuable info.
Glad it helped ya'll. Remember to include valve adjustment as an annual maintenance item.
thank you, Exactly the problem .
This is so true. Ive been building motors for my late model for 18 years and winning on good nights. Never thought my 14.5 BS had a valve lash issue. Ive been starting it with a jumper box all f n year. 20min an it is back like new. I used a pencil for the TDC -1/4 worked great. Thank you saved me buying a rider. One time a rain out at all three tracks was worth it.
My mower started doing this very exact thing this week. I went thru the entire electric from battery, sol and removing starter to test it. Removed plug engine spun fine, replaced plug and as soon as it hit compression it stopped. I can jump it with a jump box it does fine. Tomorrow I will do the above to the letter I will let you know. Thank you for your article. I feel sure its this.
Why do you have to continue to move the piston another 1/4 of an inch past TDC ? Is this minor movement going to make a difference ? Thanks
The way the camshaft is ground requires you to move the piston 1/4" past TDC so that you have an accurate lash. Adjusting at TDC on many Briggs engines will give you too loose an adjustment. Other makes, and some Briggs engines, can be adjusted at TDC.
Thanks for the tip. I replaced the head gasket on my craftsman lawn tractor, and afterwards it was hard to start with the symptoms you described. Did the procedure, and it works like a champ. Hardest part was finding a feeler gauge. (Went to Walmart and Ace before going to an Auto parts store).
I will try your procedure. My initial problem was a broken push rod. I put it all back together but it only ran for a minute. . . Roughly at that. One Q though; The valve shafts are grooved circumferencely. If the valve shaft is supposed to be locked in and move only within a certain range, is it possible that they can move out of place within a range of travel resulting in a gap so wide it can't be adjusted to spec.? The rocker arm then flops around all over the place. More after I try the valve lash adj. Thanks for your help.
(Engine is a B&S model 310707, type0136-E1, 16HP Sears riding mower)
The valve stems shouldn't have any lateral movement, only up and down. If the valves are moving around side to side, then the guides are probably bad and head replacement is the best fix.
I adjusted the valves to .005" at TDC and then checked the valves a 1/4" past TDC and there was no difference. The engine runs fine.
Hi, I can't really diagnose in this forum. If the engine is still hard to turn over then you may have another issue or even a worn cam. It would require a good bit of back and forth to work through it and I'm just not able to do that here. Thanks, PK.
Finally someone explains why it needs to be 1/4 past TDC.
I've always been curious.
Hey there, I am a 15 year veteran automotive technician with much experience in small gas engines. I had never ran into this one and the post helped a bunch.
Thanks!
The muffler was glowing red on my old Briggs. Is this because the valves are out of adjustment ?
Hello,New to this but how do you know what valve is the intake and what valve is the exhaust?Is the valve closest to the top of the engine the intake or exhaust?
Thanks,Nick
Hey Nick. The intake valve is usually smaller and is in the block closest to the carburater. The exhaust valve is usually larger and in the block closest to the muffler. When your asking if the valve is closest to the top... Im gonna say your describing the intake valve.
Sorry Nick. The intake is usually larger and exhaust is usually smaller. The valve your describing on the top , is the intake.
Hi, I can't do diagnostics on this site due to an agreement with another site and the format of Hubpages really doesn't allow well for it anyway. I will tell you that the muffler glowing red is probably an overly lean condition in the carburetor. Pull the spark plug and it's probably gray and ashy. This would mean you have a carburetor that needs to be rebuilt or replaced most likely, although an air leak could also cause this. There are a few other things that could be responsible but that would require more indepth diagnostics and I just can't do that here. A carburetor issue would be the most likely though.
If you look at the intake manifold, you should be able to see that it is in line with the intake valve, same for the exhaust valve and the exhaust manifold.
I have also seen the valve lash adjusted by rotating the engine until one of the valves is completely open, then adjusting the lash on the valve that is closed, then repeating for the other valve. How is this different than the method you explained? Thanks
On the Briggs engines it's important that you are beyond Top Dead Center and have the cam at the right position, which is the reason for going 1/4" past TDC. Using the method you mention, it would be difficult to know just where on the cam's profile you are. Having one valve completely open would have the other at the same position as at TDC so you would have to try to factor that in on each valve. Not that it couldn't work, just that the method in the article is the easiest way and insures both valves are adjusted in phase.
This is what you've stated..."Having one valve completely open would have the other at the same position." Its impossible to have both valves "completely" open. Unless theres a problem with the engine timing.
