OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines Briggs and Stratton Quantum 45 - Popping!

I managed to aquire another fairly decent Briggs & Stratton Quantum 45 Engine on an alloy Rover Base. I stole this photo from my last post however it is exactly the same engine. I have not got the codes on me right now however I will put them up when I can.
The mower would start when I got it so:
1) Thoroughly cleaned the carby and replaced the bottom washer as it was leaking fuel.
2) Fixed the primer system as it was blocked up with gunk.
3) Replaced the air filter as the old one I don�t think had ever been replaced.
4) New Spark plug/ blades etc
5) Thoroughly cleaning the mower with compressed air to remove all dirt build up
After a couple of minor adjustments to the carby it usually starts first or second pull after priming. I went and mowed my lawn however I get this strange popping sound from the exhaust. The sound is only light and to most probably not noticeable however I want to make sure it is spot on.
From what I have read to date I am thinking a leaky exhaust valve might be the culprit as the next thing on my list was checking the tappit clearances. Could someone please confirm the clearances for me and what would normally be considered acceptable for an engine of this age noting I don�t expect it to be perfect.
I know how to check the tappit clearances but if we get into fixing valves I will need some guidance please as I haven�t done this before.
The engine idles and has a nice full speed sound.

That is a 120000 series engine, so the valve clearances are intake 0.005-0.007", exhaust 0.007-0.009". As you said, any problem that matches your symptoms is more likely to be with the exhaust valve than the inlet. Remember that one possibility is a loose exhaust valve seat, which may not show up in the clearance measurement.

Remember that exhaust popping can be due to lean mixture, incorrect ignition or valve timing, or weak ignition. Bear in mind also that side valve mower engines usually miss an occasional shot, especially at light load or idle, though this should not be accompanied by an exhaust pop.

Model 12F802 Type 0875-01 Code 95082359
When I changed the plug and it did run a little better but the light popping sound was still evident once the engine warmed up. So I went on to remove the exhaust and checking the tappet clearances.
Exhaust - .15mm (.0059inch�s)
Inlet- .04mm (my smallest gauge) fit in however not all the way. � (.00157 inches)
I was very careful with my measurements and ensured both the inlet and exhaust clearances were measured with the engine a smidgen before TDC.
I would like to clarify the engine now starts on first pull and runs pretty well. It idles nicely and has smooth full speed. I have not put the engine under a heavy load yet.
Like I said I haven�t pulled valves out before however I have watched video�s of the process that included valve lapping and grinding.
I await your thoughts.

First, your engine was made on 23 August 1995, which is long before the Chongqing plant opened - and initially it made only horizontal shaft engines anyway.

Here is your Illustrated Parts List:
http://bsintek.basco.com/BriggsDocumentDisplay/18hruJXHnfBhU7y.pdf

The inlet tappet clearance is clearly inadequate, and the valve may not even close fully after warmup. The exhaust is also below specification. I suggest you should remove the valves, lap them into the seats, and grind the ends of the stems slightly to increase the tappet clearance, but just to the required levels, no more. Be careful: the ends of the stems must be accurately kept square to the axis of the stems.

Inspect the valve seat inserts, both for cracks and looseness. The iron inserts are just riveted by slightly upsetting the aluminium around them, and it is not especially unusual for them to come loose if the engine has been substantially or repeatedly overheated. If this happens it is fairly common practice to indent the aluminium around the outside of the seat slightly, usually with a center punch, to deform the aluminium to bring it into contact with the outside of the seat. However don't do this unless the seat is loose: Briggs does not approve of doing it at all. If it has to be done, let's talk about it more before you actually do it. There is a much better way than by using a center punch. Also, it is important to end up with the seat square to the valve stem, and this probably won't happen if you just rivet it with a punch.

Thanks Grumpy,
I dont have a valve spring compressor so could you explain the process behind correctly removing the valves and springs without damaging the engine. I am told it can be done however it is a bit fiddly.