My statement reflects that if one valve is completely open, then the other valve would be at the same position as it would be at TDC, meaning it would be closed. Not that both valves are completely open. The inherent problem with using this method on Briggs engines is that the valve needs to be in phase on the cam with the piston 1/4" past TDC. The method of adjusting valves with one fully open and adjusting the other valve is an old method used on many pushrod engines. You simply bring one valve fully open, rock the crank back and forth to find the loosest spot in the other valve's lash and then adjust. Then repeat for the other valve. People have been using this method for decades on all sorts of engines, works pretty well for multi-cylinder engines when you don't have a book to tell you what cylinder corresponds to the other.
I had the same problem with my 16.5 Hp Briggs engine.
The instructions here are excellent and helped me solve the problem.
However, not before frying a starter motor.
I bought a new starter from dbelectrical.com ($43)and my tractor starts like new.
great post. Had to attempt it about 4 times before i finally made progress. In the end i wound up setting the exhaust at .008 instead of .006 and the intake i set at .005 instead of .004. After setting it this way my flywheel finally purred over nice and smooth without the help of a jump start from the trusty F100. This is something i will make sure to keep on eye on from now on. Btw, on my troybilt (2002, 18 hp ovc) my exhaust valve was on top as i faced the engine. I could tell because the exhaust pipe was attached at the top, to the left of the valve, so i assumed it to be exhaust.
You said in the end you adjusted the valves at .008 and .005 . What else did you find wrong ? Your saying that the engine was hard to turn over. The valve adjustments being out that little wont make the engine hard to turn over.
Just for your information... .002 of an inch is less than a piece of paper. Not very much.
.002" is enough to make a difference with the compression release. Although setting the valves tight is preferable to loose for performance issues. That said, not all OHV Briggs engines use the .004 and .006" specs, just most. I wrote this referring to Lawn Tractors specifically, which when having OHV Briggs engine, will have Intek engines 99% of the time and use the .004-.006" lashes. If in doubt, just refer to the Briggs chart at this link, http://www.nnyparts.com/briggsspec.pdf
I think your referring to .004 and .006 as being the limits. .004 being the smallest and .006 being the largest setting. Like I said before... I adjusted both valves at TDC to .005" (middle setting) and the followed your proceedure of turning the crank another 1/4 " past TDC and my original settings of .005" were the same. I found no difference.
If .002" is going to make a difference why are the specifications between .004 and .006 ? These specs mean your adjustments can be either .004 or .006 or anything in between. Which is a .002" difference. This is why I set mine at .005" It runs fine.
.002" is enough to make a difference when you are adjusting valves. Set the intake lash at .002" when the engine likes .004" and most engines will start popping as the intake begins to open several degrees early.
Valve lash is about when the valve starts to open. Too loose and the valve opens late, too tight, the valve opens early. This is why most people have to try several times before they get it right. The reason you turn the engine 1/4" past TDC is because of the cam's profile. This is how Briggs designed the system, and how the valves on these types engines are adjusted. The slight rise in the profile that creates the compression release does wear which will change the measurements, which is another reason why you adjust at 1/4" past TDC.
Your missing my point. If the manufacturer is giving me specifications like .004" .006" for the valve adjustments it means a minimum of .004 and a maximum of .006 . When I went to school they told me there is a .002" difference between those 2 measurments. Why is the manufacturer saying .002" wont make a difference but your saying it will ?
Think about it like this. As the lobe on the cam turns and the lifter follows it, the lobe begins to lift the lifter. If your lash is too tight, the valve will be opening early on the lobe's travel. If it's loose, it will open late on the profile.
From .004-.006" is the amount of lash required to open the valve at the proper time on the lobe. Add .002" to the gap and the valve will open many degrees later on the lobe than if it were at .006". The lobe has a very sharp rise once you enter the ramp so even .002" will be a large change. From .004-.006" in this case starts the valve opening at the proper point on the cam lobe.
Valve lash is an adjustment made to control the opening and closing timing of the valves. The valves have to start to open and close within a narrow window on the cam's profile. Most people think valve lash is set to control the total lift on the valve, but it's not, even though lift is affected. It's all about the proper timing of the valves.
The manufacturer does say .002" makes a difference, attend any Briggs training school and they will cover clearances repeatedly. .004-.006" is the acceptable range in this instance. .007" would be out of spec.
Yes .002" would make a difference if the adjustment was "out of specifications" like for example .007" like your saying. Im not saying that though. What Im saying is I can adjust the valves at .004",.005",.006" or anything in between .004" and .006" (which is a .002" difference) and according to the manufacturer they say this is fine because these are the actual specs. their recommending. What your telling me is this .002" difference is not acceptable. Tell me what you would set the valves at if the recommended specs. were between .004" and .006"
Bob, you said, "You said in the end you adjusted the valves at .008 and .005 . What else did you find wrong ? Your saying that the engine was hard to turn over. The valve adjustments being out that little wont make the engine hard to turn over." and then you said, "Just for your information... .002 of an inch is less than a piece of paper. Not very much.
So you are directly contradicting yourself. You can try to recover by changing your story but you really need to accept that there is a proper procedure and that being precise matters. It's most likely that you are not being precise and therefore aren't really sure what your actual lash is.
This will be the last post of yours that appears on this page no matter how many different names you try to use. I have not allowed some of your other posts to appear because of your tone and am afraid that you bear the distinction of being the first I've had to do this to. This is not a forum to argue, it's an information forum. If you would like to adjust your valves however you like, feel free. However Briggs has a particular method they feel should be used and they would tend to know best. I've explained why the valves are set the way they are set repeatedly and that's not my recommendation, it is how Briggs designed the engines.
Attend a few Briggs training classes, and feel free to argue with them and try to convince their Engineers they don't know how their engines are put together. I've been attending factory small engine training since the mid-eighties and have yet to find a reason to tell a manufacturer that they don't know how to adjust their valves.
Cheers to that. Small engine mechanic and was needing exact specs.. Thanks Ill post 2morow and report results!!!
for the record pk, im not billy bob. I'm not gonna argue with somebody whose info saved my bacon and my dough. For whatever reason a slight adjustment to the formula was what it took for me. Had it not been for your posts, i would have been a sad sad pupy about my troybilt. You just keep doin what you're doin and i'll be happy to read what your doing...cheers to ya!!!!
Sorry BillyBladez, I didn't mean for it to sound like you were Bob. Bob has used several different usernames to try and get posted and continues to try and make insulting comments everyday. I was letting his posts without the name calling and insults through but enough is enough. Good luck with your TroyBilt.
I BOUGHT THIS ENGINE USED AND IT IS 16HP ohv. WHEN I PUT THE ENGINE ON THERE, I CHECKED THE VALVE LASH.IT HAD VERY LITTLE.I couldn't even get a .002 gage to slide between the rocker and the pistom was up right.This engine is running good.Should i just it .004 and 006 what it calls for or leave it like it is? Kneed some help THANKS
Robert, I'm afraid I cannot do diagnosis on this site. But, if this is an Briggs Intek engine, then use the link to the Briggs spec sheet, http://www.nnyparts.com/briggsspec.pdf, and check your model number against that chart. The reason for using the factory adjustments is to prevent excess wear and ensure your valves are properly timed. Outside of the factory numbers, even by .001", will add wear to the cam and rockers as well as reduce performance. At .002" lash, the valves are opening too early and staying open too late, IF .002" lash is outside of the Briggs specs.
This will cause higher fuel usage as well as a loss of power. On a 16hp engine you may not easily notice the difference, however put it on a dyno and you're probably losing 1/2 to 3/4 hp, which in a small engine is quite a bit. Also air-cooled engines are very dependent upon proper valve and ignition timing for cooling and the prevention of carbon build-up.
For maximum performance on a stock engine, adjust the valves to the tight side of the specification and adjust your ignition timing by making sure that the coil is no more than the thickness of a dollar bill that's been folded three times. By having the coil tight to the flywheel like that, you advance the spark by maybe a couple of degrees which improves the fuel burn.
Remember, the lash adjustments are to ensure the valves are opening and closing at the proper time on the cam lobes. Too loose on the valve that provides the compression release and you also will either have no compression release or it will be reduced, causing unnecessary wear on the starter.
When i ajust the coil to flywheel to.010 is this not the airgap ?
Ignition timing is keyed to the crankshaft and flywheel.
You can change the spark timing slightly by changing the air gap of the coil to the flywheel. Tight to the flywheel gives you a little advance, further away from the flywheel retards the spark. On a stock engine, gaining that little advance improves performance. On most Briggs engines you can't back the coil up far enough to keep it from firing, but the spark may be late enough to cause hard starting and make it run a little weak.
The position of the flywheel on the crankshaft is the primary timing. On a performance engine we will either use offset keys to adjust advance or not even bother with a key and just adjust it according to dyno results. The key is nothing more than an alignment tool, it doesn't hold the flywheel in place, but offset keys make adjustment easier.
You typically won't see any advantage to advancing the flywheel on a stock engine, but by tightening the coil to the flywheel you may. The gains are small, but every little bit helps.
Any one know the valve lash on this model 2bu706 16hp ohv. I cant seem to find it on the chart. I appreciate the help. THANKS
Robert is that the First part of the Family number or the Model number? If that's a Briggs engine, the model number is stamped into the valve cover or on the sheet metal.
electricsky 2 years ago
Thank you for your article