I will have to buy some valve grinding paste. I have a bench and hand grinder so that should cover me off with grinding the valves. I also have the lapping tools.

Any pointers you can provide would be greatly appreciated as this one will be a learning experience for me.

Ihope this engine works out!

Removing the valves is simple in principle but a bit fiddly in practice. With the small Briggs engines the valve retainers are normally of the keyhole type, thank heavens, and the valve springs are very weak (small valves and low maximum speed makes this possible). Essentially you need to push the retainer up while holding the valve from rising, thus compressing the spring, which leaves the valve stem projecting through the retainer. You can then push the retainer sideways across the keyhole, to the larger, off-center hole. Once you do that, stop compressing the spring, and the retainer will be pushed off the stem by the spring. You sometimes need to hold the valve down while pushing the spring up. The cylinder head needs to be off, to give you access to the valve head so you can hold it down.

Putting it back together is more fiddly than taking it apart, because you have to push the off-center hole over the valve stem, then push it sideways until the stem aligns with the smaller hole, despite the whole area being full of fingers so you can't see what you are doing. Joe Carroll has found the answer to this. You use two ordinary cable ties to hold the valve spring compressed while you install the spring and then put the retainer onto the valve stem. At that point you cut the two cable ties, freeing the spring.

A valve spring compressor is not needed for mower engines because the springs are so weak they can easily be compressed with two fingers and a thumb. If it weren't for that piece of good luck you'd have to use a special side-valve spring compressor, a simple example of which can be made out of a small piece of 16 gauge plate with a hacksaw and a drill.

Giday Jaffa J and Grumpy, I use both of these tools and on small side valve engines I use the smaller of the tool mostly,it makes for a more easier and less fiddly job and the larger valve-spring compressor on larger engines like ride-on mowers.

Thanks roebuck, I looked at the lower one and misunderstood it initially, I thought it was the type that works by levering against the engine block. However it is cleverer than I thought. I now believe you rotate the retainer until the keyhole is pointed away from you, then just press the upper side of the tool against the underside of the retainer, lift it slightly, then pull it toward you. That will move the keyhole across to the release position so it falls off when you relax the upward pressure. As long as the valve spring is weak, it should be dead easy. You can put it back on just by reversing the procedure. You do need to carry out a spot-facing operation to make the tool though, so making their own might not suit people who don't have suitable cutters.

I once bought a heavy cast iron version of the upper one, on a scale suitable for rather large engines, but threw it out after a few years, having never used it. (It came in a load of mixed blessings I bought at a clearing sale for a couple of bucks.) In this case, Briggs recommends the upper one, which is their part number 19063, for all of their side valve engines:

Thanks Guys, I appreciate the comments.
I didn�t get a lot of time on the weekend as I was away most of the weekend however I did get time to pull the head off and have a look inside the cylinder to check its general condition. I am confident I will be able to get the valves off without too much trouble.
Secondly I have been doing some reading about the grinding of valves. Some suggest using a hand file and some suggest using a grinder. I personally would be more comfortable using a hand file to ensure the valve end is kept square and to ensure I don�t take too much off the valve end.
Do you have any recommendations on the valve grinding method to ensure I don�t take too much off the valve end? I would like to further understand the process behind remeasuring the clearance after grinding. Will I have to completely reinstall the valves, valve springs & retainers before checking the clearance or do you have a trick for this process.

Photo 1 � a shot of the valve chest open
Photo 2 � Shot of the engine with the head off (note the carbon build up on the cylinder) It was fairly soft should be easy to clean
Photo 3 � A shot of the engine head and gasket (which should be easily re-usable, no sign of leaks from what I can see.

There is no way you could have much effect on the tip of a valve stem by attacking it with a hand file. The stem is nearly as hard as the file and much, much tougher. You could do the job with a hand-held diamond lap, or with an oil stone, but I think a bench grinder is quicker and will do a better job. There are two key considerations.

First, the tip of the stem has to be square to the axis of the stem.
Second, you must not allow the stem to heat up.

The easiest way to do the job, though don't expect a TAFE instructor to like it, is to clamp a short piece of steel about 10 mm square with a straight edge, to the grinder rest, so the edge is exactly square to the side of the wheel. Then slide the valve stem along the edge of the piece of steel so that it bears lightly against the side of the wheel. Using the side of the wheel is strictly forbidden of course, not for safety reasons but because you can only groove it once, then you can't dress it without wasting a whole lot of wheel. However for this type of job I personally do it that way. Remember, only light brief contact between stem-tip and wheel, then stop, long before the stem starts to heat up. Remove any trace of a burr from the stem tip with an oilstone. Then wipe the stem to make completely sure there is no abrasive on it (from the grinding wheel) and try the valve in the engine, just by dropping the valve into the guide, holding the valve head down with your fingers, and checking the tappet clearance with a feeler gauge. Don't bother to assemble the spring and retainer, it wouldn't help, it would just waste time. Then if the clearance is still insufficient, go back to the grinder and touch the wheel lightly for another couple of seconds. Repeat as many times as necessary to get just the right tappet clearance.

Remember, you need to inspect the valve seat inserts to check that they have not been moving in the cylinder casting.

Last night I removed the valves, valve springs and retainers without too much trouble. It was a little bit fiddly however nothing that couldn�t be handled with a couple of small screwdrivers.
I firstly inspected the valves where is makes contact with the valve seat and no pitting or cracks were present. It was actually in relatively good condition.
The valve seats looked good as well. They were not loose or pitted in any way from what I could see.
Your explanation of how to remove the valves was spot on, so thanks for that.
To be comprehensive I again measured the tappet clearance before removing the spring noting how sensitive the position of the piston can be in relation to the tappet clearances. The clearance of the inlet valve was .10mm (.003inch�s) which is still below minimum specification (.005-.007). Remeasuring the exhaust valve did not change. It came in at .15mm (.0059 inch�s).
We spoke in a previous thread about the measurement of tappet clearances, that being positioning the piston a little past top dead centre on the compression stroke. Do you have any extra hints to make sure I have the positioning as close as possible?
Secondly I need to confirm whether or I should lap the valve before I grind the stems?

First, you should measure the tappet clearances with the crankshaft on the compression stroke and rotated clockwise (viewed from the flywheel end) until the piston has descended 6 mm down the bore from its highest position (Top Dead Center). It would not matter if you moved the piston down the bore a bit less or a bit more than 6 mm: the inlet valve should be properly seated by TDC, though sometimes the tappet clearance is a bit scant until you get a bit past TDC, then nothing happens to the valves until the piston is more than halfway down, when the exhaust valve starts to open. However Briggs says 6 mm down, as a standard practice for all engines including those with decompressors, so it is best to follow that instruction.

Second, when you lap the valves you remove a small amount of metal from both valve head and valve seat, to get a smooth continuous contact surface between the two. Removing metal moves the valve downward onto the seat, which reduces the tappet clearance. Hence you should always measure the clearance after you finish the lapping job and are satisfied with the outcome.

Thanks for the concise explanation regarding the engine being at TDC on the compression stroke
Last night I lapped and ground the valves. Both clearances are now within the desired range.
I also cleaned the valves and cylinder head of excess carbon. The valves sit in the seats nicely and do not wobble from side to side.
I don�t expect re-installing the valves will be difficult noting Joe�s trick with the cable ties however I just wanted to make sure I hadn�t missed anything before I go and re-install the valves, springs, retainers, chest cover and cylinder head.
Once you confirm I will go ahead and finish the job tonight! Lets hope it works.

Remember that we haven't proved that the problem was due to valve leakage. Leakage fit the symptoms, especially in view of the very small tappet clearance, but the proof of the pudding is in the eating. The next step is to put it together and see how it behaves.

Remember to use a decent gasket on the valve chest cover/PCV valve. I re-use old ones if they are in good condition, but Briggs does not approve of my miserliness. However a slight leak in that gasket will only cause oil leakage, no other change in the engine's behaviour.

Well it ended up taking a little time and fiddling to get the valves, retainers and springs back in however Joe�s trick with the cable ties worked just about perfectly. To help out others I found using 3 ties evenly spaced around the spring helped ensure the retainer didn�t pop out of position when I cut the first cable tie. A pair of long nose side cutters would also make the job a little easier however I don�t see the investment as being worthwhile.
I checked to make sure the valves were not sticking before re-installing the valve chest cover and engine head. I also re-checked the clearances with the springs installed and they fell within the correct tolerances.
After putting the exhaust back on, it fired up on the first pull. I let it run for about 10 odd minutes (idle and full speed) to see if I got the same popping sound as previously noted and������.. no popping from the exhaust. Just the occasional side valve misfire.
I won�t count my chickens until I put the engine under some reasonable load however I am fairly confident the engine should be good to go.
Whilst I now have a reasonable understanding of completing a valve grind and lapping job I am still curious how this removed the popping sound from the exhaust. I am assuming it is due to the valves not opening and closing as they should however I wouldn�t mind satisfying my own curiosity if you would care to share.
Again I really appreciate your help with this one.

Popping from the exhaust usually comes from either incorrect pressures, or lack of exhaust valve sealing. Wrong pressures can imply things like the wrong camshaft timing, the wrong ignition timing, abnormally slow combustion, or high back-pressure in the exhaust port.

If the engine starts properly and runs properly except for the odd pop here and there, the main things to focus on are poor valve sealing, or high back pressure. High back pressure would come from something like a clogged muffler. When the muffler is clogged the power is way down, and the exhaust valve will have been very hot. Overheated exhaust valves have no carbon on them, and are a distinctive flat grey-black colour.

Before we got as far as looking for some of those issues, you reported fairly substantially wrong tappet clearances. It can't be expected to run properly like that, tight tappets are very likely to cause leaky valves, and it isn't difficult to fix the tight tappets, so the first step is to fix them. If the problem had persisted after that fix, it would have been time to remove the muffler, inspect it, and run the engine without it to see if the problem went away. If it hadn't been the muffler, we would have continued down the list of possible causes until we found it. Meanwhile we'd have been making it a better engine.

So, a trade secret: sometimes I'm not actually sure the first thing we look at will turn out to be the problem. I just try to start with the things that are both the most likely to be the issue, and the easiest to fix. Most car mechanics did the same thing, in the days before OBD (on-board diagnostics, a feature of all modern powertrain computers) and plug-in electronic diagnostic testers that read the OBD records and report the fault conditions that have been recorded while the car was being driven by its owner.

Thanks Grumpy. I appreciate the detailed explanation.
I gave the mower a good test on the weekend and didn�t encounter any problems at all. It is starting first pull from cold so the primer system is now working spot on.
Grinding and lapping the valves in this instance seems to have solved the problem.
The mower has now been fully serviced and runs spot on for a mower of this age. This actual engine is really smooth running and relatively very quiet.
I think this thread can be closed.
PS. I have not forgotten about the Suzuki we were working on. I have been following your other thread and have the relevant parts soaking in carb cleaner and ready to be re-assembled tonight!

Thanks for the update, and the good news. My experience with Briggs engines is similar to yours: reliable, smooth running, easy to start, and generally easy to service and repair. Whether fortunately or un-, I took and interest in OHV Hondas a couple of years ago, which led me to dispose of my Briggs mowers and get a couple of Hondas. To me, they are as big an improvement over a side valve Briggs as the Briggs is over a Kirby-Lauson. As I've said in another thread, it's a bit like comparing a Commodore with a BMW: the Honda is way more money for something that is functionally the same, it just runs so well it makes me feel good. I wouldn't contemplate buying one in running order though, the value for money just isn't there.

I'll close this thread. If anyone needs to post to it, just send a PM to a moderator.